446,663

PATENT SPECIFICATION

Application Date: Sept. 4, 1934. No. 25498/34.

Complete Specification Left: May 17, 1935.

Complete Specification Accepted: May 4, 1936.

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PROVISIONAL SPECIFICATION

Improvements in and relating to Television Systems

I, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, do hereby declare the nature of this invention to be as follows:-

The present invention relates to television systems of the kind in which pictures and synchronising signals are transmitted along a single channel, the synchronising signals being in the "whiter than white" direction. That is to say the synchronising signals extend from a value corresponding to full white in the picture in a direction corresponding to an increase in brightness of the picture. A picture signal corresponding to full white can be given a definite meaning because in the case of transmitted light (from a film for example) it is the signal produced when a completely transparent part of the picture is scanned and in the case of reflected light it is the signal produced when a perfectly reflecting part of the object is scanned. Furthermore, the amplitude of full white may be determined arbitrarily, if desired, by means of a suitable amplitude limited device which only permits picture signals of amplitude less than a predetermined value (to be called full white) to pass.

The synchronising signals are usually in the form of impulses occurring in the intervals between the scanning of successive strips of the object and they may be of two forms. In the first case the signal may be caused to assume at the end of each strip a value corresponding to full black and the synchronising signal may be superimposed upon this black signal. In the second case the synchronising signal follows on immediately at the end of the strip and therefore rises from whatever value the picture signal may have at the end of the strip. In either case the direct and low frequency signal components, representing absolute values of average brightness and slow changes thereof, may either be transmitted through the system or suppressed (by the use of amplifiers incapable of handling these low frequencies for example). In the latter case the missing components may or may not be re-inserted at a desired point or at desired points. When the D.C. and low frequency components are present it is possible to assign definite limiting signal amplitudes to "black", "white" and the peak of synchronising signals, but where these components are absent this cannot be done and in consequence it is necessary to make the synchronising pulse of greater amplitude to ensure correct synchronisation under all conditions of average picture brightness.

The signals above described may be used to modulate a transmitter and it can be arranged that minimum carrier amplitude represents either full black or the peak of the synchronising impulse. The latter is preferred because interfering pulses will only black out the reproducing beam at the receiver.

If signals of the kind above referred to are applied to the control electrodes of a receiving cathode ray tube or to some other form of picture reconstituting device, periods of very bright illumination will occur during the times when synchronising pulses are present. With certain forms of reconstituting device it is readily possible to obscure the light during these periods but with outer forms of reconstituting device such as cathode ray tubes it is not possible to do this.

It is accordingly an object of the present invention to provide means whereby the bright illumination above referred to may be avoided.

According to the present invention a receiver for use in a television system of the kind specified is provided with means whereby for picture signals having an amplitude exceeding a predetermined value (picture white) the light is maintained at a low value. Thus there may be provided, in addition to the usual channel feeding picture signals to the reconstituting device, a second channel containing a limiting device, the arrangement being such that for signals below said predetermined value the second channel is inoperative but for signals above this value the second channel applied to the device a signal effectively opposite in sense to that applied through the first channel. The signal in the second channel can be arranged to annul that in the first channel under these conditions.

It may be arranged that a single device serves to provide auxiliary signals which reduce the illumination in response to picture signals exceeding a predetermined amplitude and to separate the synchronising signals from the picture signals. Delay means may be provided to ensure that the synchronising signals cannot produce a bright illumination before the counteracting darkening means have had time to operate. The delay means may for this purpose be inserted in the main picture signal channel before the reconstituting device and after the point of connection of the darkening means. Suitable storing or delaying means may also be provided to cause the darkening effect to persist slightly after the cessation of a synchronising signal.

It is preferably arranged that the D.C. and low frequency picture signal components are present in the signals fed to the darkening means and to the means for separating the synchronising signals from the picture signals.

The invention will be described by way of example with reference to the accompanying drawing in which two circuit diagrams embodying the present invention are shown.

Referring to Fig. 1, which shows a simple circuit, picture signals (with or without the D.C. component) together with the synchronising signals are applied across terminals 1 from a suitable receiver.

