US2133422A - Television and the like system - Google Patents

Description

0a. 18, 1938. o. BROWNE 2,133,422

TELEVISION AND THE LIKE SYSTEM Filed Feb. 14, 1934 INVENTOR CECIL o. B WNE- BYI7KZML ATTODNITV Patented Oct. 18, 1938 UNITED STATES PATENT OFFICE TELEVISION AND THE LIKE SYSTEM Great Britain Application February 14, 1934, Serial No. 711,083

' In Great Britain February 15, 1933 12 Claims.

The present invention relates to television and the like systems.

It is well known in television, picture transmission and the like, to scan the object to be transmitted in a series of parallel lines and to arrange that there occurs, between the scanning of successive lines, a short interval during which synchronizing signals are transmitted. Thus the transmitted signals comprise trains of picture signals separated by synchronizing signals.

The synchronizing signals are utilized at the receiver to maintain a scanning device in synchronism with that at the trasmitter.

It has been found that a difficulty arises in certain systems of this kind, in that the synchronizing signals tend to be delayed in relation to the picture signals and the synchronizing signals may therefore arrive at the receiver scanning device during the reproduction of the picture signals instead of in intervals between the picture signals. To avoid this it has been necessary either to increase the interval between the scanning of successive lines or else to mask ofl a part of the reproduced picture.

It is an object of the present invention to remove or reduce this difficulty.

It is a further object of the present invention to provide a method of transmitting and receiving images of objects wherein there are generated at a transmitter synchronizing signals and trains of picture signals having intervals between successive trains, each synchronizing signal being transmitted simultaneously with a train of picture signals and means being provided in the transmission channel for delaying or advancing the synchronizing signals relative to the picture signals.

The invention will now be described, by way of example, as applied to a film television system, reference being made to the accompanying drawing in which Fig. 1 shows the wave form of signals transmitted in a known form of television system,

Fig. 2 shows the wave form. of signals transmitted in accordance with one form of the present invention,

Fig. 3 illustrates, in greater detail, a portion of the curve shown in Fig. 2,

Fig. 4 is a circuit diagram of television transmitting apparatus arranged and adapted to operate in accordance with the present invention, and

Fig. 5 is a circuit diagramof television receiving apparatus adapted to receive the signals transmitted by means of the apparatus of Fig. 4,

The scanning of the motion picture film (which is not illustrated) may be carried out in any of .the known ways, but preferably the film is moved vertically downward at a uniform speed past a horizontal illuminated slit. An image of the 'strip of the film at any instant illuminated is swept, by one of a number of mirrors carried on a rotating mirror drum, over an apertured photo-electric cell, thereby scanning this strip or line and generating in the cell picture signals corresponding to the light and shade of the line scanned. The drum is rotated about a vertical axis and all the mirrors lie in planes parallel to this axis. The arrangement is such that, after one mirror has swept its image over the cell aperture, a short interval occurs before the next mirror commences to sweep an image over the cell.

In this manner there are generated in the photo-cell a series of trains of picture signals ofthe kind illustrated in Fig. 1.

During the scanning of each strip of the film, the mirror which is performing the scanning function also sweeps an image of a second illuminated slit over a second apertured photoelectric cell thereby generating in this cell an electrical pulse known as a line pulse. The duration of this line pulse is made approximately equal to or less than the interval between successive trains of picture signals, that is to say, less than the interval l shown in Fig. 1.

In a manner which will be described later each line synchronizing pulse is converted into a symmetrical train of pulses and superimposed upon the picture signals as shown in Figs. 2 and 3.

When one complete frame of the film has been scanned in this way, a rotating shutter becomes operative to intercept the light from the two slits above referred to for a time about equal to that required for the scanning. of five lines. Slightly before this time, during the scanning of the film image, a member rotating with the shutter uncovers a third illuminated aperture 01 slit and allows light therefrom to fall upon a third photo-electric cell generating in this cell what is called a frame synchronizing pulse. The duration of the frame pulse is made approximately equal to or less than the time during which the shutter is operative.

Both the line and frame pulses are converted in any convenient way, one of which will be described later, into trains of oscillations at a subcarrier frequency, that is to say at a frequency lower than that of the carrier on which the transmission is to be effected and higher than the highest frequency of importance in the picture signals. The duration of these trains are equa to the durations of the line and frame pulses respectively from which they are derived.

