[mythtv-users] Quick survey regarding passthrough device settings

Greg Oliver oliver.greg at gmail.com
Tue Apr 20 13:35:16 UTC 2010


On Tue, Apr 20, 2010 at 8:23 AM, Jean-Yves Avenard <jyavenard at gmail.com> wrote:
> Hi
>
> On 20 April 2010 23:12, Greg Oliver <oliver.greg at gmail.com> wrote:
>> Without knowing how mythtv "mixes" for different outputs, will it be
>> possible to *never* mix and just let our alsa configs handle the
>> channel mappings?  I have found it to be very good as far as lag on my
>> slow cpu goes.  I remember from a thread a while ago (I believe it was
>> you) stated that the mythtv mixer was faster than alsa, so I guess
>> that would be better?
>
> Not having a separate passthrough device isn't going to change in any
> way what you can do with ALSA.
>
> regarding downmixing, my guess is that it will be much faster to do so
> in mythtv than doing it via ALSA config.
>
> mythtv decodes the audio, process the audio, and re-encode the audio
> if required.
>
> ALSA will have to decode the audio once again (converting all samples
> to float) run matrices operation, then reconvert all samples from
> floats again.
>
> So you're adding an extra layer.
>
> Especially since the conversion to and from float is going to be
> extremely fast on x86 architecture thanks to foobar SSE ML code....
>
> If you want ALSA to do the whole remix, you'll simply have to set
> mythtv for the maximum number of channels your soundcard can process
> (so 5.1 or 7.1) even if you only have stereo setup ; so ALSA will
> receive all audio channels to work with

Sounds like I'll want mythtv to do it then.  That is an ideal solution
- can't wait!

>>
>> I'm not exactly sure what alsa is doing software-wise when I use the
>> route and ttable syntax to accomplish it currently either :)  I just
>> know it finally works for any video I throw at it - that makes us
>> happy.
>
> The whole idea of doing it in myth, is that you wouldn't have to play
> with any of the alsa settings which are far from easy to play with
>
>>
>> Also, I assumed the pcm entry I created would show up in the list of
>> devices to select in mythtv setup, but in fact it does not.  I had to
>> type it manually (wasted a long time there thinking I had alsa setup
>> improperly - REALLY long time)  :)
>
> in 0.23 and earlier, the list of audio devices is hard coded.
>
> in trunk, this is automatic, it shows all devices as seen by ALSA
> (alsa only currently)

Ahhh

>> Where does mythtv get that list, and do you know how I can get my
>> slave populated there?
>
> which version of mythtv are you running?
>
> if trunk, it's from ALSA libraries

0.23-fixes.  I'll start using trunk on one of my setups so I can play
with this stuff.  The audio re-write is making me happy :)

Thanks again for the explanations!

-Greg


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