US2147266A - Television and the like transmission system - Google Patents

Description

Feb. 14, 1939.

W. S. PERQWAL ET AL TELEVISION AND THE LIKE TRANSMISSION SYSTEM 'Filed Aug. 3, 1954 5 Sheets-Sheet l 3 Sheets-Sheet 2 Filed Aug. 3, 1954 w. s. PERCIVAL ET AL I u v? TELEVISION AND THE LIKE TRANSMISSION SYSTEM Feb. 14, 1939.-

Feb. 14, 193 w. 5.,PERCIVAL ET AL 9 79 TELEVISION AND THE LIKE TRANSMISSION SYSTEM Filed Aug. 3, 1934 5 Sheets-Sheet 3 0. BR DIVA E Patented Feb. 14, 1939 UNITED STATES PATENT OFFICE TELEVISION AND" THE LIKE TRANSMISSION SYSTEM pany of Great Britain Application August 3, 1934, Serial No. 738,262 In Great Britain August 8, 1933 3 Claims.

The present invention is concerned with television and the like transmission systems of the type in which scanning at the receiver is controlled by impulses generated in dependence upon a scanning operation at the transmitter.

Certain television systems which employ a cathode ray tube at'the receiver to reconstitute the transmitted image; are of this type. In such systems the scanning movement of the cathode ray beam at the receiver is usually eifected by means of electrical oscillations of substantially saw-tooth wave form, one set of such oscillations serving to deflect the beam in one direction over afluorescent screen, and the other, which is generally of a. widely different frequency, serving to cause deflection in another direction, usually at right-angles to the first. An oscillation having a suitable wave form is one which comprises a substantially straight gradually rising portion, during which picture reproduction takes place, followed at once by rapidly falling portion, or return stroke.

The two sets of saw-tooth oscillations are preferably generated at the receiver, synchronism being maintained with the corresponding scanning motions at the transmitter by generating at the transmitter two series of synchronising impulses of different characteristics, separating the two series of synchronising impulses from one another at the receiver, and employing one series to control the generation of one set of saw-tooth oscillations and the other to control the generation of the other set of oscillations. 'In an example, a synchronising impulse, known as a line impulse,

may be generated at the end of each line of the object scanned at the transmitter, and a further series of impulses, known, where the object to be transmitted is a moving picture film, as frame impulses,-may be generated at the frequency at which complete frames of the film are scanned.

The synchronising impulses may be employed at the receiver to control the generation of sawtooth oscillations by means of two saw-tooth wave generators. These generators may each comprise a reservoir condenser which is charged at a uniform rate through a resistance connected to a suitable source of potential, and means, operatively controlled by the synchronising impulses, for periodically discharging the reservoir condenser. The discharging -means may comprise a thermionic valve,the grid circuit of which comprises a condenser which is normally negatively charged tosuch an extent that the grid is strongly negatively biassed, and the anode-cathode path ofthe valve is substantially insulating; the charge on the grid is arranged to leak away gradually through a leak resistance, and it is also arranged that the synchronising impulse arrives at such a time, and in such a sense, that it drives the grid rapidly in the positive direction to such an extent that the valve becomes conducting, and the reservoir condenser is enabled to discharge through the valve. A reaction coupling is usually provided between the grid and anode circuits of the valve and this coupling has the eifect of increasing the rate at which the grid becomes more positive and when the reservoir condenser has discharged, causes the grid to assume again its extreme negative potential, thus completing the cycle. An arrangement of this kind is known as a blocking oscillator. The time constant of the condenser and leak resistance in the grid circuit is usually made such that, in the absence of synchronising impulses, the blocking oscillator would oscillate freely at. a frequency slightly lower than the frequency of the synchronising impulses.

Now it is found desirable tocontrol the line frequency blocking oscillator, which generally operates at a considerably higher frequency than the frame frequency oscillator, by means of synchronising impulses of peaked Wave-form and short duration in order that the return stroke may be initiated as abruptly as possible. It will also be apparent that the line synchronising impulses must be of sufficient amplitude to drive the grid of the blocking oscillator valve sumciently far in the positive direction to trigger off the blocking oscillator and so render the anodecathode path conductive. The frame, or low frequency impulses, on the other hand, are usually made of longer duration than the high frequency impulses, and are given a less peaked wave form. The reason for this is explained below.

