US2143875A - Multiplex facsimile printer system - Google Patents

Description

Jan. 17, 1939. c. w. HANsl-:LL

MULTIPLEX FACSIMILE PRINTER SYSTEM Filed Dec. ll, 1934 2 Sheets-Sheet l INVENTOR C W. HANSELL ww QE Jan. 17, 1939.

C. W. HANSELL MULTIPLEX FACSIMILE PRINTER SYSTEM Filed DeC.

2 Sheets-Sheet 2 ATTORNEY Pamesa han, 1939 PATENT GFFICE MULTIPLE FACSIMILE PRINTER SYSTEM Clarence W. Hansell. Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 11, 1934, Serial No. 756,926

6Claims.

This invention relates to improvements in multiplex facsimile printer systems.

An object of the present invention is to provide a system for transmitting a facsimile reproduction of a single line of type by means of a multiplex system employing a plurality of channels, each channel having individual thereto an element for scanning the message.

A further object of the invention is to obtain a simple form of scanning system, wherein any desired kind of tape such as ordinary paper, foil, metal or the like, may be employed.

A feature of the invention resides in the use A of multiple brushes, or needle contacts for scanning the message, there being at least one of such contacts individual to a channel. In one embodiment there is employed a pair of contacts for each channel.

In general, the present invention provides a multiplex facsimile system in which a single line of type can be transmitted and recorded by a single passage of the transmitter and receiver tapes through the respective channel equipments.`

- face the message characters in electrically conductive letters, and the entire width of the line is arranged to be explored by the ten brushes as the tape is passed underneath them. Each brush has individually connected to it a source of oscillations of a particular frequency, preferably an audio frequency, whereby the current through each brush is varied in dependence upon contacting the electrically conductive letters. The currents through the ten brushes, which may be ten different audio frequencies required for transmission, or direct currents which control ten audio frequency currents, are then impressed on a common line circuit and closed through a transmitter for transmission over the ether, if a radio circuit is used, or over aline to a distant receiver if a. land line circuit is used.

In order to reduce undesired fading due to the e'ect of carrier amplitude uctuations,'and to reduce harmonic distortions which cause interference between channels, it is proposed to eliminate the carrier at the transmitter and to send out both side bands, and at the receiver to reintroduce the carrier before detecting the signal waves.

In receiving, the individual audio frequencies corresponding to each channel are separated by means of filters, and the two rectified currents corresponding to each channel combine to reproduce the message at a scanning head, which may, if desired, be identical with that used at the transmitter. 'I'hus both side bands may be used without introducing distortion due to phase fluctuations, and benefit is obtained from the amplitude diversity effect of using two side bands on each channel.

I'he `advantages of using such a multiplex facsimile printer arrangement 4as outlined above are as follows:

1. There is no need for synchronizing equipment to keep the terminal omce transmitting and receiving equipment in synchronism, as must be done in the full page facsimile system. It is only necessary to run the receiving tape at approximately the same speed as the transmitting tape, and this may readily be judged by eye and set by hand.

2. 'I'he messages sent out by the multiplex facsimile system are relatively secret, even without the provision of any special secrecy equipment. However, the messages may be made still more secret by providing like cams at transmitter and receiver to continuously vary the relative positions of the ten transmitting and receiving points and by frequently changing the assignments of the radio' channels among the ten scanning points.

3. By using the multiplex principle, the keying speed on each channel is reduced to a small fraction of that required to scan a given area in a given time by the single channel method. If ten channels are employed, then the keying speed, or modulating frequency, of each channel is reduced to 10% of that required when a single channel is used. For this reason, by the use of suitable equipment, the echo phenomena caused by multipath transmission in the radio circuit has less distorting effect upon the reproduction. In addition, the reduced speed of each channel permits the use of relatively simple transmitting and receiving equipment. At the transmitter, for example, there may be used the new photocells manufactured by the Weston Electrical Instrument Co. to operate relays and control audio channels directly without requiring any vacuum tubes or battery supply. At the receiver, ordinary electromagn'etically operated vibrators will record as rapidly as will be needed for most circuits.

