In our study of the nature of video modulation, we have seen that the video signal is essentially unidirectional in nature, with its AC component superimposed on a varying direct current component. This DC component represents the average shading or background of the picture, and is lost when ever the video signal passes through a coupling capacitor. The text and illustrations explain the manner in which the DC component is restored or re-inserted at the television transmitter. At this time we will consider the manner in which DC restoration is accomplished in the receiver.
Figure 231 illustrates the necessity for restoration of the DC component. We have again selected, as an example, a black bar against a white background (Figure 231A). The video modulating signal of the transmitter for two successive horizontal lines is shown in Figure 231B. When the video signal of Figure 231A is passed through a capacitance-coupled video amplifier, the DC component is lost and the signal appears as an alternating voltage (Figure 231C) around an average value (the average of the white and black areas or about 25% of maximum signal in this example). If a signal of this type is applied to the picture tube grid, as in Figure 231D, all of the picture components on the negative side of the average signal, including the sync pulses, will extend into the cut-off region. The white back ground will be rendered as a dark gray.
If the DC component is restored, the operating point on the picture tube characteristic is established at its proper gray value.
This places the black level (75% of maximum signal) just at the cut-off grid bias as shown in Figure 231E. The problem of proper DC restoration is thus resolved to a method of maintaining the blanking, or black level, of the video signal, at the voltage which corresponds to the light cut-off bias value of the picture tube grid.
The blanking level which acts as the reference point for the DC component, and the DC component itself, are present in the video signal as it appears across the detector load resistor. This level is a fixed value to which the picture carrier returns at the end of each horizontal line. If the entire video system, from the detector load to the picture tube grid, is direct coupled (without the use of coupling capacitors) the DC component is established automatically. While such a direct coupled circuit is capable of good performance, it is not always convenient in application.
Fig. 231. The Effect of the DC Component of the Video Signal on Image Reproduction.
THE DIODE DC RESTORER: The method of DC re-insertion, most frequently employed, is the use of a diode directly coupled to the picture tube control element or grid. Figure 232A shows the essential circuit elements of the diode restorer. A typical video output system incorporating diode restoration is shown in Figure 233.
Fig. 232. DC Restoration by Diode Rectification of the Video Signal.
The operation of the diode restoration circuit is shown in the waveform diagrams of Figure 232B and C. Figure 232B shows three horizontal lines of a video signal which has passed through a capacitance-coupled video amplifier, and consequently has lost its DC component. In this case, the DC to be restored is the value from the AC axis to the tip of the sync pulse, or 25 volts. Let us follow through several cycles of operation of the restorer:
1. During the interval from "a" to "b" the polarity of the video signal is such that the cathode of the diode is positive with respect to its anode. The tube will not conduct. (Reg ion "f" to "g" of Figure 232C.)
2. When the voltage reverses, during the interval from "b" to "c" of Figure 232B, 25 volts will appear across the diode in the correct direction for conduction, quickly charging capacitor C1. The change of voltage across the capacitor during this interval is shown in Figure 232C from "g" to "h".
3. Since the time constant of the diode coupling network (C1- R1) is .05 seconds, the charge on capacitor C1 has decreased very little during the horizontal line interval from "c" to "d". (Note: The voltage drop across the capacitor has been exaggerated at "i" and "j", for illustration.)
4. Conduction occurs again due to the sync pulse tip ''d" which charges the capacitor to the peak value of the signal.
Figure 233 shows the complete circuit including the video output tube, the picture tube input circuit, and the fixed bias adjustment circuit (brightness control).
Fig. 233. A typical Video Output System with Diode Restoration.
