Synch Code Detector (ATV-Squelch)

Often my ATV-receiver is just monitoring the repeaters output. With no activity there, I have a steady noise on the audio-channel. So I turned the volume down. When someone tries to come over, (often from far away, sometimes with a different audio-subcarrier), I just missed him, because it makes me tired of watching "nothing" while there's no activity and listening to the noise all day long.

So,- some kind of squelch has to be found. This very squelch should detect not the noise on the soundcarrier (usual LF-squelch), but the line frequency (pulse code) of a video, which is 15625 Hz with the european PAL standard. When a weak signal of that frequency is detected, a relais is to be switched. You then can, for example, switch your television on, put a loudspeaker into operation, ring a bell or let your VCR start a recording.
For a repeater it can be an essential item for switching the output-transmitter automaticly on, when a video-signal is detected on the input... The main advantage is, that only a video-signal puts the system into operation,- no Packet, no phone or no non-modulated carrier.

Based on an old photo-copy by DL1ZAM from the German ATV-Club-Magazine (TV-Amateur: 61/1986), the circuit was re-designed and prints were drawed:

Circuit of the "ATV-Squelch"

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Some remarks:

For the needed +8V an additional small standard voltage regulator (7808) is used. Note that Tantal capacitors should be used.
(In general the three-terminal regulators tend to oscillate with usual electrolytic capacitors).

• All non-polar capacitors should be MKM-types and all polar capacitors Tantal-types.
  
• The potentiometers are those fine-adjustable ones with ten turns.
  
• If the shown 12V-relais isn't available: any other comparable can be taken,- the layout possibly has to be changed.

Adjusting the board is quite simple:

• Connect a signal-generator to testpoint 1 (TP1) and put a sinus of 15625 Hz with 0.5 Vss in. Adjust the 100 Ohm potentiometer to maximum voltage.
  
• Connect a frequency counter to testpoint 2 (TP2) and adjust the 5kOhm potentiometer until you read 15625 Hz (must being done without a signal on video-in).

If you think the resolution of the here shown layout for a pcb-board is to low, you can send me a request and I shall E-mail you the layout as a plot-file (HPGL).

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