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

John Welch jrw3319 at gmail.com
Mon Aug 18 13:49:47 UTC 2008


On Sun, Aug 17, 2008 at 7:19 PM, Michael T. Dean
<mtdean at thirdcontact.com> wrote:
> 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
> _______________________________________________

Mike, would you be willing to post your ALSA config file?  Now that I
know the setup I can use to get passthrough working I'm confident I
can always go back to this point, but I would be interested in having
things configured with a proper ALSA config file so that I don't have
to specify these cryptic (to me, at least) devices in myth, mplayer,
xine, etc.

Thanks for your help.
John


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