From the ALSA wiki
|The FAQs have been reorganized. The new location of this question is FAQ#Can I use multiple soundcards with JACK / in sync|
Can I use multiple soundcards with JACK / in sync?
If your soundcard doesn't have a S/PDIF / ADAT / wordclock in then it's basically not possible. Apparently Delta 44 cards can be synced in the kernel, but that's currently only implemented under Windows XP.
If you don't need the cards to run in sync then you can use a MultipleCards asoundrc to configure them to appear as one device, but this will not work with jack, or if you need sample sync between the cards.
If you do have wordclock then linking the wordclock ports (with appropriate t-pieces and terminators) and setting the master/slave controls on the card should lock the clocks and using a MultipleCards asoundrc should make them work as one card.
If you have ADAT or S/PDIF and your card can take its master clock from the input then you might be able to do the same thing, by linking the digital output of the master card to the digital input of the slave card, but if your card does resampling (e.g the Soundblaster Live) it may be difficult, or impossible to persuade it to take its clock from the digital in.
Where can I find an explanation of syncing, wordclocks, sample rate wander, etc? Different soundcards/audio devices don't sample at exactly the same rate, because they are based on oscillators or crystals which won't be exact? How does software record this, then? The hardware interrupts the software whenever it has a full frame and dumps it in, even though it won't be perfectly accurate, and the software just strings each frame together and assumes that the sample rate is accurate? So if a sound is recorded on one sound card and played on another it will be at a slightly different frequency? Here is the thread I was reading about that led me to look for info on this page.
Is there an area of this wiki that explains this sort of stuff? I imagine it would be worthwhile to add if not. This is even more confusing when you consider USB audio, which I believe polls at 1 kHz, so if two USB devices were used at the same time their clocks would be out of sync but their polling frequency would not, meaning some samples would get dropped? I know I have used Cool Edit in windows with two USB devices and noticed no problems...
Many USB audio devices run an second a phase lock loop that is adjusted by the arrival of the USB data packets this information is used to adjust the audio clock to stay in sync.