US2251525A - Secret television system - Google Patents

Description

A. H. ROSENTHAL SECRET TELEVISION SYSTEM Aug. 5, 1941.v

Filed April 29, 1946 3 Sheets-Sheet l Aug. 5, 1941. A. H. ROSENTHAL m 2,251,525

SECRET TELEVIS ION SYSTEM Filed April 29, 1940 3 Sheets-Sheet 2 Aug. 5, 1941- A. H. ROSENTHAL 2,251,525

SECRET TELEVIS I 0N SYSTEM Filed April 29, 1940 s Sheets-Sheet s I fii'y] I Patented Aug. 5, 1941 SECRET TELEVISION SYSTEM Adolf Heinrlc Rosenthal, Kensington, London England Application April 29, 1940, Serial No. 332,319 In Great Britain March 6, 1939 7 Claims.

This invention relates to the secret transmission of television and like signals.

According to the invention there are provided at the transmitter station means for generating electrical frame synchronizing oscillations, means for utilizing these oscillations to effect scanning of a pattern to generate irregular electrical line synchronizing oscillations, a picture transmitter adapted to be controlled by both line and frame synchronizing oscillations, means for transmitting the frame synchronizing oscillations to a receiving station, means atilthe receiving station for utilizing the received frame synchronizing oscillations to scan a corresponding pattern to generate corresponding line synchronizing oscillations, and means for utilizing the frame and line synchronizing oscillations to control a picture receiver.

By the terms picture transmitter and picture receiver are to be understood those devices in which an optical or electronic image is scanned for the production of picture signals and picture signals are converted into a visible image respectively.

The term synchronizing oscillations as herein used is intended to cover both oscillations in the form of impulses spaced from each other in time and arranged to synchronize the generation of time base oscillations, as well as complete time base oscillations, e. g. saw tooth or sinusoidal oscillations arranged to be directly applied, after suitable amplification if desired, to such scanning units as cathode ray deflector coils.

By giving the patterns for generating the line synchronizing oscillations an irregular configuration, secrecy can be safeguarded, since the impulse can only be imitated at a receiving station possessing such a pattern. I

Certain embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which Fig. 1 shows a transmitting arrangement, Fig. 2 shows a form of scanning pattern,

Fig. 3 shows a receiving arrangement for reconstituting a picture transmitted by the arrangement of Fig. 1,

Figs. 4 and 5 show means for producing regumitter in the frame scanning direction. These frame synchronizing impulses also control the generation of line synchronizing impulses as follows:

The beam of an auxiliary cathode ray tube 4 scans a fluorescent screen 5 along a straight path at the frequency of and under the control of the frame synchronizing impulses. The moving light spot thus produced is imaged by a lens 5a onto and scans a transparent screen structure 6 carrying a system of opaque parallel lines intersecting the straight path of the light spot of a number equal to the number of scanning lines in the picture. The distance between adjacent opaque lines vary arbitrarily about a mean value along the straight path. A portion of such a screen structure is shown in Fig. 2, in which the opaque lines are indicated at H. A photocell 1 is arranged to receive the light through a lens la: from the moving spot that passes through the screen structure. The modulation of the intensity of this light produced by the opaque lines gives rise to electrical impulses in the circuit 8 of the photo-cell. These impulses constitute the line synchronizing impulses and will vary in their timing in accordance with the spacing of the opaque lines. The circuit 8 also includes a time base for producing saw tooth oscillations which are synchronized by the impulses and applied to the deflector plates 9 to control the line scanning in the transmitter tube.

The picture signals produced in the transmitter tube, and the frame synchronizing signals are fed to the transmitter I0 and are transmitted to the receiving station.

At this station, shown in Fig. 3 the picture signals from the receiver, synchronizing signal separator and amplifier l2 control the beam intensity in a picture receiver cathode ray tube l3 in the usual way. The frame synchronizing signals control the frame scanning oscillator M which deflects the beam in the tube l3, and, in addition, controls the generation of line syn,- chronizing impulses in a device l5, l6, l1 similar to the device 4, 5, 6, I used for generating the line synchronizing impulses at the transmitting station shown in Fig. 1. The transparent screen H with its opaque lines in this device is a photographic duplicate of the screen 6 used at the transmitting station. The line scanning osci1- lations thus produced in the time base circuit l8 and synchronized with respect to the impulses derived from the photo-cell I la in the receiver the transmitter as soon asthe correct phase relationship is established between the frame frequency oscillations at transmitter and receiver.

It is desirable that the time base for the scan of the patterns should be linear or at least governed by the same time function at the transmitter and receiver. A diiference in slope of the time characteristics of the two time bases will result in a gradual drift of the starting points of consecutive picture lines, giving a slanting appearance to the vertical edge of the received picture at the side where the line scanning starts. This can easily be adjusted by operating the tuning control of the frame time base.

