US2378388A - Recording and reproducing device - Google Patents

Description

June 19, 1945. 5. J. BEGUN 2,378,388

RECORDING AND REPRQDUCING DEVICE Filed Jan. 1, 1942 4 Sheets-Sheet l 6 M fquAuzzR MODULATOR AMPLIFIER FILTER AMPLIHER BP'HLTER June 19, 1945.

5. J. B EGUN RECORDING AND REPRODUCING- DEVICE Filed Jan. 1, 1942 4 Sheets-Sheet 2 88w Ra June- 1 9, 1945. s. BEGUN 2,378,333v

RECORDING AND REPRODUCING' DEVI CE" Filed Jan; 1, 1942 4 Sheets-Sheet 3 ffi 3 I ZTTORNEY June 19, 1945. 5. J. BEGUN 2,378,388 I RECORDING AND REPRODUCINGI DEVICE Filed Jan. 1, 1942 4 Sheets-Sheet 4 ATTORNEY.

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Patented June 19, 1945 RECORDING AND REPRODUCING DEVICE Semi Joseph Begun, Cleveland Heights, Ohio, as-

slgnor to The Brush Development Company, Cleveland, Ohio, a corporation 01' Ohio Application January 1, 1942, Serial No. 425,304

31 Claims.

This invention relates to systems and methods for studying transient phenomena and more particularly to such systems and methods for recording on a recording medium a transient circuit condition and utilizing the record of the transient for studying the characteristics of the transient. v

Among the objects of the invention are novel systems and methods of the foregoing type which operate to make a continuous obliterable record of the circuit conditions under study on an endlessly-operating cyclically-effective recording medium, such as a magnetic recording medium, so that the circuit conditions are continuously recorded; the continuous recording operation being accompanied by a continuous obliteration of the recordings and the obliterating action being stopped upon the occurrence of a transient condition so as to retain on the recording medium a record of a signal corresponding to the transient occurrence and the recording operation being discontinued after the transient occurrence has been recorded on the medium in such manner as to enable the study of the transient occurrence by cyclically reproducing the record and utilizing the reproduced record for operating a cyclically-operative indicating device, such as an oscilloscope, which presents an image corresponding to the reproduced signal upon each cycle of the recording medium; such systems and methods which are effective for studying low frequency and direct current transients by modulating with the signal which is to be recorded a source of carrier oscillations of a frequency which is reproducibly recordable on the endless magnetic recording medium, so as to provide a modulated carrier signal which is continuously recorded on the endless recording medium and obliterated immediately thereafter, the obliterating action being stopped upon the occurrence of a transientthe reproduced recorded modulated signal being modulated so as to cyclically provide a demodulated reproduced signal corresponding to the transient signal which is utilized for operating the cyclically operative indicating device; and various other novel and desirable features forming part of such systems.

The foregoing and other objects of the invention will be best understood from the following description of exemplifications thereof, reference being had to the accompanying drawings where- Figure 1 is an illustration of my invention in block diagram form.

Figure 2 is an illustration of another form of my invention, also in block diagram form.

Figure 3 is a diagrammatic illustration of a modulated carrier current and a magnetization curve.

Figure 4 is an illustration of the circuit of Figure 2, showing the transient recording circuit in detail and the trigger circuit and the transient reproduction circuits in block diagra form and connected to the transient recordin cuits in block diagram form.

Figure 6 is an illustration of the circuit used for reproducing the transient, and showing the recording and trigger circuits in block diagram form and connected to the reproducing circuit.

Figure '7 is a plan view of a magnetic head in its open position.

Figure 8 is a plan view of the magnetic head in its closed position.

Figure 9 is a diagrammatic illustration of a timing device utilizing my transient recording device.

My invention comprises means and a method for recording a transient signal'on a storage medium and repeatedly reproducing it therefrom atv regular intervals to establish a wave, each cycle of which is substantially a facsimile of the original transient. The device may use any recording medium, and suitable means for recording the transient thereon and reproducing the transient therefrom. I prefer to use a medium which can be used over and over again such, for example, as a magnetic tape or disc, etc.

Such preferred embodiment is illustrated in Figure 1, in which is shown means for a method of storing a transient by electrically recording it on an endless magnetic tape l0, means for and a method of reproducing the recorded transient from the recording medium and presenting it visually by repeatedly reproducing it on an oscillograph I I or other indicating device; and associated circuits for controlling the operations of the 40 means and method. Electrical recording may inelude electromagnetic, photoelectric, or any other process or device utilizing electricity in the recording and/or reproducing process.

The'endless tape I0 is driven about rollers 12 and 13 by a motor l4. A magnetic obliterating head I5 and a magnetic recording head l6 are associated with the tape Ill. The obliterating head I5 is adapted to apply a steady saturating flux (such as the flux derived from battery H) to the tape ID as the tape passes, and to bring each incremental portion of the tape to a state of substantially uniform saturation. With the switch 23 closed in its downward position the recording head It is connected to a signal recording circuit which may include a source I 8 of polarizing current. The transient signal to be recorded on the tape I0 is applied at the input terminals I9, 20, of the recording circuit. Included in the recording circuit is an amplifier 2| and an equalizer 22. The equalizer 22 is to provide substantially constant current to the recording head IS. The instantaneous current which is in the recording head I6 is given by the superposition of the instantaneous values of the transient signal current and the polarizing current. As the saturated tape passes the recording head a magnetic pattern corresponding to the current in the recording head is recorded on the tape in the well-known manner, and as soon as the transient has been recorded the obliterating and recording process are stopped by automatically or manually opening'switches 23 and 28 so that the recorded transient will not be erased or have further transients superimposed upon it. A playback head separate from the recording head l6 may be used in association with the endless tape III by connecting it to an equalizer 24, an amplifier 25, and the oscilloscope Ii vide the switch 23 for switching the head I6 from the recording circuit to the playback circuit.

After the transient, which may occur only once,

-is recorded on the endless tape ill, the tape can be driven around the rollers l2, l3 and, with switch 23 thrown to cause the head IE to act as a playback head, each complete transit of the tape will cause the signal recorded on the tape to produce a trace of the transient on the oscillograph I I. In this manner repeated reproduction of the transient signal on an oscilloscope II, in which the horizontal sweep of the oscilloscope is synchronized with a point on the tape ill, will produce a sufiiciently bright and sustained trace to enable a person to study the transient, and no time need be lost between the recording and playback operations other than the time necessary to throw a switch.

If the transient is of an acoustical nature it may be converted by means of an electromechan- With the above described method it is diflicult to reproduce the recorded signal from the magnetic medium if the signal has low frequency components as the rate of change of flux in the magnetic head is low for the low frequency components. For the above described means and method of recording this makes necessary the use of considerable equalization for low frequencies with consequent introduction of phase shift and background noise. With this method the quick interruption of the recording and obliterating current by throwing switches 23 and 28 after the signal transient is on the tape causes an undesired transient to be recorded on the tape which is apt to interfere with the desired signal transient.

In order to obviate these undesirable conditions I use a signal modulated carrier current to record on the tape. ihis method permits the recording without phase distortion of frequencies substantially down to zero frequency corresponding to direct current, and can be arranged to avoid introduction of undesirable transients caused by switching operations. Several methods of modulating a carrier current to record on the magnetic medium may be used; among them being amplitude modulation, frequency modulation and phase modulation. A detailed description of one carrier method of recording may be found in my application entitled Apparatus and method for magnetic recording, Serial No. 399,909, filed June 26, 1941.

