US2215175A - Television synchronizing impulse separator circuit - Google Patents

Description

Sept. 17, 1940. D J N s 2,215,175

TELEVISION SYNCHRONIZING IMPULSE SEPARATOR CIRCUIT Filed Feb. 28, 1939 2 Sheets-Sheet l I To REPRODUCER flea! r ll CA THODE voLTs AAA/00E VOLTS SYNCHQON/Z/NG PULSES INVENTOR D. J. FE W/N G5 ATTORNEY 'P 17, 1940- D. J. .FEWINGS 2,215,175

TELEVISION SYNCHRONIZING IMPULSE SEPARATOR CIRCUIT Filed Feb. 28, 1939 2 Sheets-Sheet .2

we v P m ATTO R N EY T BY I Patented Se t. 17, 1940 UNITED STATES TELEVISION smcnaomzmo IMPULSE SEPARATOR omcUrr David John Fewings,

signor to Radio Corpo poration of Delaware Application 3 Claims.

This inventioni'elates to synchronizing impulse separator circuits for main television receivers.

It is usual in modern television systems to effect synchronization of scanning action at trans- 6 mitter and cooperating receiver, by transmitting synchronizing impulses at the ends of the scanning lines and at. the end of the framing periods,

' these impulses being separated out from the video signals at the receiver and used to control 10 the time base circuits or other means at said receiver for securing scanning action. In order to iacilitateeseparation oi the synchronizing impulses at the receiver it is usual to make them blacker than black, 1. e., to transmit them in the same direction as black video signals, but to make them of larger plitude than black video signals. This en es the synchronizing signals to be separated outby amplitude separation methods and various knownseparating circuits, some employing biased diodes, others employing valves biased to cut-off or limiting valves, have been proposed for the purpose. The diode type of separating circuit is, however, the most satisfactory by reason of the fact that the char- 'acteristic curve of region "of cut-ofi and the cut-off is sharp.

The invention is illustrated in and explained in connection with the drawings wherein:

, Figure 1 shows a known system for separating picture signals and synchronizing impulses.

Figure 2 shows a characteristic curve of a diode, and

Figures 3 th pulse separator circuits in ac invention. v a

Figure 1 of the drawings shows diagrammatically a known amplitude separating circuit of the diode type. ,Here A'r'epresents the last valve of the normally-provided video amplifier of a television receiver; The anode of the valve A is D. C. coupled over lead WC to the Wehnelt cylinder (not shown) or. equivalent control electrode of the normally,provided cathode ray 'reproducer tube. The video signal at the anode of the valve A is such that white signals are in the positive direction and synchronizing pulses in the negative direction. The anode ofv the valve A is connected to the'cathode of a diode B whose anode is connected through a resistance R to a point PRT on a potentiometer resistance PR .connected across the high tension source. This arrange- -ment will give a measure of separation since it the bias on the diode is such that it just cuts on on a "black" signal, a blacker than blac sigrough 6 show synchronizing imcordance with this a diode is very steep in the V Ohelmsford, England, as-

ration of America, a cor- February 28, 1939, Serial No. 258,863 Great Britain March 4, 1938 ml, 1. e., a synchronizing pulse, will cause current to flow through the diode whereas, for any signal between black and white" the diode will pass no current. Separated synchronizing pulses can be taken on by way of lead SP and utilized in any 1 known way. The action is represented in conventional graphical manner in Figure 2 of the drawings which shows a diode characteristic curve with cathode voltages varying from a few volts more positive to a'few volts more negative 10 than its anode voltage.

The serious defect of the known arrangement of Figure 1 arises from the fact that the video signals in modern television systems include a D. C. component (corresponding to "mean pic- 15 ture brightness or background; which varies 'very substantially from time to time so that a bias setting which is quite satisfactory for one scene may be badly wrong for the next, with the result that, when the bias is wrong; separation is incomplete and video signals may pass through the diode and interfere with the correct functioning of the time bases.

According to this invention in its broadest aspect, an amplitude separating circuit in a televisionapparatus for separating synchronizing pulses from video signals or smaller amplitude, said circuit being of the type wherein separation is effected by means including an electron discharge device arranged to operate over a sharply curved part of its anode current anode voltage characteristic and to which the video signals and synchronizing impulses are applied is characterized by the provision of means for rendering the voltage applied across said device substantiallyindependent of slow variations in the video signals,

In Figure 3-this is not a preferred embodiment-the cathode of the diode B is tapped at CP upon a'resistance C in the anode circuit ofthe valve A. The anode of valve A .is connected to. the anode of diode B through resistances D and R in series and the junction point between D and R isconnected to the cathode of valve A (earth) through a condenser E. Separated synchronizing signals are fed to wire SP via condenser F.-

With this arrangement the voltage drop in the resistance C causes the diode anode to be slightly less positive than the diode cathode i. e. the diode so is biased back. The condenser E is large enough to by-pass video signals to ground but not large enough materially to effect slow changes of voltage: such as variations in the D. C. com- .ponent of the video signals; The resistance D is large enough to prevent any appreciable adverse efiect of condenser E upon picture quality.

The circuit of Figure 3, though a marked improvement on that of Figure 1, has two defects which render it not to be preferred. First the condenser E is necessarily of such capacity asto anodel When a scene is faded out this chargeis lost, the positive bias on the diode anode is reduced and if the total video signal is not of sufficient amplitude synchronism may be lost. Both these defects are reduced to negligible proportions in the preferred arrangement of Figure 4.

