Subject: Re: [ecasound] full-duplex sync
From: Kai Vehmanen (k_AT_eca.cx)
Date: Sun Oct 14 2001 - 16:57:18 EEST
On Fri, 12 Oct 2001, Jeremy Hall wrote:
[ecasound's multitrack-sync]
> It works until a person wants to use the live signal as a base to work
> from. The track drift I have noticed seems to be related to the
> frames_per_cycle and if there are many tracks that were recorded
> separately, the drift can get in there because the person is listening to
> the output of ecasound and playing or singing with it.
It should specifically work when using a live signal as base!
Let's take a simple multitrack setup:
ecasound -a:mon -i:track1.wav -o:/dev/dsp \
-a:rec -i:/dev/dsp -o:track2.wav
Goal of the multitrack-sync code is to make sure that the first sample
written to track2.wav was recorded from /dev/dsp at the same time as the
first sample from track1.wav came out from your soundcard's output.
It's not critical that the two /dev/dsp instances are started in sync (*).
All that matters is that track1.wav and track2.wav are synced.
But of course, as I said in my earlier post, ecasound's mt-sync code is
not perfect, and there's room for improvement. I'd be interested in
hearing about how the current CVS-version of ecasound works for multitrack
recording. I've recently made two specific changes to the sync-code (one
in 2.1dev0, and another in the yet-to-be-released 2.1dev2). I've done
quite a lot of recording with these, and with good results. If you make
test recordings, please take a note of the following line printed by the
ecasound engine:
(eca-engine) sync fix is xxx usecs.
On my machine, the sync-fix is usually between 20-50 usecs, which is not
much. One sample at 44100Hz takes ~23usec, so the sync-fix is just 1-2
samples. But the delay might be much longer on non-SMP machines.
In any case, without support from the hw-level, there's always a small
error (in modern computers a _lot_ happens during 20usecs). But for
practical (musical) purposes, delays below 1ms (1000usecs, ~44 samples)
are not noticable.
(*) On the other hand it _is_ important that the /dev/dsp instances
run at the exactly same _speed_. It's because of this requirement
that using multiple consumer soundcards to record +2 channels
doesn't work (at least not reliably).
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