Figure 2 illustrates a form of my invention adapted to record transients in direct current circuits or circuits normally carrying no current, and using the method detailed in the above-clescribed patent application. Examples of transients which this particular form of the apparatus is adapted to record are the current in spot welders and the charge or discharge of condensers, etc.

The input terminals I9, 20 are connected to the circuit in which a transient is expected, such for example, as across a low resistance in the circuit of an impact welder. The motor il drives the roller l2 to cause the endless magnetic tape III to continuously rotate about rollers l2 and I! at a uniform velocity in the direction shown by the arrow. The time of one cycle of the tape loop preferably is slightly longer than the transient which may, for example, be of about one-tenth of a second duration, thereby establishing on the tape loop a record of the normal circuit conditions and a record of the transient condition. An oscillator 26 is tuned to a frequency somewhat higher than the highest frequency component to be recorded. When the highest frequency component of the signal is in the neighborhood of 500 cycles per second a carrier frequency on the order of 2000 cycles per second is satisfactory. The oscillator is connected to and caused to excite an obliterating amplifier 21, which in turn supplies 2000 cycle current to the obliterating head l5. As the tape l0 passes the obliterating head l5 each incremental portion of the tape is subjected to a 2000 cycle alternating magnetic force which is sufficiently strong to saturate the part of the tape closest to the coils. As each incremental portion of the tape leaves the coils the magnetic field active on it is gradually reduced and thus the successive increments of the tape are brought to an unmagnetized state. In this condition the tape is ready to have a new record iliipressed on it at the magnetic recording head The oscillator 26 also supples 2000 cycle carrier current to the magnetic head IE, but before reachingthe magnetic head I6 the carrier current passes through a modulator 29 to which is also connected the transient signal input terminals I9. 20. In the modulator 29 the transient signal modulates the 2000 cycle carrier current from the oscillator, and the signal modulated carrier then passes to an amplifier 30. The modulator 29 is of the balanced type arranged to balance out the signal and pass the modulated carrier. A high pass filter 3I- may be connected in the recording circuit between the amplifier 30 and the magnetic head iii. For recording transients whose highest frequency components are in the neighborhood of 500 cycles per second the high pass filter may be designed to cut off somewhat below 1500 cycles. The exact point at which the high pass filter cuts on. depends upon the frequency range of the transient to be recorded and upon the chosen carrier current frequency. The filter 3i is not essential but is useful in taking out hum and the residual transient signal which may have gotten through the modulator 29. If no transient is being recorded the 2000 cycle alternating flux set up in the magnetic head I6 by the carrier current causes a 2000 cycle magnetic wave to be recorded on the moving tape Ill. Upon nearly completing a. circuit of rollers I2, I 3, the recorded wave is removed by the obliterating head l5.

When a transient signal is applied to the signal input terminals I9, 20, the carrier current is modulated by the transient and a corresponding modulated magnetic carrier wave is recorded on the tape. To prevent obliteration of the transientmodulated magnetic carrier wave, and to avoid superimposing the carrier on the recorded wave after the tape H] has completed a cycle around the rollers l2, l3, a transientpperated control circuit is energized to block the obliterating amplifier 21 and to open the recording circuit after suitable time intervals.

The gain of the amplifier 30 which feeds the magnetic head I6 is preset to swing the recording flux set up in recording head l6 by the carrier current, over the proper part of the magnetic curve of the tape. (This is explained in detail in connection with Figure 3.) The strength of the transient signal input may be adjusted by a potentiometer 32 connected across the signal input terminals 19, 20.

The transient operated control circuit consists of a trigger device 33 connected to the transient signal input terminals I9, 20, the obliterating amplifier 21, switching means 34, and sometimes, if desired, to a switch 41 connected to the tape drive motor I4. The trigger means is actuated by the incoming transient to cause the obliterating amplifier 21 to be blocked thereby preventing the obliterating head I from erasing the record of the transient which is made on the moving tape I 0, and the actuation of the trigger means 33 also causes the switch means 34 to be thrown to break the recording circuit to the magnetic head I6 and to make a playback circuit from the magnetic head Hi to the indicating instrument II. If desired the trigger circuit may loe used to break the circuit to the motor l4 and stop the tape l0 immediately after a transient has occurred. Thus, shortly after a transient has been recorded on the tape I0, the entire device automatically becomes inoperative due to the action of the trigger means.

For applications in which it is not desired to have an automatic control of the trigger circuit, an external control 4| is provided. The external control may be a push button or other similar device for initiating the action of the trigger device 33 to effect the same results as the actuation of the trigger device 33 by the incoming transient signal.

After the transient is recorded on the tape II] in the form of a modulated carrier magnetic recording, it is ready to be observed by repeatedly reproducing the tape record on an oscillograph II or other indicating instrument. From the magnetic pickup head I B the reproduced signal goes through an amplifier I30 then through a band pass filter 36 which is designed to pass, in addition to the carrier, the side bands corresponding to modulation frequencies up to about 500 cycles. The signal then passes to the demodulator 31 where a rectifier detects the output of the band pass filter and the carrier and its harmonics are filtered out, and then to an indicating instrument such as an oscillograph I I.

The output from the demodulator consists of a direct current component upon which is superimposed a wave composed of a series of transients each one being substantially a facsimile of the original transient, and following at regular time intervals such that it can be viewed on a cathode ray oscillograph.

The oscillograph H shown may be a standard commercial cathode-ray oscillograph, such as type TMV122-B sold by the Radio Corporation of America, as described in its instruction book IB-23,339, published in 1937. As is well known, such standard cathode-ray oscillographs are equipped with a synchronizing switch which is used to control the synchronization of the horizontal time-axis sweep with the investigated signal. The synchronizing switch usually has at least two positions, namely, an internal synchronizing position, in which the time-axis sweep is automatically synchronized with the investigated input signal, and an external synchronizing position, in which the time-axis sweep is synchronized with an external synchronizing signal.

The number of repetitions per second at which the recorded transient is repeated on the oscillograph II depends on the number of tape cycles per second. For example, this frequency of repetition may be ten transient facsimiles per second on the oscillograph. The time-axis sweep of the cathode-ray may be internally synchronized with the reproduced, demodulated and illtered signal, which is impressed upon the oscillograph II, or the tape joint 49 or a nick in the tape maybe used as a trigger for synchronizing the time-axis sweep of the cathode-ray.

In order to record on the steel tape l0 an elec-' tric current is used to set up a proportional magnetizing force which is applied to the tape l0. Therefore, as the electric current varies in accordance with the instantaneous value of the signal to be recorded, the magnetizing force impressed on the tape varies in proportion to the original signal.

The relationship between the magnetizing force H and the resultant magnetic induction B for increasing and decreasing values of H is expressed graphically by a hysteresis loop, and for increasing values of H starting with a demagnetized tape it is expressed by a virgin magnetization curve. Every magnetic material has a particular virgin magnetization curve and hysteresis loop which identifies the magnetic characteristic of the material.

From hysteresis loops it is obvious that magnetic effects are not reversible, and that the magnetic induction obtained from a given magnetizing force depends upon the previous magnetic history of the material on which the magnetizing force is impressed. Accordingly, with a tape upon which previous magnetic records have been made it is necessary to produce a substantially uniform condition either of saturation or demagnetization in order to "erase the magnetic record.

In a. magnetic tape it is necessary to obliterate the previous record before a new record is put on the tape or mixed records are apt to result. This obliteration may be obtained by applying a magnetizing force to the tape which is suificiently high in value to cause an induction which is in the saturation range on the magnetization loop of the material. That is, in the neighborhood of point S1 or S2 in Figure 3, in which the heavy line 38 represents a typical virgin magnetization curve for a material which has been demagnetized. The obliteration may also be obtained by applying over a relatively large area" of the moving tape an alternating magnetizing force derived from an alternating current, and gradually reducing the magnetizing force to zero to reduce the residual magnetism to substantially zero.

