[pulseaudio-discuss] On surround channel nomenclature and remixing

[Alexander E. Patrakov]

Hello.

[Note: the whole e-mail is purely theoretical. I still don't have any 
surround-sound system in my room. A salesman told me that I should buy 
5.1 speakers, not 7.1, because the TV is centered on the long side of 
the room, and thus not enough space exists behind the listening area. I 
agree with this argument - so will not be able to test what I am talking 
about anyway.]

I tried to play surround-sound files using various players and look how 
PulseAudio remixes things, and found some strange results.

First, let's start with a simple third-party 5.1 AC3 file:

http://distribution.bbb3d.renderfarming.net/audio/bbb3d_sunflower_soundtrack_surround.ac3

If you play it using ffplay, vlc, or with anything gstreamer-based, this 
will yield the following channel map:

front-left,front-right,front-center,lfe,rear-left,rear-right (Surround 5.1).

If you use "mpv --channels 6", then it will yield this instead:

front-left,front-right,front-center,lfe,side-left,side-right

Now let's see what happens in PulseAudio when a user has 5.1 speakers.

With ffplay, vlc, and anything gstreamer-based, each channel's sound 
comes to the correct speaker. With mpv, rear-left and rear-right 
channels get remixed from other channels (if remixing is enabled), which 
is bad. If remixing is disabled, then rear speakers stay silent, and the 
sound in the corresponding channels gets lost. Obviously, the ffplay 
result is correct here.

And now let's see what happens if a user has 7.1 speakers.

With ffplay, vlc, and anything gstreamer-based, PulseAudio will send the 
original signals to the channels mentioned in the channel map, and will 
remix side-left and side-right outputs from the remaining channels. With 
mpv, it will send the last two channels of the input stream to side-left 
and side-right speakers, and remix rear-left and rear-right. That's 
unless the user disables remixing.

If the user disables remixing, then, with ffplay, side channels will 
remain silent, and with mpv, rear channels will be silent. However, I'd 
guess that the mpv result is correct here. See 
http://www.thx.com/consumer/home-entertainment/home-theater/surround-sound-speaker-set-up/ 
- the "rear" channels have moved between 5.1 and 7.1 layouts. What 5.1 
calls "rear" is actually "side" in 7.1 terminology.

See also http://www.razersupport.com/gaming-audio/razer-tiamat-71/ - in 
particular, the answer to the "What is the difference between 5.1 and 
7.1 surround sound?" question.

How should PulseAudio deal with this nomenclature mess?

-- 
Alexander E. Patrakov