US2212933A - Television system - Google Patents

Description

Aug; 27, 1940. Q FAUDELL 2,212,933

TELEVIS ION SYSTEM Filed Jan. 18, 1938 2 Sheets-Sheet l lNVE/VTOR C. L FA UDELL BY -v ATTORNEY 27, 1940- c. L. FAUDELL 2,212,933

TELEVIS ION SYSTEM Filed Jan. 18, 1938 2 Sheets-Sheet 2 //V l ENTOR c. L. FAZELL ATTORNEY Patented Aug. 27, 1940 UNETE 2,212,933 TELEVISION SYSTEM Charles Leslie Faudell,

near Stoke Poges, Engpany of Great Britain Application January 18, 1938, Serial No. 185,493 In Great Britain JanuaryZO, 1937 5 Claims.

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 trans- 5 mitter.

.In the specification of British Patent #455,375, the problems involved in obtaining synchrcnisrn 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 duratiombu-tof the same amplitude. In certain of the arrangements described in the specification, a condenser is charged 5. through a resistance and shunted by a valve which is positively biased, and is, therefore, con-. ducting 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 pro-, ducing them but of greater amplitude in the case of the longer frame pulses. By applying hoth derived saw-teeth pulses. to an amplitude selects ing device the pulses of larger amplitude operate 25 to produce pulses which are applied to the frame deflecting apparatus. i

In one arrangement described in the SPECifiCQm tion referred to, an inductance is inserted in series with a resistance in the anode lead oi the v 40 .up across the resistance suddenly changes and the saw-tooth voltage pulse thus produced is taken ofl? from terminals, one of which is con.- nected to the junction between the inductance and-the resistance and the other to the cathode of the valve. The saw-tooth pulses due to the frame synchronising pulses will be of larger amplitude than those due to the strip or line synchronising pulses and will operate frame wavesive to the smaller amplitude saw-tooth pulses derived from the line synchronising pulses,

The object of the present invention also is to provide improved line and frame pulse s'eparat ing circuits in which irregular operation due to form deflecting apparatus which is not resp-Qn he effect of adjacent, pulses upon each othe i avoided or reduced. 7 I

According to the presentinvention, converting means for producing derived fros and strip pulses from frame and strip sy nchrgmising' include an inductance through'vvhichpasses cur: rent controlled by a controlling device, a unilateerally conducting device being operatively connected with this inductance in such a manner that "current normally flowing through said unilaterally conducting device ceases to flow on the application of frame synchrjo ising pulses' to the mwl ins d v e the r v d rame ulse bein obtaine irom the ou put termina of he unilaterally conducting device.

v l 1 In a t qu uli ran ements embod n the invention the controlling device a thermie onic valve which is normally conductive but is r de e no -c ndu ive on th ap icat on to 7 pulse to. h t erm c'ni e ve, The indu tance i onnected in series w t a re istan e, ime o an o t indu nce and res s a ce 9 m b n tion being e com a d ith he dim Of a r me nc on sin u s The inductance may be connected directly in the anode lead of the thermionic valve and the unilateral y wnductins i e ma o si t f diode valve, the anode of which is connected to the end of the inductance remote from the anode of the valve, a positive potential beingapplied to the cathode of the diode such that current flows through the 10d? until a frame synchronising pulse isapplied to -the thermionic valve. The di-' ode may be provided with a. second anode conanode of said thermionic valve in order to provide 'a short circuit path across the inductance on the conclusion of a frame synchronising pulse and so to damp oscillation due to the coil which mightotherwise occur.

In a modification of the present invention an inductance and a unilaterally conductmg device constitute ashunt path across a controlling device. The controlling device maybe a thermionic valve which is normally non-conductive but is nected to the end of the inductance adjacent the rendered conductive on the application to it of either frame or strip synchronising pulses, and the unilaterally conducting device may be a'diode valve the cathode of which has applied to it a positive potential such that current flows through the diode until a frame synchronising pulse is applied to the valve. A resistance may be connected between a source of voltage and the anode of the thermionic valve or a tapping point in the inductance. In any of the arrange-- ments employing a screen grid thermionic valve as a controlling device, strip frequency pulses derived from strip frequency synchronising pulses applied to the control grid of said valve may be taken from the screening grid. of the valve.

In order that the invention may be more clearly understood and readily carried into effect, some circuit arrangements embodying the invention will now be described more fully by way of example with reference to the accompanying drawings in which:

Figs. 1, 2, 4, 5 and 6 are alternative forms of circuit, and

Fig. 3 represents transient voltage impulses.

