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

[Marton Balint]

On Wed, 29 May 2024, Jan Ekström wrote:

> On Mon, May 27, 2024 at 10:31 PM Marton Balint <cus at passwd.hu> wrote:
>>
>>
>>
>> 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.
>>
>
> If we explicitly go for a non-standardized names then it probably is
> an even better idea to add "///< " comments marking what we actually
> based these new enum entries on with regards to matching ISO 23091 and
> IEC 62574 / BS.2051 identifiers (even if we leave out the approximate
> azimuth and vertical degrees out of it, as that is a whole separate
> mess).
>
> I am planning on attempting to add such for at least some of the enum
> values we have for the existing ones, but for these new ones we should
> have these from the get-go.

Sure, OK, I have no problem with adding comments about the origins of the 
channel. And we should ask MediaArea to add a new column to their table, 
the FFmpeg channel names :)

Thanks,
Marton

>
> Jan
>
>> 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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