Circuit Notebook 77 - Black (Blanking) Level Clamps

This subject has arisen because of the problems some local ATV Amateurs have experienced when receiving signals from 24 cms FM ATV transmitters that use an oscillator controlled by a phase-locked loop circuit.

It was particularly noticeable with some early transmitter modules [1] where their restricted low frequency response resulted in variations in the black level with picture content, causing some monitors and receivers to display poor frame lock and picture roll.

By examining the received waveform on a 'scope, locked to the field rate, it can be seen that a period of peak white video causes the black level and the sync pulses to be depressed. But, during the field blanking where no video is present, the black level rises. The field sync separator circuit in some monitors cannot cope with this change of level and the result is poor frame lock. I have two different monitors, a Philips HCS 31, which is unaffected and a Matsui 1436 TV that rolls uncontrollably.

The answer is, I suppose, to correct the problem at the transmitter. You will have seen suitable modifications and read various discussions about this in CQ-TV. However, the symptoms can be eliminated at the receiver by 'clamping' the video signal before it reaches the monitor. Some satellite receivers have a built in 'clamp' for this purpose.

Method of Operation

The principle is to repetitively stabilise one point of the video waveform to a reference voltage, e.g. 0 volts. To do this, we need to arrange for an electronic switch, to switch on for a short period at a precise point on the video waveform and connect the signal to 0 volts. The point chosen is at 'blanking level' on the line blanking waveform (where the colour burst sits). The 'text book' line sync and blanking waveform is shown in Fig.1.

Experimental 'Clamp' Circuit

The circuit is shown in Fig.2. The input signal is fed through C5 to the output amplifier IC2. The input video signal is also fed to the sync separator IC1, LM 1881. This generates several output pulse signals one of which, from pin 5, is correctly timed to coincide with the point at which we want to clamp the video signal.

This pulse is inverted by TR1 and fed to TR2. TR2 operates as a switch which clamps the signal line to 0 volts at the correct point on the video waveform. An inductor is included in the circuit to avoid shorting out the colour burst. The video waveform and associated 'clamp' pulse waveform is shown in Fig.3.

How it works

Consider then, at the end of one line the switch operates and clamps the video blanking level to 0 volts at that instant. Because of poor low frequency response, hum or other disturbance, the blanking level may no longer be at 0 volts by the end of the next line. When the clamp operates again any error voltage, which may be present, drives a current back through C5 until the resultant stored voltage cancels the error. Thus, any error occurring during a line period is automatically corrected at the end of that line and provides a clean video output signal with the blanking (black) level clamped to 0 volts.

The upper trace, in Fig.4, shows an input video signal suffering from poor low frequency response and the lower trace shows the same signal after passing through the 'clamp' circuit.

The question we are asking ourselves is, should an ATV repeater contain a 'clamp' to clean up incoming signals or should the incoming signals be repeated warts and all?

References

[1] G1MFG's ATV Modules Reviewed, Brian Kelly GW6BWX, CQ-TV 192, pages 9 -11

[2] A Processing Amplifier, Bryan Dandy G4YPB, CQ-TV 136, pages 74 - 76

[3] Processing & Distribution Amplifier, John Goode, Best of CQ-TV, page 79

[4] D.C. Clamp Circuits, S. W. Amos & D. C. Birkinshaw, Television Engineering, Vol. 4, Pages 75 - 80

[5] Television Measurements, Margaret Craig, Engineering Manager, Tektronix Inc.

Figures

Fig.1. Line sync and blanking waveform (TV Line Sync)

Fig.2. Experimental black level 'clamp' circuit (Black Level Clamp)

Fig.3. Video waveform and 'clamp' pulse (Clamp pulse 1)

Fig.4. Video input and 'clamped' output signals (Video clamp 1)