[mythtv-users] Digital audio issues

[Michael T. Dean]

On 02/16/2008 05:32 PM, Richard Shaw wrote:
> On Feb 16, 2008 11:27 AM, Michael T. Dean <mtdean at thirdcontact.com> wrote:
>> On 02/13/2008 01:07 AM, Justin wrote:
>>     
>>> I have my mythbox (using the latest SVN) connected to my mixer via a
>>> digital optical cable.  I have an asoundrc file created according to the
>>> documentation found here:
>>> http://mythtv.org/wiki/index.php/Configuring_Digital_Sound
>>>
>>> I am using the mixed-digital.
>>>
>>> I also have AC3 and DTS to SPDIF passthrough enabled.
>>>
>>> Sound works fine for livetv, recordings, and mythmusic.  However there
>>> is a quirk in mythvideo.  I have some ISO movies, when I watch them the
>>> sound is fine unless I try to fast forward, pause, or rewind.  As soon
>>> as I unpause or finish fast forwarding or rewinding I get digital
>>> noise.  In order to restore the sound I have to go back to the main menu
>>> and restart the dvd.
>>> Does anyone know what might cause this?
>>>       
>> You've set the Internal player to use AC-3/DTS passthrough and you've
>> set the passthrough device to default or ALSA:mixed-digital, which won't
>> work.  You need to change the passthrough device to something else (i.e.
>> ALSA:digital may work (depending on your particular sound card), if not,
>> use ALSA:iec958 --do not use the one in the combobox that says,
>> "ALSA:iec958:{ AES0 0x02 }", as it's different (and a broken
>> configuration)).
> Strange, I have mine set with the {AES0 0x02} and it still works fine. Mine
> is a nForce3 chipset...

It's not a full IEC 958 status specification, so it only works if the 
previous state of the port is close enough to correct.  Depending on the 
types of audio you play back and how playback changes the iec958 status, 
explicitly specifying a partial status can put the card into a state 
where the port won't properly initialize for AC-3/DTS passthrough or 
leave it in a state where it can't play back other audio later.  Also, 
the specification won't work with all cards.

However, generally, ALSA will just "do the right thing" once you've 
configured digital audio properly if you don't try to tell it what to 
do, so it's not necessary to specify it.  Until Myth allows better 
control over the ALSA subsystem, it's generally better to leave the 
iec958 status specification out as it can introduce an extra layer of 
complexity.

Mike