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

["Jan Ekström" <jeebjp@gmail.com>]

On Mon, May 27, 2024 at 10:31 PM Marton Balint <cus@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@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.

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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