Referring to Fig. 1 of the drawings, the incoming signals are applied to the control grid of a screen grid valve I through a coupling condenser 2 shunted by a leak resistance 3. The control grid of the valve I is so biased relative to the cathode by means not shown in the drawing that no anode current flows in the absence of signals. The signals are applied to the valve I in such a sense that only the synchronizing signals drive the control grid 'suficiently positive to cause the valve to conduct. An inductance 4 and resistance 5 are connected in series in the anode lead of the valve I and have a time constant which is long compared with the duration of a frame pulse.

The anode of a diode valve I is connected to the junction 6- between the inductance 4 and resistance 5, and the cathode of this diode is maintained at a constant potential of the order of 60 volts, derived from a resistance 8 connected across the high tension source of supply of 250 volts. The positive potential at which the cathode of the diode is maintained is such that a considerable current normally flows through the diode.

During the occurrence of a line pulse, increasing current flows in the valve I and the current through the diode I decreases owing to the decreasing voltage at the anode of the diode, the sum of the valve and diode currents being substantially constant. On account of the short duration of the line pulse, and the long time constant of the anode impedance, the current in the valve does not rise to a value greater than that initially flowing in the diode, so that the latter remains conducting throughout, and the voltage of its anode only varies slightly. On the occurrence of a frame pulse of long duration, however, the current in thevalve increases to such an extent that the potential of the junction 6, and thus the anode of the diode I, fall below the potential of the cathode of the diode. The diode is thus rendered non-conductive.

Thus, assuming that there is no stray capacity. the voltage waveform appearing at the junction 6 will take the form of a series of triangular pulses of negative sense corresponding to the frame pulses. These saw-teeth pulses are fed through the condenser 9 to a scanning oscillator circuit.

In the arrangement shown in Fig. 2 of the drawings, the diode cathode is connected through the winding of a transformer I0 coupled to the scanning oscillator, to a suitable positive potential as before. In this arrangement the diode remains conducting during the occurrence of line pulses and is rendered non-conducting at the occurrence of frame pulses so that a voltage pulse of considerably greater amplitude is induced in the secondary winding.

Theoretically, any values for the inductance 4 and resistance 5 to give a suitable time constant may be selected, but owing to the distributed capacity of the coil 4, a tuned circuit is formed having a definite decay period so that after each synchronising pulse, the time taken for the pulse on the anode of the valve to return to zero will depend upon the natural frequency of this tuned circuit and its clamping. In a system employing interlacing, the return to zero must, in order that the interlacing should be accurate, be effected within a half line period.

The waveforms shown in Figure 3 of the drawings serve to illustrate the effect of the selection of inductances of different values.

The waveforms represented at a, band 0 correspond with the use of inductances 50,- 10 and 2 henries respectively, while that shown at d is obtained when a damped inductance of 2 henries is employed. Only the waveform shown at cl is really satisfactory. It thus appears that the inductance 4 must be small and suitably damped, and a coarse iron core may be employed to provide suitable damping. Again, the resistance 5 should be high. The damping may be improved by connecting the anode of the diode I to a tapping point in the resistance 5, although in such a case there will be some loss of pulse amplitude.

In the arrangement shown in Figure 4 of the the steady potential of the diode cathode at the l;

conclusion of a pulse, there is a direct connection of the anode of the valve through the diode to the large condenser II, thus preventing large amplitude oscillation. It Will be seen that this arrangement ensures complete isolation from any deleterious effects between adjacent synchronising pulses and the operation of the separating circuit is quite regular. The arrangement shown in Figure 4 is found to be effective even if the inductance 4 is undamped. It is advantageous to employ a lightly damped inductance having small self capacity.

