Configuration of hsync / vsync values for X

Polytropon freebsd at
Sat Jun 8 20:21:43 UTC 2019

Currently I'm experimenting with ATV (amateur radio television)
and I'd like to put a computer output signal to the transmitter.
This has been no problem in the past (with an old laptop and
a VGA->Video converter called "Creative TVCoder External").
Even though I get a signal on a video screen (a b/w TV with a
video input), the transmitter result is garbled. My assumtion,
also judging from the slightly distorted control signal on the
TV, is that there's something wrong with the VGA signal, and
probably something "too high" because of too "modern" devices
or settings.

I could easily get X to output 640x480, which is what the
converter will happily accept and which is fully sufficient
for my needs. However, according to xvidtune, I have a
horizontal sync of 31.47 kHz and a vertical sync of 59.94 Hz.
What is the recommended way to control those values for X?
Let's say, I want to tune down vsync to 60? I think I need
to "reduce signal quality" in order to get the transmitter
(or our local relay's digital enconder?) to accept my signal.
The retransmitted signal is all "out of sync", while normal
camera input to the transmitter is correctly retransmitted
by the relay (as expected).

Dealing with modelines has been standard knowledge decades
ago, but during all the years I forgot so many things... ;-)

Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...

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