The two sets of synchronising impulses are generally mixed at the transmitter and fed to the same transmission channel, and separated from one another at the receiver by means of filter circuits, for example. It will be apparent that the line impulses, which are of peaked wave form and short duration, are characterised by very much higher frequencies than are the frame impulses, which are of less peaked wave form and longer duration. The line impulses may thus be freed from the frame impulses by means of a high-pass filter, for example, which passes a sufficient range of high frequencies to preserve the peaked wave form of the line impulses. The separation of the low-frequency impulses may be effected by means of a low pass filter. Now the duration of each low-frequency synchronising impulse may be equal to the time taken to scan several lines of the picture, and a certain number of high-frequency synchronising impulses will thus be superimposed upon each low-frequency impulse. If this is so, it is apparent that.

the transmission channel, which may for example be a carrier wave, must be capable of accommodating amplitudes up to the sum of the amplitudes of the highand low-frequency impulses. If any limitation of amplitude is imposed, the high-frequency impulse may be of insufficient amplitude at the receiver to control efficiently the oscillation of the high-frequency blocking oscillator, and a gap-effect, due to the high-frequency blocking oscillator oscillating freely at its own natural frequency for one or more whole periods before it is once more held in synchronism, may be introduced. Any attempt to avoid this effect by reducing the amplitude of the low-frequency impulses may introduce the possibility of unreliable synchronisation of the low-frequency blocking oscillator. -It is an object of the present invention to provide a method and means for overcoming the difliculties set out above. Other advantages accruing from the invention will appear from the remainder of this specification.

The present invention accordingly provides, in a television or the like transmission system, the method of synchronising two scanning motions at a receiver with two scanning motions of different frequencies at the transmitter which comprises generating at the transmitter, a series of impulses having corresponding points in their wave form recurring at the frequency of one of said scanning motions, certain impulses, or sets of successive impulses, being of longer duration than the remainder, and, at the receiver, utilising the whole of said series of impulses to control one scanning motion and the said impulses of longer duration to control the other scanning motion.

According to a feature of the invention, the impulses, or sets of successive impulses, of longer duration recur at the frequency of the scanning motion of lower frequency.

According to a further feature of the invention, a television or the like transmitter comprises means for generating a series of synchronising impulses of which certain impulses or sets of successive impulses are of longer duration than the remainder, the time interval between corresponding points in the wave forms of successive impulses being substantially constant, and dependent upon the frequency of one of two scanning motions of different frequencies at the transmitter.

In a preferred form of the invention, the synchronising impulses are all of the same amplitude, and sets of three successive impulses, at suitable regular intervals, are made of longer duration than the remainder. Preferably, also, all the impulses have an initial steeply rising portion, which serves, in systems such as those referred to above in which blocking oscillators control the operation of scanning at the receiver, to initiate the return strokes of the higher frequency sawtooth oscillations. When the impulses are of this nature, it is arranged that the time interval between the steeply rising portions of successive impulses is substantially constant. In a system employing blocking oscillators at the receiver, the individual impulses of each set of impulses of longer duration may be integrated, for example by a condenser, at the receiver and caused to set up a. potential which is sufficiently large to initiate the return stroke of the low-frequency saw-tooth oscillation.

The following advantages which are introduced? by the invention will be at once apparent.

Firstly, the amplitude which the synchronising impulse channel must be made to accommodate is reduced, and may, in fact, never exceed the amplitude of the high-frequency impulses alone. Thus, if the transmission channel is a radio frequency carrier wave, considerable economy in the design of the modulating and transmitting apparatus may be effected. Secondly, the possibility of the occurrence of the gap effect mentioned above is reduced, since synchronisation by the method of the present invention may be made very reliable.

The invention will be described with reference to the accompanying diagrammatic drawings, in which Fig. 1 is a partly diagrammatic drawing of a television system showing in plan view apparatus forming part of a television transmitter according to the present invention.

Fig. 2 is a view in side elevation of the part [4 of the apparatus shown in Fig. 1.

Fig. 3 is a view in front elevation of a further part I of the apparatus shown in Fig. 1.

