- snd-usb-audio - SPDIF clock source mixer control for Digidesign Mbox

[Damien Zammit <damien.zammit@gmail.com>]

Hi all,

As some of you may remember, the Digidesign Mbox has a multiple endpoint
interface and does not follow the standard USB audio spec.
I am pretty sure that the first interface 0 is for firmware upgrades
only, and the second interface has 2 endpoints one for playback and one
for capture.

I have heard that the SPDIF output is always on, but there is a special
command to change the clock source to external sync with SPDIF.

I think I have found the correct USB command for changing the clock
source of the device to SPDIF external sync.  I tried to use a pair of
the devices in JACK using a pcm_multi configuration and syncing one of
them to the other with a RCA cable, but unfortunately I get lots of
XRUNs in JACK and I am wondering if there is a reason for this?

Can someone who knows about the pcm_multi stuff shed some light on why I
might be getting thousands of XRUNs when I use two identical USB
soundcards that are (hopefully) synced to each other?

I have written a non-standard mixer control for this so alsamixer can
select the external clock source.

Basically, the command to set SPDIF sync seems to include a sample rate
command, but the sample rate is set to zero for SPDIF sync.  Could JACK
be resetting the sample rate and overriding my command perhaps?  How
would I override the ALSA driver code so that the sample rate is not
reset by JACK to 48000 instead of 0 but only when the mixer control sets
the flag?

Damien