It will be seen that the load resistor of the diode restorer (R6) is ยท in series with the grid-to-cathode circuit of the picture tube, and thus any bias set up by the diode acts in series with a fixed bias determined by the setting of the brightness control (R7). The AC component of the video signal is superimposed on this combination bias through coupling capacitor C2, in such polarity that the blanking signals and sync pulses drive the tube to cut-off (black region). The DC component is applied in positive polarity to the grid and acts to reduce the fixed negative bias (from R7), to a degree determined by the average level of the alternating video signal (background illumination). The diode restorer can be classified as a peak rectifier of long time constant (affected by changes in video level which occur at a rate lower than approximately 50 cycles). The vacuum tube diode may be replaced by the 1N34 germanium diode as shown in the alternate circuit of Figure 233. Since the crystal diode has a lower dynamic resistance than the vacuum tube type, it produces a more secure clamping action at the sync tip level, and also tends to produce an even alignment of sync tips, thus improving the scanning control action.
DC RESTORATION BY GRID RECTIFICATION: The video output tube can perform the dual functions of providing the voltage swing for picture tube operation, and the restoration of the DC component. In this case, the grid and cathode act as a diode, as in the familiar grid leak detector. A circuit embodying this method is shown in Figure 234. Its operation is as follows:
1. The excursions of the video signal in the positive direction, caused by the sync pulses, produce a current now into capacitor C1 via the grid-to-cathode path. 2. A charge of a value equal to the sync pulse tips accumulates on the capacitor. 3. During the active picture interval, the grid circuit is non-conductive and the capacitor starts to discharge through resistor R9. 4. The voltage thus established across R9 acts as a DC component which may be considered as adding to the AC signal. The amplified signal, with its added DC, is impressed on the picture tube input.
RESTORATION BY RECTIFICATION IN THE PICTURE TUBE INPUT CIRCUIT: Figure 235 shows a method of using the picture tube grid to-cathode as a diode for restoration of the DC component. This method has been called "AC stabilized brightness control". A self-bias (in the absence of signal) is established across resistors R4 and R5 or a total picture tube beam current. This bias tends to correct automatically for changes in brilliance, due to power line or high voltage variations. The video signal is coupled to the cathode through capacitor C2, and the DC bias, whose peak value is established by the synchronizing pulse tip, is set up across resistor R4 in the same manner as that described in the preceding text covering diode restoration.
Fig. 235. DC Restoration by Self Bias in Picture Tube Grid Circuit.
Fig. 234. DC Restoration by Grid Circuit Rectification in the Video Output
Stage.
Fig. 236. Use of a Triode for DC Restoration and Sync Separation.
DC RESTORATION EMPLOYING A TRIODE: Figure 236 shows a circuit in which a triode is employed for the dual purpose of DC restoration and sync separation. This circuit will be recognized as similar to that of Figure 95C. The cathode bias developed across circuit R2-C3, due to plate rectification, is determined by the peak value of the synchronizing pulses, and this bias constitutes the DC component. A more detailed description of the C2 performance of this circuit was given.
DC RESTORATION BY DIRECT COUPLING FROM THE VIDEO DETECTOR TO THE PICTURE TUBE: in the general discussion of DC restoration, mention was made of the fact that the entire picture, including the DC component, is present in the output of the video detector. If this signal is impressed on the picture tube input through a system which is entirely direct coupled, no restoration is necessary. Figure 237 shows such a direct coupled circuit. Several unusual features will be noted: 1. In order to avoid a great difference of potential between the heater and cathode of the picture tube, the entire "B" supply system is below ground.
The plate and screen circuits of the video amplifier tube (T1) return to ground and the cathode is brought to a point 150 volts negative with respect to ground. The picture tube grid circuit is returned to potentiometer R6, which acts as a brightness control, and establishes the grid at the proper point so that the black or pedestal level occurs at the beam cut-off or black point. 2. The sync signals are taken off at the junction of the plate load resistor and one of a pair of shunt peaking coils. This is ...
Fig. 237. DC Restoration
by Direct Coupling from the Video Detector to the Picture Tube.
... done to reduce the capacitance loading of the sync system on the video amplifier, and thus extend its high frequency response. 3. Since no capacitors exist in series with the signal path, the time constant of operation is extremely low, and restoration action occurs line by line rather than over a number of lines, as in the other methods described in this section. This means that a direct coupled system is capable of very rapid accommodation of changes in background illumination.