[pulseaudio-discuss] Revisiting 5.1/7.1 channel positions

[Arun Raghavan]

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?

Regards,
Arun

[Tanu Kaskinen]

Post by Arun Raghavan
Hey folks,
I've written up a quick analysis of the channel positions we
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