501,966

PATENT SPECIFICATION

Application Date: June 7, 1937. No. 14728/37

Complete Specification Left: April 13, 1938.

Complete Specification Accepted: March 7, 1939.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Television Systems

We, ALAN DOWER BLUMLEIN, of 37, The Ridings, Hanger Lane, Ealing, London, W.5, and ROLF EDMUND SPENCER, of 8, Courtfield Gardens, West Ealing, London, W.13, both British subjects, do hereby declare the nature of this invention to be as follows:

This invention relates to television transmission systems and has particular reference to such systems in which a carrier wave is modulated by mixed signals obtained from separate scanning cameras. Two scanning cameras may be employed, for example, in a system in which it is desired to superpose the matter to be transmitted upon a separate background.

In connection with such systems a circuit arrangement has previously been proposed which permits a foreground scene to be superposed upon a background scene in such a manner that the background scene is obscured by the foreground scene. The circuit includes a normally operative camera for scanning the background, which camera is switched off during periods in which another camera scanning the foreground is providing picture signals. The foreground scene is arranged in front of a black surface, the foreground camera being continuously operative and the background camera normally operative, superposes the scene it scans upon the black region of the foreground scene.

It is preferable to use a white surface behind the foreground scene, and to arrange that signals representative of the white surface are not transmitted from the foreground camera. Such an arrangement is preferable because in the former case it is not practicable to ensure that the whole surface of the foreground object is sufficiently illuminated as not to produce a black area which would be treated by the separating circuit in the same way as the black of the surface behind the foreground, with the result that the background scene will appear with the dark part of the foreground scene. In the case of a white surface behind the foreground scene, it can be arranged that this surface is always brighter than any portion of the foreground scene, so that separation of the wanted and unwanted foreground signals can be effected.

The object of the present invention is to provide a system of television transmission in which an illuminated surface is arranged behind a foreground scene to be transmitted, the illumination rendering the said surface brighter than any part of the foreground scene, and in which the illuminated surface is prevented from appearing in a combined picture of the foreground scene superposed upon a background scene.

According to the present invention a television transmission system comprises two scanning channels, one containing signals representative of a foreground scene with a bright white rear surface, and the second representative of a background scene and wherein means are provided for selecting signals from the first channel representative of the white of the rear surface which is effectively the peak white value, and developing from these selected signals two control signals, one of which is effective to control the background channel, and the other is effective to control the foreground channel in such a manner that the foreground channel is operative and the background channel is inoperative in the absence of said white signals in the foreground channel and vice versa.

According to a further feature of the invention the white signals selected from the first foreground channel are lengthened before they are used to develop control signals which effect switching in the channels.

According to a further feature of the invention, the signals in the foreground channel are delayed between the point of selection of the signals representative of the peak white value and the point of application of the control signals so that this channel is made operative slightly after a change from the peak white value to the value of the foreground scene, and is make inoperative slightly before the change of signals from the foreground value to the peak white value. The functions of the lengthening of the signal representing the peak white value of the control channel and the delay in the signals in the foreground channel ensures that small errors in timing, together with a finite time required for a change of signalling strength from that representing the peak white value to the foreground value or vice versa, do not cause white edges to appear around the foreground scene due to the peak white signals representing the white rear surface.

In order that the nature of the invention may be more clearly understood, a part of a television transmission system embodying the invention will now be described by way of example with reference to the accompanying drawing which is a block diagram illustrating certain features essential to the carrying out of the invention.

Referring to the drawing, 1 Is the input terminal to the foreground channel, 2 is the input terminal to the background channel, and 3 is the tapping point from the foreground channel to the control channel. One line of an incoming signal is represented by the waveform 4 and consists of a peak white pulse, a vision signal and the second peak white pulse, the vision signal corresponding with an object in the foreground scene and extending over about the middle third of the line. The black and white levels are indicated at 5 and 6 respectively. One line of the background signals is represented by the waveform 7 and consists of continuous vision signals representing the background, and for the purposes of explanation the effective height of the vision signals is shown to be the same in the foreground and background channels.