Referring now to Fig. 4, the trains of picture signals, of the wave form illustrated in Fig. 1, are amplified in the thermionic valve 2, the pulses generated at the line frequency are applied, as indicated by the arrow 3, across the primary winding of the transformer 4 and the longer pulses generated at the frame frequency are applied, as indicated by the arrow 5, to the primary winding of the transformer 6. An oscillator of any known or suitable hind, adapted to oscillate at the sub-carrier frequency is connected, as indicated by the arrow I, across the primary winding of the transformer 8.

The secondary winding of the transformer 4 is connec ed across a potentiometer resistance 3, a suitable portion of this resistance, that between the sliding contact I and the point I I, being connected in the grid-filament circuit of a thermionic valve I2. neced, through resistance I3, the parallel tuned circuit I4 and resistance I5, to the positive terminal of a source of current (not shown) the negative terminal of this source being earthed. Grid bias for the valve I2 is supplied by the battery I6 shunted by the condenser II. In

shunt with the anode-filament path of the valve I2 is a condenser I8 in series with an inductance I9.

The secondary winding of the transformer 6 is connected across a potentiometer resistance 2|] a suitable portion of which is connected in the grid-filament circuit of a thermionic valve 2|. The anode of valve 2| is connected through circuit I4 and resistance I to the positive side of a source of current and grid bias for valve 2| is supplied by battery 22 in shunt with which is a condenser 23.

The filaments of the valves I2 and 2| are earthed as also is one end of the secondary winding of transformer 3, the other end of this winding being connected to points II and 24, so that the secondary winding of transformer 8 is connected in the grid-filament circuits of both of valves I2 and 2|.

The anode of valve 2| is connected through condenser 25 to the grid of a third thermionic valve 26, the output of which forms the commencement of the transmission channel. Circuit 21 comprises a grid leak 28 for valve 26, and

a condenser 29 shown in dotted lines which represents the'stray capacity of valve 26.

The operation of the arrangement is as fol lows:-

The valves I2 and 2| are biased negatively to such an extent that in the absence of line and frame synchronizing pulses respectively, no currents flow in these valves. When a line synchronizing pulse is applied to the "primary winding of transformer 4 the potential of the grid of valve I2 is raised positively with respect to the filament until current flows in the anode circuit; on account of the oscillations of sub-carrier frequency applied to transformer 8, however, a train of signals of sub-carrier frequency flows in the anode circuit of valve I2, the duration of this train being equal to the duration of a line synchronizing pulse. The shunt circuit I8, I3 is designed to by-pass the line pulse frequency with about its first ten harmonics so that the line synchronizing signals supplied to the grid f valve The anode of valve I2 is com I 26 from valve I2 consist of symmetrical trains of sub-carrier frequency.

' In a similar manner the frame pulses applied to the transformer 6 operate to raise the potential of the grid of valve 2| above its normal negative value so that current flows in the anode circuit, whilst the sub-carrier oscillations applied to transformer 8 have the effectof chopping this current at sub-carrier frequency. The frame pulse components are not removed, however, since they serve a useful purpose at the receiver which will be described later.

The parallel tuned circuit I4 is tuned to th sub-carrier frequency and is designed to have a high impedance at this frequency relative to that of the circuit 21 and resistance I5 so that all components of sub-carrier frequency are removed from the picture signals. The output of valve 26 for about three lines of the picture scanned is thus somewhat as shown in Figs. 2 and 3.

The arrangement may also be considered inthe following light.

The valve 2 operates as a generator connected across a first impedance I5 and either of valves I2 or 2| operates as a generator connected across impedances I4 and I5 in series, whilst an impedance 21 is connected in shunt with impedances I4 and I5. Because at the sub-carrier frequency, the impedance I4 is high compared to that of either of impedances I5 and 21, the trains of subcarrier frequency supplied to the grid of valve 26 by valve I2 or 2I are amplified and the components of sub-carrier frequency in the picture signal are removed.

