US2297612A - Television and like system - Google Patents

Description

Sept. 29, 1942.

Filed May 26, 1939 U E R Y m M W ZR V m 5 E A m M Patented Sept. 29, 1942 TELEVISION AND LIKE SYSTEM Charles Leslie Faudell, Stoke Poges, England, as-

signor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application May 26, 1939, Serial No. 275,787 In Great Britain May 31, 1938 3 Claims.

The present invention relates to television and like systems.

In the majority of television and like picture transmission systems it is common practice to transmit the signals representative of the pictures at amplitude levels which are outside the amplitude levels of the synchronising signals. For example in one well known system of television transmission the picture signals are transmitted with the brighest picture points corresponding to the peak signal amplitude and the darkest points corresponding to approximately 30% of the peak amplitude. The synchronizing signals are interspersed with the picture signals and comprise pulses which reduce the amplitude of the transmitted signal from the 30% level to zero level. In the television receiver it is necessary to control the scanning generators by means of the synchronising pulses which must be free from the picture signals in order to prevent the latter from upsetting the synchronisation. and advantage has been taken of these differences in amplitude to effect such separation.

Various types of separators have been suggested for the purpose of separating the picture signals from the synchronising pulses. For example, it has been suggested to connect a suitably biassed diode valve in series between the anode of a thermionic valve amplifying television signals and the scanning generators so that the diode valve only conducts during synchronising pulse periods. This method suffers from a number of disadvantages; for instance, the separating diode has a considerable capacity between its electrodes which tends to transfer the picture signals of higher frequency to the scanning generators, thus interfering with correct synchronisation and in addition is detrimental to the vision signals due to the by-passing effect at the higher frequencies. It is also necessary to supply the heater current to the diode Valve by means of a transformer of low capacity in order that the high vision frequency signals are amplified Without loss, which would otherwise result due to the effective capacity between the cathode and heater of the diode shunted across the anodecathode path of the amplifying valve feeding the diode. The amplitudes of the synchronising pulses are also-restricted owing to the power loss in the diode system and the fact that the resistance in the anode circuit of the thermionic valve supplying the signals has to be low in value in order to overcome the effects of stray capacities.

It is an object of the present invention to pro? vide an improved amplitude separator with a view to avoiding these difficulties.

According to one feature of the present invention there is provided a method for the amplitude separation of synchronising signals from picture signals which comprises biassing a uni-directional conducting device connected in shunt with a portion of an impedance network to which the mixed signals are applied in such a manner that the state of conduction of said uni-directional device is changed during synchronising pulse periods.

More specifically the invention comprises applying to a thermionic valve the combined picture signals and synchronising pulses, said valve being associated with an impedance shunted by a diode valve, the arrangement being such that the diode is rendered non-conducting during the synchronising pulses, whereby the amplitude of the synchronising pulses is increased compared with the amplitude of the picture signals.

According to another feature of the present invention there is provided a circuit comprising a thermionic valve to which the combined picture signals and synchronising pulses are applied and an impedance shunted by a diode valve associated with said thermionic valve, which diode valve is biassed such that it is rendered non-conducting during synchronising pulse periods for separating the synchronising signals from the picture signals.

The change in the state of conduction of the diode valve may be utilised to increase or decrease the gain of said thermionic valve to which the combined picture signals and synchronising pulses are applied. 7

The present invention will now be described with reference to the drawing wherein:

Figure 1 shows one form of the present invention, and

Figure 2 shows a modifying form.

In the accompanying drawing Figure l illustrates a portion of a television receiver, the valve 2 may be either an anode bend detector or a vision frequency amplifier to which the picture and synchronising signals are applied. In the case of the vision frequency amplifier it is assumed that the D. C. component is present in the signals as applied to the grid of the valve. In the anode circuit of valve 2 are connected in series two resistances 3 and 4 and the picture signals for application to the picture reproducer, as for example, a cathode ray tube, are taken from the terminal 5 which is connected to a source of positive potential, as is also the screening grid of the valve 2. The synchronising signals are taken from the junction of resistance 3 and 4 and are applied through the coupling condenser 6 to the grid of a valve I which is used for amplifying and shaping the synchronising pulses. The valve I is provided with a suitable grid resistance 8. Across the source of supply for the thermionic valves is connected a resistance potentiometer comprising resistances 9 and I0, the resistance 9, which is preferably variable, being shunted by a condenser II. For the purpose of affecting the separation of the picture signals from the synchronising pulses a diode valve I2 is connected between the junction of resistances 3 and 4 and the junction of resistances 9 and If the anode of the diode I2 being taken to this latter junction. In the anode circuit of valve l is connected a resistance I3 which is taken to a suitable positive potential. A condenser M is connected between the anode of valve i and the source of negative supply and the frame synchronising pulses are derived across this condenser. In the screen grid circuit of the valve I is connected a transformer winding I5 which is used for the purpose of supplying line synchronising pulses to the line scanning generator. Should the line scanning generator be a blocking oscillator with a reaction transformer, the winding I5 can be an extra winding on the blocking oscillation transformer. Any other suitable method of coupling, as for example, by the use of a resistance and condenser, may be employed for coupling valve I to the line scanning generator.