Subsequent magnetization in accordance with a signal will have a 3-H relationship in accordance with the virgin magnetization curve 33 of the material. The decrease in strength of the magnetizing force is obtained for each portion of the tape by removing that portion of the tape farther and farther from the center of the magnetizing force. This is the method of obliteration employed in the preferred form of my apparatus.

Continuous alternating current is applied to the coils of obliterating pole pieces which are positioned near the moving tape. This causes a rapidly reversing diffuse magnetic flux to be impressedthrough an area of the tape which is large in comparison to the area through which the recording flux is impressed, and as each por tion of the tape moves away from the pole pieces which are the center of the flux distribution, the strength of the field on each such portion of the tape decreases gradually to zero. The pole pieces of the obliterating head have their like magnetic poles toward each other. However, the use of alternating current in the coils causes the magnetic polarity of each pole to alternate; at one instant the north poles are toward each other, the next instant the south poles aretoward each other. This establishes a magnetic field about each pole piece which bucks the other magnetic field and results in a relatively large leakage or diffuse magnetic field through which the tape moves during the obliterating process- Each portion of the tape which has been removed sufficiently far from the obliterating poles is now reduced to substantially zero residual magnetism and is ready to have a magnetizing force corresponding to a signal impressed on it. This is done by the recording head indicated generally by the reference character It and comprising two pole pieces, one of which is surrounded by a coil. In the coil there is a current corresponding to the signal to be recorded on the tape. The current establishes a varying magnetizing force in the pole piece which in turn establishes varying magnetic flux density in successive portions of the tape.

Figure 3 diagrammatically illustrates my preferred method of recording a low frequency transient on a magnetic materlalwhich has previously been demagnetized to substantially its virgin state, and comprises modulating a carrier frequency current 39 by the transient signal to establish two envelopes 40, and applying to the tape l magnetizing forces having successive peak values corresponding to both of the envelopes 40, and by adjusting the modulation of the carrier current by the signal to cause it to be at all times below 100 per cent.

Recording a signal on a tape introduces considerable distortion due to non-linearity of portions of the magnetization curve of the tape material unless steps are taken to cause the recording to be only on the substantially linear portions of the magnetization curve. I effect recording on the substantially linear portions of the magnetization curve of the tape material by applying to the coils a current comprised of a carrier current modulated by a signal to be recorded, the highest value of the modulated carrier current being insufficient to cause magnetic saturation in the tape l0, and the percentage of modulation of the carrier by the signal being at all times substantially less than 100 per cent.

I have found that for most carrier frequencies the same oscillator can be used as a source of both carrier and obliterating current. This eliminates the need for an extra oscillator and makes a more simple device. However, if the need arises it is to be understood that the obliterating current and the carrier current may be produced by separate sources (such as oscillators) and that the obliterating current frequency may be of any value which will give a suflicient number of reversals of polarity to reduce the residual magnetism in' the tape to substantially zero.

In Figure 3 the line 38 represents a magnetization curve for a material which may be used as a magnetic tape. The curve comprises two substantially straight portions 232 and 233 and three curved portions 234, 235, and 238. The curved portion 236 lies between the two substantially straight portions 232 and 233 and represents nonlinearity between the magnetizing force and the induced fiux density for small values of the magnetizing force. From zone 231, which is substantially the junction of curved line 236 with line 232, to zone 238, which is substantially the junction of line 232 with curved line 234, the line 232 is substantially straight. This means that for each unit increase in magnetizing force H between zones 231 and 238' there is substantially a proportional increase in the flux density B in the tape I0 from 231" to 238". Above zone 238 partial saturation takes place and B does not increase on a substantially constant ratio with H. Below the H axis the same effects take Place for magnetizing forces created by currents of the opposite polarity. Zone 239 represents the junction of lines 236 and 233; and zone 240 represents the junction between lines 233 and 235. Zones 231, 238, and 239, 240 may be referred to as critical zones or points as they define substantially the extremities of the substantially straight portions of the magnetization curve.

In order that a signal recorded on the tape Hi can be reproduced without considerable distortion it is necessary that the maximum magnetic flux induced in the tape during each cycle of the modulated carrier current correspond to a point which lies on the substantially straight portion 232 or 233 of the magnetization curve of the material.

Figure 3 further illustrates how the magnetizing forces H corresponding to a modulated carrier current induce maximum magnetic flux densities in the tape l0 which correspond to points on the straight portions 232 and 233 of the magnetization curve of the tape material, and how distortion in the reproduced signal may be reduced by preventing the maximum flux densities for each cycle in the tape from corresponding to points on the curved portions 234, 235, and 233 of the magnetization curve of the material. This may be referred to as recording on the substantially straight portions of the curve. In recording a substantially symmetrical transient on a substantially demagnetized tape by my modulated carrier current system, the greatest dynamic range can be obtained by adjusting the normal peak amplitude of the unmodulated carrier current to have a value which corresponds to a magnetizing force H having a value which lies substantially midway between the value of the forces corresponding to zones or points 231' and 238'. The value of this force is represented by the point having the reference character 245. Point 248 represents the value of this force for the opposite polarity of the carrier current. The minimum amount of distortion due to recording on the curved portion of the magnetization curve is obtained by adjusting the percentage of modulation of the carrier current to a value which is at all times sufllciently below 100%: that is, with the normal peak amplitude of the carrier current adjusted substantially midway between zones or points 231' and 238 for one polarity and between zones or points 239 and 240 for the other polarity, the peak value of the magnetizing force produced by each cycle of the modulated carrier current will not be greater than the value of the magnetization force H represented by the points 238 and 240, nor less than the value of the magnetization force H represented by the points 231 and 233. I have found that 50 to 60 per cent modulation is satisfactory for some magnetic materials. By this process the maximum values of the flux densities, in the tape for each carrier cycle correspond to points which lie only on the substantially straight portions of the magnetization curve of the material, and therefore distortion of the signal reproduced from the envelope of the carrier is reduced to a minimum. The ma netizing force H corresponding to the amplitude of the peak of each cycle of the modulated carrier current is always less than the value of zones or points 236', 240, to prevent recording on the curved portions 234, 235 of the magnetization curve, and is always greater than the value of zones or points 231', 239' to prevent recording on the curved portion 236 of the magnetization curve. Depending upon the material used for the endless tape the size of the curved portions 234, 235, and 236 varies and the degree of curvature of the substantially straight portions varies. For each kind of tape it is possible to adjust the amplitude of the unmodulated carrier current to cause the normal peak value of each cycle to fall substantially half way between points 231' and 238 and between 238' and 240', and to adjust the amount or percentage of modulation of the carrier current by the signal to be recorded to cause the peak value of each cycle of the modulated carrier current to lie between points 231' and 238 for one polarity of the carrier current, and between points 239' and 240' for the other polarity of the carrier current.

In certain applications, the transients to be recorded will all be of the same general nature and may consist of unidirectional pulses. A typical example is the transient voltage developed across a resistor when a condenser is discharged therethrough. In such cases, the amplitude of the unmodulated carrier may be so adjusted that the resulting magnetizing forces, corresponding to the peak values of the wave, fall near points 237 and 233 and the transient circuit so connected that the carrier is modulated upwardly toward but not exceeding points 238 and 240; or downward modulation may be used with unmodulated carrier peaks producing magnetizing forces falling near points 238 and 240. For cases between symmetrical transients and completely unsymmetrical transients, intermediate carrier amplitudes may be used. By following this procedure thus substantially straight portion of the virgin magnetization curve is used to the maximum advantage.