Figs. 4 and 4a together constitute a circuit diagram of apparatus for use in a further form of television transmitter according to the invention, the terminals llll to I06 in Fig. 4 being connected to the correspondingly numbered terminals in Fig. 4a, and the diagrams of Fig. 5 illustrate the method of operation of the circuit of Figs. 4 and 40,.

Referring now to Figs. 1, 2 and 3, and more particularly in the first place to Fig. 1, a motion picture film I is moved uniformly. and vertically downwards past a horizontally disposed slit 2 the length of which is equal to the width of the pictures on the film. This slit 2 serves to analyse each picture into strips and will therefore be called a first scanning aperture. The strip of film which at any instant lies in the first scanning aperture 2 is illuminated by light from a suitably disposed are 3 with the aid of a lens system 4 and light which passes through the film l and first scanning aperture 2 is reflected, from one of ninety plane mirrors (of which two are indicated at 5) disposed upon a rotating mirror drum 6, on to a plane vertical scanning screen I situated about six feet from the mirror drum 6. The mirrors 5 are all mounted tangentially about the surface of the drum 6 and the latter rotates about a vertical axis. Close to the drum is placed a spherical lens 8 adapted to focus an image of the film strip in the first scanning aperture 2 on to the scanning screen 1. In the screen is a second, slit-like, scanning aperture 9 (Fig. 3) behind which is disposed a photo-electric cell In and, as the mirror drum 6 rotates, images of successive strips of the film which appear in the first scanning aperture 2 are translated over this second scanning aperture 9; one of these images is indicated at II in Fig. 3. Corresponding electric picture signals are generated in the photo-electric cell In and these are applied to a suitable amplifier 5B.

synchronising impulses are also generated at the transmitter and are mixed with the picture signals for transmission over the same channel as the picture signals to the receiver. The synchronising impulses are generated in the following way:-

Parallel with the first scanning aperture 2, but slightly out of line therewith, is disposed a second aperture I2 (in the form of a slit) which is illuminated uniformly by light from a source-I3. Of this aperture I2 a portion, which is shown in broken lines in Fig. 2 is' normally obscured by a rotatable shutter I4 and another portion I5 shown cross hatched in Fig. 2, is at all times unobscured by the shutter I4. The unobscured portion I5 of this aperture I2 will be called the line aperture and the whole of the aperture I2 will be called the frame aperture.

Images of the line aperture I5 are translated, by means of the mirrors of the rotating mirror drum 6, over a line slit I6 (Fig. 3) disposed in the scanning screen I above and slightly to one side of the second scanning aperture 9. The apertures 2 and I2 are placed out of alignment in order that the images thereof shall not pass over the same path on the screen I. One of the line aperture images is indicated at H in Fig. 3. Behind the line aperture is disposed a synchronising photocell 5'I (Fig. 1) and its associated synchronising amplifier 5B. As the image I? passes over the line slit IS a line synchronising impulse of short duration and peaked Wave form is generated.

Usually the line aperture I5 is so disposed, relatively to the mirrors of the mirror drum 6 and the line slit I8, that the images of the line aperture, such as I? in Fig. 3, cross the line slit I6 during the intervals between the passages of the images Ii over the second scanning aperture 9. In this case line synchronising signals are generated in between the generation of the trains of picture signals corresponding to successive lines.

Geared to the film sprocket (not shown) which draws the film I past the first scanning aperture 2 is the rotating opaque shutter I4, the axis of which is disposed perpendicularly to the plane of the film I. The gearing between the film sprocket and shutter is such that while one complete picture on the film moves past the first scanning aperture 2, the shutter I4 performs half a revolution.

Referring now more particularly to Fig. 2, the shutter I4 comprises a central disc I8 and an outer annular ring IS, the ring being spaced apart from the disc by two arms 25 and 2i which are symmetrically disposed with respect to the centre of the disc I8. The edge of the disc I8 bears two symmetrically disposed notches 22 and 23 and the outer edge of the ring I9 also bears two symmetrically disposed notches 24 and'25. The gearing between the film sprocket and the shutter is such that, for every half revolution of the shutter M, an arm 28 or 2I obscures the first scanning aperture 2 (and therefore of course the second scanning aperture 9) with respect to light from the are 3, whilst a notch 24 or 25 uncovers the whole of the frame aperture I2. The covering of the scanning aperture 2 and the uncovering of the frame aperture I2 commence simultan ously substantially at the end of the scanning of each frame and last for the same period of time.