4. By using a single line of type sent and received, with, let us say, paper tape, very little,

if any, scanning of blank space is required. Therefore the total intelligence transmitted per unit frequency band per unit of time is better than that obtained with the full page facsimile 5 system. Even on fairly closely written messages the gain may easily be about two to one, and, on the average, considerably more.

5. The facsimile method, as distinguished from code operated printers, is relatively free from errors, and even when letters are mutilated such fact will be recognized. Incorrect, unrecognizable errors and substitutions are practically absent. With the terminal equipment which may be used, the probability of whole letters being missed is greatly reduced by staggering the pickup and recording points lengthwise of the tape. For a letter to be missed entirely, a fade or interference must last for the time required to pass several letters through the equipment.

80 6. The operation of the multiplex facsimile communication system does not require operators specially trained to be eflicient in the Morse or Continental telegraph codes. It is only necessary to employ skilled typists to type messages on tape at the transmitter and to copy them on message blanks at the receiver. 'Ihis should make possible a considerable reduction in central oiiice personnel.

'7. If it is assumed that the power of the trans- N mitter, or its full modulation swing, used on one channel is 100%, then if ten channels are multiplexed on it the amplitude of the modulation allowable on each channel for ideal conditions is 10%, and the power used per channel is only 1%.

35 In other words, multiplexing theoretically reduces the power per channel in proportion to the square of the number of channels. Actually, with the multiplex facsimile scheme, considerable allowance may be made for the diversity in phase of the 40 audio modulating currents and the timing of the marking intervals on the channels. That is, the system may be so set up that all the channels never mark, and all audio frequency currents are never at their peak value with like polarity at the 45 same time, so that one might have a considerable increase in the amount of modulation per channel due to the fact that the modulations do not all add together in phase at the same time, and very seldom, if ever, would all channels be modu- 50 lated at once. By using the equipment without any particular regard to' the phase relations of the ten tones, the amplitude per channel can be set at one-quarter the amplitude which could be allowed for a single tone. This corresponds to 55 a power per channel of 6.25% instead of 1% of that allowable when all the modulation is concentrated on a single tone. Since in addition to time diversity in the channels there may be` tolerated occasional slight overmodulation, the power 60 per channel may be increased to 10% without causing undue diiculties. Since there are ten channels, the sum total of modulation therefore may amount to 100% of that obtainable on av single channel. In other words, the power ei- 65 ciency of the multiplex system may be set up to be approximately the same as the eiciency of a is approximately six times that of a single tone amplitude modulated transmitter.

9. At the receiver, use of automatic tuning control with the phase or frequency modulated circuit permits the increase in selectivity of the re- 5 ceiving system to a point where there is admitted a frequency band only great enough to include the useful modulation.

The foregoing advantages will be more readily apparent from a reading of the following de- 10 tailed description which is accompanied by drawings wherein:

Figs. 1 and 2 illustrate two different embodiments of transmitting circuits for multiplex facsimile printer systems embodying the principles 15 of the present invention, the differencein the two circuits being mainly in the kind of tape used and the manner of sending out phase modulated waves instead of amplitude modulated waves.

Fig. 3 illustrates one form of receiving appa- 2o ratus which can be used to receive the phase transmitting waves.

In Fig. l there is shown a multiplex facsimile transmitter system for ten channels. A multitone generator l of any well known form is ar- 25 ranged to generate ten different audio frequencies fl, f2, f3, etc., one for each channel, which frequencies are individually passed by sending vlters 2, to scanning contacts 3, which may be brushes or needles arranged to bear down on a tape 4. vTape 4 is comprised of a non-conductive fine tissue having written or printed thereon the message in conductive letters, this tape being passed underneath the scanning contacts. Each channel is shown as having a separate contact 3 associated with its filter and another contact such as 3 which is in a common circuit with 4 the other channels, whereby when the two contacts of each channel are connected together upon engaging a letter on the tape, an obvious 40 circuit will be closed to send out the audio frequency characteristic of the channel over line 5, which line is common to all the channels.