Referring now to Fig. 5, which illustrates apparatus adapted to receive the signals transmitted with the aid of the apparatus of Fig. 3 and to reconstitute images of the transmitted pictures upon a cathode ray tube, the composite signal is applied, after detection and, if necessary, amplification, to the grid-filament circuit of a thermionic valve 30. The anode of the valve 30 is connected, through a resistance 3| to the positive terminal of a source of current (not shown) and the filament is earthed. In parallel with the anode-filament path of the valve are connected in series, a condenser 32, a parallel tuned circuit 33 tuned to the sub-carrier frequency and circuit 34 consisting of a condenser and resistance in parallel.

The cathode of the cathode ray tube (not shown) is earthed and the grid or modulator electrode of the tube is connected to a point 35 between the circuits 33 and 34. The circuits 33 and 34 form two arms of a bridge, the other two arms of which are constituted by a resistance 36 and a parallel tuned circuit 31 tuned to the subcarrier frequency.

The impedances of circuits 33 and 31, at the sub-carrier frequency, are high compared to those of the resistance 36 and circuit 31 so that the component of sub-carrier frequency is substantially eliminated from the signals developed across the circuit 34 which are applied to the cathode ray tube. These signals thus consist of trains of picture signals together with pulses at the frame frequency.

The delay of the sub-carrier signals relative to the picture signals (produced by the circuits tuned to the sub-carrier frequency) is of the order of a few micro seconds and whilst this is sufflcient to bring the line synchronizing signals into the intervals between trains of picture signals, has relatively little effect upon the position of the frame signals relative to the trains of picture signals.

The pulses of frame frequency which were retained in the transmitted signals may therefore be used to black out the picture in between frames.

Alternatively the pulses of frame frequency may be removed from transmitted signals and the timing of the generation of frame pulses at the transmitter be so chosen that the frame synchronizing signals of sub-carrier frequency occur, at the receiver, in the intervals between pictures.

Since the impedance of resistance 36, at the sub-carrier frequency is low compared to that of the circuit 31, the trains of synchronizing signals of sub-carrier frequency are selected from the composite signal and passed on to the grid of valve 38 where they are amplified before being detected, separated and applied to their appropriate synchronizing units (not shown).

It is in the synchronizing units that the major portion of the delay of the frame and line synchronizing signals relative to the picture signals occurs.

Each set of synchronizing signals is used in any known or suitable manner to control the generation of a saw tooth wave, one of these waves serving to control the deflection of the cathode ray in one co-ordinate and the other to control the deflection in the other co-ordinate.

If the moment at which the line signals are generated at the transmitter in relation to the picture signals has been correctly chosen, the return strokes of the line scanning movements will occur in the intervals between the reproduction of successive lines.

In Fig. 6 there is shown a receiver circuit arrangement somewhat similar to that shown in Fig. 5, like parts in the two figures being given like reference numbers. In the case of Fig. 6 however, the parallel tuned circuit 33, which serves to eliminate the component of sub-carrier frequency from the picture signals also serves to select the sub-carrier signals and pass them on to the amplifying valve Bil.

The present invention is also applicable where the picture signals are delayed more than the synchronizing signals, the synchronizing signals in this case being generated and transmitted later than if the delay did not take place.

The invention as set forth above will be seen to provide means for in effect, advancing or retarding the synchronizing signals in relation to the picture signals to compensate for delay occurring at another part of the system.

In some forms of television and the like systems,

synchronizing signals are generated at the transmitter and are used to control the scanning both at the transmitter and at the receiver. An example of such a system is one using a cathode ray tube for generating the picture signals at the transmitter. In applying the present invention to such a system, where the synchronizing signals are delayed in transmission more than the picture signals or vice versa, means are provided either at the transmitter or at the receiver for suitably advancing or retarding the synchronizing signals relatively to the picture signals so that the delay introduced in transmission is compensated.

I claim:

1. A method of transmitting and receiving images of objects wherein there are generated at the transmitter station synchronizing signals and trains of video signals having intervals between successive trains comprising transmitting simultaneously the synchronizing signals witha train of video signals, forming each synchronizing signal from a train of signals of a sub-carrier frequency, and delaying or advancing the synchronizing signals in time relationship relative to the video signals in the transmission channel.

2. The method claimed in the preceding claim comprising in addition the step of removing from the video signals that component thereof which is of the sub-carrier frequency.