In'Figure 4 a choke K is inserted in series with resistance D between resistance R and the anode of valve A and the junction of D and R is connected to ground through a condenser J in series with a potentiometer resistance. The adjustable tap on the potentiometer resistance is connected to the grid of valve G in whose anode circuit is included a resistance H. The anode of valve G is connected to the anode of the diode through a condenser I in series with a resistance R. As will be seen the valve G reverses the phase of the reduced video signals arriving at the anode of the diode by way of K, D and R and the reversed phase signals are applied to the diode anode by way of I and R at such strength as just to cancel out the unreversed signals arriving at said anode through K, D and R. The

strength of the compensating, reversed signals applied by way of I and J may be adjusted by adjusting the grid potentiometer through which the valve G is fed. The valve G will not efiect slow changes of D. C. potential on the anode of the diode, which changes can occur freely with and will accompany corresponding changes on the diode cathode. The actual D. C. voltage across the diode will, therefore, remain substantially constant over normal fluctuations of anode current in the valve A. To give a practical case, resistance C may be about 100 ohms so that the change of bias due to fluctuations of anode current through it, will be very small. If the diode separator shown is followed by a limiter device (not shown but fed over wire SP) such changes as may occurwill be of no consequence in any event. If desired, a large condenser L may be connected between the anode-of valve A and the tap CP.

Figure 5 shows another preferred e'mbodiment differing from Figure 4 maiiily in that the capacity coupling provided by condenser J in Figure .4, is replaced by transformer coupling by Y means of a transformer M whose primary is inserted in series between D-and R and whose secondary is in the grid circuit ofvalve G. The

transformer connections are such that the phase at N and at the grid of valve G is the same.

A I Another, but minor, difference between Figures 4 and 5 is that in thelatter figure condenser I is adjustably tapped on resistance H instead of using a grid'poten'tiometer for the valve G. The advantage of Figure 5 over Figure 1 is that the condenser J being eliminated there can be no D. C. bias-developed at point N by virtue of video signals;

- Figure 6 shows another preferred embodiment in which the valve G ofFllures4 and 5 is elimisistance R to the anode declare that what I claim nated. Here a transformer M'whose primary is, as in Figure 5, inserted betweenD and R, has its secondary connected end of the secondary being connected to earth through an adjustable resistance Q and a con-- denser P ih series.

former secondary is 0. The connections The other endof the transearthed through condenser of the transformer are such that the phase of the signals appliedto R is re-' versed as compared to the phaseat point N. If the transformer turn ratio is of the order of 2 to 1 it is possible, by adjusting Q, to secure the at one end through re-, of the diode, the same 1 result that the signals applied through R and.

R to the diode anode cancel one another out.

Having now particularly described and ascertained the nature ,of my said invention and in what manner the same is to be performed. I

1. A television system including an amplifier tube having a cathode, a control electrode and an anode, means including a load resistance for maintaining said anode positive with respect to said cathode, means-for applying picture signals and synchronizing impulses to the control electrode of said amplifier tube, a resistance and a diode, means for connecting one end of the resistance to the anode of said amplifier tube,- -means for connecting the other end of the resistance to the diode anode, means for connecting the diode cathode to a point along said amplifier tube load resistance, a dischargetube' including a cathode, a control electrode and an" anode means for coupling the control electrode of said discharge tube to a point along said resistance, means including another resistance for maintaining the anod of said discharge tube positive with respect to its cathode, means including a condenser for connecting the anode of said discharge tube to the diode anode, and an output circuit including the diode anode whereby synchronizing impulses alone. may be present in the output circuit of substantially uniform intensity irrespective of the signal levels applied to said amplifier tube. v

2. A television system including an amplifier tube having a cathode, a control electrode and an anode, means including a load resistance for maintaining said anode positive with respect to said cathode, means for applying picture signals and synchronizing impulses to the control electrode of said amplifier tube, a resistance and a diode, means for, connecting one end of the resistance to the anode of said amplifier tube,.

means for connecting the other end of the resistance to the diodeanode, means for connecting the diode cathode to a point along said amplifler tube load resistance, a dischargetube including a cathode, a control electrode'and an anode, means' including a condenser for coupling the control electrode of said discharge tube synchronizing signal" impulses "of substantially:

uniform intensity may" be .derived from them; putcircuit. 1

';3. A televisionsystem including an amplifier tube having a cathodefa control electrode and an anode, means including a load resistance for maintaining said anode positive with respect to said cathode, means for applying picture signals and synchronizing impulses to the control electrode of said amplifier tube, a resistance and a diode, means for connecting one end of the resistance to the anode of said multiplier tube,

means'for connecting the other end of the resistance to the diode anode, means for connecting the diode cathode to a point along said amplifier tube load resistance, a discharge tube including a cathode, a control electrode and an anode, means including a transformer for 0011-.

pling the control electrode of said discharge tube to the current flow in the resistance connected between the anode of saidamplifier tube and the anode of said diode, means including a further resistance for maintaining the anode of said discharge tube positive with respect to its cathode,

means including a condenser for connecting the anode of said diode to a point along said further resistance, and an output circuit including the anode of said diode whereby synchronizing signals of substantially uniform intensity may be present in said output circuit.

DAVID JOHN FEWINGS.

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