Owing to the irregular timing of the line synchronizing impulses, the length of the scanning lines both at transmitter and receiver will be different for different lines. To avoid this disadvantage, the actual picture area in the transmitter tube is so arranged that the shortest of the scanning lines just completely cover the picture area in the line direction and the excess length of the longer lines falls outside the picture area. Alternatively the amplitude of the saw tooth current effecting the line scan may be limited to a constant value by a saturated valve circuits l9 (Fig. 1) and 20 (Fig. 3), thus limiting the amplitude of the deflection to a value equal to the extension of the picture area in the line direction.

The variation in the time intervals between the start of successive line scans would, further, result in a varying spacing between successive lines, assuming the frame scanning speed to be constant. Means may therefore be provided both at the transmitter and receiver to adapt the frame component of the picture scan to the varying line scanning intervals. This can be done in various ways.

Thus as illustrated in Fig. 4 there can be provided separate correcting frame deflecting means comprising for example plates 2| which are so energized as to produce a deflection field oppo site in direction to the frame deflection field proper and of a strength proportional to the amount by which the length of the line just being scanned exceeds the dimension of the picture area in the line direction. To achieve this, the

line scanning current may be fed to the correcting deflecting means 2| through circuit 22 including a valve so biased as to pass only amplitude in excess of that required for scanning the complete picture area. Instead of providing separate correcting deflectors, it is possible to superimpose the correcting currents (or voltages) from the circuit 22 on the frame scanning currents (or voltages) applied to the main deflectors.

Alternatively, the line scanning voltages may be utilized to charge to equal amounts a condenser which feeds the frame deflecting means, so that the scanning lines will be displaced from each other in the frame scanning direction by equal steps. This can be done as shown in Fig. 5 in which is illustrated a receiving tube I3. The line scanning voltages are applied from to the condenser 41 through a saturated diode valve so biased that it cuts off the parts of the line voltages corresponding to the varying excess lengths of the lines. The condenser is discharged through another valve 46, which may be of the type known under the registered trademark Thyratron, by the frame synchronizing signals to effect the frame fly-back.

By thus adapting the frame scan to the varying line scanning periods a proper interlaced scanning can be effected notwithstanding the variations in these periods.

The number of lines in the pattern may be a submultiple of the number of, scanning lines, say one tenth, in which case the scanning frequency in th auxiliary cathode ray tube would be equal to ten times the frame frequency, and thesame pattern would be repeatedly scanned for each frame.

The scanning path in the auxiliary cathode ray tube may be circular, the lines of the pattern then being arranged radially on a circular screen, and the scan being effected by two sinusoidal deflections perpendicular to each other and with phase displacement.

In Fig. 6 is illustrated a circuit arrangement for effecting such sinusoidal scanning in the auxiliary cathode ray tube 4, either at the receiver or transmitter. Oscillations from the generator 23 which in'the case of the transmitter is the generator 2 and in the receiver is the circuit [2 are fed to a circuit 24 from which are produced two sinusoidal currents the one being 90 out of phase with respect to the other. These oscillations are fed to two pairs of deflector plates 25 and 26 in the tube 4, the beam of which then executes 'a circular motion over the screen 5 and scans the transparent screen 611..

Several systems of lines may be provided on the same screen to increase the degree of secrecy. In the case of circular scanning these systems are preferably disposed concentricallyyand the scanning spot is caused to,change over from one system to another by simply altering the amplitude of the two oscillations deflecting this beam. For example in the arrangement shown in Fig. 6, two transparent screens 6a and 6b are arranged concentrically, and the change from one to the other may be affected by an amplitude control in the circuit 24. Where straight scanning is used, the line systems can be arranged in bands adjacent to one another in a direction perpendicular to the line scanning direction, that is in the direction perpendicular to the plane of the paper in Figs. 1 and 3, and the beam can be deflected on to one or other of the bands at will.

Alternatively, the light spot scanning may be replaced by an electronic scanning of a pattern screen having suitable properties, as illustrated in Fig. 7. The screen 21 consists of a metal capable, under the operating conditions, of a high secondary electron emission, and narrow radial lines are inscribed in a carbon deposit on its scanned side, the number of these lines being equal to the number of scanning lines in the picture. Th distance between adjacent lines vary arbitrarily about a mean value along the circular path. The plate is incorporated in a circuit 28 including an anode 29 of the tube 4, the circuit 28 being such that each time the cathode ray beam strikes one of the carbon lines, which has a lower secondary electron emission than the metal, an electrical impulse is produced in this circuit. These impulses constitute the lines synchronizing signals which are used as shown in Figs. 1 or 3 and the time intervals between them will vary in accordance with the spacing of the carbon lines. Two or more such screens may be included in corresponding to the screens 6a and 6b of Fig. 6.

