[pulseaudio-discuss] Revisiting 5.1/7.1 channel positions

[Tanu Kaskinen]

On Wed, 2018-12-05 at 16:39 +0530, Arun Raghavan wrote:
> 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

According to those illustrations, 5.1 surround channels are exactly the
same thing as 7.1 side channels (based on the suggested 90-110 degree
angle). So it would be logical to simply always use the side channels
instead of the rear channels with 5.1, no need for introducing any new
channel positions. Then there would be no ambiguity.

Clients for some reason sometimes use the side channels and sometimes
the rear channels with 5.1 content. If that indicates a bug, the bug is
in the clients (well, also in our 5.1 sink channel maps, because we use
rear instead of side).

> 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
> ).

That pdf defines these things:
"surround on side", angle 90 degrees
"surround on side in rear", angle 110 degrees
"surround in rear", angle 150 degrees

The Dolby illustrations have just one pair of speakers for 90-110
degrees in both 5.1 and 7.1 configurations, so to satisfy the ETSI
table, a new channel wouldn't be needed further behind what we now call
"rear" (which is what you seem to be suggesting), but we would have to
split the current "side" channels into "side" and "side in rear". I
don't see any practical reason for doing that separation. If we have
any real problem, it's that we sometimes incorrectly play 5.1 content
to the rear speakers (in 7.1 configuration), not that we don't
distinguish between two slightly different side channel positions.

-- 
Tanu

https://www.patreon.com/tanuk
https://liberapay.com/tanuk