Detailed description of recording circuit Figure 4 is a detailed circuit of a transient signal recording means somewhat similar to the recording means of Figure 2 but differing in that a single amplifier is shown, and relay 34 does not make the playback circuit.

,Plate power for the various tubes detailed in Fig. 4' is supplied by the power supply P1 which may be of conventional design.

The modulator 29 comprises a pair of pentode tubes 15, 16 with plate and screen voltage stabilized by a glow tube voltage regulator 11. Carrier signal supplied by oscillator 26 is applied in opposite phase to the grids of the two modulator tubes by center tapped transformer 64. The transformer secondary 66 is tuned to the oscillator frequency by shunt condenser 18. The transient signal derived from potentiometer 32 connected across input terminals I9 and 20 is supplied in the same phase to both modulator grids through the connection and the center tap of the transformer secondary 66. Bias for the modulator tubes is produced by the plate and screen current flowing through resistor 19. The plates of the modulator are connected in push-pull fashion to the center tapped primary I45 of transformer 85. The secondary of this transformer supplies the modulated carrier to amplifier 30. Since the transient signal is applied to the two grids in the same phase while the plates of the tubes are connected in push-pull fashion, the transient signal itself cannot appear at the secondary of the output transformer, provided that the modulator circult is properly balanced. Presence of the transient signal itself in the recording circuit is undesirable as it may interfere with proper recording of the modulated carrier. Rather than provide means for accurate balancing, a high pass filter 3| is employed at the output of the amplifier to filter out any residual transient signal due to modulator unbalance.

The oscillator 26 employs a dual triode 55. The oscillator circuit proper is of the resistance stabilized Hartley type and employs one triode section 56 of the tube 55. The frequency determining elements are tapped inductance 59 and condenser 60. Resistor BI is the stabilizing resistor and 63 is a blocking condenser. The plate circuit is supplied through choke coil 42. The tube is biased by means of the cathode resistor 5| which is bypassed by condenser 52. The second section 51 of the tube 55 is employed as a buffer amplifier. Its grid is connected through blocking condenser 46 to the plate end of the oscillator coil 59 and has a bias path to ground through grid leak 48. In the plate cathode circuit are connected three impedances, plate resistance 44, bias resistor 45, and primary 65 of the modulator input transformer 66. The plate voltage for both sections of the tube is stabilized by the same regulator tube 11 that functions for the modulator. The amplified carrier voltage developed across resistance 44 is supplied to the obliterating amplifier 21. Carrier current flowing through primary 65 supplies carrier signal to the modulator. Resistor 45 supplies bias for the buffer tube and also introduces some degeneration.

The amplifier 30 is a two stage resistance capacity coupled type of conventional design. A fixed voltage divider comprising resistors 16 and H is provided at the input to select a suitable portion of the modulated carrier output of modulator 29. The output of the second stage amplifier tube 81 is coupled to high pass filter 3| through a suitable matching transformer BI and the output of the filter is supplied via switch or relay 34 to recording head I6. The high pass filter 3| is of a simple configuration, comprising two condensers 88 and inductance 89.

The carrier frequency supplied by the oscillator, and the frequency response characteristics I 05 which is not energized during the recording process.-

A trigger relay circuit (to be described later) is provided to block the amplifier to cut off the obliterating current a short time after the transient occurs and just before the recording of the transient modulated carrier reaches the obliterating head. This is done by causing the trigger circuit to energize relay I05 which breaks the ground connection for grid resistor I and con- I nects the resistor to a negative bias tap I09 of powe pply P1.

head and is connected to the recording circuit only as long as the trigger relay switch 34 has not been actuated by trigger means 33 to cause it to become inoperative. Shortly after a transient signal is applied to the signal input terminals I9, 20 the trigger relay becomes operative to cause the magnetic head to be disconnected from the recording circuit to prevent superimposing extra signals in the desired recorded transient. The actuation of the trigger circuit also blocks the obliterating amplifier which prior to the coming of the transient has been continuously erasing the record of the carrier on the tape I0. Blocking the obliterating amplifier prevents the amplifier from erasing the transient signal which has just been recorded on the tape.

The use of an obliterating current which gradually reduces the magnetism in the tape III to zero prevents a transient from being recorded on the tape due to the sudden blocking of the obliterating current, and the use of the modulated carrier current recording process prevents a transient from being recorded on the tape due to the sudden disconnection of the recording head from the recording circuit.

Obliterating amplifier Figure illustrates the details of the obliterating and trigger circuits in connection with the recording and playback circuits which are shown in block form.

During the part of the recording process in which the unmodulated carrier is recorded on the tape I 0, that is, during the time that the apparatus is in operation in anticipation of the occurrence of a transient, the coils of the obliterating head I5 are energized by an alternating current of suflicient strength to saturate magnetically the portions of the tape immediately adjacent the head. For convenience, this obliterating current is obtained from the oscillator 26, a single stage amplifier 21 being interposed to obtain sufllcient current and to isolate the head from the oscillator. The obliterating amplifier 21 comprises a power pentode tube 91 whose plate is coupled to the obliterating head by means of transformer I02. Condenser I04 tunes the obliterating head circuit to the frequency of the oscillator so that maximum current may be obtained. Plate current for the tube 91 is supplied by power supply P1 which may be of conventional design. Normal operating bias is provided by cathode resistor IOI which is bypassed by condenser III. Oscillator 26 energizes control grid 98 through the connection 95 and blocking condenser 96. Grid 98 is maintained at the proper negative bias potential by the direct current path to ground comprising resistor 99 in series with the parallel combination of resistor I01 and resistor I00. Resistor I00 is grounded through the contacts of relay stop the obliterating process.

This bias is sumcient to stop the flow of plate current to tube 01 and thereby Condenser I08 is connected into this bias circuit to slow down the blocking process to prevent the recording or an undesired transient which might result if the obliterating process were stopped suddenly.

The obliterating head I5 has two pole pieces, ll6and II6, surrounded, respectively, by coils II! and H8. The two coils are connected in series opposing relationship so that at any instant the adjacent ends of the pole pieces have the same polarity, thus causing a difiuse magnetic field in the neighborhood of the tape.

Details of trigger circuit Simultaneously with the recording of the transient signal on the moving tape I0, the transient actuates a trigger circuit shown in detail in Figure 5 to cause (after a certain time delay) the obliterating head to stop erasing" the signal which is on the tape, and to cause the magnetic head I6 to stop recording on the tape. The interval between the time when the first part of the transient signal is put on the tape I0 and the tim when the obliterating and magnetic heads I5 and I6 cease to be operative should be slightly less than the time interval for one cycle of the tape.l0 around the rollers I2 and I3. This assures that there will be no superposition of another signal on top of the already recorded transient signal, and if the tape is long enough the entire transient will be recorded. In other words, the trigger circuit synchronizes the recording cycle with the time period in which the transient occurs; and in addition governs the length of time of the recording cycle, this time being a function of the speed and the length of the endless tape. It is essential that overlapping in the recording process be avoided, meaning that the recording period should be stopped before the tape I0 has completed a cycle around the rollers I2 and I3. Y

A gas tetrode tube I 33 has in its plate circuit the coils of relay I05 which when energized blocks the obliterating amplifier and relay I62 which interrupts the recording process. Normal- 1y, this tube is non-conducting and the relays I 05 and I62 are not energized. Its control grid I58 is connected through variable resistance I6I to the cathode of another gas tetrode I32 which also normally is non-conducting. Grid I58 is bypassed to ground by condenser I60. When the gas tetrode I32 becomes conducting the voltage drop across cathode resistor I 51 applies a positive bias to grid I58 thus "firing gas tetrode I33 and thereby energizing relays I05 and I62. Resistor I 6| and condenser I60 introduce a time delay, depending on the values of these two components so that the relays are energized at a predetermined interval of time after the firing of tetrode I32. This tube is "fired by the transient signal and the time delay provided by resistor IN and condenser I60 is so adjusted that the obliterating and recording processes are stopped just as the tape loop has completed the transient recording cycle.