If it is desired to apply the invention to television transmitters of the type employing mechanically moving scanning members, it is not always possible to control these members with the irregular synchronizing impulses on account of their inertia. In this case the picture signals produced by the regular scanning of the mechanical apparatus are stored and scanned by electronic means which will respond to the irregular synchronizing signals. For example the arrangement shown in Fig. 8 may be used. A moving fllm 3| is scanned by a beam of light from the source 32 which is swept over the fllm 3| by the mechanical scanning member 33. The scanner 33 is driven by the synchonous motor 35, synchronized by the oscillator 36, the impulse from which are regular. The picture signals generated in the photo-cell 31. placed behind the fllm 3| are amplified at 38 and fed to the modulating electrode of a double cathode raytube 39, having two cathode ray guns l and 4|. The tube 39 includes a mosaic storage screen 42. The oscillator 36 also feeds saw tooth impulses to the deflecting plates 43 of the gun 40, so that the beam from this gun writes on the screen 42 the signals corresponding to a picture line of the fllm 3i. The gun 4| roduces a beam of electrons which follows the beams from the gun 40 and which is deflected by the irregular signals from the cathode ray tube arrangement I, 5, 6, 'I previously described. This beam from the gun 4| generates picture signals in the output circuit 44 of the tube 39. In this case the tube 39 is the picture transmitter as previously defined.

If it is desired to apply the invention to television receivers employing mechanically moving scanning members, somewhat similar methods can be used for controlling these receivers by the irregular synchronizing signals resulting from the pattern, these methods being described in detail in United State patent application Serial No; 280,426, flied June 21st, 1939.

Instead of or in addition to a variation of the time intervals between successive synchronizing signals, a variation in the shapeof the signals may be introduced. Thus, if these signals represent complete time base oscillations, changes in their wave form, such as deviations from a linear character, may be introduced periodically or aperiodically. This may be achieved, for example, by means of a pattern such as is illustrated in Figure 2 but modified by the provision of a suitable graduated shading in each section of the patterns between the lines ll.

claim:

1. Apparatus for the transmission of television and like signals comprising a generator of electrical frame synchronizing oscillations, a scanning device adapted to, be controlled by said frame synchronizing oscillations for scanning a pattern to generate irregular electrical line synchronizing .oscillations, a picture transmitter adapted to be controlled by both line and frame synchronizing oscillations, means for transmitting the frame synchronizing oscillations to a receiving station, a scanning .device at the receiv-' ing station adapted to be controlled by the received frame synchronizing oscillations for scanning a corresponding pattern to generate corresponding line synchronizing oscillations, and

means forautilizing the frame and line synchronizing oscillations to control a picture receiver .2. Apparatus according to claim lTwherein said scanning device comprises a cathode ray tube having a fluorescent screen, means for defleeting the beam along a path on said screen cent spot on said screen to scan a transparent pattern and to generate line sychronizing oscillations in an output circuit connected with said device.

3. Apparatus according to claim 1 wherein said scanning device comprises a cathode ray tube, a secondary emitting electrode therein, and means associated with said tube for causing the beam thereof to scan said electrode, said pattern being in the form of variations in the secondary emitting properties of said electrode.

4. Apparatus for the transmission of television and like signals comprising a generator of electrical frame synchronizing oscillations, a scanning device adapted to be controlled by said synchronizing oscillations for scanning a pattern to generate irregular electrical line synchronizing oscillations, limiting means comprising a frame synchronizing oscillations, means for transmitting the frame synchronizing oscillations to a receiving station, a scanning device at the receiving station adapted to be controlled by the received frame synchronizing oscillations for scanning a corresponding pattern to generate corresponding line synchronizing oscillations, and means for utilizing the frame and line synchronizing oscillations to control a picture receiver.

5. Apparatus for the transmission of television and like signals comprising a generator of electrical frame synchronizing oscillations, a scanning device adapted to becontrolled by said synchronizing oscillations for scanning a pattern to generate irregular electrical line synchronizing oscillations, a picture transmitter adaptedto be controlled by both line and frame synchronizing oscillations, means for adapting the speed of frame scanning in said picture transmitter tothe time intervals between the starts of successive line scans, means for transmitting the frame synchronizing oscillations to a receiving station, a canning device at the receiving station adapted to be controlled by the received frame synchronizing oscillations to scan a corresponding pattern to generate corresponding line synchronizing oscillations, and means for utilizing the frame and line synchronizing oscillations to control a picture receiver, and means for adapting the speed of frame scanning in said picture receiver to the time interval between the starts of successive line scans.

6. Apparatus according to claim 5, wherein said adapting means comprise a condenser, a saturated valve so biased that in operation it cuts oil the parts of the line voltage applied thereto which correspond to the varying excess lengths of the lines. said condenser being arranged to feed said frame deflecting meansand to be discharged once every frame period by a second valve fed with the frame synchronizing signals.

7. Apparatus according to claim 5, wherein said adapting means comprise auxiliary frame deflecting means, and means for energizing said auxiliary deflecting means by electrical oscillations derived from the line synchronizing oscillations.

ADOLF HEINRICH ROSENTHAL.

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