Tape 4 may be made of paper upon which is placed any suitable conductive solution, for example, potassium, ferricyanide, and sodium chloride. For that matter the surface may be rendered conductive by marking with an ordinary lead pencil. The tape may consist of a thin foil mounted upon a stout paper backing and, if so, the surface of the foil should be covered with a non-conductive fine tissue and the message placed upon this tissue. In such case a special solution of graphite may be used for rendering the tape electrically conductive where marked by the stylus or pen. When the metal foil is used the foil may serve as a common contact for all the scanning points.

Instead of using paper or a metal foil, an endless metal tape may be employed. This tape may be passed through a liquid solution, which, when dried, leaves an insulating lm on the tape. Then, when writing or typewritten matter is put on the lm by marking with an electrically concluctive element such as graphite, in the same liquid as the insulator, the insulating material will be pushed and Washed aside and its place taken by conducting material. The tape may then be run under the electrical contacts, its message sent, and subsequently it may be washed, returned to the insulating solution, and the whole process repeated.

It is preferred that the metal tape be of a color which is as different as possible from the marking material so as to make the record readily visible to the operator. For economical reasons, however, a polished steel tape is desired. The solution for the insulating and marking material should preferably be in water and slightly alkaline to prevent corrosion of the tape. 'Ihe insulating material might be made of such elements as water glass, sugar, alum, borax, cadmium, sulphide, camphor, dextrin, starch, Egyptian blue, glue, mucilage, etc.

The marking fluid might be a colloidal solution of any conducting material which is chemically stable in water. For example, carbon, tungsten, silver and platinum might be used. If desired, the marking may be done with a solution which reacts chemically with the material in or on the paper to deposit or precipitate out a conducting material. If desired, solutions ,of deliquescent material, for example a solution of deliquescent salts-which retain moisture and are conducting, might be used for passage under the transmitter contacts. In the latter case coloring matter would be added to the solution in order y to make it legible on the transmitting tape. As

an alternative to coloring the salt solution, substances might be put in which would decompose due to the passage of electrical current to form avisible record. The record would then be developed by the current flowing through it as itA passed under the transmitting contact points. The record so formed would thus represent a true copy of the message as sent through the transmitter and so would be of value as a check on the functioning of the transmitter. This may be accomplished, for example, by using iron scanning electrodes and a marking fluid containing potassium iodide or a combination of potassium ferricyanide and sodiumchloride.

The tape may be impressed with the message characters either by writing or by a typewriter whose keys hit upon a ribbon placed above the tape and the thin tape then passed under the sending contacts. If desired, thev tape may be wound around a drum 6, as shown in Fig. 2, which drum is rotated to move the type past the scanning contacts. In using an arrangement such as shown in Fig. 2, it is preferred to use metal tape of the type mentioned above, whereupon it is only required that there be one scanning Contact 3 associated .with each channel, the other contact 3" in common with all of the channels being in contact with the tape through the drum, whereby closure of both contacts will occur when 3 engages a conductive symbol or letter on the tape.

As the tape passes beneath the sending con` tacts, there is produced a complex wave representing the sum of the waves of frequencies fI f2,

.f3 etc. at any instant, depending upon the number of channels closed through by the configuration of the letter at that instant, which complex wave is transferred over line 5 through a suitable audio amplification means such as 1, and then through a land line 8 leading from the transmitter tothe receiver in the case of wire transmission, or to a radio transmitter in the case of radio transmission.

At the radio transmitting station there is a source of carrier frequency 9 which preferably is controlled by a crystal I0. The carrier energy is supplied symmetrically to a push-pull modulator II, as shown in Fig. 1 of the drawings, in consequence of which the carrier Nis eliminated from the output I2, and only the twoy side bands and the original modulating frequencies are produced, which are then amplified by power ampliauasvs one of the side bands are eliminated, the other side band being amplified by the power amplifier I 4.

'I'he system of Fig. 2 is very similar to that of stead of a paper or metal foil tape, and phase or frequency modulation is employed by means of suitable well known apparatus, herein conventionally indicated by box I6. It is preferred to employ the transmitter systeln of this figure instead of that of Fig. 1, because of the greater amount of power which may be obtained over each channel.

