[LAU] Using multiple sound cards with jack

[Len Ovens]

On Thu, 30 Mar 2017, Bill Purvis wrote:

> I currently use qjackctl to control Jack, but would like to be able to use 
> more than
> one sound card at a time, or at least be able to switch from one to another 
> without
> having to restart jack. Currently, the QjackCtl Connect window shows a single 
> stereo
> audio output referred to as 'system'. Is it possible to have more than on 
> audio output
> here?

Yes and no. Now I can ask some questions :)

Are all of the cards hard synced? (Do they use the same media clock?)

Do they have to be accessable at the same time? Or is it good enough to 
switch audio cards on the fly without stopping jackd?

How much do you care about quality? (is it ok if some of the audio is 
resampled?)


Best quality = just one audio interface with a stable clock. 
Apparently Digidesign manufactured an audio interface with such a bad 
internal clock it actually sounded better with a master clock.

next best quality = more than one interface synced with word clock with 
very short cables.

next best quality = AVB networked audio interfaces

lowest quality = resample to sync

In all cases there are better and worse options. As I stated above even 
internal clocks can be bad. The Digidesign 192 interfaces are just one 
example (see 
http://pinknoisemag.com/pink-papers/pink-paper-002#aestheticrise )

resampling is very common in the broadcast industry where all the AES3 
inputs can be set to resample (in the same way video frame storage is used 
to sync remote video sources). Can you hear the difference? I don't know. 
I personally think that if you are going to have more than one card used 
for output, resampling will not hurt. I would prefer to keep my inputs 
synced.

Anyway zita-ajbridge will allow a second or third card to be added to 
jack. With or without resampling.

ALSA will allow two cards known to be in sync (word clock used) to be 
used as one card.

--
Len Ovens
www.ovenwerks.net