It is the nature of gas tetrode I32 to fire and become conducting upon application of positive potential to its control grid I56 but not to fire upon application of a negative potential. To insure operation of the trigger at the proper time for transients of either polarity, a full wave rectifier is interposed between the transient input terminals I9, 20 and the control grid I56. The transient signal is applied through blocking condenser I46 to grid I40 of dual triode tube I34 connected as a phase inverter of conventional design. The plates I36 and I43 are coupled through blocking condensers 20I and 202 to grids I39 and I42 of the dual triode I35 which is connected as a full wave biased detector. The plates I31 and MI are connected to power supply P1. The grids I39 and I42 are both biased to cut off through the connection of grid resistors I I2, II 3 to a negative bias tap on power supp-1y P1. Due to the large negative bias no plate cathode current flows in tube I35 so that there is no voltage drop across cathode resistor I66.

When a transient occurs either grid I39 or I42 becomes less negative causing plate current to flow in its section of the tube. This current produces a voltage drop across resistor I66 which in turn fires trigger tube I32. As explained before, as soon as tube I32 fires a voltage is applied to the time delay network I60, I6I and after a suitable time interval the voltage of the grid I58 rises sufiiciently to fire tube I33, thus energizing relays I and obliterating and recording processes. During the time taken to build up a firing potential at the grid of tube I33 the transient is being recorded on the tape loop in the form of a transient modulated carrier wave and the operation of relays I05 and I62 finally takes place to prevent obliteration of the recorded transient and to prevent superimposing additional carrier signal on the recorded transient.

In order to reproduce the transient from the moving tape I0, a switchA is thrown from the recording position R to the playback position P. The switch A has six blades I2I, I22, I23, I24, I25, and I26 as shown in Figures 4 and 5. In the recording circuit shown in detail in Figure 4, the actuation of the switch A causes blade I2I to break the plate supply to oscillator 26 which is through the choke coil 42, and causes the blade I22 to break the connection from the recording circuit to the magnetic head I6 and make a connection from the magnetic head I6 to the playback circuit. The playback circuit may include a Dre-amplifier indicated generally by the reference character I3I, and positioned between the magnetic head I6 and the amplifier 30. The amplifier 30 is adapted to amplify the transient modulated carrier signal both before it is recorded on the tape I0 and while it is being played back from the tape. The blade I23 of switch A is adapted to control the input to the amplifier 30. In the recording position the input to amplifier 30 is from the modulator 29 and in playback position the input is from the pre-amplifier I3I. The switch blade I24 is located between-the amplifier 30 and the high pass. filter 3| and is adapted to direct the output of the amplifier 30 to the magnetic head I6 when it is in the recording position and to the band-pass filter 36 of the playback circuit when it is in the playback position. The blade I26 is located in the plate I62 which stop the supply to the two trigger tubes. In the recording position the switch closes the plate circuit so that the tubes may be fired by the transient. In order that the tubes may be restored to the non-conducting condition before making a new recording, the switch I26 is arranged to open the trigger tube plate circuits when it is turned to the playback position. This restores the contacts of relay I05 to their original condition, removing the blocking bias from the obliterating amplifier 21. To prevent obliteration when this takes place, a switch blade I is located in the plate circuit of obliterating amplifier tube 91 to open this circuit during playback.

Reproducing circuit detail-F g. 6

After the transient signal modulates the carrier current and is recorded on the tape I0, and after the trigger circuit has operated to stop further recording and further obliteration, the transient signal, a facsimile of which is recorded on the tape, is played back from the tape to an oscilloscope II or other indicating device by means of the playback circuit. Repeated reproduction of the transient on the indicating instrument will give an observer sufficient time to see the transient even though it originally was a very rapid one.

For the reproduction of the transient, the magnetic head I6, which is the recording head during the recording process, becomes a playback head due to switch I22 connecting it to the playback circuit through line 205. It is obvious that separate recording and playback heads may be associated with the tape I0 if it is so desired. Actuation of the switch blade I22 from terminal R to terminal P connects the magnetic head I6 to the playback circuit which consists of the pre-amplifier I3I, the playback amplifier I30. (which may be the same amplifier as the recording amplifier the band pass filter 36. and the demodulator and low pass filter 31, to the recording or indicating instrument II. It is apparent that the single amplifier, which is here indicated by reference character 30 in the recording circuit and I 30 in the reproducing circuit, may be two separate amplifiers if desired.

The pre-amplifier I3I may employ a single tube I10, and is coupled to the recording and reproducing amplifier I30 by a blocking con denser HI and a resistor I12. The plate of the tube I10 is connected to power supply P1 through coupling resistor I69 and filter resistance I68. and the screen grid I99 is connected to the power supply Pl through resistor I61 and is by-passed to ground by a condenser 209. A filter and isolating condenser 204 by-passes to ground the junction of resistors I61, I68, I69. The signal input from the magnetic head I6 to the preamplifier I3I is through line 205 to control grid 206 in the tube I10. Bias for tube I10 is provided by cathode resistor 208 which is by-passed by condenser 2I0. The tube 86 in the amplifier I30 (Fig. 4) has a grid resistor I13 which, together with the resistor I12, establishes a voltage dividing network to cause the same voltage to be supplied to the amplifier I30 during reproduction as was applied to the amplifier during the recording process. The output of the amplifier I 30 is applied through connection I92 to the single T-section band pass filter 36 which is comprised of two series arms each having an inductance I14 and a condenser I15, and a shunt arm having inductance I16 and a condenser I11.

For a carrier frequency of 2000 cycles per sec- Resistor I98 is provided to maintain grid nd and transient signal frequency components up to 500 cycles per second, the pass band of the filter should extend from about 1500 cycles per second to about 2500 cycles per second.

The band pass filter 36 is connected to the demodulator 31 by means of a transformer I18, the secondary winding of which is connected to the plates of a double diode rectifier tube I19. The output of the double diode tube I19 is connected through a resistance I80 to a low pass filter comprised of two M-derived sections I8I and I82. There are two series branches and three shunt branches in the filter. The. first series branch is comprised of inductor I83 and a condenser I 84, and the second series branch is comprised of inductor I85 and a condenser I86. The first shunt branch has a condenser I81, the second shunt branch has a condenser I88, and the third shunt branch has a condenser I89. the filter is applied across a resistor I93 and then to output terminals I94 and I95, to which may be attached an oscilloscope, oscillograph, or other indicating device lg. One of the sections of the low pass filter has ts frequency of maximum attenuation at the carrier frequency 2000 cycles and the other section has its frequency of maximum attenuation at the second harmonic of the carrier, 4000 cycles. Adjusting the filter sections I8I and I82 in this manner eflectively suppresses the carrier frequency and its second harmonic and causes only the transient signal to be applied to the output terminals I94 and I95.

External trigger controL-Fig.

