Re: spdif output and mono audio

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On 30/12/06, Andrew Lyon <andrew.lyon@xxxxxxxxx> wrote:
> Hi,
>
> I am using iec958 spdif output on gigabyte ga-965-ds3 motherboard with
> onboard hda-intel, kernel 2.6.18.6 alsa 1.0.13, I dont have a
> asound.conf or asoundrc. Most of my sound is coming from mythtv (ac3
> and dts passthru enabled), this works really well provided I run the
> following commands (just once) before running myth:
>
> amixer set IEC958 mute
> aplay -D spdif /usr/share/sounds/alsa/Front_Center.wav
> amixer set IEC958 unmute
>
> I dont know why that is necessary but without it I get silence or
> "helecopter" noises, I think the aplay command actually errors as
> well, it certainly doesnt output any sound,  which is even stranger..
>
> Anyway thats not the problem, the problem is that a small number of
> programmes that I watch have mono audio, when myth attempts to play
> them back I get the following error and no sound:
>
> 2006-12-26 16:22:46.585 Opening ALSA audio device 'iec958'.
> 2006-12-26 16:22:46.586 AudioOutput Error: Channels count (1) not
> available: Invalid argument
> 2006-12-26 16:22:46.586 AudioOutput Error: Unable to set ALSA parameters
>
> I understand why this happens, iec958 does not support mono audio,
> either its simply not possible or not supported on my chipset, Ive
> tried changing options in myth but it doesnt make any difference, it
> always tries to set the channel count to 1.
>
> I have asked on the myth mailing list but nobody has been able to
> help, I wonder if anything can be done from the alsa side of things?
> Perhaps alsa could be configured to accept mono audio on the iec958
> and to resample it to 2 channel?
>
> Andy
>

It is a problem with myth.
myth should open difference ALSA devices depending on the type of
sound being played.
I.e. for AC3/DTS is should open the iec958 device.
For PCM (stereo or mono) it should open the plug:iec958 device.
The plug bit does the resampling and mono->stereo conversions.

James

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