[pulseaudio-discuss] Revisiting 5.1/7.1 channel positions

[Arun Raghavan]

On Wed, 5 Dec 2018, at 3:59 PM, Tanu Kaskinen wrote:
> On Wed, 2018-12-05 at 15:08 +0530, Arun Raghavan wrote:
> > Hey folks,
> > I've written up a quick analysis of the channel positions we
> > currently support, and what I think makes sense:
> > 
> >   https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/merge_requests/55#note_85318
> > 
> > The summary is that "Rear Left/Right" is currently being used in
> > place of what should be "Left/Right Surround", and we do not have any
> > channel  position between that position and "Rear Center", which is
> > needed for common 7.1 configurations (what would be "Rear Left/Right
> > Surround").
> > 
> > To add to this, the PulseAudio channelmap header is incorrect in that
> > we say that "Side Left/Right" should correspond to Dolby "Surround
> > Left/Right" when they are separate (they are further forward than
> > "Surround Left/Right". Yhis can be corrected easily enough as it's
> > just a documentation comment.
> > 
> > My proposal is to add a "Rear Left/Right of Center" position to
> > represent the missing positions. At the ALSA level, it would
> > correspond to RLC/RRC.
> > 
> > Any comments?
> 
> If I understood correctly, you're proposing that we should have three
> surround channel pairs (side, surround and rear) instead of two (side
> and rear). What practical problem would this solve?
> 
> I'm aware of the problem that some 5.1 streams use side and some use
> rear in their channel map specification (I don't know if there's any
> good reason for this), and up until very recently we didn't handle the
> side case properly. However, Alexander fixed this:
> https://gitlab.freedesktop.org/pulseaudio/pulseaudio/commit/73156649e76ac4000931990edcdcb3be31aade7b

In this case, the problem is that rear left/right means different things based on the content.

For 5.1 content, you will use FL, FR, FC, RL, RR, LFE.

Now when you add 2 more channels for 7.1 content, those are supposed to be further *behind* the 2 surround channels of 5.1, but we do not have such channels. Which means we need to represent 7.1 content as FL, FR, FC, SL, SR, RL, RR, LFE.

So Rear Left/Right means different things based on the content, rather than have a somewhat fixed notion. This is illustrated in:

  https://www.dolby.com/us/en/guide/dolby-atmos-speaker-setup/5-1-2-setups.html
  https://www.dolby.com/us/en/guide/dolby-atmos-speaker-setup/7-1-2-setups.html

Also, notionally, for Dolby/DTS side left/right is a distinct set of channels from surround left/right, though I'm not aware of whether content that provides individual streams for those two sets of channel exists (see page 82 of https://www.etsi.org/deliver/etsi_ts/102100_102199/102114/01.05.01_60/ts_102114v010501p.pdf).

Cheers,
Arun