For receiving the signals sent out over the system of Fig. 1, any suitable arrangement may be employed wherein a carrier wave is reintroduced at the receiver and the audio frequencies characteristic of the various channels filtered out to record the signals on a suitable scanning head which preferably is identical with that used on the transmitter. One suitable way of accomplishing this is disclosed in my United States Patent No. 1,751,584, to which attention is invited.

Fig. 3 illustrates the preferred form of receiver for receiving phase modulated waves transmitted over the circuit of Fig. 2. This apparatus comprises a special phase modulation telephone receiver having two sets of audio frequency filters and two sets of rectifiers or thyratrons operated from the output of the receiving filters. The operation of this receiver is as follows:

The incoming phase modulated radio frequency energy picked by antenna 40 is increased in power ina high frequency amplifier tube 22, and the amplified energy is beat down to an intermediate frequency in a detector 23, which intermediate frequency energy is further amplified in amplifier 24. The output of amplifier 2l is then divided into two portions, one of these portions being the carrier wave and the other being the side frequency waves produced by the multiplex modulation of the carrier at the distant transmitter. The carrier wave is separated out from the complex signal by means of a neutralized piezo-electric crystal filter 25 whose selectivity is so great that in the output there appears only the unmodulated carrier. This carrier is then amplified and limited in limiter 26 so that its strength will remain constant regardless of variations inthe strength of the carrier picked up by the antenna. The limiting tends to eliminate the effects of carrier fading. After limiting, the carrier, whose strength is made relatively large, is split into two componentshaving equal, strength but differing in phase by 0 or 180, one of thesev components being applied co-phasially to the control electrodes of the detector 21, and the other component being applied co-phasially to the control electrodes of detector 26. 'I'ne strength of carrier introduced into the two detectors is made relatively large.

Another portion of the intermediate frequency energy is taken from the output of amplifier 2l and applied to high and low pass filters 29 and 30. These filters separate those frequencies lying above and below the carrier frequency into two independent circuits. The carrier may or may not be permitted to pass through the filters but this is not essential since if carrier energy does pass through it will reach the two detectors 21 and 28 with a strength very much less than that applied from the limiter 26. Assuming that Fig. 1, except that herein a steel tape is used inthere are upper and lower side frequencies produced by the multiplex modulation divided into two circuits, the energy of one group of side frequencies is applied to both pairs of detectors 21 and 28 in like phase relation. The other set of side frequencies is applied to the two detectors 21 and 28 in reversed phase relation. Under these conditions, taking into account the fact that detectors 21 and 28 are supplied with carrier energy in 0 or 180 phase relation, the outputs from the two detectors 21 and 28 will differ in audio phase by 0 or 180. The outputs from the two detectors are passed through transformers 3| and 32, each of which has two secondary windings. One secondary winding of each transformer is connected in series with a similar winding on the other transformer with like polarity, and the other secondary winding of each transformer is connected in series with the corresponding winding of the other transformer with reversed polarity.

Under these conditions, if the conductive connections from the low pass lter 30 are interrupted, it can be seen that the energy from high pass filter 29 will produce outputs from both detectors 21 and 28 which will add together in one of the serially connected output transformer cir'- cuits but oppose each other in the other serially connected output transformer. If, on the other hand, the energy from high pass lter 29 is instead interrupted but that from filter 30 allowed to pass then there will be obtained output energy in the secondary circuits of the output transformer in the other of the two serially connected output circuits. In other words, in each of the two output circuits energy is obtained corresponding to only one of the `side bands produced by the modulation. When the modulation consists of intermittent audio tones of different frequency produced by the scanning of the tape at the transmitter one should normally find a corresponding audio modulation in both of the serially connected output circuits. However, if radio conditions are such that one side frequency or side band has faded out or been reduced in amplitude the corresponding side frequency or frequencies on the other side of the carrier frequency may still bevpresent. Only one side frequency corresponding to any one modulated frequency needs to be present in the signal picked up by the signal in order that there may be a corresponding output in one or the other of the serially connected output circuits. If desired, the series of keyed tone frequencies in each output circuit may be taken, separated from one another by means of band passlters and then the energy of each tone converted into a direct current. A similar treatment may be given to the several tones in the other output circuit. Under fading conditions the tone outputs in either of the output circuits may fail fairly frequently but only occasionally will be outputs from both output circuits fail simultaneously. In other words, there is a diversity effect against fading in using the two side bands independently. If it is attempted to detect the modulation with the ordinary detector scheme wherein both the side frequencies are used toproduce each audio output, very frequent failures will occur due to the phase relation of the carrier Wave varying with respect to the side band waves during transmission over the radio circuit.