3. The method of transmitting and receiving images of objects wherein there are generated at a transmitting station both synchronizing signals and trains of video signals having time intervals between successive trains which includes the steps of producing synchronizing signals in the form of a train of signals of a subcarrier frequency, producing video signals, limiting the time durationof the synchronizing signals to a period at least no greater than the duration of the intervals of time between successive trains of the video signals, removing from the video signals that component thereof which corresponds to the sub-carrier frequency, and con trolling in the receiver the delay or advance in time of occurrence of the synchronizing signals relative to the video signals in order to place the synchronizing signals in the time intervals between successive trains of video signals.

4. In television apparatus wherein video and synchronizing signals are transmitted upon a single carrier frequency, means for producing the synchronizing signals in the form of a wave train of energy of a sub-carrier frequency, means for removing from the video signals that component thereof which is of the sub-carrier frequency, means for superimposing the synchronizing signals upon the video signals during a scanning of an elemental strip of the object of which the electro-optical representation is to be produced, and means for controlling the time period at which the produced synchronizing signals are effective in the receiver to cause the synchronizing signals to appear in the time intervals between successive trains of video signal energy.

5. The system claimed in the preceding claim wherein the time control system for controlling the synchronizing signals in time of occurrence comprises a first and. second impedance connected in series, a thermionic valve, and a third impedance in parallel with said first and second impedances and connected across the output of said thermionic valve, means for applying the video signals across the first named impedance and means for applying the synchronizing signals across the first and second impedance, the said second impedance being of high value compared to that of the first impedance measured at radio frequency and said-second impedance being in the form of a tuned circuit tuned to the sub-carrier frequency.

6. Means for receiving television signals in the form of a wave train of energy representing video signals having intervals between successive trains and synchronizing signals in the form of trains of oscillations of a radio frequency superimposed upon the video signals and wherein each interval between successive trains of the video signals is at least equal to the duration of one of the trains of radio frequency oscillations comprising means for receiving the modulated carrier, means for separating from the received carrier modulation oscillations of a sub-carrier frequency, a pair of series connected circuits wherein one of said circuits is of high impedance relative to the second at the sub-carrier frequency, whereby synchronizing signals are controlled as to time in order to delay or advance the effectiveness thereof to time periods separating successive trains of the video signals.

7. A method of transmitting and receiving images of objects wherein there are generated at a transmitter synchronizing signals and trains of picture signals having intervals between the successive trains comprising the steps of generating synchronizing signals and picture signals simultaneously, and delaying or advancing the synchronizing signals relative to the picture signals prior to the transmission of said signals.

8. A method in accordance with claim '7 wherein there are generated synchronizing signals oi duration substantially equal to the duration of one of the intervals between successive trains of picture signals.

9. Apparatus for transmitting images of objects in the form of trains of picture signals having intervals between successive trains and synchronizing signals in the form of trains of oscillation of a radio frequency comprising means for generating picture signals, means for generating synchronizing signals of a radio frequency simultaneously with the generation of said picture signals, means for compensating for advancing or delaying of. said synchronizing signals relative to the picture signals at a receiver, and means for transmitting said advanced or delayed synchronizing signals superimposed upon said picture signals.

10. Apparatus according to claim 9 wherein means are provided for generating said radio frequency signals during the scanning of a strip of said object.

11. Apparatus for transmitting images of objects in the form of trains of picture signals having intervals between successive trains and synchronizing signals in the form or trains of oscillation of a radio frequency comprising means for generating picture signals, means for generating synchronizing signals of a radio frequency simultaneously with the generation of said picture signals, means for compensating for advancing or delaying of said synchronizing signals relative to the picture signals at a receiver comprising first and second impedances connected in series, a third impedance connected in parallel therewith, a thermionic valve having anode, cathode, and control electrodes, said control electrodes being connected to said third impedance, means for applying picture signals across the first impedance, means for applying synchronizing signals across the first and second impedances, and means for transmitting said advanced or delayed synchronizing signals superimposed upon said picture signals.

12. A method in accordance with claim 7 wherein there are generated synchronizing signals of a radio frequency of a duration less than the duration of one of the intervals between successive trains of picture signals.

CECIL OSWALD BROWNE.

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