[pulseaudio-discuss] [PATCH] fix profile configurations for IEC/SPDIF outputs

pl bossart bossart.nospam at gmail.com
Mon Jun 21 06:58:33 PDT 2010

> I appreciate that the mapping "iec958-surround-40" may be irrelevant
> seeing as spdif only support stereo PCM, but the I don't really follow
> the "duplicates what PA could do" bit. Can you explain a bit?

Surround40 only modifies the channel mapping to generate 4ch.
PulseAudio could do this natively if the sink was declared with 4ch.

>> and the former is a lossy codec that will decrease the audio quality
>> while increasing the workload on your computer. You are probably
>> better off sending stereo and letting your receiver doing the
>> post-processing.
> Hmm, I don't follow here. What if your source is 5.1? Are you saying we
> should downmix the 5.1 source to stereo and then send the stereo over
> spdif then let the receiver try and do the best it can with it, e.g. via
> DPL?

If your source is 5.1, it came from an encoded AC3/DD file. What you
want is the passthrough mode. In your configuration, you decode first,
pass the data to PulseAudio and then reencode. The only case where
this would make sense is if you wanted to mix your alert tones and
beeps with the music track, not a compelling use case IMO.

> To me that doesn't make sense. I'd rather encode my 5.1 source as DD/AC3
> and send the whole thing over spdif. But that said, if it's a really bad
> method of working, then I'd say get the replacement implemented before
> removing potentially useful bits without any timetable for it's replacement.

I don't disagree, I just wanted to see which ones made sense and who
needed those profiles...I really think if this is needed we could
implement the a52 encoding in PulseAudio and avoid relying on ALSA

> So how do you propose to send 5.1 streams over spdif without using pure
> passthough if not via a DD/AC3 encoder of some sort? Are you able to
> implement DDL or DTS?

I'll send you a snippet of code to see how to format your ac3 files to
play them in passthrough mode.

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