Subject: Re: [linux-audio-user] Multiple (delta xx) cards
From: Björn Pfeiffer (bjoern_pfeiffer_AT_gmx.de)
Date: Tue Jul 15 2003 - 17:45:35 EEST
On Sun, Jul 13, 2003 at 12:46:53PM -0700, Mark Knecht wrote:
> >
> > I don't understand this whole 'drift' thing. If I'm able to listen to a
> > previously recorded drum track and record a guitar track over it (in
> > sync with the recording) with either card, how is it possible for them
> > to be out of sync with each other?
>
> Peter,
...
> I hope this helps clarify the physical reasons that this is difficult.
>
> There is a software reason also. Linux/Alsa will onlt interrface to a
> single clock, so clocks being generated by two sound cards are not something
> that Alsa is designed to handle.
:-/
->
!
Would not a software/driver-solution to synchronize the
clocks/adjust the drift be handy here? At least theoretically,
somthing similar to ntp.
This would need
1: a way to measure the clock-difference exact enough
2: a way to delay the the stream running over the hardware
within the driver within very small units.
3: ... ?
But I doubt that this idea could get far, probably timing would
become a rather statistical than exact procedure in this region.
And expensive too. Also, maybe clocks are not even stable over
time (temperature, etc.) and would have to be remeasured
periodically. Also, I bet this idea has been around earlier and
if possible we would already have it realised.
The psycho-acoustic boundary for discerning discrete acoustic
events/clicks is about 1/20 s, AFAIR, so 50 ms would have to be
beaten to satisfy the ear.
Björn
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