The peak white signal pulses from the foreground channel are selected by a filter represented by the rectangle 8 and are passed to a delay network represented by a rectangle 9 giving a total delay of 1 n, the interval n being indicated on the signals represented at 10. Tappings on the delay network 9 give outputs of delay by ½ n and 1½ n. The resultant outputs of the network 9 are fed to a mixer 11 and then to a limiter 12 to produce a control signal 13 which has peak white pulses each longer by the period n than the original peak white pulses. The signals 13 are converted to push-pull signals by the stage 14 and applied in different phase ot the foreground and background channels.

The signals in both these channels have been delayed by networks 15 and 16 respectively, by a period n and have been amplified at 17 and 18 respectively, to increase the strength of the vision signals. In the foreground channel signals have also been limited at 17 to remove some of the surplus of the peak white, so as to prevent overloading of subsequent apparatus.

It will be seen that the signals in the background channel are now delayed by the amount lying between the maximum and minimum delay used for the control channel, so that the control signals overlap at their beginning and end of the peak white signals in the foreground channel. The delay to amplification of the background channel is included to retain symmetry of the apparatus, and is not necessary for the correct functioning of the circuit. The control signals 19 are then fed to a mixer 20 in the foreground channel in such a phase as to drive the peak white signals in the foreground channel in the black direction beyond the amplitude representing the black signal. Similarly the inverse control signals 21 are applied to a mixer 22 in the background channel and drive the signals in that channel beyond the black amplitude during the period that the foreground channel is not driven back. The signals in each channel are then applied to limiters 23 and 24 in which they are limited to an amplitude representative of black amplitude. The signals from both channels are then fed to a mixer 25 in which they are mixed to give a complete picture represented by the waveform 26 containing both foreground and background signals.

It will be seen that the mixing of the control signals with the channel signals and the subsequent limiting is in reality only a method of switching these channels on and off and if preferred some device such as an hexode switching valve may be employed instead.

It will be noted that there is an overall unused delay of n/2 and this could be avoided by reducing the delay in the foreground and background channels to n/2 and reducing the delay in the control channel to n and taking off the signals from the beginning and end of the delay network. The same result would be produced but this method would not allow any quick adjustment in hand. It is preferred to arrange for an excessive delay in the main channels so that the amount of overlapping of the control signals can be adjusted by movable tappings, the tappings in this case being half-way between minimum and maximum.

The arrangement described with reference to the drawing does not allow for any difference of delay in the channels which can, of course, be compensated for in the delay networks. In the absence of any delay a slight white ‘whisker’ would occur at one or both sides of the foreground object due to the fact that all signals are slightly rounded and in selecting the peak white signals the topmost and so the shortest portion of these signals is selected. The periods of delay are adjusted so as to remove any white edges round the foreground objects. Adjustment of the delay period to a value greater than that necessary to illuminate the white edges will tend to encroach on the correct shape of the foreground object. It is advisable to develop very sharp signals from the push-pull stage 14 and to preserve as much as possible of this sharpness in the mixer circuits so that the remnant portions of the two channels fit as accurately as possible. The frequency range of the limiter circuit 12 in the control channel, the push-pull amplifier 14 and the mixers 20 and 22 may with advantage be of the order of twice that of the general channels preceding and following.

The system described can be used for other purposes than superposition. Thus by feeding square topped signals into the limiter circuit 12 in the control channel the channels can be switched on and off so that parts of the observed picture comes from one channel and parts from another. Line frequency signals will produce vertical division of the final picture and frame frequency signals will produce horizontal division. A convenient method of obtaining such signals for producing a fading effect from one picture to another by a moving line across the received picture, is to provide a limiter having considerable amplification in the control channel, so that given a saw-tooth input it will in effect produce a stepped wave. If a saw-tooth input a line frequency is then added to a variable D.C. bias potential, a vertical change-over can be made from one channel to another, the position of the change-over varying with the bias potential. Such a limiter can conveniently be made of a number of stages of push-pull amplification D.C. coupled as described in the Specification of co-pending Application No. 18597/36 (Serial No. 482,740).

