Re: [Cellar] Last call on FLAC specification

Martin Leese <martin.leese@stanfordalumni.org> Fri, 30 September 2022 16:24 UTC

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From: Martin Leese <martin.leese@stanfordalumni.org>
Date: Fri, 30 Sep 2022 10:24:40 -0600
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Subject: Re: [Cellar] Last call on FLAC specification
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On 9/30/22, Martijn van Beurden <mvanb1@gmail.com> wrote:

> The RF64 specification actually defines a lot more channel mask bits than
> just SPEAKER_ALL. It does also define downmixes, control samples and
> bitstreams.

I am confused.  (This is not unusual.)  I am
looking in Recommendation ITU-R BS.2088-1
(10/2019), and don't see any mention of
either SPEAKER_ALL or downmixes.

> The document says: Another “#define”, “SPEAKER_ALL” turns on
> all
> loudspeakers (channels). In other words, a channel that must be played on
> all speakers.

Then I agree SPEAKER_ALL would be a poor
addition.

> I agree with adding 0x0, but I'm not sure these RF64 defines would be a
> good addition. I'd rather stick to what is defined in WAVEFORMATEXTENSIBLE.
>
> Furthermore, the Microsoft specification allows for a channel mask having
> more bits set than channels (MSBs must be ignored) and less bits than
> channels (in which case last channel(s) are to be treated like they do not
> belong to a speaker and ignored). I don't like the idea of allowing an
> overdefined channel mask, and I'm also not sure whether it is a good idea
> to encourage playback applications to just ignore certain channels.
>
> Any thoughts?

There are situations where encouraging
playback applications to just ignore certain
channels is appropriate.  An example is three-
and four-channel Ambisonic UHJ files where
the first two channels are stereo compatible.
(The extra channels allow decoding to
surround.)  Replay in stereo needs to ignore
channels 3 and 4.  However, I believe this is
better handled by specifying a downmix
including channel weights of 0.0.

Specifying a downmix could be handled in a
structured VorbisComment or a FLAC
METADATA_BLOCK_APPLICATION, so maybe
it doen't need to be in the FLAC spec.  But it
should be somewhere.

Here is my 2011 proposal for a downmix chunk
in WAVE-EX files:
https://members.tripod.com/martin_leese/Audio/stereo_mix_proposal.html
https://members.tripod.com/martin_leese/Audio/StereoMix_chunk.html

Regards,
Martin
-- 
Martin J Leese
E-mail: martin.leese  stanfordalumni.org
Web: http://members.tripod.com/martin_leese/

[Cellar] Last call on FLAC specification Martin Leese
Re: [Cellar] Last call on FLAC specification Martijn van Beurden
Re: [Cellar] Last call on FLAC specification Martin Leese
Re: [Cellar] Last call on FLAC specification Martijn van Beurden
Re: [Cellar] Last call on FLAC specification Martin Leese
Re: [Cellar] Last call on FLAC specification Martijn van Beurden
Re: [Cellar] Last call on FLAC specification Martin Leese