[linux-audio-user] Multiple (delta xx) cards

[Björn Pfeiffer]

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