456,135

PATENT SPECIFICATION

Application Date: April 3, 1935. No. 10395/35.

Complete Specification Left: March 24, 1936.

Complete Specification Accepted: Nov. 3, 1936.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Television and like Transmitting Systems

We, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, and ERIC ARTHUR NIND, a British Subject, of 10A, Gordon Road, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:-

The present invention relates to television and like transmission systems of the kind in which synchronising impulses are transmitted in the intervals between trains of picture signals, the transmission being by modulated carrier wave.

In such systems it is known to be desirable to arrange that the carrier shall be reduced to zero at the peaks of the synchronising impulses. This is, however, often difficult of achievement because of the curvature of the bottom bend of the characteristic of most valves used as modulates. This necessitates what may be an undesirably large amplitude of synchronising pulse to reduce the anode current to zero.

The present invention has for its object to provide means whereby these difficulties can be overcome.

According to the present invention in a system of the kind referred to, ca carrier wave is modulated by picture signals (with or without synchronising signals superimposed) at a point in a radio frequency amplifier system, and there is applied, earlier in the radio frequency amplifying system, additional modulation consisting of synchronising signals only, the additional modulation serving to ensure that during the synchronising signals the carrier is reduced to substantially zero.

Two circuit arrangements illustrative of the present invention are shown in the accompanying drawing.

In Fig. 1, picture signals are applied to the grid of a sub-modulator valve 1. These signals are in the form of trains of signals representative of picture values interspersed with interval signals corresponding to picture black. The interval signals occur in the intervals between the scanning of successive line and successive complete frames of the object. Synchronising signals are preferably, but not essentially, superimposed upon these interval signals in the blacker than black sense. It is arranged that the direct current components of the picture signals is present in the signals applied to the valve 1. The picture signals are in such sense that an increase in brightness makes the anode of valve 2 more positive.

The anode of the valve 1 is coupled through a suitable bias battery to the grid of a modulator valve 2, the anode of which is connected to the centre point of a transformer 3 and through a resistance 8 to earth.

A carrier oscillation is applied at terminal 4 to the input of an amplifier 5 which may comprise any desired number of stages. The output of the amplifier 5 is coupled through a final stage of amplification 6 to the modulated carrier output 7. The potential differences developed across resistance 8 by the picture signals from the modulator 2 serves to modulate the carrier so that the output at 7 is a carrier modulated with the picture signals.

The synchronising signal amplitude from the modulator 2 is however insufficient to reduce the carrier amplitude to zero.

Synchronising impulses are applied to the grid of a valve 9 in such a sense as to make the grid less negative. The anode of the valve 9 is connected though a condenser 10 at the centre point of a transformer 11 on the input side of amplifier 5 and through a resistance 12 to earth. The synchronising pulses are arranged to make the upper end of resistance 12 and hence the grids of the first valves of amplifier 5, so negative that the anode current in these valves is reduced to zero. It will be evident that the amplitude of the pulses necessary to achieve cut-off at the first stage is much less than the amplitude which would be necessary at the resistance 8. Thus irrespective of whether the synchronising signals are present in the signals applied across resistance 8, the carrier is reduced to zero by the potential difference developed across resistance 12.

The channel extending between transformers 1 and 3 can without difficulty be made capable of passing the side band width necessary to represent the synchronising signals.

If desired the synchronising pulses applied to the valve 9 may be obtained by separating them from the picture signals by an amplitude selection where these pulses have been mixed with the picture signals at some earlier point. This can be done by so biasing the valve 9 that anode current flows only during the synchronising pulses.

The slight phase difference which may occur between the modulation points 8 and 12 has not been found noticeable in practice. However, any slight delay which may occur between points 12 and 8 may be compensated for if desired by delaying the input the point 8 from modulator 2 relatively to the input to point 12 from valve 9 with the aid of a suitable delay network.

Instead of modulating the carrier at the point 12 with synchronising signals, these signals may be arranged to control the operation of one of more absorption valves upon the circuit between points 4 and 3. The absorption valve of valves may be arranged effectively in parallel with a part of this circuit, such as the primary winding of the output transformer from the amplifier 5, and may be normally insulating. The synchronising signals may be arranged to render the valve or valves conducting.

An example of a circuit of this kind is given in Fig. 2 in which like parts have the same references as in Fig. 1. Picture signals with synchronising signals in the intervals are applied to terminals 13 and 14 in antiphase. The required phase opposition can be obtained with the aid of a reversing valve of otherwise.

The signals are thus applied to two valves arranged in push pull. One of these, namely the valve 2, acts as before as the modulator valve whilst valve 15 has its anode connected to the grid of an absorption valve 16. This valve is effectively in parallel with the primary winding of the transformer 3.

A synchronising pulse makes the anode of valve 15 more positive and the absorption valve 16, which is arranged to be normally insulating is thus rendered conducting. The carrier input to the amplifier 6 is thus reduced to zero. An acceptor circuit if offering a low impedance to the carrier frequency is arranged between the grid and cathode of the valve 16.