The effect of covering the scanning aperture 2 is, of course, to prevent the generation of picture signals in the photo cell I 0.

The frame aperture I2 is arranged to be uncovered for a time about equal to that necessary to scan three lines, and during this time three images of the frame aperture are swept over the line slit I6 at the screen I. One such image is shown at 26 in Fig. 3. I In this manner three electrical impulses with short intervals between them are generated in the synchronising photocell 51 at the end of the scanning of each complete picture on the film and each of these three impulses is of longer duration than one line synchronising signal although of substantially the same amplitude. The three separated broad impulses succeed one another at the line scanning frequency.:.;

The synchronising signal thus generated is in the form of a series of impulses having corresponding points in their wave forms, for example, the initial steeply rising portions, recurring at the line frequency. Sets of three successive impulses at regular intervals are of longer duration than the remainder, and the intervals between the initial steeply rising portions of the first impulses of these sets is substantially constant and dependent upon the frame scanning frequency.

Preventing the generation of picture signals in the photo-cell ID at the end of the scanning of each complete picture is equivalent to giving to the picture signals (during this time) a wave form corresponding to black or nearly black. The;

picture signals and the synchronising impulses are transmitted to a receiver over one channel and the synchronising signals are preferably inserted into the channel in the opposite, or what may be called the blacker than black direction, from the picture signals. In other words, the picture and synchronising signals are inserted into the transmission channel on opposite sides of a datum line corresponding approximately to black in the picture. The transmission channel may be constituted by a carrier wave, in which case the outputs from amplifiers 56 and 58 are fed (in opposite senses) to a modulator 59 associated with a source Gil of carrier oscillations and an aerial-earth system 6|.

At the receiver, after detection of the received signals in a radio receiver 82 associated with an aerial-earth system 63, the synchronising impulses and the picture signals are together applied between the cathode 64 and a modulating electrode I55 of the cathode ray tube 68, which serves to reconstitute the transmitted picture. The picture signals are arranged to make the modulating electrode more positive, while the synchronising impulses, being in the opposite sense, tend to make the potential of this electrode more negative.

The received signal, after detection, is also applied to a synchronising channel 81, and the synchronising impulses are freed from picture signals in a separator 68 of any suitable kind. All of the synchronising impulses are fed to the input circuit of a generator 59 of line frequency scanning oscillations, for example, a blocking oscillator, and serve to control the generation of a saw-tooth scanning oscillation of line frequency, the initial transient of each impulse serving to initiate a return stroke.

The impulses are also applied in parallel to a low-pass filter 'IIl terminated by a condenser II in which the sets of line frequency impulses of longer duration are integrated to form a series of pulses of the frame scanning frequency, this series of pulses being employed to control the generation of saw-tooth scanning oscillations of the frame frequency by means of a second generator I2 which may also comprise a blocking oscillator.

At the end of the scanning, or reconstitution, of each picture by means of the receiving cathode ray tube, the ray moves diagonally across the fluorescent screen to the starting point for a succeeding picture under the control of the return stroke of a saw-tooth frame frequency scanning oscillation.

During this time, each of. the three impulses of long duration which form a single frame synchronising signal may reduce the intensity of the cathode ray completely to zero but in between each of the three pulses the screen of the cathode ray tube may possibly be not quite black, and in order to ensure absolute blackness of the screen at these times, in order that the return stroke of the frame scanning oscillation shall not produce an effect on the screen, an auxiliary blackout signal is generated in between successive whole pictures and the frame signals are superimposed on this blackout signal. The blackout signal is of course inserted into the transmission channel in the blacker than black direction and consists of a broad pulse lasting for substantially the whole of the time between successive pictures. The blackout impulses may be fed to the amplifier 58 by terminals 73. By this means the intensity of the ray may be reduced to zero between the separate pulses which make up one frame synchronising signal.

