[Matroska-devel] Opus audio codec

Ralph Giles giles at thaumas.net
Thu Dec 15 01:24:55 CET 2011


Hey Moritz. Thanks for taking a look. Some comments:

On 14 December 2011 13:11, Moritz Bunkus <moritz at bunkus.org> wrote:

>                      The ID header
> they describe looks nearly fine to me: nothing that's too Ogg-centric
> apart from the "OpusHead" string.

I agree this is completely unnecessary in Matroska, but if we go this
route I think we might as well keep it. The extra eight bytes aren't
significant overhead and it's easier if the two encapsulations use
exactly the same format. I mean, the number of channels and sample
rate fields aren't necessary here either.

> So what I'd propose would be your solution 2 (no new Matroska channel
> mapping element only for this codec) with a slight modification.

The Ogg encapsulation also has a second vorbis-style comment header
with tag data. These are also redundant of course, but including both
headers exactly as they are in Ogg would match what's done for vorbis,
theora, and flac.

> Then we only have to define the channel mapping family 1 somewhere
> which should match Vorbis' channel mapping (I just don't like the
> Matroska specs referring to the Vorbis specs for something like this).

That's fine. For the record, the channel ordering for mapping family 1 is:

one channel: mono
two channels: (stereo) left, right
three channels: left, centre, right
four channels: front left, front right, rear left, rear right
five channels: front left, centre, front right, rear left, rear right
six channels: (5.1) front left, centre, front right, rear left, rear right, LFE
seven channels: front left, centre, front right, side left, side
right, rear center, LFE
eight channels: (7.1) front left, centre, front right, side left, side
right, rear left, rear right, LFE

More than eight channels doesn't have a defined ordering and should be
treated like channel mapping 255.

 -r



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