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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Copyright 2000 by Johannes Köring (DL4EBJ). All Rights Reserved.