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:
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).
The single-sided board has the dimensions of 79x49 mm.
When completed, you will recognize, that the squelch is very sensitive,- it even reacts, when the signal it hardly to be seen on TV. Pay attention to how you pick the video for the video-in without influencing the video itself...
Good luck,- any comments are appreciated !
VY 73 de Johannes (DL4EBJ)
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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J. Köring, Bahnstraße 3B, 47551 Bedburg-Hau, Germany
Copyright 2000 by Johannes Köring (DL4EBJ). All Rights Reserved.
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