[mythtv-users] Quick survey regarding passthrough device settings

Jean-Yves Avenard jyavenard at gmail.com
Tue Apr 20 13:23:51 UTC 2010


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

> 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)

> 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

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