In some applications the use of the internal automatic trigger circuit 33 may not be desirable. I, therefore, provide an external trigger circuit which may be actuated by an operator to effect the same results as the actuation of the automatic circuit insofar as blocking the obliterating amplifier and stopping the recording process are concerned. Switch I65 is manually controlled and switches the phase inverter out of the circuit and connects grid I56 of gas tetrode I32 to the external control circuit. The external trigger circuit comprises a switch I96 which may be operated byhand or by some external circuit. One terminal of the switch I96 is connected to the positive terminal I91 of a low voltage D. C. source whose negative terminal is grounded, and the other terminal of the switch is connected through switch I65 to the control grid I56 of the first gas tetrode tube I32. I56 at ground potential until the external control is actuated. Applying positive bias to the control grid I56 causes the tube I32 to fire thereby initiating the sequence of events which blocks the obliterating amplifier 21 and stops the magnetic head I6 from recording as has heretofore been described.

Modulation check-Fig. 6

I have provided a switch 200 which connects the oscilloscope II directly to the output of the amplifier 30 in order to make it possible to observe-in the oscilloscope the amount of modulation for applications where the transient can be caused at the will of an operator, such for example as an impact welder. It is possible to regulate the amount of modulation in advance of recording a' transient to be studied on the recording medium. If there is too much or too little modulation it can be adjusted before the The output of the second section I62 of transient signal is applied. The amplitude of the input transient signal can be controlled by the potentiometer 32.

Figures '1 and 8 illustrate a type of head I6 which is particularly useful for a transient recording device. It comprises a base 2I5 upon which is mounted a stationary block M6 and a movable block 2I1. Inside the stationary block 2I6 there is a coil 2! which is adapted to be electrically connected into the recording or the reproducing circuits, and inside the coil 2! is a rigid pole piece M9. The movable block 2 is attached to the base 2I5 by means of a screw 220 about which the block is adapted to pivot to establish an open position shown in Figure 7 and a closed position shown in Figure 8. The magnetic tape I0 is adapted to lie between the movable block 2" and the stationary block 2I8, and when the head I6 is in its open position may be slipped out from between the two blocks. This construction is particularly valuable for transient recording devices which may be located in a remote place such for example as a remote power supply line. The recording device may be connected to the line and left in an operating condition. If a transient should occur it will be recorded. For an installation of this type the switch 41 (Fig. 2) to stop the motor I 4 is valuable. Examination of the recording device at a later date will disclose the presence of the recorded transient and the tape can be removed from the head I6 and taken back to the laboratory for study. A movable pole piece MI is positioned within the movable block 2 I 1 and is biased toward the tape I0 by a spring 222. A spring clip 223 is provided for latching the two blocks 2| 6 and 2H together when the head I6 is in its operating condition.

Figure 9 illustrates diagrammatically a timing device utilizing my invention and adapted to record the speed of a bullet 225. The timing device comprises an oscillator 226 in a bridge circuit having four resistances 221, 228, 229, and 230. With all four resistances in the circuit there is no output to the recording head I6 as the bridge circuit is balanced. The bullet 225, upon being fired, breaks or otherwise alters the resistance 221 thereby unbalancing the bridge circuit and causing signal output to the recording head I6, which signal is recorded on the tape I0 in accordance with my invention. The bullet then breaks or otherwise alters the resistance 228 reestablishing a balance in the bridge circuit which stops the recording of the signal. The recorded transient consisting of a few cycles of a known frequency from oscillator 226, may then be reproduced on an oscillograph and the time of flight from resistance 221 to resistance 228 determined from the number of cycles of the signal on the oscillograph and the distance d between resistances 221 and 228.

The present application is directed to features of the invention disclosed and claimedherein and relating to systems and methods for studying transient phenomena, and more particularly to such systems and methods employing an electric circuit for recording on a, recording medium a transient circuit condition occurring on the circuit and utilizing the record of the transient for studying the characteristics of the transient by continuously making an obliterable record of the circuit conditions under study on an endlesslyoperating cyclically-effective recording medium, such as a magnetic recording medium, so that the circuit conditions are continuously recorded; the continuous recording operation being accompanied by a continuous obliteration of the recomings and the obliterating action being stopped upon the occurrence of a transient condition so as to retain on the recording medium a record of a signal corresponding to the transient occurrence and the recording operation being discontinued after the transient occurrence has been recorded on the medium in such manner as to enable the study of the transient occurrence by cyclically reproducing the record and utilizing the reproduced record for operating a cyclicallyoperative indicating device, such as an oscilloscope, which presents an image corresponding to the reproduced signal upon each cycle of the recording medium; such systems and methods which are eflective for studying low frequency and direct current transients by modulating with the signal which is to be recorded a source of carrier oscillations of a frequency which is reproducibly recordable on the endless magnetic recording medium, so as to provide a modulated carrier signal which is continuously recorded on the endless recording medium and obliterated immediately thereafter, the obliterating action being stopped upon the occurrence of a transient-the reproduced recorded modulated signal being modulated so as to cyclically provide a demodulated reproduced signal corresponding to the transient signal which is utilized for operating the cyclically operative indicating device; and various other novel and desirable features forming part of such systems.

The magnetic recording systems and'methods utilizing a signal modulated carrier frequency which is reproducibly recordable on a magnetic recording medium, described herein in connection with the exemplifications of the present invention, embody various other inventive features which are claimed in my copending application Serial No. 540,667, filed June 16, 1944, as a continuation-in-part of the application Serial No. 399,909, filed June 26, 1941.

It will be apparent to those skilled in the art that the novel principles of the ,invention disclosed herein in connection with specific exemplifications thereof will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific exempliflcations of the invention described herein.

I claim:

1. In a system for recording a transient circuit condition which may occur in an electric circuit at an unpredictable instant and for reproducing the record: an endlessly-operating cyclically-effective recording medium; recording means connected to the circuit in which a transient is expected for continuously recording the circuit conditions; means for obliterating the recorded signal; means responsive to an occurrence of a transient circuit condition for rendering the obliterating means inoperative; means operative a time interval thereafter for rendering the recording means inoperative so as to retain on said recording medium a record corresponding to said transient condition; and reproducing means operative to cyclically reproduce the record recorded on said recording medium.

2. In a system for recording a transient circuit condition which may occur in an electric circuit at an unpredictable instant and for reproducing the record: an endless-operating cyclically-effective magnetic recording medium; means for continuously supplying signal voltages of a frequency tions of said circuit; said recording medium being cyclically operated at a predetermined rate correlated to the frequency of said signal voltages;

recording means for continuously magnetically recording on said recording medium a record of said signal voltage and obliterating means for returning a portion of said medium to a uniform state of magnetization a relatively short time interval after a record has been made thereon; means responsive to an occurrence of a transient circuit condition on said circuit for discontinuing the action Of said obliterating means on said medium; means operative a time interval thereafter for rendering the recording means inoperative so as to retain on said recording medium a record corresponding to said transient condition; and reproducing means operative to cyclically reproduce the record recorded on said recording medium.

3. In an arrangement for recording a transient circuit condition of an electric circuit which occurs thereon at an unpredictable instance: an endlessly operating magnetic recording medium; means associated with said circuit for continuously supplying signal voltages of a frequency reproducibly-recordable on said magnetic recording medium and representative of the conditions of said circuit; said recording medium being cyclically operated at a predetermined rate correlated to the frequency of said signal voltages; recording means for continuously magneticallyrecording on said recording medium a record of said signal voltage and obliterating means for continuously obliterating said record; and means re sponsive to an occurrence of a transient circuit condition on said circuit for rendering said obliterating means ineffective and for subsequently rendering the recording means inoperative after a signal corresponding t a predetermined interval of the transient condition has been recorded on said medium.