The blackout signals may be generated at the transmitter in the following way:-

Two notches 22 and 23, Fig. 2, cut on the central disc 18 of the shutter M, co-operate with a beam of light passing through an aperture 21, or

with an image of an aperture. The aperture is covered during the generation of. picture signals and is uncovered during substantially the whole of the time between the scanning of successive complete pictures and, therefore, during the generation of the broad line frequency impulses. Light passing through the aperture 21 is refiected from a stationary reflector (not shown) into either the synchronising photo-cell or into a photo-cell associated with means for mixing the signals generated therein with the synchronising signals, so that in effect, the broad line frequency impulses are superimposed upon a broad blackout signal, the frame and blackout signals both being inserted into the transmission channel in the blacker-than-black direction.

Means are also preferably provided at the transmitter for ensuring that no damage is done to the film, due to overheating by light from the intense source 3, if the film drive fails.

The shutter i4 is driven by gear 28, which is loosely coupled to the shutter shaft 29, by means of a spring 30. A safety shutter 3| is also loosely mounted on the shaft 29. The shape of this safety shutter is as shown by the dotted lines in Fig. 2. Two arms 32 and 33 of the safety shutter are adapted to cover or uncover the first scanning aperture 2 and the blackout aperture 2? respectively, and angular movement of the safety shutter is limited suitably by stops 34 and 35. The safety shutter is counterweighted so that, when the film is not being scanned, it covers the apertures 2 and 21. The safety shutter 3i bears a friction pad 36 which is held in engagement with shutter H! by means of the collar 31' slidably mounted on shaft 29. The collar 3'! is operated by a system of levers controlled by an electromagnet 38. The electromagnet 38 and light source l3 are connected in parallel, current being supplied to them through a common switch.

Assuming the film to be stationary and no current to be flowing to the electromagnet 38 and source I3, the operation of the device, on commencing to scan a film, is as follows:-

On starting up the film drive, the shutter I4 commences to rotate, but the safety shutter 3!, not being pressed into frictional engagement with shutter l4, obscures both apertures 2 and 21. On lighting up the lamp l3, however, electromagnet 38 is energised and collar 31 is moved in such manner that the pad 36 is forced against shutter l4 and the safety shutter 3| is rotated so as to uncover the apertures 2 and 21. Thus light can only reach the film provided, simultaneously, the lamp I3 is on and the film drive is in operation.

The spring 30 operates as a mechanical filter between a driving motor for the system and shutter l4, and the friction pad 36 serves to damp this mechanical filter.

The safety shutter 3! is designed so as not to cover the frame and line apertures 12 and 15 when the film driving mechanism stops so that the generation of synchronising signals continues and, in the case where a cathode ray tube receiver is used, the ray is prevented from coming to rest and so burning a hole in the fluorescent screen during intervals between the transmission of, say, complete motion picture films.

The arm 32 of the safety shutter may, however, be extended so as to cover the part I2 of the frame aperture, leaving the line aperture l5 uncovered. In this case the high frequency synchronising signals only are transmitted when the driving mechanism stops.

If desired, the frame and line apertures l2 and i5 may be placed separately one on either side of the centre of shutter l4, two light beams being used to illuminate these apertures. In this case either the picture signals alone or both the picture and line synchronising signals may be suppressed during the generation of frame synchronising signals, suitable notches being cut on the shutter M for obscuring the appropriate apertures.

The synchronising impulses may if desired be generated electrically instead of by optical mechanical methods such as that described above. One suitable electrical method of generating synchronising impulses according to this invention will now be described with reference to Figs. 4, 4a and 5., reference being made in the first place to the diagrams of Fig. 5, which illustrate the method of operation; Figs. 4 and 4a show a suitable circuit arrangement for producing synchronising impulses of the form shown in diagram h of Fig. 5.

Referring to Fig. 5, two sets of impulses, one, which is shown at a, at the high synchronising frequency and the other (shown at b) at the low frequency, are generated in any known or suitable manner at the transmitter. It will be assumed that the impulses of both sets have a substantially rectangular wave form, and that the duration of each low-frequency impulse is equal to between two and three times the interval between successive high frequency impulses, as shown. The two sets of impulses, after amplification if desired, are mixed together in the same sense and passed to a limiting device such as a thermionic valve the grid of which is so biased that the valve only passes current when impulses of both sets appear together on the grid. Fig. 5 (0) shows the form of the signal passed to the limiting valve.

