[pulseaudio-discuss] Revisiting 5.1/7.1 channel positions

Wed Dec 5 14:01:49 UTC 2018

On Wed, 2018-12-05 at 18:59 +0530, Arun Raghavan wrote:
> On Wed, 5 Dec 2018, at 5:26 PM, Tanu Kaskinen wrote:
> > 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).
> 
> At this point, that "bug" is so ubiquitous, I don't think we can
> really change the fact that rear left/right really means surround
> left/right.
> 
> > > 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.
> 
> Based on my comment about not being able to change the fact that Rear
> in our case means "surround on side in rear", and that we already
> have a Side which effectively means "surround on side", my conclusion
> is that we need to add "surround in rear".

Ok, let's assume we can't change what channels clients use.

You seem to think that it's a mutually agreed fact that "rear" means
"side in rear" rather than "side", but I don't understand why that
would be the case. The 5.1 surround channel position is exactly the
same as the side position in 7.1, according to the Dolby illustrations.
With 7.1 "rear" means "rear", and with 5.1 it seems much more
convenient to assume that "rear" means "side" rather than "side in
rear". In any case the ambiguity remains: the meaning of "rear" is
different with 7.1 and 5.1.

I still don't understand what practical problem you're trying to solve.
Alexander already fixed the biggest issue (5.1 streams with side
channels were played really badly on 5.1 sinks with rear channels).

-- 
Tanu

https://www.patreon.com/tanuk
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