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