In the output circuit of this valve is arranged a circuit which, although it allows the pulses to build up rapidly to their maximum value, has such a time constant that it delays the fall of the pulses to their minimum value. There are thus produced in the output circuit of the limiting valve sets of broadened impulses, successive sets being separated by a time interval which is dependent upon the frequency of the original lowfrequency synchronising impulses. One such set of impulses is shown at d. The sets of broadened impulses have their tops removed (Fig. 5 (e) in a second limiting valve, and are then mixed with the. original high-frequency impulses, and the mixed .impulses, (Fig. 5 (f)) are applied to the grid of a third limiting valve. The output of this last valve contains a series of impulses, shown at g, all of which are of substantially the same amplitude; sets of successive impulses at regular intervals are of longer duration than the remainder, these setsof impulses commencing at a frequency equal to the low synchronising frequency, and all the impulses of the series having an initial steep wave front which recurs at afrequency equal to the high scanning frequency.

If desired, the final composite synchronising signal may be made to contain black-out impulses, as shown at h in Fig. 5, by mixing the broadened impulses in the output of the first or second limiting valve with the original low frequency impulses as well as the orig nal high frequency impulses, the mixed impulses being then applied to the third limiting valve.

Apparatus suitable for producing agcomposite synchronising signal of the form shown in Fig. 5 (h) by the method outlined above is shown in Figs. 4 and 4a. Low frequency synchronising im pulses of the form shown in Fig. 5 (b) are applied from an adjustable tapping point on a potential divider resistance 39 between the control grid and cathode of a triode separator valve V1, and high frequency impulses, Fig. 5 (a), are appliedfrom an adjustable tapping point on potential divider 40 to ahscreened grid separator valve V2, the impulses causing the grids of the valves V1 and V2 to becomegmore negative. The impulses are derived from sources 14 and 15 associated respectively with potential dividers 39 and 43'; the sources 14 and 75 may take any known form and may for example each comprise a rotating commutator. Thejanodes of the valves V1 and V2 are connected together through a resistance 4!, and

through a coupling resistance 42 to the positive terminal 43,of a source of anode current.

The mixedimpulses, Fig. 5 set up across resistance 42 are applied by means of a coupling condenser 44 and a potential divider resistance 45 to a screened grid limiting valve Vs, the control grid of which is connected to an adjustable tapping in the potential divider 45, and the cathode of which is connected to the negative terminal 46 of the anode current source through a bias resistance 47. The mixed impulses are applied to the valve V3 in such a sense as to make its control grid more positive, and the valve is so biased that only those parts of the mixed impulses lying above the horizontal dotted line in Fig. (6) cause anode current to flow.

The anode circuit of valve V3 comprises a circuit, constituted by a resistance 48 and a condenser 48, which has a time constant such that although the increase of current in the resistance 48 is rapid, the decay of current is relatively slow; there is accordingly set up across the circuit 48, 49 a series of broadened high frequency impulses of the form shown in Fig. 5 (d). The condenser 49 is preferably made variable so that the time constant of the circuit 48, 49, and hence the duration of the broadened pulses, may be varied.

The broadened high frequency impulses are applied, together, with low frequency impulses from potentialdivider 39, to the control grid of a second limiting valve V4 of the screened grid type, in such asense as to make the control grid thereof more negative This valve is arranged to be overloaded at the bottombenol of its characteristic by the applied impulses, and the tops of the broadened highfrequency impulses are thus removed. As has beenstated, the function of the low frequencyimpulses introduced here is to serve as black-outimpulses.

The ,valve ,V4 is connected in parallel with a screenedgrid valve V5, to the control grid of V which are applied high frequency impulses from the ,potentiaLdividerAU. The common output circuit of the valves V4 and V5 comprises a coupling condenser 55 and a potential divider resistance 51, an adjustable tapping point in which is .connectedto the control grid of a screened grid phase-reversin valve V6, the output of which is established across a coupling condenser52 in series with two resistances 53 and 54.

The fractionof the output of valve V6 set up across resistance 54 is, applied between the control grid and cathode of a third limiting valve V: of the. screened grid type, in such a sense as to make its control grid more negative. The grid base of the valve V7 is made such that when black out impulses, broadened high frequency impulses, and'the original high frequency impulses all appear on its control grid together, the valve is overloaded, and the original high frequency impulses produce substantially no change of anode current; the latter impulses are thuseliminated, and the final composite synchronizing signal set up acrossa potential divider 55 inthe output circuit of valve V7 is of the form shownin Fig. 5 (h).

