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

[Marton Balint]

On Mon, 27 May 2024, Jan Ekström wrote:

> On Mon, May 27, 2024 at 12:37 AM Lynne via ffmpeg-devel
> <ffmpeg-devel at 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`.

I guess the idea was to be consistent with existing FFMPEG channel word 
order and short names. I kind of prefer that approach, because the other 
channel names will not follow ISO 23091 anyway, so there is not much 
benefit in being consistent with ISO 23091 for these 4 newly added 
channels only, at the cost of not being consistent with existing ffmpeg 
way... So I think the patch is fine as is.

Regards,
Marton


>
> 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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