[mythtv-users] "Woodchuck" sound on TV playback

Michael T. Dean mtdean at thirdcontact.com
Sun Aug 17 23:19:18 UTC 2008


On 08/17/2008 05:37 PM, Allen Edwards wrote:
> On Sun, Aug 17, 2008 at 2:34 PM, Allen Edwards wrote:
>> The issue for me with setting up an ALSA configuration file is that
>> ALSA will take the bitstream, decode it, manipulate it per the
>> configuration file, then encode it (poorly) before sending it to the
>> output.
>>     

No, it doesn't.  ALSA doesn't speak AC-3 or DTS.  The /only/ difference 
between having a (proper) config file and not having one is the 
difference between specifying the IEC958 status bits in the 
application's settings (Myth's "Passthrough output device") or in the 
ALSA config.

>> I really wanted ALSA to do nothing, I just wanted the spdif
>> information sent unaltered to the output so that my external decoder
>> could deal with the pure bitstream.
>>
>> The other problem I read is that people end up with bit rate specific
>> files.  You really don't want ALSA changing the bit rate for any
>> device.
>>     

All your above complaints (manipulating the bitstream and doing 
"bit-rate specific" stuff) have to do with the case where you're sending 
PCM data (not AC-3/DTS) out the sound card's digital output and your 
sound card needs 48kHz PCM input.  The "digital" logical device 
specified by the ALSA config file in the wiki uses ALSA's automatic 
conversion plugin (plug) to convert the format, samples, and rate of any 
data it receives to a form that's usable by the sound card (i.e. 48kHz 
PCM data, for a large majority of sound cards available today).  The 
"mixed-digital" logical device does exactly the same, but also uses the 
dmix plugin to allow other processes to use the sound card at the same 
time and "mix" the audio to both play at once.

Note that if you delete the ALSA config file, you use the ALSA-specified 
default device, which uses plug and dmix (i.e. is an identical 
configuration to that of "mixed-digital").  The difference is that you 
no longer have the option for "digital."  However, in either case, the 
ALSA drivers (technically, the plugins) are manipulating the audio so 
that it's in a format that works for your sound card.  Note that if it 
doesn't do this, the audio does /not/ work for your sound card, so you 
don't get sound (or you get chipmunks or you get screeching or ...).

>> The goal is bit true audio...

A lofty goal with run-of-the-mill computer sound cards.  Perhaps I 
should say, "A lofty goal with AC'97 and Intel HD Audio."

> I should have added, my understanding was that newer versions of ALSA
> have built in default configurations that work well so it is not that
> you don't need a config file, but that the built in one is a good one
> to use.  Am I wrong?

Yes, newer versions have made the default device for "software" sound 
cards use plug and dmix.  But, see above.

BTW, you did see me mention that the ALSA config on the wiki does /not/ 
specify any passthrough stuff, right?  Therefore, I'm not saying that 
people should use the one on the wiki for passthrough (or, at least, 
that they would need to modify it, as I've done for mine).

Mike


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