[LAU] different soundcards for in/out in alsa vs. jack

Len Ovens len at ovenwerks.net
Mon May 27 18:33:25 CEST 2019

On Mon, 27 May 2019, Max wrote:

> Hi list,
> I know that having different soundcards for in and out will lead to 
> clock deviations and is generally not a wise thing to do, but my 
> question is differently:
> Surprisingly it works quite well with ALSA, but not so much with jack.

With Alsa? or with pulse? Most linux desktop audio uses pulse and looks 
like alsa to the application. pulse does SRC where needed. For just alsa 
such as audacity, recording and playback are separate. The application 
could also treat inputs and outputs with different callbacks. however, 
while this would work, the two streams would drift away from each other 
over time making things like mutitracking impossible... or at least two 
tracks would not line up from the beginning of a song to the end.

> Both is from Pd. Jack has a plethora of settings, so this could have to 
> do with it and make my blanket statement above too simplistic, but I 
> wonder is there a general design in jack which makes it harder to rund 
> different cards for i/o or do i have the wrong settings? Where to look at?

Jack is made for a different prupose. SRC causes artifacts in the audio 
and so is not used. Jack expects the same audio may be routed as both in 
and out and mixed. In order for this to happen without throwing away some 
of the audio or changing is sample rate, everything has to be in sync. 
Jack can deal with inputs and outputs on separate devices that are synced 
using wordclock if they are combined into one psuedo device. Or one card 
can be used as jack master and another device can be added through an SRC 
jack client such as alsa_in/out or zita-ajbridge.

Len Ovens

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