490,150

PATENT SPECIFICATION

Application Date: March 4, 1937. No. 6409/37.

(Patent of Addition to No. 455,375: dated Jan. 15, 1935.)

Complete Specification Left: Feb. 1, 1938.

Complete Specification Accepted: Aug. 10, 1938.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Television Systems

We, ELECTRIC & MUSICAL INDUSTRIES LIMITED, of Blyth Road, Hayes, Middlesex, a British Company, and ALAN DOWER BLUMLEIN, British subject, of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:-

This invention relates to television systems and is particularly concerned with circuits for obtaining synchronism between the operation of scanning at a receiver with that at a transmitter.

In the Specification of Patent No. 455,375, the problems involved in obtaining synchronism are discussed, and alternative synchronising pulse separating circuits are described, such circuits being for use in television systems employing frame pulses of relatively long duration and line pulses of relatively short duration, but of the same amplitude. In the arrangements described in the specification, a condenser is charged through a resistance and shunted by a valve which is positively biased, and is, therefore, conducting in the intervals between pulses. The condenser charges during pulses and a saw-tooth voltage waveform is produced, the pulses of which are of duration equal to that of the pulses producing them but of greater amplitude in the case of the longer frame pulses. By applying both derived saw-teeth pulses to an amplitude selecting device the pulses of large amplitude operate to produce pulses which are applied to the frame deflecting apparatus.

The Specification No. 455,375 only discloses circuit arrangements which depend upon the components having shorter time constants between pulses than during pulses. Actually the time constant between the pulses is much shorter than the gaps between them.

The object of the present invention is to provide a pulse separating circuit which will give correct separation for interlacing frame pulses while employing time constants which are longer during the intervals between pulses than during pulses.

According to the present invention, a television receiver of the type set forth in the Specification of Patent No. 455,375 includes a circuit arrangement for effecting selection from mixture of line and frame synchronising pulses which are of differing duration, of the frame pulses which exceed the duration of the line pulses, said circuit arrangement including an impedance and a condenser charged from a source of potential, a device controlling the charge in said condenser, the time constant of said impedance and condenser being greater than the intervals between the pulses, but rendered effectively short by means of unilaterally conducting device such as a diode valve.

The device controlling the charge in the condenser may be a thermionic valve, to the input circuit of which the mixture of pulses to be separated are applied.

In order that the nature of the invention may be more clearly understood, an impulse separating circuit operating in accordance therewith, will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 is a circuit diagram, and

Figs. 2 and 3 show explanatory wave-forms.

Referring to Fig. 1 of the drawings, the valve 1 which although represented as a triode, may be a tetrode or a pentode, is arranged to be self-biassing, and a resistance 2 is connected between the grid and cathode of the valve for this purpose. Picture signals and synchronising signals are together fed to the grid of the valve 1 through a condenser 3, the synchronising signals being in the positive sense and of sufficient amplitude to cause anode current cut-off in the valve 1 for all picture signal amplitudes. A resistance 4 is connected between the anode of the valve and the source of high tension voltage and a condenser 5 is connected across the resistance 4. The circuit connections and method of operation up to the point marked A in the diagram are typical of known arrangements for separating synchronising pulses.

Diode valves 6 and 7 are shown the anode of one and the cathode of the other connected to the anode of the valve 1 and derive their operating potentials from the high tension source supplying the charging current for the condenser 5.

The operation of the circuit will be described with reference to Figs. 2 and 3 of the drawings, the former showing a typical synchronising signal waveform comprising three line synchronising pules l followed by two frame pulses f.

Fig. 3 shows the two curves A and B drawn with reference to a line CD representing maximum high tension voltage. The curve A represents the voltage obtained at the point A in the circuit shown in Fig. 1 independently of any apparatus following that point in the circuit.

On the application of a synchronising pulse to the grid of valve 1, the valve is rendered conductive and the condenser tends to charge negatively, as shown by the straight falling portions of curve A, and in the intervals between the synchronising pulses the potential of the anode of valve 1 tends to return towards the H.T. voltage as shown by the rising curved portions of the curve A. The time constant in this instance is such that the discharge of the condenser occupies a time somewhat greater than that of a line period.