It is interesting to note that if the phase relation of the carrier with respect to side frequencies is varied continuously, then the beat frequency outputs of detectors 21 and 28 to either output circuit will vary sinusoidally and differentially in amplitude but can never oppose in either output circuit because of the phase relation between beat frequency outputs. 'Ihe vector sum of the two detector outputs in either output circuit remains constant even though the phase relation of the carrier ls continually changing. For this reason there is no need for the usual fixed relation in phase between carrier and side frequencies. Therefore, large carrier phase variations in transmission do not produce the usual modulation fading. Also, if desired, the carrier at the transmitter may be suppressed and its place taken by a non-synchronous carrier generated and introduced at the receiver.

Instead of converting each tone frequency from each output circuit to direct current in the ordinary manner, itis preferred to `use these energies to control a pulsating direct current more suitable for actuating the vibrator of a multiplex facsimile recorder. Assuming, for example, that one of the multiplexed channels employs a keyed tone frequency of 1000 cycles, then under normal conditions 1000 cycle energy will appear in both of the receiver output circuits. If band pass filters 33, 34 will pass 1000 cycles, then keyed 1000 cycle energy will reach the grids of two thyratrons or grid controlled glow discharge tubes 35, 36. The presence of audio excitation on the grids of the thyratrons will cause the gas Within the tubes to be ionized when anode voltage is applied. This will permit anode current to pass. If the anodes of the two thyratrons are excited differentially by means of 60 cycle alternating current from the transformer I1, as shown, then so long as a 1000 cycle tone is present in both output circuits both tubes will carry current on the positive half of the 60 cycle voltage applied to their anodes. As a result, a pulsating direct current which is modulated at the rate of cycles will be applied to the coil of the recorder vibrator I8 and will cause the vibrator to record marks on a suitable recording tape corresponding to the markings on the transmitter tape at the sending station. If either side frequency in the incoming signal fades out, the recorder vibrator will still be operated if the opposite corresponding side frequency is still present, though the recording will ybe perhaps slightly less efficient. A pair of audio frequency band pass lters corresponding to filters 33, 34, will be used for each tone frequency used in the multiplex facsimile transmission. Of course, if desired, the tubes 35, 36 may be of the high vacuum type or they may be replaced by oxide rectiers or any other suitable means for controlling a'direct or a pulsating current, which can be used for recording purposes. Preferably ten or twelve transmitting channels and ten .f or twelve recorder vibrators would be used. The recorder vibrators Amay print 'the message on a paper tape by tapping a typewriter ribbon or a strip of carbon paper, or by any other means by which electrical energy may be used to make markings on the paper.

It will. be noted that the output `of the special receiver noted above gives two side bands which are .composed of the ten transmitted tones in a single complex Wave. The audio filters 33, 34 are really wave analyzers,there being one pair for each of the two tones, so that eachl of the complex side bands are resolved into ten component tones; therefore, since there is an upper and a lower complex side band, there will .be an upper and a lower tone side band for each transmitted tone. Fig. 3 shows only one of ten units composed of audio filters 33, 34, Thyratrons 35, 38, and recorder vibrator I8. As explained above, the audio filter passes one tone side band. One tone side band is impressed on the grid of one Thyratron and the other tone side band is impressed on the grid of the other Thyratron. Thus either side band will cause a half wave rectified current to flow through the recorder vibrator Winding in pulses 60 per second. Each pulse causes the vibrator to move against a light spring.

the spring lifting the vibrator between impulses. The ten vibrators are arranged so as to account for that element of the signal mark corresponding to the tone which was produced by scanning a single element at the transmitter. The vibrator engages a carbon paper tape or typewriter ribbon, which rests in contact with a white paper tape, to reproduce the signal mark scanned.

