The Problem

A few years ago I bought an impressively cheap CMI-8738 based sound card from Trust, it cost £15 I think. Ever since I have been trying to find a way to use it reliably under Ubuntu linux for digital recording from our Akai DPS-12.

The root cause is that as soon as you enable the S/PDIF input, the card stops delivering sample buffers via Alsa unless there is a valid S/PDIF signal present (ie: the DPS-12 is connected and switched on). This is fine except that we are using jackd at the bottom end of the audio stack, and jackd doesn't like a card that doesn't sample regularly, it crashes out with a watchdog error.

We could remember to switch the DPS-12 on before the PC every time we switch the PC on... not gonna happen unless someone intends to record something, or we can live with no digital input (as we have been doing for a number of years).

The Solution

..for which I owe Mr Torben Hoen a beer: alsa_in

It's so darn simple - leave jackd running with output only (S/PDIF and analogue are fine and nice low-latency), then in my audio startup script, run alsa_in to read samples from the S/PDIF input and pretend to be the standard capture backend for jackd (ie: call it 'system').

The magic is that alsa_in is happy to work with no input, it just delivers empty buffers to jackd unless the S/PDIF signal is provided, thus samples arrive from the CMI-8738, whereupon alsa_in delivers those to jackd - genius :)

/me dances a little jig :)

AshbySoft: CMI-8738 vs Jack (last edited 2011-04-17 17:39:24 by Phlash)