On the application of the frame pulses to the grid of valve 1, the anode potential tends to fall further than in the case of the line pulses, and to pass a limiting value a at which point the frame scanning apparatus is operated. It will be seen that the time taken for the potential of point a to be reached, depends upon the amount of charge in the condenser left over from the preceding line synchronising pulse.

If the synchronising pulses are arranged to effect interlacing, some frame pulses will occur mid-way between line pulses and others will occur at the same time as line pulses. Due to the different positions of alternate frame pulses with respect to the line pulses, there are different charges in the condenser at the occurrence of alternate frame pulses so that the times after the beginning of the frame pulses at which the potential of the condenser rises to the value required to fire the blocking oscillator generating the deflecting wave form are different, resulting in one frame scan being a little late with respect to the other and inaccurate interlacing.

The curve B represents the voltage across the condenser 5 resulting from the connection of the diode valves 6, 7 as shown. The condenser 5 is prevented by the diode 6 from discharging to the H.T. potential represented by the line CD, the cathode of the diode being connected to the positive terminal of the high tension source through a tapping point 8 in a potential divider including resistances 9, 10 and 11, the point 8 being decoupled by the condenser 12.

During the interval between the line synchronising pulses, the potential of the anode of valve 1 therefore rests at the value indicated by the straight horizontal portions of the curve B. On the occurrence of a synchronising pulse the potential falls as in curve A, and at the cessation of a synchronising pulse the potential rises. If the diode 6 were not connected as shown, the curve would proceed along the dotted line shown. As soon as the potential of the anode reaches the critical value of the cathode of the diode 6, the rise of the anode potential ceases, so that its potential is maintained fixed as shown in the full line of curve B. It will thus be seen that in effect an artificial short time constant is given to the potential rise by the diode 6. On the occurrence of a frame synchronising signal the valve remains conducting for a longer period, and causes the potential of the point A to drop below the limiting voltage applied to the diode 7, the anode of which is held through the coil 13 at the potential at the tapping point 14 which is decoupled through condenser 15. As soon as the potential across the condenser 5 drops below the level b, current flows through the coil 13 which may be coupled for example, with the coils of a blocking oscillator controlling the frame scanning. It will be seen from examination of the curve B that the time at which the potential b is reached is unaffected by any remnant of the preceding frame synchronising pulses. If desired, the diode 7 may be biassed from lower potential such s that represented at b1, so that it will not conduct until the occurrence of the second, third or even the fourth frame synchronising pulse.

Dated this 3rd day of March, 1937.

F. W. CACKETT,

Chartered Patent Agent.

COMPLETE SPECIFICATION

Improvements in or relating to Television Systems

We, ELECTRIC & MUSICAL INDUSTRIES LIMITED, of Blyth Road, Hayes, Middlesex, a British Company, and ALAN DOWER BLUMLEIN, a British subject, of 32, Audley 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:-

This invention relates to television systems, and is particularly concerned with circuits for obtaining synchronism between the operation of scanning at a receiver with that at a transmitter.

In the Specification of Patent No. 455,375, the problems involved in obtaining synchronism are discussed, and alternative synchronising pulse separating circuits are described, such circuits being for use in television systems employing frame pulses of relatively long duration and line pulses of relatively short duration, but of the same aptitude. In the arrangements described in the specification, a condenser is charged though a resistance and shunted by a valve which is positively biased, and is, therefore, conducting in the intervals between pulses. The condenser charges during pulses and a saw-tooth voltage waveform is produced, the pulses of which are of duration equal to that of the pulses producing them but of greater amplitude in the case of the longer frame pulses. By applying both derived saw-teeth pulses to an amplitude selecting device the pulses of large amplitude operate to produce pulses which are applied to the frame deflecting apparatus.

The Specification No. 455,375 only discloses circuit arrangements which depend upon the components having shorter time constants between pulses than during pulses. Actually the time constant between the pulses is much shorter than the gaps between them.

