Circuit Notebook 83 - A Video Detector

Video Detector

This type of circuit is part of most ATV repeaters. Its function is to detect when a valid video transmission has been received and to communicate this to the repeater control system. The repeater control system can then switch from displaying identification and other captions to re-transmitting the incoming signal. It may have other uses where a video signal requires detection.

Circuit description

The circuit, shown in Fig. 1, contains the well-known LM1881N (Maplin UL75S) sync separator I.C. (U1) followed by the LM567CN (Maplin QH69A) tone decoder I.C. (U2). The input would normally be connected across the video output of a receiver that is already terminated in 75ohms. The video level would therefore be about 1V p-p. The mixed sync output of the LM1881 is passed through C4, R3 and C5 to provide a signal approximating a sawtooth at line frequency for the input of U2.

The LM567 (U2) is a general-purpose tone decoder which provides a saturated transistor switch to ground (pin 8) when an input signal is present within the passband. The internal circuit consists of an I and Q detector driven by a voltage controlled oscillator (VCO) which determines the centre frequency of the decoder. External components are used to independently set centre frequency, bandwidth and output delay. An LED is connected to the output to give a local indication of a valid signal.

Construction

The circuit was built on Veroboard stripboard and the layout is shown in Fig. 2. This is a top (component) view. A 'X' on the drawing indicates that the track has been cut on the underside. Wire links are shown as straight lines. The layout is fairly compact and miniature ceramic and electrolytic capacitors are required. A photograph of the completed unit is shown in Fig. 3.

Setting up

Connect the circuit to a +5V supply and connect a normal 1V p-p video signal to the input. Slowly rotate the pre-set control RV1 from one extreme to the other. Note the range of rotation where the LED is illuminated and estimate the centre point, set the pre-set control to this position. Remove and replace the video signal to check that the circuit functions correctly. With the component values given, the circuit has a bandwidth of about 10% (1.5kHz)

In use, when monitoring an off-air signal, the circuit indicates a 'valid signal' from P5 down to about P1 - P2 (picture just locking).

References

LM567/LM567C Tone Decoder, www.national.com/ds/LM/LM567.pdf

LM1881 Sync Separator, www.national.com/ds/LM/LM1881.pdf

Figures

Fig.1. Video detector circuit diagram

Fig.2. Veroboard layout

Fig.3. Completed video detector