''MultipleCards

Multiple Soundcards

Many people attempt to get more than two simultaneous inputs or outputs by putting more than one cheap soundcard in their system. I suppose this can be made to work, but I've yet to hear from anyone who's really pulled it off without big problems. The trouble is that each soundcard derives its sampling rate from a crystal oscillator clock. The clocks in the two soundcards will inevitably not be synchronized. You might think that 44,100 Hz is always exactly 44,100 Hz, but sadly, it's not. The soundcards will always be off by a few samples per second. Your software doesn't know this - it thinks 44,100 samples from one card represents the same amount of time as 44,100 samples from the other. As a result, the sound from one soundcard will gradually lag farther and farther behind the sound from the other soundcard. If there's any sound being sent to both cards, you may even get weird phasing effects.

There are only three possible remedies:

Use a true multichannel soundcard. See the ProfessionalSoundcard? section.
Use cards that can be sync'ed to an external clock, and drive one card with the other card's clock. I do not know what (if any) cards that support synchronization work with any of the linux drivers. There is an interesting card in development that has this feature. It was called Nightingale, I think; anyone have a URL?
Compensate for the discrepancy in software. I don't know of any existing linux software that does this, nor do I know how it could be done. The topic does occasionally come up on the linux-audio-dev mailing list, but I don't think anyone's actually dealt with it.

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