The object of the present invention is to provide a pulse separating circuit which will give correct separation for interlacing frame pulses while employing time constants which are longer during the intervals between pulses than during pulses.

According to the present invention in a modification of a television receiver according to claim 1 of the Patent No. 455,375, the converting means for producing derived frame and line pulses from frame and line synchronising pulses includes a condenser charged from a source of potential, a resistance connected across said condenser and a device for controlling the charge in said condenser, the time constant of said resistance and condenser being greater than the intervals between the synchronising pulses but rendered effectively short by means of a unilaterally conducting device such as a diode valve which serves to maintain a predetermined voltage across the condenser in the intervals between pulses. The device for controlling the charge in the condenser may be a thermionic valve to the input circuit of which the mixture of pulses to be separated is applied.

In a particular circuit arrangement embodying the invention the resistance and condenser are connected in the anode circuit of the controlling valve and a diode valve or the equivalent is connected between a source of potential and the anode of the controlling valve in such a manner that the diode is only rendered conducting when the charge in the condenser reaches a predetermined value. The derived frame pulses are applied to a saw-tooth waveform generating circuit through a diode valve the cathode of which is connected to the anode of a controlling valve and the anode of the diode is connected to a source of positive potential through a coil coupled with the coils of a blocking oscillator circuit.

In order that the invention may be more clearly understood, and readily carried into effect, an impulse separating circuit operating in accordance therewith, will now be described by way of example with reference to the drawings left with the Provisional Specification in which:-

Fig. 1 is a circuit diagram, and

Figs. 2 and 3 show explanatory wave-forms.

Referring to Fig. 1 of these drawings, the valve 1 which although represented as a triode, may be a tetrode or a pentode, is arranged to be self-biassing, and a resistance 2 is connected between the grid and cathode of the valve for this purpose. Picture signals and synchronising signals are together fed to the grid of the valve 1 through a condenser 3, the synchronising signals being in the positive sense and of such amplitude that while they will cause anode current to flow in the valve 1 the anode current will be cut off at all picture signal amplitudes. A resistance 4 is connected between the anode of the valve and the source of high tension voltage and a condenser 5 is connected across the resistance 4. The circuit connections and method of operating up to a point marked A in the diagram are typical of known arrangements for separating synchronising pulses.

Diode valves 6 and 7 are shown having the anode of one and the cathode of the other connected to the anode of the valve 1 and derive their operating potentials from the high tension source supplying the charging current for the condenser 5.

The operating of the circuit will be described with reference to Figs. 2 and 3 of the drawings, the former showing a typical synchronising signal waveform comprising three line synchronising pulses 1 followed by two frame pulses f. Fig. 3 shows the two curves A and B drawn with reference to a line CD representing maximum high tension voltage obtained at the point A in the circuit shown in Fig. 1 independently of any apparatus following that point in the circuit.

On the application of a synchronising pulse to the grid of valve 1, the valve is rendered conductive and the condenser tends to charge negatively, as shown by the straight falling portions of curve A, and in the intervals between the synchronising pulses the potential of the anode of valve 1 tends to return towards the H.T. voltage as shown by the rising curved portions of the curve A. The time constant in this instance is such that the discharge of the condenser occupies a time somewhat greater than that of a line period.

On the application of the frame pulses to the grid of valve 1, the anode potential tends to fall further than in the case of the line pulses, and to pass a limiting value a at which point the frame scanning apparatus is operated. It will be seen that the time taken for the potential of point a to be reached, depends upon the amount of charge in the condenser left over from the preceding line synchronising pulse.

If the synchronising pulses are arranged to effect interlacing, some frame pulses will occur mid-way between line pulses and others will occur at the same time as line pulses. Due to the different positions of alternate frame pulses with respect to the line pulses, there are different charges in the condenser at the occurrence of alternate frame pulses so that the times after the beginning of the frame pulses at which the potential of the condenser rises to the value required to fire the blocking oscillator generating the deflecting wave form are different, resulting in one frame scan being a little late with respect to the other and inaccurate interlacing.

