Re: [Cellar] Last call on FLAC specification

[Martijn van Beurden <[e-mail]>]

Op do 29 sep. 2022 om 17:42 schreef Martin Leese <
[e-mail]>:
>
> 1.  Section 9.6.2, Page 22, the line:
>     WAVEFORMATEXTENSIBLE_CHANNEL_MASK=0x12004.
>
>     should read:
>     WAVEFORMATEXTENSIBLE_CHANNEL_MASK=0x12104.

Thanks!

>
> 2.  Section 9.6.2, Table 7, Page 20.  FLAC files do not
>     necessarily contain speaker feeds.  Examples of such
>     files are a collection of audio stems or an Ambisonic
>     B-Format recording.  Could we therefore please include
>     a Channel description of SPEAKER_NONE with:
>     WAVEFORMATEXTENSIBLE_CHANNEL_MASK=0x0
>
>     Given the way Table 7 is organised, It is not immediately
>     clear to me how to include this.


I don't know what the FLAC reference implementation or ffmpeg do in this
regard, but the channel mask is copied directly from Microsoft's
WAVEFORMATEXTENSIBLE structure. See here:
https://learn.microsoft.com/en-us/previous-versions/windows/hardware/design/dn653308(v=vs.85)

It says

> If, for example in a multi-channel audio authoring application,
> no speaker location is desired on any of the mono streams, the
> dwChannelMask should explicitly be set to 0. A dwChannelMask of
> 0 tells the audio device to render the first channel to the
> first port on the device, the second channel to the second port
> on the device, and so on. This also means that if the device
> doesn't know how to process the raw audio streams, it should
> not accept the multi-channel stream with a dwChannelMask of
> 0. A device like a digital mixer or a digital audio storage
> device (hard disk, ADAT, and so on) might want to accept
> formats without particular speaker locations.

Perhaps this and other comments about the channel mask should be added to
the FLAC specification, rephrased.

Seeing how much ambiguity was in the original RIFF/WAV specification, it is
nice to see these details are very well defined actually.