[pulseaudio-discuss] Passthrough support and sink channel count

[Anssi Hannula]

10.09.2013 09:08, Tanu Kaskinen kirjoitti:
> On Mon, 2013-09-09 at 22:14 +0300, Anssi Hannula wrote:
>> Hi,
>>
>> I was looking at using the PA passthrough support (and adding support
>> for TrueHD, DTS-HD), but it seems there is another issue that has to be
>> resolved first:
>>
>> Pulseaudio passthrough (with ALSA) does not currently work unless a
>> stereo profile is in use.
>>
>> If a HDMI surround profile is in use, the device seems to be permanently
>> in a 6 channel mode. As passthrough of e.g. 48kHz AC-3/DTS requires the
>> device to opened in 2 channel 48kHz mode, pa_sink_input_new() will
>> always fail with "Could not update sink sample spec to match passthrough
>> stream".
> 
> The HDMI surround profile is a 6-channel profile, so it's expected that
> you can't open the device in 2-channel mode. If you want 2 channels, you
> need to activate a 2-channel profile.
> 
> It would probably be good to change the profile automatically when a
> passthrough stream appears.

That could seem a bit confusing to users ("Why does the HDMI sink switch
to stereo mode if I'm playing a surround DTS stream?"), but OK, better
than nothing of course.

I guess the profile would switch back when the stream ends? Otherwise it
wouldn't seem very nice as the user would have to re-set it after every
passthrough stream.

>> How should this be solved?
>>
>>
>> (also, TrueHD and DTS-HD MA will require a 768kHz IEC 60958 frame rate,
>> which in ALSA is 8 channel 192kHz)
> 
> This will then require a 8-channel profile to be activated.
> 
>> BTW, shouldn't/couldn't the sink channel count be auto-selected from
>> EDID in case of HDMI instead of having to select profiles?
> 
> I suppose the HDMI device can be opened in many channel configurations,
> so I guess your question is about what should be the default channel
> count. I don't know what information EDID contains, but if it contains
> just the maximum supported number of channels, I'm not so sure it makes
> sense to default to a surround setup. Stereo mode is a safer default.

It also contains a speaker mask.

All stereo systems (i.e. TVs) that I've seen seem to have the supported
number of channels and/or speaker mask set to 2 channels (not a large
sample size, though).


-- 
Anssi Hannula