The curve B represents the voltage across the condenser 5 resulting from the connection of the diode valves 6, 7 as shown. The condenser 5 is prevented by the diode 6 from discharging to the H.T. potential represented by the line CD, the cathode of the diode being connected to the positive terminal of the high tension source though tapping point 8 in a potential divider including resistances 9, 10 and 11, the point 8 being decoupled by the condenser 12. The diode 6 is thus biased so that it becomes conductive only when the voltage at the anode reaches a certain value as the condenser 5 tends to discharge to the high tension potential.

During the interval between the line synchronising pulses, the potential of the anode of valve 1 therefore rests at the value indicated by the straight horizontal portions of the curve B. On the occurrence of a synchronising pulse the potential falls as in curve A and at the cessation of a synchronising pulse the potential rises. If the diode 6 were not connected as shown, the curve would proceed along the dotted line shown. As soon as the potential of the anode reaches the critical value of the cathode of the diode 6, the rise of the anode potential ceases, so that is potential is maintained fixed as shown in the full line of curve B. It will thus be seen that in effect an artificial short time constant is given to the potential rise by the diode 6. On the occurrence of a frame synchronising signal the valve remains conducting for a longer period, and caused the potential of the point A to drop below the limiting voltage applied to the diode 7, the anode of which is held through the coil 13 at the potential at the tapping point 14 which is decoupled through a condenser 15. As soon as the potential across the condenser 5 drops below the level b, current flaws through the coil 13 which may be coupled for example, with the coils of a blocking oscillator controlling the frame scanning. It will be seen from examination of the curve B that the time at which the potential b is reached is unaffected by any remnant of the preceding frame synchronising pulses. If desired, the diode 7 may be biassed from a lower potential such as that represented at b1, so that it will not conduct until the occurrence of the second, third or even the fourth frame synchronising pulse.

While the valve 1 has been shown to be of the triode type, it will be understood that a valve of the screen grid or pentode type may be employed, and in such a case the line frequency pulses derived from the line frequency synchronising pulses and fed to a line frequency saw-tooth wave-form generating circuit may be taken from the screening grid of the valve.

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 modification of a television receiver according to claim 1 of Patent No. 455,375, wherein the converting means for producing derived frame and line pulses from frame and line synchronising pulses includes a condenser charged from a source of potential, a resistance connected across said condenser and a device for controlling the charge in said condenser being greater than the intervals between the synchronising pulses but rendered effectively short by means of a unilaterally conducting device such as a diode valve which service to maintain a predetermined voltage across the condenser in the intervals between pulses.
  2. A television receiver according to claim 1 wherein said device for controlling the charge in said condenser is a thermionic valve to the input circuit of which the mixture of pulses to be separated is applied.
  3. A television receiver according to claim 2 wherein said resistance and condenser are connected in the anode circuit of said controlling valve and a diode valve or the equivalent is connected between source of potential and the anode of said controlling valve in such a manner that said diode is only rendered conducting when the charge in the condenser reaches a predetermined value.
  4. A television receiver according to claim 2 or 3 wherein said derived frame pulses are applied to a saw-tooth wave-form generating circuit through a unilaterally conducting device, which is rendered conductive only on the occurrence of a frame synchronising pulse.
  5. A television receiver according to claim 2 or 3 wherein said derived frame pulses are applied to a saw-tooth wave-form generating circuit through a diode valve or the equivalent which is rendered conductive only on the occurrence of a frame synchronising pulse, the cathode of the diode being connected to the anode of the controlling valve while the anode is connected to a source of positive potential.
  6. A television receiver according to claim 5 wherein the anode of said diode is connected to said source of positive potential through a coil which is coupled with the coils of a blocking oscillator circuit constituting said saw-tooth wave-form generating circuit.
  7. A television receiver according to any one of claims 2 to 5, wherein said controlling valve is of the screen grid type and line frequency pulses derived from line frequency synchronising pulses applied to the valve are taken from the screening grid of said valve.
  8. The modification of the invention claimed in Patent No. 455,375, substantially as described with reference to Fig. 1 of the drawings filed with the Provisional Specification.

Dated this 31st day of January, 1938.

F. W. CACKETT,

Chartered Patent Agent.

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Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. – 1938.