The grid 4 of a cathode ray tube is fed through a resistance 3 from a potentiometer 2. The resistance 3 is much higher than the impedance of the circuit from which it is fed. The signals are applied in such a sense that the whites and synchronising signals make the grid 4 more positive. The signals are also fed through condenser 5 and shunt leak 6 to the grid of valve 7. The screen grid voltage of this valve is adjusted so that the grid base (negative voltage for cut-off) is about equal to or less than the amplitude of the synchronising signals (full white to synchronising) applied to its grid. The time constant of the leak 6 and condenser 5 is such that the valve 7 automatically biases itself so that the D.C. component is present on its grid and so that only synchronising signals cause anode current to flow. This anode current will produce a negative voltage pulse on the anode of the valve 7 and on the grid 4, thus blacking out the cathode ray. The resistance 3, or the maximum setting of the potentiometer 2 must be such that the negative pulse from the anode will not tend to obliterate the positive synchronising volts that produce it.

Fig. 2 shows a more complex circuit, intended for operation with a transmitter which transmits the D.C. modulation component. Modulated high frequency signals from the circuit 8 are rectified by a diode 9 co-operating with a condenser 10 and a resistive potentiometer 11. The modulation is supposed to be in such a sense that "black" is represented by an increase in carrier amplitude so that the rectifier as shown will give negative voltage corresponding to the high carrier of "black". If the modulation were in the reverse sense the diode 9 would be inverted. 12 is an H.F. choke, on the far side of which a conductive connection is taken to the grid of valve 13 which is either so adjusted or operated (for example by adjustment of screen voltage) that no anode current flows for carrier values representing picture signals but only for the low or zero carrier representing the synchronising signal. The output of the valve 13 is led to any amplifying valve 14 (shown as a pentode) whose output is applied to a resistance in the circuit of the cathode 15 of the cathode ray tube. The synchronising signals will cause current to flow in the anode circuit of the valve 13, thus producing on the grid of the valve 14 a negative pulse which will give on the cathode 15 of the cathode ray tube, a positive pulse which blacks out the beam. The grid 16 is fed with picture signals from the potentiometer 11. It must be arranged that the synchronising signal produces a bigger pulse on the cathode 15 than on the grid 16, preferably much bigger. As an added refinement a delay network 17 may be added. This network preferably has a cut-off well above the highest picture frequency but serves to delay the picture signals slightly, so that the black-out effect from the valve 14 can operate well before the synchronising signal arrives at the grid 16. This prevents any short flash being produced by the beginning of the synchronising signal even if its wave front is not ver abrupt. A condenser 18, bridged across part of the anode resistance of the valve 13, delays the end of the pulse in the synchronising signal applied to the cathode 15. This ensures that despite the delay network 17, the synchronising signal has ceased to appear at the grid 16 before the pulse on the cathode 15 has died out sufficiently to be ineffective. Alternatively a further delay network may be inserted in place of the condenser 18, or the condenser 18 may be inserted across part of the resistance feeding the cathode 15. If desired, the valve 13 may also be used for separating synchronising signals, these signals being taken out at a point such as 19 to the synchronising mechanism.

Dated this 4th day of September, 1934.

REDDIE & GROSE,

Agents for the Applicant,

6, Breamís Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in and relating to Television Systems

I, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

The present invention relates to television and like systems, and more particularly but not exclusively to systems of the kind in which picture and synchronising signals are transmitted as a composite signal, along a single channel, the synchronising signals occurring in intervals between trains of picture signals; the synchronising signals may be in the "whiter than white" direction, in which case, if the whole composite signal is fed at the receiver to a reconstituting device such for example as a cathode ray tube, they produce spurious white flashes at the receiver, or they may be in the "blacker-than-black" sense. In both cases, spurious white flashes may be produced at the receiver by intense interference, whether or not the synchronising signals are fed to the reconstituting device, and the present invention aims to provide means for eliminating or reducing these flashes.

In the case in which they are in the whiter-than-white direction, the synchronising signals extend from a value corresponding to full white in the picture in a direction corresponding to an increase in brightness of the picture. A picture signal corresponding to full white can be given a definite meaning because in the case of transmitted light (from a film for example) it is the signal produced when a completely transparent part of the picture is canned, and in the case of reflected light it is the signal produced when the most perfectly reflecting part of the object is scanned. Furthermore the amplitude of full white may be determined arbitrarily, if desired, by means of a suitable amplitude limiting device which only permits picture signals of amplitude less than a predetermined value (to be called full white) to pass.

