[mythtv] Ticket #2539: Mac OS X doesn't correctly handle AC3 Passthru

[Andrew Kimpton]

MythTV wrote:
> #2539: Mac OS X doesn't correctly handle AC3 Passthru
>
>  Note that if you enable AC3/SPDIF Passthru in the settings but do NOT have
>  a SPDIF connection to an external decoder you're quite likely to hear
>  potentially damaging to your hearing (depending on level) amounts of
>  'noise' (undecoded AC3 data) through either your headphones, built-in
>  audio or analog external speakers.
>
>   

I tried unsuccessfully to change the audio_passthru setting in the 
audiooutput cases in scenarios where there is no supported AC3 output 
hardware (Mac OS X can report whether the built-in connector  on a 
Macintosh is being used for analog 2 channel audio or fibre-optic  
SPDIF), but failed to find a mechanism to do this.

It looks like the 'enable/disable passthru' logic is being handled in 
the depths of the NVP and that by the time the audiooutput class has 
been constructed it's all 'too late' to make the change.

I guess a second set of (cross-platform) changes would be to add some 
logic in NVP and new code in AudioOutput to detect for the availability 
of a correctly configured AC3/SPDIF output and configure appropriate 
(effectively rendering the setting irrelevant ?). I'm fine with such 
changes and implementation on OS X but I've no idea if such detection is 
even feasible on Linux with ALSA or OSS ? Can the /dev entry be probed 
to detect whether there is/is not really a SPDIF connector ?

Andrew 8-)