In order that the receiver may be made more efficient, it is preferred to apply an automatic tuning means for correcting the receiver adjustments and holding optimum conditions for reception. In doing this a phase detector I9 may be used for detecting the phase relation between radio frequency energies of the carrier before and after the crystal filter 25. Several different methods are well known in the art for varying the value of a direct current by means of the phase relation between two alternating current energies. Any one of these schemes may be used in the detector I9. A direct current output is taken from detector I9 which varies in strength or polarity or both in accordance with the phase relation of radio frequency energies across input and output of the crystal filter. The direct current energy is then used in any one of severalwell known schemes for varying the frequency of the first beating oscillator in the receiver. When connections and adjustments are made correctly, any change in the frequency of the intermediate frequency energy will be accompanied by a phase shift in the radio frequency energy on the two sides of the crystal filter and this phase shift will automatically react upon the first beating oscillator to reduce the change in intermediate frequency. Thus, once the receiver is adjusted to the transmitter, it will keep itself correctly adjusted without manual manipulation.

It is to be understood that the invention is not limited to the precise arrangements of parts shown since various modifications may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. In combination in a receiving circuit, a pair of filters, means for receiving signals and for applying the modulations derived from the side bands of said signals to said filters, a pair of gasfilled electron discharge devices each being individual to a filter and each having a cathode, grid and anode, the grids and cathodes of said respective devices being connected to said filters, the anodes of said devices being connected together through a coil, a vibrator having a winding and an armature responsive to energy in said winding, a connection from one terminal of said vibrator winding to a point intermediate the ends of said coil, and a connection from the other terminal of said winding to said cathodes, and means for applying alternating potentials to said coil.

2. In combination, in a receiving circuit, a pair of audio frequency by-pass filters, means for receiving signals and applying the modulations derived from the side bands of said signals to said filters, a pair of gaseous discharge devices each having an input and an output circuit, a connection from one of said input circuits to one of said filters, and a connection from the other of said input circuits to the other filter, means for connecting said output circuits together and to a recording instrument, and means for applying periodically varying potentials to said output circuits in opposite senses.

3. In combination, in a receiving circuit, a pair of filters, means for receiving signals and applying the modulations derived from the side bands of said signals to said filters, a pair of electron discharge devices each having an input and an output circuit, connections from said input circuits to said filters, means for connecting said output circuits together and to a utilization circuit, and means for applying potentials varying periodically at an audio frequency rate to said output circuits in opposite senses 4. In combination in a receiving circuit, a pair of filters, means for receiving signals and for applying the modulations derived from the side bands of said signals to said filters, a. pair of electron discharge devices each being individual to a filter and each having a cathode, grid and anode, the grids and cathodes of said respective devices being connected to said filters, the anodes of said devices being connected together through a coil, a vibrator having a winding and an armature responsive to energy in said winding, a connection from one terminal of said vibratorwinding to a point intermediate the ends of said coil, and a connection from the other terminal of said winding to said cathodes, and means for applying alternating potentials to said coil.

5. In combination in a receiving circuit, a pair of audio frequency filters, means for receiving signals and for applying the modulations derived from the side bands of said signals to said filters, a pair of gas-filled electron discharge devices each being individual to a filter and each having a cathode, grid and anode, the grids and cathodes of said respective devices being connected to said filters, the anodes of said devices being connected together through a coil, a vibrator having a winding and an armature responsive to energy in said winding, a connection from one terminal of said vibrator winding to a point intermediate the ends of said coil, and a connection from the other terminal of said winding to said cathodes, and means for exciting the anodes of said devices differentially.

6. In combination in a receiving circuit, a pair of filters, means for receiving signals and for applying the modulations derived from the side bands of said signals to said filters, a recording instrument in the form of a vibrator, a winding' for said vibrator, a pair of electron discharge devices each having an anode and a grid, means for connecting the anodes of said devices to said winding and the grids of said devices each to a. different filter, and a source of pulsating energy for exciting said anodes differentially.

CLARENCE W. HANSELL.

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