Audio output over HDMI #7
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At COSCUP the idea was to hook up their own recording solution behind ours, and we'd just send them a copy of what we mixed. For audio, this was a bit of an issue, as they were counting on having the audio embedded into what we feed out of the HDMI.
Some notes on what was done until now:
The quickest/easiest thing that I was able to look into was to use
dsnoopdevices and thenalsaloopfrom the audiomixer to the audio device of the HDMI output. It seemed to work, but we were not able to verify it.Doing this in the
ffmpeginsink.shwould induce too large a latency. We did not test this, though.We do not seem to have a way to wire the mixer into the radxa to do this directly.
Also note that this MUST be configurable, otherwise this would create feedback loops in a lot of places, as so many projectors and screens have some kind of built-in sound.
I got the latency down to about 5ms with pipewire, but this is on top of whatever the HDMI receiver gives and the USB buffering, so realistically it is about 25ms which is a lot
25ms is 8.2m distance, which should be livable/acceptable, I've played guitar with that latency. We should test.
Slap me if I'm wrong; but isn't the part of the latency we actually care about the difference between the video and audio over HDMI? The video already has some latency, as long as the audio is in sync with that, we should be good? From what I understand, that would be 5ms which I guess is pretty much in sync?
If the HDMI audio gets used for PA then the absolute latency also matters. Like the COSCUP setup.