Again various types of signal, together with a D.C. component having different waveforms may be injected at the position of the limiter circuit 12 so as to produce different types of fading effect from one picture to another.

The arrangement described may again be used in conjunction with a third camera channel, the third camera being connected to the input of the limiter circuit 12 in the control channel. By placing a silhouette in front of the third camera the background channel may be switched into operation with a portion of the picture not covered by the silhouette and the foreground channel for the portion of the picture covered by the silhouette or vice versa. In similar manner variable objects may be placed in front of the third camera so that variable superposition effects of the two channels are obtained. Thus a fade over from one picture to another may be made by scattering ink on a white surface in front of the third camera, the new picture channel being switched into operation at the points made black by the ink dots.

In the case of the circuits described it is very advantageous that the D.C component of the picture signals be present at every limiting or mixing operation, since the amount of signal representing peak white in the foreground picture is dependent upon the width of the foreground object and if this width varies, unless the D.C. component is present, it will be impossible to select the peak white pulses with any certainty. Similarly the mixed peak white pulses will vary in length, and unless there is correct D.C. transmission through this channel the subsequent limiting operation will be affected as also will the generation of the pulses of correct amplitude. The D.C. transmission may be effected by employing a D.C. coupled amplifier throughout or the D.C. may be re-inserted by well-known means prior to any point of amplitude limitation.

Dated this 7th day of June 1937

F. W. Cackett

Chartered Patent Agent

Complete Specification

Improvements in or relating to Television Systems

We, ALAN DOWER BLUMLEIN, of 37, The Ridings, Hanger Lane, Ealing, London, W.5, and ROLF EDMUND SPENCER, of 8, Courtfield Gardens, West Ealing, London, W.13, both British subjects, do hereby declare the nature of this invention and in what manner the same is to be performed and ascertained in and by the following statement:

This invention relates to television transmission systems and has particular reference to such systems in which a carrier wave is modulated by mixed signals obtained from separate scanning cameras. Two scanning cameras may be employed, for example, in a system in which it is desired to superpose the matter to be transmitted upon a separate background.

In connection with systems employing superposed pictures a circuit arrangement has previously been proposed to the Specification of Patent No. 417,282, which permits a foreground scene to be superposed upon a background scene in such a manner that the background scene is obscured by the foreground scene. The arrangement includes a normally operative camera for scanning the background, which camera is switched off during periods in which another camera scanning the foreground is providing picture signals. The foreground scene is arranged in front of a black surface, the foreground camera being continuously operative and the background camera which is operative except when switched off during the occurrence of foreground signals, superposes the scene it scans upon the blank region of the foreground scene. In the specification referred to, it was also proposed to use a highly reflecting rear surface to the foreground scene.

It is preferable to use a white surface behind the foreground scene, and to arrange that signals representative of the white surface are not transmitted from the foreground camera. Such an arrangement is preferable because in the former case it is not practicable to ensure that the whole surface of the foreground object is sufficiently illuminated as not to produce a black area which would be treated by the separating circuit in the same way as the black of the surface behind the foreground, with the result that the background scene will appear with the dark part of the foreground scene. In the case of a white surface behind the foreground scene, it can be arranged that this surface is always brighter, as by back projection, than any portion of the foreground scene, so that separation of the wanted and unwanted foreground signals can be effected. It has been proposed in the Specification of Patent No. 455,785, to illuminate the rear surface of a foreground scene more intensely than the foreground.

The object of the present invention is to provide a system of television transmission in which superposed pictures are transmitted and received without confusion due to overlapping of the superposed pictures, and further, to provide a system in which a received picture shall contain portions of scenes selected by different scanning cameras.