The operation of this circuit is as follows:

Assuming, as an example, that the television signal has a waveform of the Marconi-EMI type, which is illustrated in the Wireless World for October 4th, 1935, page 373, the signal is applied 7 picture signals are obtained across the resistance 7 3 in the anode circuit of valve 2 and are applied to the picture reproducer. When the synchronising signals are applied to the valve 2 and cause the current of this valve to decrease below a certain value the potential drop across the resistance 4 decreases to such an extent that the oathode potential of diode I2 rises to a value more positive than the anode potential of the diode I2. This has the effect of rendering diode I2 nonconducting and a synchronising pulse is applied via condenser 6 to the grid of valve I.

It will be apparent that the resistance of the diode I2 in the conducting state, that is, when the picture signals are being received, is very low and effectively short circuits the resistance 4. The effective anode impedance of valve 2 during the picture periods is thus the resistance 3. By making resistance 3 of a suitably low order the very high frequency picture signals are not affected by the shunt capacities across this resistance. During the synchronising pulse periods, however, the effective anode impedance of valve 2 comprises resistances 3 and 4 in series, and by making resistance 4 about two or more times as great as resistance 3 it is possible to amplify the synchronizing pulses considerably more than the picture signals.

The separated synchronizing pulses are applied to the valve 1 in the positive sense and by a suitable choice of the coupling condenser B and resistance 8 it is possible to bias valve 1 by means of its grid current so that this valve only conducts during synchronising pulse periods. This has the effect of improving the shape of the synchronising pulses. The condenser I4 in conjunction with the anode resistance I3 of valve 1 serves to integrate the line and frame pulses and to produce frame pulses which are larger in amplitude than the line pulses. The amplitudes of the line pulses are so small that substantially only frame pulses affect the frame scanning oscillator. The line pulses are taken from the transformer winding I5 to the line scanning generator. The anode-cathode impedance of valve '1 is arranged to be large compared with the resistance I 3 so that when a line synchronising pulse is applied to valve I the condenser I4 is only discharged slightly and the condenser is recharged to its maximum value in approximately half a line period. On the application of a frame synchronising pulse to valve 1, however, the condenser I4 is discharged to a greater extent owing to the longer duration of the frame synchronizing pulse compared with the duration of the line synchronising pulse. The large amplitude frame pulses across the condenser I 4 can be applied directly to the frame scanning oscillation generator or via an amplitude separator.

It will be apparent that the method of separation of picture signals from synchronising pulses described will also function in systems in which the synchronising pulses have amplitudes which are greater than the amplitudes of the picture signals. In this case the signals should be applied to the grid of valve 2 in a reverse sense so that the synchronising pulses are applied in a negative sense similarly to those oi the Marconi-EMI type already discussed. Should it be necessary to apply the signals with the synchronising pulses in a positive sense to terminal I it will be necessary to reverse the connections of the diode valve I2, that is, the anode of the diode l2 would be connected to the junction of resistances 3 and 4 and a suitable biassin'g potential would be applied to the cathode of the diode I2.

Since the diode valve 12 is connected in shunt with the resistance 4 and the valve is conducting during the picture periods the capacity of the electrodes of this valve are ineffective to spoil the picture signals and in addition the capacity of the heater windings do not affect the picture signals.

The bias on the diode valve I2 can be adjusted by means of the variable resistance 9 so that the valve I2 only insulates during the synchronising pulse periods.

Interference may also be eliminated by grid current in the valve 1 by making the amplitude of the synchronising pulses applied in'this valve sufficiently great. It is preferable in this case to make resistance 8 of a suitably low value, say, five times the value of resistance 4.

