[pulseaudio-discuss] Decode SPDIF compressed formats

Tanu Kaskinen tanuk at iki.fi
Mon Jul 24 19:38:57 UTC 2017

On Sat, 2017-07-22 at 21:02 +0200, Denis Shulyaka wrote:
> 2017-07-20 17:06 GMT+02:00 Tanu Kaskinen <tanuk at iki.fi>:
> > No, I don't think there's currently any code to read the status bits
> > from the alsa mixer.
> > 
> But in general, if one wants to passthrow unmodified uncompressed PCM data
> from S/PDIF-in to HDMI, shouldn't he know the source sample rate to
> correctly open the output device?


> Do I understand it correct that the only way to implement this scenario
> with PA is to indirrectly measure the sample rate by comparing the
> pa_bytes_to_usec() result against the actual time passed?

I'm not sure what you mean by "implement this scenario with PA". Are
you talking about modifying PA code itself? If so, it would seem best
to modify the alsa source so that it reads the spdif status bits and
configures its sample rate accordingly.

If you use an external program to record from the spdif source, you
might get pretty good results if the program reads the status bits from
alsa and then configures the recording stream accordingly. As long as
there's just one recording stream connected to the source, the source
should change its sample rate depending on the application's stream

> If the source is opened with 48000 sample rate, but produces the data at
> 44100 rate, will I receive a buffer underflow error on the source?

No, I don't think the source will have any complaints. It runs at
whatever rate the sound card produces data. It's just that the data
will be incorrectly labelled as having 48kHz rate, so it will sound
wrong when played back, and if module-loopback is involved, it probably
won't work properly if the source is running way too slowly.



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