4. In an arrangement for recording a transient circuit condition of an electric circuit: an endlessly operating magnetic recording medium;

means for continuously supplying signal voltages 'of a frequency reproducibly recordable on said magnetic recording medium and representative of the conditions of said circuit; said recording medium being cyclically operated at a predetermined rate correlated to the frequency of said signal voltages; recording means for continuously magnetically recording on said recording medium a record of said signal voltage and obliterating means for returning a portion of said medium to a uniform state of magnetization a relatively short time interval after a record has been made thereon; and mean responsive to an occurrence of a transient circuit condition on said circuit for discontinuing the action of said obliterating means on said medium and operative a time interval thereafter for discontinuing the action of said recording means on said medium.

5. A device for recording a transient which may occur at an unpredictable instant comprising, in combination, an endless recording medium, means for driving said endless medium, recording means connected to the circuit in which the transient is expected for continuously recording the circuit conditions, means for obliterating the recorded signal, and means responsive to acircuit transient condition for rendering the obliterating means inoperative and for rendering the recording means inoperative after the transient condition has been recorded on the recording medium.

6. The method of visually presenting on an oscillagraph a non-repetitive wave which comprises the steps of: electrically recording the nonrepetitive wave on an endless medium, repeatedly reproducing said record from said medium to establish a cyclic signal the frequency of which corresponds to the rate of repetition of said record of said non-repetitive wave, and controlling the oscillograph by said cyclic signal, the rate of occurrence of said record of said non-repetitive wave being sufliciently high that together with the persistence of the observers eye the trace on the oscillograph corresponding to one cycle of said repetitive wave appears substantially steady.

7. The method of visually presenting on an oscillograph a wave of short duration which comprises'the steps of: electrically recording the short wave on a short endless medium, repeatedly electrically reproducing said record from said short endless medium to establish a cyclic signal hav ing a frequency which equals or exceeds the rate of repetition of said record of said short Wave, and controlling the oscillograph by said reproduced signal, the rate of repetition of said record of said short wave being sufficiently high that together with the persistence of the observer's eye the trace established by said oscillograph appears substantially steady.

8. A device for visually presenting on an oscillograph the wave form of a non-repetitive electrical transient comprising; endless signal storage means, electrical recording means in association with said signal storage means for recording thereon said non-repetitiv wave, means for repeatedly rotating said endless signal storage means at a substantially constant rate of speed, electrical reproducing means in association with said signal storage means for reproducing the record thereon upon each rotation thereof to establish a cyclic signal the frequency of which corresponds to the rate of repetition of said record of said non-repetitive wave, means connecting said electrical reproducing means to said oscillograph for controlling the oscillograph in accordance with said cyclic signal, the rate of occurrence of said record of said non-repetitive wave being sufiiciently high that together with the persistence of the observers eye the trace on the oscillograph corresponding to one cycle of said non-repetitive wave appears substantially steady.

9. A device for visually presenting on an oscillograph the wave form of an electrical transient of short duration comprising, endless signal storage means, electrical recording means in association with said signal storage means for recording thereon said wave form of short duration, means for repeatedly rotating said endless signal storage means at a substantially constant rate of speed, the period of rotation of said storage means being comparable to the duration of said electrical transient, electrical reproducing means in association with said signal storage means for reproducing the record thereon upon each rotation thereof to establish a repeating wave, means connecting said electrical reproducing means to said oscillograph for controlling the oscillograph in accordance with said repeating wave, the rate of occurrence of said repeating wave being sufficiently high that the trace on the oscillograph appears substantially steady.

10. A device as set forth in claim 9 in which said transient of short duration is less than one second long.

11. A device for use in recording a transient signal comprising, in combination, an endless magnetic record material, means for driving said record material, magnetic recording means associated with said record material and connected to a circuit in which said transient is expected, said recording means being adapted to record on said record material a signal corresponding to the normal and abnormal conditions of the circuit, magnetic obliterating means continuously obliterating th recorded signal during normal conditions of the circuit, means operable upon the occurrence of an abnormal condition for rendering said obliterating means inoperative whereby a signal corresponding to the obnormal condition recorded on said record material is retained. and means operable upon the occurrence of the obnormal condition and after a time delay for rendering said recording means inoperative to prevent a. subsequent record from being superimposed on said recorded transient signal.

12. A device for recording a transient signal comprising, in combination, endless record material, means" for cyclically driving said record material, recording means associated with said record material and connected to a circuit in which said transient is expected, said recording means being adapted to record on said record material a signal corresponding to the conditions of the circuit, means for continuously erasing said record during normal conditions of the circuit, switching means operable upon the occurrence of an abnormal condition for rendering said erasing means inoperative whereby a signal corresponding to the abnormal condition recorded on said record material is retained, and switching means operable upon the occurrence of the abnormal condition and after a time delay of a duration no greater than the length of time for one cycle of the record material for rendering said recording means inoperative to prevent a subsequent record from being recorded on said record means.

13. A device for recording a transient comprising in combination, an endless recording medium, means for cyclically driving said recording medium, recording means associated with said recording medium and adapted to be connected to a circuit in which a transient might occur for recording the circuit conditions on said medium, obliterating means for destroying th recorded record after a short time interval, means operable upon the occurrence of a transient in the circuit for rendering said obliterating means inoperative, and means operable upon the occurrence of said transient for rendering said recording means inoperative after a short time delay 14. A device as set forth in claim 13 in which said recording means includes a source of carrier current and means for modulatin said carrier current by the transient.

15. A device for visually presenting on an oscilloscope the wave form of a non-repetitive electrical transient comprising: endless magnetic tape means, magnetic means for recording the entire non-repetitive transient on said magnetic tape means, means for cyclically driving said magnetic tape means, magnetic reproducing means actuated by the record of the non-repetitive transient during the cyclic rotation of the endless tape means, said magnetic reproducing means controlling said oscilloscope means to repeatedly show the transient wave form, the rate of reproduction of the wave form being sufiiciently high, and the persistence of the screen and of the human eye being such that the trace on the oscilloscope screen appears substantially steady.

16. A device for converting a non-repetitive signal transient into a substantially constant sig- 'nal which can be seen on an oscilloscope unit comprising, an endless magnetic tape, means for magnetically recording the entire non-repetitive transient on said magnetic tape, magnetic reproducing means actuated by the record on the magnetic tape and connected to the said oscilloscope unit for producing on the oscilloscope a visual trace of said entire non-repetitive transient, and means for repeatedly rotating said endles tape to cause said visual trace to repeatedly appear on said oscilloscope, the rate of repetition of said trace being sufliciently high, and the persistence of the screen and of the human eye being such that the trace on the screen of the oscilloscope unit appears substantially steady.

17. A device for use in recording a transient signal comprising in combination, an endless record member, means for driving said record member, recording means associated with said record member and connected to a circuit in which said transient is expected, said recording means including a source of carrier current and mean for modulating said carrier current by the signal in said circuit whereby said recording means records continuously on said record member, obliterating means associated with said record member for continuously restoring said record member to the condition it was in prior to recording, first switching means operable upon the event of a transient in said circuit for rendering said obliterating means inoperative, and second switching means operable upon th event of the transient in said circuit for rendering said recording means inoperative.