The arrangement of the valves 2 and 15 in push-pull has a number of advantages. For example, cross neutralisation to remove the effects of inter-electrode capacity will be employed, the neutralising condensers 18 and 19 being provided for this purpose. Furthermore, any resistance in the battery 20 is ineffective in modifying the A.C. wave because if the balance of the push pull arrangement is perfectly adjusted, the current from the battery 20 is constant.

Dated this 3rd day of April, 1935.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in and relating to Television and like Transmitting Systems

We, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, and ERIC ARTHUR NIND, a British Subject, of 10A Gordon Road, Ealing, London, W.5, 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 transmission systems of the kind in which synchronising impulses are transmitted in the intervals between trains of picture signals, the transmission being by modulated carrier wave.

In such systems it is know to be desirable to arrange that the carrier shall be reduced to zero at the peaks of the synchronising impulses; this is, however, often difficult of achievement because of the curvature of the bottom bend of the characteristic of most valves used in the modulated amplifier stage, the curvature necessitating what may be an undesirably large amplitude of synchronising pulse to reduce the anode current of the modulated amplifier valve, and hence the transmitted carrier, to zero.

The present invention has for its object to provide means whereby the difficulties mentioned above can be overcome.

The present invention provides a method of transmission by modulated carrier, for use in television and like systems, which method comprises generating a carrier oscillation, modulating the amplitude of said carrier in a modulated amplifier by trains of picture signals having intervals between trains, and modulating the amplitude of said carrier by synchronising signals at a secondary modulating point which is between the carrier oscillation generator and the modulated amplifier, modulation at the secondary modulating point being effect in such a manner that the amplitude of the transmitted modulated carrier is equal substantially to zero during said intervals.

The picture signals may modulate the carrier in such a sense that an increase in picture brightness corresponds to an increase in carrier amplitude, and the synchronising signals may also be fed to the modulated amplifier in such a sense as to tend to reduce the carrier amplitude during the intervals between trains of picture signals.

The invention further provides apparatus for carrying the methods of transmission according to the invention into effect.

The invention will be described, by way of example, with reference to the accompanying drawing, in which Figs. 1 and 2 illustrate two methods of carrying the invention into effect; reference will also be made to Fig. 2 of the drawing filed with the Provisional Specification.

In Fig. 1, picture signals, which are arranged to contain their direct current component, are applied to the grid of a sub-modulator valve 1. These signals are in the form of trains of signals representative of picture values, interspersed with interval signal corresponding to picture black. The interval signals occur in the intervals between the scanning of successive lines and successive complete frames of the object. Synchronising signals are preferably, but not essentially, superimposed upon these interval signals in the blacker-than-black sense, and the picture signals are arranged to be in such sense that an increase in brightness makes the control grid of valve 1 more positive.

The anode of the valve 1 is coupled through a direct-current coupling comprising a suitable bias battery to the grid of a modulator valve 2, the anode of which is connected to the centre point of a transformer 3 and through a resistance 8 to earth.

A carrier oscillation is applied at terminal 4 to the input of an amplifier 5 which may comprise any desired number of stages. The output of the amplifier 5 is fed to a modulated amplifier stage 6, the output of which is taken from the modulated-carrier output terminal 7. The potential differences developed across resistance 8 by the picture signals from the modulator 2 serve to modulate the carrier in stage 6 so that the output at terminal 7 is in the form of a carrier wave modulated with the picture signals, an increase in picture brightness corresponding to an increase in carrier amplitude.

The synchronising signal amplitude from the modulator 2 is, however, assumed to be insufficient to reduce the carrier amplitude to zero.

Synchronising impulses are applied to the grid of valve 9 in such a sense as to make the grid less negative. The anode of the valve 9 is connected through a condenser 10 to the centre point of the secondary winding of a transformer 11 arranged at the input side of amplifier 5, and through a resistance 12 to earth. The synchronising pulses are arranged to make the upper end of resistance 12, and hence the grids of the first valves of amplifier 5, so negative that the anode current in these valves is reduced to zero. It will be evident that it can be arranged that the amplitude of the pulses necessary to achieve cut-off at the first stage is much less than the amplitude which would be necessary at the resistance 8. Thus, irrespective of whether the synchronising signals are present in the signals applied across resistance 8, the carrier amplitude is reduced to zero by the potential difference developed cross resistance 12.

The channel extending between transformer 11 and 3 can without difficulty be made capable of passing the side band width necessary to represented the synchronising signals.

If desired, where the synchronising pulses have been mixed with the picture signals at a point preceding valve 9, the synchronising pulses applied to the valve 9 may be obtained by separating them from the picture signals by an amplitude selection. Such an amplitude selection may be so effected by so biasing the valve 9 that anode current flows only during the synchronising pulses, the synchronising signals being applied to valve 9 in such sense that they make the grid of this valve less negative.

