[mythtv-users] Myth, Jack & resampling

Dave Badia dbadia at gmail.com
Fri Jul 4 14:47:34 UTC 2008


On Fri, Jun 27, 2008 at 12:20 AM, Alen Edwards <
allen.edwards at oldpaloalto.com> wrote:

>
> The docs say that JACK outputs to ALSA.
>
Which doc are you referring to?  My understanding was that JACK only used
the ALSA drivers and not the ALSA plugins.
Perhaps I mis-understood and the doc you are referring to would help me.



> I think that ALSA is resampling up to 48kHz.  Even if you give it
> digital in and want digital out, it will decode and encode.  It
> resamples in the process.  My understanding is that it also does not do
> a very good job and that it is easy to hear the difference between
> passthrough and resample mode.  Have not tried it as I could not get the
> wiki to work and gave up when I read it wasn't going to give me what I
> wanted anyway.  When you put ALSA in passthrough as you did when you
> switched it to iec958, it does not resample.


Yup, and that's the central point of my confusion.  I thought by using
passthrough (that is, passing -dalsa -diec958 to jackd) that no resampling
would take place whatsoever.   But, the frontend logs shows resampling is
indeed taking place -> 2008-06-26 22:14:14.693 AO: Using resampler. From:
44100 to 48000

Some futher investigation shows that the Using resampler log output is
coming from mythtv/libs/libmyth/audiooutputbase.cpp, so it appears that it's
the myth code that's performing the resampling, not ALSA.   hmmm

Thanks for the help
Dave
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