Fwd: Re: [AVT] Re: New I-Ds for Ogg technologies (Vorbis over RTP, Ogg file format, Mimetypes)

["Michael Smith" <msmith@labyrinth.net.au>]

Oops, resending - hit 'reply' instead of 'reply to all'...

Allison Mankin <mankin@psg.com> said:

> Silvia, AVT WG, Chairs,
> 
> We IESG passed draft-walleij-ogg-mediatype-08.txt as a Proposed
> Standard today.  It had had a four week Last Call some time back and
> was not changed, other than now having the internet-draft available to
> document the fileformat.  Congratulations!
> 
> The accompanying i-d, draft-pfeiffer-ogg-fileformat, was on the agenda
> as well and Harald Alvestrand raised a technical question that needs
> to be resolved before we can progress it as Informational.  Harald's
> written question:
> 
>   Technical omission (I think): The document says that the BOS page
>   "contains information to identify the codec type and any additional
>   information to set up the decoding process.  The format of that page
>   is therefore dependent on the codec and therefore MUST be given in
>   the encoding specification of that logical bitstream type."

I'd suggest a rewording of "and any information ..." to "and MAY contain
information to set up the decoding process". Vorbis, for example, contains
some but not all of the setup data in this header (BOS page) - most of the
setup information is in the 3rd header packet (the codebook packet). This
makes it clearer that Ogg doesn't require ALL setup info in the BOS page.

Also, the format does not mandate the following, but I think perhaps we should
considering strongly suggesting it (it's done by all existing Ogg-encapsulated
codecs, and is very useful generally): (something like this) "The BOS page
SHOULD also contain information about the type of the output (decompressed)
media - for example, for audio, it should contain sample rate and number of
channels" (that needs a bit of editorial attention, but I think it'd be a
worthwhile addition).

> 
>   It's pretty clear that OGG players can identify which codec is being
>   used. But this document does not say which bytes of the BOS page can
>   be used to identify the codec. (first 4 bytes? first 16 bytes? up to
>   the first NUL?)
> 

Again, based on vorbis as an example (for the spec doesn't explicitly state
this), any leading part of the data (i.e. packet contents, so these are not
the absolute leading bytes in the stream, since there is the page header
first) in this first page. Vorbis uses the first 7 bytes (0x01, 0x76, 0x6f,
0x72, 0x62, 0x69, 0x73 -- which is a leading 0x01 followed by the ascii values
for "vorbis"), but the intent is to not have a single fixed-size identifier.
We should, however, specify a _maximum_ length for this).

Mike

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