Subject: Re: [ecasound] full-duplex sync
From: Jeremy Hall (jhall_AT_uu.net)
Date: Fri Oct 12 2001 - 22:25:39 EEST
Hi,
In the new year, Kai Vehmanen wrote:
> > Things that may be "non-noticeable" to you are noticeable to us.
> > What we care about is !exact! sync between input and output.
>
> Ecasound can't guarantee sample-level sync. Achieving
> it requires that capture and playback are started
> _exactly_ at the same time. And this can be only
> done with hw-level support. Without limiting
> ecasound to some specific sync-API, this is not possible.
>
> So ecasound uses a different approach. It starts
> devices independently, but records the exact time
> of triggering. Using this information, ecasound
> can fix possible sync-offsets when writing to
> output files. This method works and requires very
> little support from the driver-level.
>
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.
> I suggest you take a look at ardour (ardour.sourceforge.net).
> Ardour has always been ALSA-only and thus it can
> more easily take advantage of ALSA's features. Hw-sync
> support was added to the new ALSA 0.9/0.6 version, but
> I don't know if Ardour uses it yet. But it's worth
> checking.
>
It does appear to use it. I seem to recall a lively discussion on
alsa-devel between Abramo and Paul on the subject.
_J
> --
> http://www.eca.cx
> Audio software for Linux!
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