The circuit of Figs. 4..and 40!. has not been described, in fulldetail, many of the elements such as the feed resistances, decoupling resistances, and coupling. and (decoupling. condensers not being referred to specifically. These components do not, however, play an important functional part in thezoperation of the circuit from the pointof View. of the presentinvention, and the manner inwhich: theyareconnected is well understood by those versed in the art.

It may also be pointed out here that although the limiting devices employed in Figs. 4 and 4a take the form of thermionic valves arranged to be overloaded at the bottom bend, other limiting devices may be employed if desired. Other suitable devices by way of example are diode rectifiers so biassed that substantially no current flows until the applied potential difference exceeds a certain value, and triode valves arranged to limit applied potential diiferences by permitting the passage of grid current.

The composite synchronizing signal is mixed with the picture signals and fed to a suitable transmission channel such as a carrier wave for example, the synchronizing signal being arranged to modulate the carrier in the blacker-thanblack sense.

At the receiver, the synchronizing signal is freed from picture signals by means of a suitable limiting valve, for example, and the whole series of impulses is fed to a high frequency blocking oscillator. The impulses serve to control the generation of saw tooth scanning oscillations by the blocking oscillator, the initial transient, that is to say, the steep wave front of each impulse serving to initiate a return stroke. The whole series of impulses is also fed. to a low pass filter terminating in a condenser which serves to integrate each set of impulses of longer duration to produce, eifectively, a low frequency impulse which serves to initiate the return stroke of a low frequency blocking oscillator.

The picture signals and synchronizing signal are applied together to the modulating electrode of the receiving cathode ray tube, and the blackout impulses serve the function already described of ensuring that the ray is reduced substantially to zero in the intervals between complete pictures.

In television systems such as those described above which employ two blocking oscillators at the receiver, it has hitherto been necessary to provide means such as a filter or bridge circuit to prevent the high-frequency blocking oscillator from being triggered off by the low-frequency impulses. It will be seen that, with the present method of synchronization, the low-frequency impulses have no separate existence, and the possibility of false actuation of the high-frequency blocking oscillator is avoided.

We claim:

1. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, the provision of a source of impulses at the higher of said frequencies, these impulses having durations short compared with the intervals between them, a source of impulses of the lower of said frequencies, these impulses being of such a duration that a plurality of said higher frequency impulses occurs during each lower frequency impulse, a first circuit, means for feeding impulses from both said sources to said first circuit in the same sense, a limiting device forming part of said first circuit and arranged to pass only those higher frequency impulses which are superimposed on lower frequency impulses, a second circuit having a time constant which is long relatively to the duration of said higher frequency impulses associated with said limiting device for increasing the durations of impulses from said limiting device, a third circuit, means for mixing the broadened impulses and impulses of the higher frequency from said source in the same sense in said third circuit, and a second limiting device forming part of said third circuit and so arranged as to remove those of the un-broadened higher frequency impulses which are superimposed upon broadened impulses.

2. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, the provision of a source of impulses at the higher of said frequencies, these impulses having durations short compared with the intervals between them, a source of impulses of the lower of said frequencies, these impulses being of such a duration that a plurality of said higher frequency impulses occurs during each lower frequency impulse, a first circuit, means for feeding impulses from both said sources to said first circuit in the same sense, a limiting device forming part of said first circuit and arranged to pass only those higher frequency impulses which are superimposed on lower frequency impulses, a second circuit having a time constant which is long relatively to the duration of said higher frequency impulses associated with said limiting device for increasing the duration of impulses from said limiting device, a third circuit, means for mixing the broadened impulses and impulses of both the higher and lower frequencies from said sources in the same sense in said third circuit, and a second limiting device forming part of said third circuit and so arranged as to remove those of the un-broadened higher frequency impulses which are superimposed upon broadened impulses.

3. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, apparatus as claimed in claim 2, characterized in that at least one of said limiting devices comprises a thermionic valve so connected and arranged as to be overloaded at the bottom bend of its anode current-grid voltage characteristic.

WILLIAM SPENCER PERCIVAL. CECIL OSWALD BROWNE.

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