Subject: Re: [ecasound] Consistent buffer overrun problem (?)
From: Kai Vehmanen (k_AT_eca.cx)
Date: Thu Feb 26 2004 - 19:45:39 EET
On Thu, 26 Feb 2004, Arno van Amersfoort wrote:
> I've set up a Fedora/Planet CCRMA system to record with 4 soundcards (8
> channels) simultaniously. And yes, I know, everybody recommends not to
> do this because of sync problems but I'm kinda stoburn ;-). Anyway, I
[...]
> seems to work a little now. I can record about 2m45 without any problems
> or error messages but after that ecasound starts to fill my screen with
> "warning! playback overrun - samples lost!" messages.
[...]
> minutes). I don't really understand what the real cause is. Can somebody
This is caused by the very reason why people recommend against using
multiple cards: clock drift.
Although all cards nominally run at the same speed (for example 44100 or
48000 samples a second), in practise they each run at slightly different
speeds. Unfortunately Ecasound (and other apps) only know the nominal
speed, so they cannot easily compensate for the speed difference.
Now what happens is that sooner or later, the soundcard with the fastest
clock will run out of data and you'll get xruns errors. Similarly a slow
client will block the engine as it takes longer than other to process the
given samples.
In theory you could try to measure the clock differences in software and
try to compensate, but in practise it is easier to just buy a real
multichannel card.
-- http://www.eca.cx Audio software for Linux!
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