18. A device as set forth in claim 17 in which said record member is cyclically driven and the second switching means operates to render the recording means inoperative after a time delay which is equal to or slightly less than the time for one cycle of said record member whereby the transient is retained on the record member,

19. In a system for reproducing a characteristic image of an electric transient signal: an endless recording medium; means for providing a carrier current of a frequency reproducibly recordable on said recording medium; said recording medium being cyclically operated at a predetermined rate correlated to the frequency of the carrier current; modulator means for causing said signal to modulate said carrier current and providing a modulated signal; said modulator means being arranged to substantially balance out the said transient signal; recording means for recording said modulated signal on said recording medium; said recording means including means for amplifying said modulated signal and filter means interposed in the path of the amplified modulated signal and designed to pass a range of frequencies including the frequency of the modulated carrier current and side bands corresponding to said transient signal and to restrict the passage of other frequencies so that the signals of the passed frequency range are efiectively recorded; reproducing means including amplifying means for reproducing said modulated signal from said recording medium and for amplifying the reproduced signal; said reproducing means including filter means de-' signed to pass a range of frequencies including the frequency of the modulated carrier current and side bands corresponding to said transient signal and to restrict other frequencies and demodulator means for demodulating the reproduced signals passed by said filter means and so correlated to the other elements of the system that the demodulated reproduced signals correspond in a predetermined manner to said original transient; and visual indicating means actuated by said demodulated reproduced signals; the rate of the cyclical operation of the recording medium being correlated to the characteristics of the indicating means so that in response to actuation by said demodulated reproduced signals it produces a visually observable characteristic image of said transient signal.

20. In a system for reproducing a characteristic image of a transient signal: an endless recording medium; means for providing a carrier current; said recording medium being cyclically operated at a predetermined rate correlated to the frequency of the carrier current; modulator means adapted to combine said carrier current and said transient signal and cause said signal to modulate said carrier current and provide a modulated signal; said modulator means being arranged to substantially balance out the said transient signal; recording means including means for amplifying said modulated signal and means for recording said modulated signal on said recording medium; reproducing mean including means for reproducing said modulated signal from said recording means and means for amplifying said reproduced signal; said reproducing means including means for demodulating said modulated signal so as to provide an image signal corresponding to said original transient signal; and visual indicating means actuated by said image signal; the rate of the cyclical operation of the recording medium being correlated to the characteristics of the indicating means so that in response to actuation by said demodulated reproduced signals it produces a visually observable characteristic image of said transient signal.

21. In the method of recording a transient circuit condition which may occur at an unpredictable instant on an electric circuit subjected to predetermined normal circuit conditions, the procedure of continuously supplying signal voltages of a frequency reproducibly recordable on a magnetic recording-medium and representative of the conditions of said circuit; continuously magnetically recording on a cyclically operating endless magnetic recording medium a record of said signal voltages and continuously obliterating said record; stopping further obliteration of said record upon the occurrence of a transient circuit condition; and stopping further recording a predetermined time after the obliteration of the record was stopped.

22. In the method of observing a transient signal, the procedure of: providing a carrier current of a frequency reproducibly-recordable on a magnetic recording medium; producing a modulated signal by modulating said carrier current by the transient signal; recording the modulated signal on a magnetic recording medium which is cyclically operated at a frequenc correlated to the carrier frequency; cyclically reproducing from the recording medium the recorded modulated sign-a1 and demodulating the reproduced signal; and controlling an indicating device in accordance with said cyclicall reproduced demodulated signal.

23. In the method of observing a transient signal, the procedure of providing a carrier current of a frequency reproducibly-recordable on a magnetic recording medium; producing a modulated signal by modulating said carrier current by the transient signal; recording the modulated signal on a magnetic recording medium which is cyclically operated at a frequency correlated to the carrier frequency; cyclicall reproducing from the recording medium the recorded modulated signal and demodulating the reproduced signal; and controlling a trace-producing electron beam by the cyclically reproduced demodulated signal so as to provide a visually observable trace.

24. In the method of observing a signal, the procedure of: providing a carrier current. of a frequency reproducibly-recordable on a magnetic recording medium; producing a modulated signal by modulating said carrier current by the signal; recording the modulated signal on a magnetic recording medium which is cyclically operated alt-"a, frequency correlated to the carrier fre- .providea visually observable trace.

25. In a system for studying a transient signal; an endless magnetic recording medium; a source of carrier current of a frequency reproduciblyrecordable on said recording medium; said mag- I netic recording medium being cyclically operated at a predetermined rate correlated to the frequency of said carrier current; modulator means for causing said signal to modulate said carrier current by said transient signal and providing a correspondingly modulated signal; recording means for magneticall recording on said medium a signal corresponding to said modulated signal; reproducing means including demodulating means for cyclically reproducing from said medium the recorded modulated signal and for demodulating the reproduced signal so as to cyclically provide a demodulated reproduced signal corresponding to the transient signal; and cyclically-operative indicating means cyclically actuated by said demodulated reproduced signal for presenting said reproduced signal upon each cycle of said medium.

26. In a system for studying a transient signal: an endless cyclically-operating magnetic recording medium; a source of carrier current of a frequency reproducibly-recordable on said medium; modulator means adapted to modulate said carrier current by said transient signal for providing a correspondingly modulated carrier current; magnetic recording means arranged to continuously record on said medium a magnetic si nal corresponding to the said carrier current and operative upon the occurrence of a transient to record on said medium a modulated signal corresponding to the said modulated carrier current; magnetic obliterating means spaced from said recording means for continuously obliterating the recorded signal so that said endless medium always includes a short medium portion corresponding to the space between the recording and obliterating means upon which the record has been obliterated and a medium portion upon which a record has been made; means actuated upon the occurence of a transient to first render said obliterating means inoperative and to render, a relatively short time interval thereafter, said reproducing means inoperative so as to retain on said medium a record of a modulated signal corresponding to the transient signal; oscilloscope means; and reproducing means including demodulating means associated with said medium and interconected with said oscilloscope means for cyclically reproducing the signals recorded on said medium during each entire cycle and for cyclically impressing on said oscilloscope means unmodulated or modulated si nals corresponding to the signals reproduced from said medium so as to indicate overmodulation of the carrier by comparing the recorded and unrecorded portions of the medium.

27. In the method of recording a transient cincuit condition of an electric circuit, the procedure of: continuously supplying signal voltages of a frequency reproducibly-recordable on a magnetic recording medium and representative of the conditions of said circuit; continuously magnetically recordin on a cyclically-operating endless magnetic medium a record of said signal voltages; obliterating each record element recorded on the medium prior to the end of the cycle during which it was recorded; stopping obliteration of said record upon the occurrence of a transient circuit condition which it is desired to record; and stoppin further recording a predetermined time after stopping the obliteration.

28. In a system for studying a transient signal: an endless magnetic recording medium; a source of carrier current of a frequency reproduciblyrecordable on said recording medium; said magnetic recording medium being cyclically operated at a predetermined rate correlated to the frequency of said carrier current; modulator means for causing said signal to modulate said carrier current by said transient signal and providing a correspondingly modulated signal; recording means for magnetically recording on said medium a signal corresponding to said modulated signal; reproducing means including demodulating means for cyclically reproducing from said medium the recorded modulated signal and for demodulating the reproduced signal so as to cyclically provide a demodulated reproduced signal corresponding to the transient signal; and oscilloscope means cyclically actuated by said reproduced demodulated signal for presenting said signal upon'each cycle of said medium, the cyclical rate of said medium being sufficiently high so that a substantially steady picture of said signal is produced on said oscilloscope.

' 29. A system as defined in claim 28 in which said transient signal is a direct current signal.

30. A system as defined in claim 28 in which said carrier current has a frequency sufficiently low that when modulated by an alternating transient signal the resulting upper side band is within the range which can be magnetically recorded and reproduced. 1

31. A system as defined in claim 28 in which said carrier current has a frequency sufliciently high that when modulated by a very low or direct current transient signal the resulting lower side band is within the range which can be magnetically recorded and reproduced.

SEMI JOSEPH BEGUN.

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