Circuit Notebook 92 - A Simple Peak Video Detector

The original purpose of this circuit was to monitor the amplitude of 625 line monochrome video being fed into an A-D converter. This was part of a 625 line to NBTV Converter (Peter Smith G4JGU, CQ-TV 189, page 15) which allowed the level of the incoming video to be monitored and so avoid over-driving the A-D Converter. The circuit, which is shown in Fig.1, may be considered in three parts.

DC Restorer

The incoming video is capacitively coupled through C1 to the emitter of TR1. The base is held at approximately +2V by R2 & R3. Negative-going sync pulses cause TR1 to conduct which results in the bottom of the sync pulses being clamped at about +1.4V. R4 is included to stabilise the clamping action.

Level Detector

The level detector is formed by one half of the dual comparator IC1, LM393. The clamped video is fed to pin 3. A DC voltage from RV1, the 'Set Level' control, is applied to pin 2. Providing that the video signal voltage is lower than that at pin 2, then the output of the comparator is held at 0V.

If the video signal at pin3 exceeds the voltage at pin 2, then the comparator output becomes open circuit and the voltage at pin1 rises to +2.5 volts, determined by R7 & R8.

Monostable

The second half of IC1 is connected to form a monostable (one-shot) having an active period of about 20ms. In the static condition, pin5 is held at +2V by R9 & R10 and the output at pin 7 sits at +5V and is AC coupled to pin 5 by C2. When the input at pin 6 rises to +2.5V (as it will do if the video signal exceeds the threshold set by RV1) then the output of IC1b drops to 0V, thus lighting the LED. This negative change is passed through C2 to R9 & R10, thus maintaining the active condition until C2 is discharged through R7 & R8. The active period is about 20ms which ensures that the LED is on long enough to be visible. After the active period, the output of IC1b becomes open circuit and C2 is re-charged through R11, R7 & R8.

Setting up

Assume that the video signal to be monitored is 1V p-p.

  1. Disconnect the video input signal. Measure the voltage at TR1 emitter using a high resistance voltmeter. Note the value, typically +1.4V.
  2. Measure the voltage at IC1a pin 2 and adjust RV1 so that the voltage is exactly 1V higher than that at TR1 emitter, i.e. +2.4V.

That's it, reconnect the video input, video signals above 1V p-p will be detected and indicated by the flashing LED.

Limitation

As with many things, if it appears to be too good to be true, then it probably is. Due to the speed limitation of the LM393 (about 300ns) the circuit will not respond to a colour sub-carrier if it exceeds the set level. Also, very narrow text/lines may not be detected. Accepting these limitations, the circuit could possibly replace an oscilloscope for the basic amplitude monitoring of a video signal.

Figures

Fig.1.Fig.1. A Simple Peak Video Detector