Referring now to Fig. 5 of the drawings, the incoming signals are applied to the control grid of a screen grid valve I which is so biased relatively to the cathode by means not shown in the drawings and the signals are applied in such a sense that only the sychronising signals drive the control grid sufficiently positive to cause the valve to conduct. A resistance I4 is connected in the anode lead of the valve I and a choke coil I5 is also connected directly to the anode lead I8 and to a terminal I9 of a frame frequency oscillator not shown in the drawings. The anodeZII of a diode I! is connected to the terminal I9 and the cathode of the diode is maintained at an intermediate positive potential by means not shown in the drawing. When valve I is non-conducting the diode I I is conducting and current flows through the choke I5 from the lead I8 to the anode 20 of the diode. When the impedance of the valve I is reduced to a low value by the applincation of a positive synchronising pulse to its grid, its anode falls to a very low potential and a large voltage is set up across the choke I5 and causes a constant rate of decrease of the current flowing through it into the diode ii. If the positive pulse applied to the grid of the valve i is of short duration, suificient time may not elapse for the current through the choke to fall to zero and to cause the diode to become non-conducting. A positive pulse of longer duration such as a frame pulse, will result in the diode bceoming non-conducting and negative pulses will therefore 'be set up at the anode of the diode I! only in response to those positive pulses applied to the grid of the valve i which exceed a certain predetermined duration. The values of the-resistance M, the inductance of the choke coil l5 and the voltage applied to the cathode of the diode I! will determine the duration of a positive pulse applied to the grid of valve I which will result in the diode l1 becoming non-conductive and these values may be selected accordingly. The form of the pulses set up at the terminal [9 may be modified by connecting a condenser 22 between the anode 20 of the diode and earth. This condenser will have no effect on the operation of the circuit as already described because during the decrease of current through the choke coil E5, the condenser 22 is efiectively short circuited by the low impedance of the diode and is therefore inoperative. Additional integration of the pulses can be obtained by the insertion of another condenser 23, which serves to delay the fall of potential of the anode 58 when the valve is made non-conducting, but the use of this extra condenser is also not essential to the operation of the circuit.

A more detailed form of circuit is shown in Fig. 6 of the drawings. The connections for takingofi the line synchronising pulses are shown added to those of Fig. 5 and include a resistance 25 of 500 ohms and a condenser 25 of .5 microfarads connected between the screen grid of valve I and a terminal 28 of a line frequency oscillator not shown in the drawings. The screen grid is also shown connected to a source of high tension voltage through a resistance 30 of .5 of a megohm. The positive biassing potential applied to the cathode of the diode is derived automatically by means of a resistance 32 of 50,000 ohms shunted by a condenser 33 of four microfarads inserted in the cathode lead. A condenser 35 of ,0005 mi.- crofarad is connected directly across. the valve i, but no condenser equivalent to the condenser .22 in Fig. 5 is included as the stray capacities of. a frame frequency blocking oscillator connected to the terminals 3'! and 38 through a condenser ls of .01 microfarad, render the insertion of a con-' denser unnecessary.

The operation of this circuit is similar to that described with reference to Fig. 5, the synchronising pulses again being applied in the positive sense to the control grid of valve l. Instead of the direct current coupling to the grid of valve i shown in Fig. 5, an alternating current coupling is shown in Fig. 6, a condenser being inserted in the grid lead with a shunt resistance to earth,

the limiting valve 1 in this case constituting a self re-established bias limiting device. Again, instead of the single anode diode shown in Fig. 5, a double diode is shown in Fig. 6, the diode being self biassing by virtue of the resistance 32. The time constant of the resistance 32 and condenser 33 is arranged to be long compared with a line period. The second anode 20a of the diode serves to damp the oscillation of the choke coil l5 in a manner similar to that described with reference to the arrangement shown in Fig. 4 of the drawings. I 7

In a modified circuit arrangement the resistance 14 instead of being connected-directly to the anode of the valve i may be connected to a tapping point in the choke coil i 5, the tapping being, for example, mid-Way between the ends of the choke coil. 7

While in Fig. 6 the derived strip pulses are taken from the screening grid of the valve 1, these derived pulses can be taken from the anode of that valve. In this case the condenser 35 will be omitted in order to. retain the sharp wave front, and in order to reduce the effects-of interference, a iurtherpulse amplitude limiting valve may be inserted.

In operating certain. saw-tooth waveform generating circuits includinggas discharge valves, the synchronising pulse should preferably be applied in the positive sense. Each or both of the anodes in the diode H can be replaced by the control grids of a pair of phase reversing valves which are biased to a potential such that the control grids operate in substantially the same manner as the anodes of the diode H. A further advantage is that the lower region of the synchronising pulse which is most liable to contain interference will be further limited andagain the possibility of cross coupling will be reduced.

I claim:

1. In a television receiver wherein a sin le signal series of line and frame synchronizing'immined negative bias on the control electrode with respect to the cathode, means for applying the received synchronizing impulses and picture signals to said control electrode to render said tube conducting throughout the duration of each synchronizing impulse only, a diode including a cathode and an anode, means for connecting the anode of said diode to the junction of said resistance and inductance, means for maintaining the cathode of said diode positive with respect to the cathode of said separator tube by apredetermined amount, whereby the current drawn by said diode may be caused to vary by reason of the voltage drop in the separator tube load circuit when synchronizing impulses are applied to said ube, and an output circuit coupled to said diode.

