As explained earlier, relative relationship of ac signal to the blanking and sync pulses remains same with or without the dc component. Furthermore, brighter the line, greater is the separation between the picture information variations and the associated pulses.
As the scene becomes darker, the two components move closer to each other. It is from these relationships that a variable bias can be developed to return the pulses to the same level which existed before the signal was applied to the RC network.
DC Restoration with a Diode. A simple transistor-diode clamp circuit for lining up sync pulses is shown in Fig. The V CC supply is set for a quiescent voltage of 15 V. In the absence of any input signal the coupling capacitor ‘C’ charges to 15 V and so the voltage across the parallel combination of resistor R and diode D will be zero. Assume that on application of a video signal, the collector voltage swings by 8 V peak to peak.
The corresponding variations in collector to emitter voltage are illustrated in Fig. The positive increase in collector voltage is coupled through C to the anode of diode D, turning it on. Since a forward biased diode may be considered to be a short, it effectively ties (clamps) the output circuit to ground (zero level). In effect, each positive sync pulse tip will be clamped to zero level, thereby lining them up and restoring the dc level of the video signal.
In the case under consideration the diode will cause the coupling capacitor to charge to a peak value of 19 V. However, during negative excursion of the collector voltage the capacitor fails to discharge appreciabl y, because the diode is now reverse biased and the value of R has been chosen to be quite large.
The average reverse bias across the diode is – 4 V, being the difference between the quiescent collector voltage and peak value across the capacitor. Note that as the input video signal varies in amplitude a corresponding video signal voltage appears across the resistor R and it varies in amplitude from 0 to – 8 V (peak to peak). This, as shown in Fig., is the composite video signal clamped to zero.
Similarly as and when the average brightness of the scene varies the capacitor C charges to another peak value thereby keeping the sync tips clamped to zero level. Reversing the diode in the restorer circuit will result in negative peak of the input signal being clamped to zero.
This would mean that the dc output voltage of the circuit will be positive. The video signal can also be clamped to any other off-set voltage by placing a dc voltage of suitable polarity in series with the clamping diode.
It was assumed while explaining the mechanism of dc restoration that charge across the coupling Capacitor C does not change during negative swings of the collector voltage. However, it is not so because of the finite value of RC.
The voltage across C does change somewhat when the condenser tends to discharge through the resistor R. Another aspect that merits attention is the fact that whenever average brightness of the picture changes suddenly the dc restorer is not capable of instant response because of inherent delay in the charge and discharge of the capacitor.
Some receivers employ special dc restoration techniques but cost prohibits their use in average priced sets.
This, being positive, reduces the net negative voltage between the grid and cathode and the scene then moves to a brighter area on the picture tube characteristics.
Any decrease in average brightness of the scene being televised will have the opposite effect and net grid bias will become more negative to reduce background illumination of the picture on the raster.
Thus the diode with the associated components serves to restore the dc content of the picture signal and the difference in potential between ‘X’ and ‘Y’ serves as a variable dc bias to change the average brightness of the scene.