[pulseaudio-discuss] Passthrough support and sink channel count

[Tanu Kaskinen]

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.

> 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.

-- 
Tanu