The synchronising signals are usually in the form of impulses occurring in the intervals between the scanning of successive strips of the object. The picture signal may be caused to assume, at the end of each strip, a value corresponding to full black, and the synchronising signal may be superimposed upon this black signal in the desired direction. When the synchronising impulses are transmitted in the whiter-than-white direction, the synchronising impulse may follow on immediately at the end of the strip and may therefor rise from whatever value the picture signal has at the end of the strip. In any case, the direct and low frequency signal components, representing absolute values of average brightness and slow changes thereof, may either be transmitted through the system or suppressed by the use, for example, of amplifiers incapable of handling these low frequencies. In the latter case, the missing components may or may not be reinserted at a desired point or at desired points. When the D.C. and low frequency components are present it is possible to assign definite limiting signal amplitudes to "black", "white" and the peak of synchronising signal, but where these components re absent this cannot be done and in consequence it is necessary to make the synchronising pulse of greater amplitude to ensure correct synchronisation under all conditions of average picture brightness.

The composite signal comprising picture and synchronising signals may be used to modulate a carrier wave at the transmitter, and it can be arranged that minimum carrier amplitude either represents full black or the peak of a synchronising impulse in the blacker-than-black sense; alternatively, minimum carrier may represent peak picture signal or the peak of a synchronising impulse in the whiter-than-white sense, and in these cases the advantage is gained that interfering pulses which cause an increase in carrier amplitude only black out the reproducing beam at the receiver.

As has been stated, if a composite signal of the kind above referred to is applied to the control electrodes of a receiving cathode ray tube or to some other form of picture reconstituting device, periods of very bright illumination will occur during the times when synchronising pules in the whiter-than-white direction are present. With certain forms of reconstituting device it is readily possible to obscure the light during these periods but with other forms of reconstituting device such as cathode ray tubes it is not always possible or convenient to do this.

Similar periods of bright illumination may also be caused by interfering signals, and clearly since these may occur in any position in the reconstituted picture, they cannot be obscured by any fixed mask or the like.

The present invention accordingly provides a television or like receiver which compresses means whereby, for received signals, such as synchronising signals or interfering signals having an amplitude exceeding a predetermined value (for example, whiter-than-white signals) the intensity of the light produced is maintained at a low value. Thus there may be provided, in addition to the usual channel feeding picture signals to the reconstituting device, a second channel containing a limiting device, the arrangement being such that for signals below said predetermined value, the second channel is inoperative, but for signals above this value the second channel applies to the reconstituting device a signal effectively opposite I sense to that applied through the first channel. Under these conditions, the signal in the second channel can be arranged to annul that in the first channel.

It may be arranged, in cases in which the synchronising signals are in the whiter-than-white sense, that single device serves both to provide auxiliary signals which reduce the illumination in response to picture signals exceeding a predetermined amplitude, and also to separate the synchronising signals from the picture signals. Delay means may be provided to ensure that the whiter-than-white signals cannot produce a bright illumination before the second channel has had time to provide a counteracting signal. The delay means may for this purpose be inserted in the main picture signal channel before the reconstituting device and after the point of connection of the second channel. Suitable storing or delaying means may also be provided to cause the darkening effect to persist slightly after the cessation of a whiter-than-white signal.

It is preferably arranged that the D.C. and low frequency picture signal components are present in the signals fed to the device in the second channel from which darkening signals are derived and to the means for separating the synchronising signals from the picture signals .

The invention will be described by way of example with reference to the drawing filed with the Provisional Specification, wherein Figs. 1 and 2 show diagrammatically two circuit arrangements embodying the present invention. It will be assumed in both cases that the synchronising signals re in the whiter-than-white sense, but it is to be understood that both arrangements operate equally well to reduce or eliminate white flashes due to interfering signals in the whiter-than-white sense.

Referring to Fig. 1, which shows a simple circuit, picture signals (with or without the D.C. component) together with the synchronising signals are applied across terminals 1 from a suitable receiver.