According to the present invention a television transmission system includes two scanning channels, one of which containing signals representative of a foreground scene with a rear surface which is substantially uniformly whiter than the area in the foreground scene, the second channel containing signals representative of a background scene and wherein means are provided for selecting from the foreground channel signals which are representative of said rear surface, further means being provided for developing from the selected signals, two control signals, one of which is effective to control the background channel, and the other of which is effective to control the foreground channel in such a manner that the foreground channel is operative and the background channel is inoperative in the absence of the signals in the foreground channel representing said uniformly whiter surface and vice versa, the outputs of said foreground and background channel being fed to a common channel.

In a particular transmission system embodying the invention, the signals in the foreground channel are delayed between the point of selection of the signals representative of the uniformly whiter value and the point of application of the control signals, and the duration of the selected signals is lengthened before said signals are used to develop the control signals so that the foreground channel is rendered operative slightly after a change from the uniformly whiter value to the value of the foreground scene and is made inoperative slightly before the change of signals from the foreground value to the uniformly whiter value. The lengthening of the selected signals is effected by applying them to a delay network from which are taken two tappings spaced apart by a predetermined amount, the output from the spaced tappings being combined and subsequently fed to an amplitude limiting device followed by an amplifier, and the output from said amplifier constitutes said control signals which are then applied in opposed phase to the foreground and background channels. The function of the lengthening of the signal representing the peak white value of the control channel i.e. the uniformly whiter value of the rear surface of the foreground scene, and the delay of the signals in the foreground channel ensures that small errors in timing, together with a finite time required for a change of signalling strength from that representing the peak white value to the foreground value or vice-versa, do not cause white edges to appear around the foreground scene due to the peak white signals representing the uniformly whiter rear surface.

In order that the invention may be more clearly understood and readily carried into effect, parts of a television transmission system embodying the invention will now be described by way of example with reference to the drawing filed with the Provisional Specification No. 15728/37 which is a block diagram illustrating one method of carrying out the invention.

Referring to that drawing, 1 is the input terminal to the foreground channel, 2 is the input terminal to the background channel, and 3 is the tapping point from the foreground channel to the control channel. One line of an incoming signal is represented by the waveform 4 and consists of a peak white pulse, a vision signal and a second peak white pulse, the vision signal corresponding with an object in the foreground scene and extending over about the middle third of the line. The black and white levels are indicated at 5 and 6 respectively. One line of the background signals is represented by the waveform 7 and consists of continuous vision signals representing the background, and for the purposes of explanation the effective height of the vision signals is shown to be the same in the foreground and background channels.

The peak white signal pulses from the foreground channel are selected by a filter represented by the rectangle 8 and are passed to a delay network represented by the rectangle 9 giving a total delay of 2 n, the interval n being indicated on the signals represented at 10. Tappings on the delay network 9 give outputs of delay by ½ n and 1½ n. The resultant outputs of the network 9 are fed to a mixer 11 and then to a limiter 12 to produce a control signal 13 which has peak white pulses each longer by the period n then the original peak white pulses. The signals 13 are converted to push-pull signals by the stage 14 and applied in different phase to the foreground and background channels.

The signals in both these channels have been delayed by networks 15 and 16 respectively, by a period n and have been amplified at 17 and 18 respectively, to increase the strength of vision signals. In the foreground channel signals have also been limited at 17 to remove some of the surplus of the peak white, so as to prevent overloading of subsequent apparatus.

It will be seen that the signals in the background channel are now delayed by an amount lying between the maximum and minimum delay used for the control channel, so that the control signals overlap at their beginning and end of the peak white signals in the foreground channel. The delay to amplification of the background channel is included to retain symmetry of the apparatus, and is not necessary for the correct functioning of the circuit The control signals 19 are then fed to a mixer 20 in the foreground channel in such a phase as to drive the peak white signals in the foreground channel in the black direction beyond the amplitude representing the black signal. Similarly the inverse control signals 21 are applied to a mixer 22 in the background channel and drive the signals in that channel beyond the black amplitude during the period that the foreground channel is not driven black. The signals in each channel are then applied to limiters 23 and 24 in which they are limited to an amplitude representative of black amplitude.

