[PlanetCCRMA] Re: Mulitple Jack audio interfaces (was Tascam US-122 audio problem)

Andrew Burgess aab@cichlid.com
Thu Feb 24 22:12:01 2005

>> > > If you are only correcting for clock accuracy and
>> > > using the same sample rate
>> > > (e.g. two soundcards running at 48K) then you only
>> > > need delete/insert a sample
>> > > every few 100,000 or million samples. Smooth it a
>> > > little and it shouldn't be too
>> > > bad cpu-wise.

>Aye. I dont know who wrote the original message, but its very optimistic!

I did, and I suppose I am optimistic :-)

>This is really very difficult, 

It is proving difficult but I'm not discouraged yet.

> and the results will never be that good.


>It should be possible with firewire devices, as its possible to make them
>share a common clock, but with unmodified consumer PCI devices you have no
>source of sync, the drift will be substantially more than 1 sample every

How much more substantial will it be? That seems like a high guess to me,
aren't crystals accurate to a few ppm?

>Also, its not just the aggregate drift over a long time that
>matters, its the short-time relative drift.

Yes, you have to adjust dynamically. Thermal drift of two crystals in the
same computer should be somewhat similar.

Do you think it impossible to calculate the sample rate difference
with alsa timestamps over millions of samples?

>There is a paper in this years LA conference that will describe a
>technique than can be used to solve this class of problem, with two
>out-of-sync sources.

Thanks for the pointer. Hope I'm not reinventing the wheel...