The slight phase difference which may occur between the modulation points represented by the upper ends of resistances 8 and 12 has not been found noticeable in practice. However, any slight delay which may occur between the two modulation points may be compensated for if desired by delaying, with the aid of a suitable delay network, the signals established by modulator 2 at the upper end of resistance 8 relatively to the signals fed by valve 9 to the upper end of resistance 12.

Instead of modulating the carrier at the upper end of resistance 12 with synchronising signals, these signals may be arranged to control the operation of one or more absorption valves upon the circuit between input terminal 4 and transformer 3. The absorption valve or valves may be arranged effectively in parallel with a part of this circuit, such as the primary winding of the output transformer from the amplifier 5, and may be normally insulating, the synchronising signals being arranged to render the absorption valve or valves conducting.

An example of a circuit of this kind is given in Fig. 2, in which like parts have the same references as in Fig. 1. Trains of picture signals, having synchronising signals in the blacker-than-black sense occurring in the intervals between trains are applied to terminals 13 and 14 in antiphase. The required phase opposition can be obtained with the aid of a reversing valve or otherwise.

The signals are thus applied to two valves 2 and 15 arranged in push pull, the control grids of which are biased negatively by means of battery 21. One of these valves, namely the valve 2, acts as in the arrangement of Fig 1. 1 as the modulator valve, whilst valve 15 has its anode connected to the control grid of each of two absorption valves 16 and 161, each of these valves being effectively in parallel with different halves of the primary winding of the transformer 3, the center point of which winding is earthed.

The synchronising pulses are applied in such sense that each synchronising pulse makes the anode of valve 15 less negative, and the absorption valves 16 and 161, which are so biased as to be insulating during the trains of picture signals, are thus rendered conducting, the carrier input to the modulated amplifier 6 being thereby reduced substantially to zero. An acceptor circuit 17 offering a low impedance to the carrier frequency is arranged between the grid and cathode of the valve 16.

The arrangement of the valves 2 and 15 in push-pull has a number of advantages. For, example, cross-neutralisation to remove the effects of inter-electrode capacity can be employed, the neutralising condenser 18 and 19 being provided for this purpose. Furthermore, any resistance in the anode current battery 20 is ineffective in modifying the wave form of the applied signals because if the balance of the push-pull arrangement is perfectly adjusted, the current from the batter 20 is constant.

The full-wave absorption circuit comprising valves 16 and 161 may in some cases be replaced by a half-wave arrangement, as indicated in Fig. 2 of the drawing accompanying the Provisional Specification, in which parts which appear in Fig. 2 of the accompanying drawing have the same references.

The invention is not limited to the arrangements described, and may modifications within the scope of the appended claims will occur to those versed in the art.

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

  1. A method of transmission by modulator carrier, for use in television and like systems, which method comprises generating a carrier oscillation, modulating the amplitude of said carrier in a modulated amplifier by trains of picture signals having intervals between trains, and modulating the amplitude of said carrier by synchronising signals at a secondary modulating point which is between the carrier oscillation generator and the modulated amplifier, modulation at the secondary modulating point being effected ins such a manner that the amplitude of the transmitted modulated carrier is equal substantially to zero during said intervals.
  2. A method according to claim 1, wherein said picture signals are arranged to modulate said carrier in such a sense that an increase in picture brightness corresponds to an increase in carrier amplitude.
  3. A method according to claim 1 or 2, wherein said synchronising signals are also fed to said modulated amplifier in such a sense as to tend to reduce the amplitude of said carrier during said intervals.
  4. Apparatus for carrying into effect the method claim in any of the preceding claims, comprising a source of carrier oscillations, a modulated amplifier valve adapted to be fed with said carrier oscillations and with trains of picture signals having intervals between them, and secondary modulating means for ensuring that the amplitude of the carrier oscillations applied to said modulated amplifier valve during said intervals is substantially zero.
  5. Apparatus according to claim 4, wherein said secondary modulating means comprise a secondary modulator valve arranged between said source and said primary modulator valve and adapted to be fed with synchronising signals.
  6. Apparatus according to claim 4, wherein said secondary modulating means comprise an absorption circuit adapted to be fed with synchronising signals and substantially to prevent the passage of carrier oscillations to said modulated amplifier valve during said intervals.
  7. Apparatus according to claim 6, wherein said absorption circuit comprises a thermionic valve arranged to be substantially insulating except when synchronising signals are applied to it.
  8. Apparatus according to any of claims 4 to 7, wherein there is provided a source of composite signals comprising trains of picture signals interspersed with synchronising signals, means for feeding said composite signal to said modulating amplifier valve, and means for extracting synchronising signals from said composite signal and for feeding said synchronising signals to said secondary modulating means.
  9. Apparatus according to claims 7 and 8, wherein there are provided two valves arranged in push-pull relationship and adapted to have said composite signal applied in anti-phase to their control grids respectively, one of said valves being arranged to feed said composite signal to said modulated amplifier valve, and the other being arranged to feed synchronising impulses to the valve which forms part of said absorption circuit.
  10. Apparatus for modulating the amplitude of carrier oscillations substantially as herein described.

Dated this 24th day of March, 1936.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

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