The signals from both channels are then fed to a mixer 25 in which they are mixed to give a complete picture represented by the waveform 26 containing both foreground and background signals.

It will be seen that the mixing of the control signals with the channel signals and the subsequent limiting is in reality only a method of switching these channels on and off and if preferred some device such as an hexode switching valve may be employed instead.

It will be noted that there is an overall unused delay of n/2 and this could be avoided by reducing the delay in the foreground and background channels to n/2 and reducing the delay in the control channel to n and taking off the signals from the beginning and end of the delay network. The same result would be produced but this method would not allow any quick adjustment in hand. It is preferred to arrange for an excessive delay in the main channels so that the amount of overlapping of the control signals can be adjusted by movable tappings, the tappings in this case being half-way between minimum and maximum.

The arrangement described with reference to the drawing does not allow for any difference of delay in the channels which can, of course, be compensated for in the delay networks. In the absence of any delay a slight white ‘whisker’ would occur at one or both sides of the foreground object due to the fact that all signals are slightly rounded and in selecting the peak white signals the topmost and so the shortest portion of these signals is selected. The periods of delay are adjusted so as to remove any white edges round the foreground objects. Adjustment of the delay period to a value greater than that necessary to illuminate the white edges will tend to encroach on the correct shape of the foreground object. It is advisable to develop very sharp signals from the push-pull stage 14 and to preserve as much as possible of this sharpness in the mixer circuits so that the remnant portions of the two channels fit as accurately as possible. The frequency range of the limiter circuit 12 in the control channel, the push-pull amplifier 14 and the mixers 20 and 22 may with advantage be of the order of twice that of the general channels preceding and following.

In the case of the circuits described it is very advantageous that the D.C. component of the picture signals be present at every limiting or mixing operation, since the amount of signal representing peak white in the foreground picture is dependent upon the width of the foreground object and if this width varies, unless the D.C. component is present, it will be impossible to select the peak white pulses with any certainty. Similarly the mixed peak white pulses will vary in length, and unless there is correct D.C. transmission through this channel the subsequent limiting operation will be affected as also will the generation of the pulses of correct amplitude. The D.C. transmission may be effected by employing a D.C. coupled amplifier throughout or the D.C. may be re-inserted by well-known means prior to any point of amplitude limitation.

Having now particularly described and ascertained the nature of our invention and in what manner the same is to be performed, we declare that what we claim is:

  1. A television transmission system including two scanning channels, one of which contains signals representative of a foreground scene with a rear surface which is substantially uniformly whiter than any area in the foreground scene, the second channel containing signals representative of a background scene and wherein means are provided for selecting from the foreground channel signals which are representative of said rear surface, further means being provided for developing from the selected signals, two control signals one of which is effective to control the background channel and the other of which is effective to control the foreground channel in such a manner that the foreground channel is operative and the background channel is inoperative in the absence of the signals in the foreground channel representing said uniformly whiter surface and vice versa, the outputs of said foreground and background channels being fed to a common channel.
  2. A television transmission system according to claim 1 wherein the signals in the foreground channel are delayed between the point of selection of the signals representative of the uniformly whiter value and the point of application of the control signals and the duration of the selected signals is lengthened before said signals are used to develop the control signals, so that the foreground channel is rendered operative slightly after a change from the uniformly whiter value to the value of the foreground scene and is made inoperative slightly before the change of signals from the foreground value to the uniformly whiter value.
  3. A television transmission system according to claim 2 wherein the lengthening of the selected signals is effected by applying them to a delay network from which are taken two tappings spaced apart by a predetermined amount, the output from the spaced tappings being combined and subsequently fed to an amplitude limiting device followed by an amplifier, and wherein the output from said amplifier constitutes said control signals which are then applied in opposed phase to the foreground and background channels.
  4. A television transmission system substantially as described with reference to the drawing filed with the Provisional Specification No. 15728/37.

Dated this 12th day of April 1939

F.W. Cackett

Chartered Patent Agent