Re: [FFmpeg-devel] [PATCH v4 01/10] channel_layout: add new channel positions supported by xHE-AAC

From: "Jan Ekström" <jeebjp@gmail.com>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH v4 01/10] channel_layout: add new channel positions supported by xHE-AAC
Date: Mon, 27 May 2024 17:11:23 +0300
Message-ID: <CAEu79SYfoO_NS6eAXiK1puEVFJimoNz=SWrnGAoe4vmjK2iZwg@mail.gmail.com> (raw)
In-Reply-To: <20240526213739.67158-1-dev@lynne.ee>

On Mon, May 27, 2024 at 12:37 AM Lynne via ffmpeg-devel
<ffmpeg-devel@ffmpeg.org> wrote:
>
> apichanges will be updated upon merging, as well as a version bump.
> ---
>  libavutil/channel_layout.c | 4 ++++
>  libavutil/channel_layout.h | 8 ++++++++
>  2 files changed, 12 insertions(+)
>
> diff --git a/libavutil/channel_layout.c b/libavutil/channel_layout.c
> index 98839b7250..2d6963b6df 100644
> --- a/libavutil/channel_layout.c
> +++ b/libavutil/channel_layout.c
> @@ -75,6 +75,10 @@ static const struct channel_name channel_names[] = {
>      [AV_CHAN_BOTTOM_FRONT_CENTER  ] = { "BFC",       "bottom front center"   },
>      [AV_CHAN_BOTTOM_FRONT_LEFT    ] = { "BFL",       "bottom front left"     },
>      [AV_CHAN_BOTTOM_FRONT_RIGHT   ] = { "BFR",       "bottom front right"    },
> +    [AV_CHAN_SIDE_SURROUND_LEFT   ] = { "SSL",       "side surround left"    },
> +    [AV_CHAN_SIDE_SURROUND_RIGHT  ] = { "SSR",       "side surround right"   },

Just as a note, if you want to follow the ISO 23091-3 naming for these
(BS.2051/IEC 62574 calls them just SiL, SiR), then it would be Lss,
Rss ("Left side surround", "Right side surround"). Just a word order
thing. This would then also match the Apple identifier,
`kAudioChannelLabel_LeftSideSurround`.

Also we might want to start adding comments into the enum like "///<
+90 degrees, Lss, SiL" to note the matching ISO 23091-3 and BS.2051
identifiers that this channel is supposed to match?

> +    [AV_CHAN_TOP_SURROUND_LEFT    ] = { "TTL",       "top surround left"     },
> +    [AV_CHAN_TOP_SURROUND_RIGHT   ] = { "TTR",       "top surround right"    },
>  };

So here the mapping seems to be that ISO 23091-3 calls these Lvs and
Rvs (Left, Right vertical surround) and they are mapped against
BS.2051/IEC 62574 TpLS and TpRS - except BS.2051 doesn't seem to
mention these two, it contains as U+110 Ltr and Rtr. At this point it
would be nice if someone actually has access to IEC 62574 :D .

as for the enum comment, maybe "///< +110 degrees, +30 degrees
vertical, Lvs, TpLS" ?

references:

BS.2051 https://www.itu.int/dms_pubrec/itu-r/rec/bs/R-REC-BS.2051-3-202205-I!!PDF-E.pdf
ISO 23091-3: ISO-IECJTC1-SC29_N19613_Text_of_ISOIEC_23091-3CDAmd1_Information_technology__Coding-independent_code_points__Part_3_Audio__AMENDMENT_1_Headphone_support_SC_29WG_06_N_00045.zip
(used to be public on N-documents, but now that site always requires a
login :< )
ISO 23003-3: USAC spec - contains the same listing as in 23091-3.
Some D document from 2021 on setting up audio setups that has a table
that attempts to match their names to various actual specifications
(including IEC 62574 where it mentions TpLS and TpRS):
https://www.audiosciencereview.com/forum/index.php?attachments/dolby-atmos-home-entertainment-studio-technical-guidelines-2021-05-pdf.246631/
MediaInfo's attempt at matching the different specs:
https://mediaarea.net/AudioChannelLayout

Jan
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