2. In a television receiver wherein line and frame synchronizing impulses and picture signals are received, the frame impulses being of greater duration than the line impulses and all synchronizing impulses being of greater intensity than any picture signal, comprising a picture signal separator tube having a cathode, a control electrode and an anode, a source of potential, means for connecting the negative terminal of said source to said cathode, means including a load circuit comprising a series connected resistance and inductance for connecting the: positive terminal of said source of potential to said anode, means for maintaining a predetermined negative bias on the control electrode with respect tothe cathode, means for applying the synchronizing impulses and picture signals to said control electrode to render said tube conducting upon the occurrence of each synchronizing impulse, a diode including a cathode and an anode, means for connecting the anode of said diode to the junction of said resistance and inductance, means for maintaining the cathode of said diode positive with respect to the cathode of said separator tube by an amount such that the current drawn by said diode will be reduced to zero by reason of the voltage drop in the separator tube load circuit when frame synchronizing signals are applied to: said discharge tube, and an output circuit coupled to said diode.

3. In a television receiver wherein line and frame synchronizing impulses and picture signals are received, the frame impulses being of greater duration than the line impulses and all synchronizing impulses being of greater intensity than any picture signal, comprising a picture signal separator tube having a cathode, a control electrode and an anode, a source of potential, means for connecting the negative terminal of said source to said cathode, means including a load circuit comprising a series connected resistance and inductance for connecting the positive terminal of said source of potential to said anode, means for maintaining a predetermined negative bias on the control electrode with respect to the cathode, means for applying the synchronizing impulses and picture signals to said control elec trode to render said tube conducting upon the occurrence of each synchronizing impulse, a diode including a cathode and an anode, means for connecting the anode of said tube to the junction of said resistance and inductance, means for maintaining the cathode of said diode positive with respect to the cathode of said separator tube by a predetermined amount, whereby the current drawn by said diode may be caused to vary in accordance with the voltage drop in the load circuit of the separator tube as determined by the current drawn by said separator tube, the potential of the cathode of said diode being such that the diode current is reduced to zero upon the application of a frame synchronizing impulse to said separator tube, and an output circuit responsive to the potential across said diode.

4. In a television receiver wherein line and frame synchronizing impulses and picture signals are received as a single signal series, the frame impulses being of greater duration than the line impulses and all synchronizing impulses being of greater intensity than any picture signal, comprising a picture signal separator tube having a cathode, a control electrode and an anode, a source of potential, means for connecting the negative terminal of said source to said' cathode, means including a load circuit comprising a series connected resistance and inductance for connecting the positive terminal of said source of potential to said anode, means for maintaining a predetermined negative bias on the control electrode with respect to the cathode, means for applying the synchronizing impulses and the picture signals to said control electrode to render said tube conducting only upon the occurrence of each synchronizing impulse, a diode including a cathode and an anode, means for connecting the anode of said diode to the junction of said resistance and inductance, means for maintaining the cathode of said diode positive with respect to the cathode of said separator tube by an amount such that the voltage drop in the lower circuit of said separator tube will be suflicient to reduce the diode current to zero when a frame synchronizing impulse is impressed upon said separator tube, and an output circuit responsive to the current drawn by said diode.

5. In a television receiver wherein line and frame synchronizing impulses and picture signals are received as a single signal series, the frame impulses being of greater duration than the line impulses and all synchronizing impulses being of greater intensity than any picture signal, comprising a picture signal separator tube having a cathode, a control electrode and an anode, a source of potential, means for connecting the negative terminal of said source to said cathode, means including a load circuitcomprising a series connected resistance and inductance for connecting the positive terminal of said source of potential to said anode, means for applying the synchronizing impulses and the picture signals to said control electrode to render said tube conducting only during the synchronizing impulse intervals, a diode including a cathode and a pair of anodes, means for connecting one of the anodes of said diode to the junction of said resistance and inductance, means for connecting the other anode of said diode to the anode of said separator tube, means for maintaining the cathode of said diode positive with respect to'the cathode of said separator tube, by an amount such that the current drawn by said diode will be reduced to zero by reason of the voltage drop in the load circuit of said separator tube when a frame synchronizing impulse is applied to the control electrode of said tube, and an output circuit coupled to' said one anode and said cathode of the diode.

CHARLES LESLIE FAUDELL.

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