[mythtv-users] ac3 passthrough problems

[Scott]

On Sep 22, 2006, at 5:31 AM, Thomas Börkel wrote:

> HI!
>
> Scott wrote:
>
>> Audio output device:
>> ALSA:iec958:CARD=0,AES0=0x6,AES1=0x82,AES2=0x0,AES3=0x2
>> Passthrough output device: Default
>> Enable AC3 to SPDIF passthrough: [checked]
>> Enable DTS to SPDIF passthrough: [checked]
>> Aggressive Sound card Buffering: [not checked]
>> Use internal volume controls: [not checked]
>
> It is funny, that this actually works, because with these setting,
> MythTV uses the "Default" device for AC3, not the one with the hex  
> codes.

Yes. But, my understanding of the "Passthrough output device" is that  
if it is set to "Default" (case sensitive) then it simply refers to  
the device defined by "Audio output device". This can be confirmed by  
running with mythtvfrontend -v audio > audio.debug.txt and looking at  
the device MythTV opens with ALSA.

So you're correct that mythtv uses the "Passthrough output device"  
when AC3/DTS passthrough is enabled. However, setting "Passthrough  
output device" to "Default" is a special case that tells MythTV to  
use the "Audio output device". Using the HOWTO asoundrc file the  
following alternative setup should work as well:

Audio output device:  ALSA:digital-dmix
Passthrough output device:  
ALSA:iec958:CARD=0,AES0=0x6,AES1=0x82,AES2=0x0,AES3=0x2
Enable AC3 to SPDIF passthrough: [checked]
Enable DTS to SPDIF passthrough: [checked]
Aggressive Sound card Buffering: [not checked]
Use internal volume controls: [not checked]

This config would be useful if MythTV was able to detect and route  
AC3/DTS sound through the "Passthrough output device" only as needed.  
For example, if in MythMusic sound would not be 48kHz AC3 format and  
get routed via the "Audio output device". During OTA ATSC playback  
where AC3 formats are present MythTV would route sound via the  
"Passthrough output device" instead.

Of course, I have no idea if MythTV actually works this way. I'm just  
saying, based on the definition of the two fields that it would be  
nice if it MythTV could work this way! :)

--
Scott