[mythtv] [mythtv-commits] Ticket #1608: Proper Handling of Alsa AC3-PassThru

[Martin Ebourne]

On Sat, 01 Apr 2006 16:17:05 +0200, Markus Schulz wrote:
> What have you configured as alsa output? ALSA:spdif? or similar alias?

No, I have AudioOutputDevice configured as 'ALSA:hw:1' which is direct to
the hardware.

> IIrc the iec958 is used all the time if you pass thru, cause iec958 is
> the sample format for sp/dif output. Check "aplay -L" for all pcm-output
> device strings.

No, my soundcard does not need iec958, it always sends all PCM sent to it
out through the s/pdif. Even in alsamixer there's only one control -
analogue output volume. I suspect this is more a limitation of the
snd_usb_audio driver than the sound card, but it works fine for PCM over
s/pdif and AC3/DTS passthru.

> The problem is, that we should be sure to send normal uncompressed pcm
> data to another pcm output then compressed ac3 streams.

This depends on hardware configuration and the user's preferences.

> Best way will be
> an additional input field (next to passthru checkbox) where you can
> enter the alsa-pcm-device string for ac3 streams differently to
> uncompressed pcm data.

I think this is the best solution. There appear to be several different
ways that s/pdif and passthru are implemented in ALSA, and a good many
more variations in people's setups, so I'm rapidly coming to the
conclusion that the only way to satisfy everyone is to make both
configurable independently.

> Current SVN Code only appends the iec958 specific options ( ":{ AES0
> 0x02
> }" ) to the device string you have been given. When this is ALSA:default
> and default is pcm0 Out, this option is totally wrong and results in
> errors.

Yes, I know. I submitted a patch a few days ago to workaround this fairly
recent change:
http://svn.mythtv.org/trac/ticket/1598

Although this makes the old cases work, and allows these other new cases
to work, I can now see there's other possibilities too which this does not
solve, hence the need for full control as you suggest.

Cheers,

Martin.