[pulseaudio-discuss] On surround channel nomenclature and remixing

[Alexander E. Patrakov]

17.08.2014 10:50, Tanu Kaskinen wrote:
> On Fri, 2014-08-08 at 16:33 +0600, Alexander E. Patrakov wrote:
>> 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.
>
> It's not obvious to me. It appears that there are different opinions
> about how to assign channels in 5.1 and 7.1 setups. According to
> http://en.wikipedia.org/wiki/Surround_sound there are two 5.1
> "standards" and three 7.1 "standards". Dolby thinks that 5.1 should use
> the side channels, and AC-3 is a Dolby format, so to me it sounds more
> likely that mpv is correct here.

I think that this non-obviousness and other "disagreements" come from 
the fact that you used Dolby "standard" in your e-mail as a source of 
truth, while I used the existing (inconsistent) PulseAudio conventions.

Given that, in the end, you propose a sensible solution that achieves 
what's needed, I see no point in discussing the "disagreements" further. 
They don't really exist.

>> How should PulseAudio deal with this nomenclature mess?
>
> To make audio with the Dolby channel arrangement work better, we could
> change the default device channel map to use the side channels for 5.1,
> but that will then be worse for audio that uses a non-Dolby channel
> arrangement. I have no idea what's the most common "standard".
>
> Perhaps we should add special casing to the remapping code so that if
> the source channel map is 5.1 with rear channels and the target channel
> map is 5.1 with side channels or vice versa, we don't do any remixing,
> but copy rear to side or vice versa.
>
> If consistent behaviour for 5.1 -> 7.1 remapping is desired, we could
> always feed the rear or side channels of the source to the side channels
> of the target (or to the rear channels of the target, if that's how the
> user prefers to have it).

+1 to all of the above.

-- 
Alexander E. Patrakov