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

Allen Edwards allen.edwards at oldpaloalto.com
Sun Aug 17 23:58:57 UTC 2008

On Sun, Aug 17, 2008 at 4:19 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
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Well, what I have found is that there isn't any documentation on a
wiki that works for mythtv where people want spdif output at whatever
bit rate the source has.  I found a configuration that works and have
helped several people who were totally stuck get their sound working
with this setup.  I am not an expert on this, but I have a
configuration that works, which I didn't invent, but got through

It would be nice if there was a wiki that shows how to set up a
computer for mythtv with spdif.

Actually, what I want is a setup where I will get bit true spdif (same
bit rate as the source and hopefully unaltered) and analog output.
What I do now is run two external decoders, one for 5.1 and one for
stereo.  To make matters worse, I can't get spdif from firefox as it
always gives analog output so I have a resistive combiner taking the
output of my external stereo decoder and the analog L-R of my sound
card and combine them to feed the TV.  What a mess, but it works.


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