[mythtv-users] Myth, Jack & resampling

Alen Edwards allen.edwards at oldpaloalto.com
Fri Jun 27 04:20:49 UTC 2008

Dave Badia wrote:
> So I've been playing with jack audio support in mythtv a bit....
> Sample rates are still a mystery to me.
> If I run jack with a sample rate of 44100      (jackd -R -dalsa 
> -diec958 -r44100 -p1024 -n2,   sysout shows configuring for 44100Hz, 
> period = 1024 frames, buffer = 2 periods)
> and set MusicAudioDevice to JACK:
> and I play songs in MythMusic that are sampled at 44100 (flac encoded 
> files)
> but I see this in the frontend log
> JACK tmpdir identified as [/var/run/jack]
> JACK tmpdir identified as [/var/run/jack]
> 2008-06-26 22:14:14.693 AO: Using resampler. From: 44100 to 48000
> and 48.0 kHz appears on the mythmusic UI screen.
> I confused as to why it is being resampled
> Pretty sure my source FLAC is indeed 44.1, as if I switch my audio 
> output to ALSA:iec958, the mythmusic UI screen shows 44.1kHz.
> Must admit, I'm pretty new to sample rates, etc, so I may be having 
> some fundamental mis-understanding here.
> Thanks
> Dave
The docs say that JACK outputs to ALSA.

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.

Hope this helps,


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