Quoting conrad berhörster <conrad.berhoerster(a)gmx.de>de>: Jack works only with one soundcard, or an array of sample-synced soundcards. Nothing else. You do need sample sync to make your idea work properly. Sample sync, or you dump the idea of using jackd and do resampling in the code. Here is the reason: Soundcard 1 -> Application -> Soundcard 2 Looks simple, eh? It's simple because it's wrong. The truth is this: Soundcard 1 -> Application -> Soundcard 2 @44102hz @44092hz (This is due to the fact that no two crystals are the same) What happens in this scenario? When you are running the application for a period of say, 10 minutes, the output you hear from soundcard 2 lags about 0.136 seconds. After 20 minutes, it lags about 0.272 seconds, etc. The reason for this? While soundcard 1 takes in 44102 samples per real-world-second, soundcard two outputs only 44092 samples in that time. If soundcard 2 would have a faster crystal than soundcard 1, you'd be in a heap of trouble. For every buffer you would write to soundcard2 you would get an underrun where soundcard 2 isn't fed data fast enough (altough, exact error frequency depends on the buffering scheme used). With multichannel, we mean "audio devices running in parallel or serial in the same signal chain". Either use only one soundcard (if you are recording from one and playing back on the other one, i don't see why you couldn't use one soundcard), or don't use jack and resample between the two soundcards. Altough, getting a software resampler to perfectly sync two non-synced cards is a mean feat. If you want to use two soundcards as one, the best option is to: wordclock them. That is, to make them sample-sync. Sorry. But this is how it is. Sampo