The grid 4 of a cathode ray tube is fed through a resistance 3 from a potentiometer 2. The resistance 3 is much higher in valve than the impedance of the circuit from which it is fed. The signals are applied in such a sense that the whites and synchronising signals make the grid 4 more positive. The signals are also fed through condenser 5 and shunt leak 6 to the grid of a valve 7. The screen grid voltage of this valve is adjusted so that the grid base (negative grid voltage required for cut-off) is about equal to or less than the amplitude of the synchronising signals (full white to synchronising) applied to its grid. the time constant of the leak 6 and condenser 5 is such that the valve 7 automatically biases itself so that only synchronising signals cause anode current to flow. This anode current will produce a negative voltage pulse on the anode of the valve 7 and on the grid 4, thus blacking out the cathode ray. The resistance 3, or the maximum setting of the potentiometer 2 must be such that the negative pulse from the anode of valve 7 will not tend to obliterate the positive synchronising voltage that produces it.

Fig. 2 shows a more complex circuit, intended for operation with a transmitter which transmits the D.C. modulation component. Modulated high frequency signals from the circuit 8 are rectified by a diode 9 co-operating with a condenser 16 and a resistive potentiometer 11. The modulation is supposed to be in such a sense that as the picture becomes darker, the carrier amplitude increases, so that the rectifier 9, connected as shown, will give a maximum negative voltage at its anode corresponding to the high carrier amplitude representing "black". If the modulation were in the reverse sense the diode 9 would be inverted. 12 is an H.F. choke, from the right hand side of which a conductive connection is taken to the control grid of valve 13, which is either so adjusted or so operated (for example by adjustment of screen grid voltage) that no anode current flows for carrier values representing picture signals but only for the low or zero carrier representing the synchronising signal. The output of the valve 13 is fed to an amplifying valve 14 (shown as a pentode) whose output is applied to a resistance in the circuit of the cathode 15 of a cathode ray tube. The synchronising signals will cause current to flow in the anode circuit of the valve 13, thus producing on the grid of the valve 14 a negative pulse which will give, on the cathode 15 of the cathode ray tube, a positive pulse which blacks out the beam. The grid 16 is fed with picture signals from the potentiometer 11. It must be arranged that the synchronising signal produces a bigger pulse on the cathode 15 than on the grid 16, preferably much bigger. As an added refinement a delay network 17 may be added. This network preferably has a cut-off well above the highest picture frequency, but serves to delay the picture signals slightly, so that the black-out effect from the valve 14 can operate well before the synchronising signal arrives at the grid 16. This prevents any short flash being produced by the beginning of the synchronising even if its wave front is not very abrupt. A condenser 18, bridged across part of the anode resistance of the valve 13 delays the end of the pulse in the synchronising signal applied to the cathode 15. This ensures that despite the delay network 17, the synchronising signal has ceased to appear at the grid 16 before the pulse on the cathode 15 has died out sufficiently to be ineffective. Alternatively a further delay network may be inserted in place of the condenser 18, or the condenser 18 may be inserted across part of the resistance feeding the cathode 15. If desired, the valve 13 may also be used for separating synchronising signals, these signals being taken out at a point such as 19 to the synchronising mechanism.

Many other modifications of the invention within the scope of the appended claims will readily occur to those versed in the art.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-

  1. A television or like receiver wherein there are provided means whereby, for received signals having an amplitude exceeding a predetermined value, the intensity of the light produced is maintained at a low value.
  2. A receiver according to claim 1 for use in receiving signals in which synchronising signals are in the whiter-than-white sense, wherein said means include a device from which synchronising impulses free from picture signals can be derived.
  3. A receiver according to claim 1 or 2, wherein received signals are fed to a reconstituting device through first and second channels, said second channel including a limiting device adapted to pass only signals of amplitude greater than said predetermined value, and being arranged to apply to said reconstituting device signals of opposite sense to those applied thereto through said first channel.
  4. A receiver according to claim 3, wherein there are provided delay means for delaying the signals fed through said first channel relatively to signals fed through said second channel.
  5. A receiver according to claim 3, wherein storing or delaying means are provided for increasing the duration of the effect of signals fed through said second channel.
  6. A receiver substantially as herein described with reference to the drawing filed with the Provisional Specification.

Dated this 17th day of May, 1935.

REDDIE & GROSE,

Agents for the Applicant,

6, Breamís Buildings, London, E.C.4.

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Leamington Spa: Printed for His Majestyís Stationery Office, by the Courier Press. - 1936.