Referring now to the circuit of Figure 2, which also illustrates a portion of a television receiver, the combined picture and synchronising signals are applied to the control grid of a'pentode valve I so that the valve I acts as 'a self-establishin D. C. amplifier, as described in the'specification of U. S. Application Serial No. 720,205 the synchronising pulses being applied in the positive sense, through a capacity I9 and leak resistance 20. The applied signals establish D. C. potentials at the control grid of the valve I with consequent production of potentials at the anode of valve I6 which are 180 degrees but of phase with the applied signals. The anode of valve 16 is loaded with a resistive impedance Consisting of the resistances 11, 2|, 22 and 23, a diode valve [8 bridging this impedance between the anode of valve [6 and the junction of the resistances 22 and 23. The anode of the diode I8 is connected to the anode of valve [6 and the cathode of the diode I8 is suitably biassed due to the potential applied between the end terminals of resistance 2! and resistance 23. The junction of resistance 2| and 22 is connected directly to the screen electrode of valve IS.

The biassing potentials applied to the screen electrode of valve I6 and to the cathode of the diode I8 are chosen so that when the potential of the control grid of valve [6 is above a predetermined value, for example that potential corresponding to the application of the synchronising pulse, the anode current of the valve I6 is sufficient to cause the anode potential of valve 6 to fall to a chosen value, above which chosen value the diode l8 becomes conducting.

The operation of the circuit is as follows:

The signals applied to the control grid of valve It would, in the absence of diode i8, result in amplified signals appearing at the anode of valve 16, the tips of the synchronising pulses corresponding to zero anode potential and the threshold black level corresponding to some positive potential. made to correspond to an anode potential of valve I6 of 30 volts, and picture signals will then correspond to an anode potential of more than 30 volts while synchronising pulses will correspond to an anode potential of less than 30 volts.

The diode valve is may be biassed so that it is rendered conducting when the potential of its anode falls below 30 volts. On the arrival of a synchronising pulse, the diode I8 is thus rendered non-conducting and by arranging that the arrival of a synchronising pulse results in zero anode current in the valve 16 throughout the period of the synchronising pulse a constant potential is applied at the terminal 24 from whence the line and frame deflecting circuits may be actuated by the leads 25 and 28. A picture signal, however, renders diode I8 conducting so that no actuating potentials are applied to leads 25 and 26 during the periods of arrival of picture signals.

The arrangement described thus results in sep aration of the picture and synchronising signals and, to obtain good separation, the resistance ll should be considerably greater than the impedance of the diode IS. The picture signals are fed directly from terminal 24 to the junction of resistances 22 and 23 during picture periods, which path has no capacity in shunt therewith.

It is not essential to employ a self-establishing arrangement in connection with valve [6, and direct D. C. coupling may be employed if desired.

This threshold black level may be r I claim:

1. In television apparatus wherein composite signals are received which comprise video, line synchronizing and frame synchronizing signals, and wherein said synchronizing signals are outside of the amplitude range of said video signals, means for separating said signals comprising a thermionic tube having anode, cathode, and at least one control electrode, means for impressing said signals onto said thermionic tube to control the current in the anode-cathode circuit thereof, a potentiometer arrangement connected in the anode-cathode circuit of said thermionic tube, a uni-directional conductor connected substantially in parallel with at least a portion of said potentiometer whereby signals attaining a predetermined amplitude are partially short-circuited by said uni-directional conductor and whereby a definite division may be made between said synchronizing signals and said Video signals, means for impressing said synchronizing signals onto an integrating circuit, and separate output means electrically connected to said integrating circuit whereby said line and frame synchronizing signals may be separately removed from said apparatus.

2. Apparatus in accordance with claim 1, wherein said potentiometer unit comprises a pair of resistive members serially connected and whereby one of said resistive members has a value considerably greater than the other of said resistive members.

3. In television apparatus wherein video, line synchronizing and frame synchronizing signals are received, and wherein said synchronizing signals are outside of the amplitude range of said video signals, a first thermionic tube having anode, cathode and at least one control electrode, means for impressing said signals onto the control electrode-cathode path of said thermionic tube, potentiometer means connected in the output circuit of said thermionic tube, a time constant circuit, uni-directional conducting means connected substantially in series with said time constant circuit, said series circuit being connected substantially in parallel with at least a portion of the resistance in said potentiometer, a second thermionic tube having anode, cathode and at least one control electrode, means for impressing at least a portion of the voltage variations across at least a portion of said potentiometer onto the control electrode-cathode path of said second thermionic tube, an integrating circuit connected in the output circuit of said second thermionic tube, and separate output means connected to said second thermionic tube for separately removing the line and frame synchronizing signals.

CHARLES LESLIE FAUDELL.

Images