[vorbis-dev] clarifications on comments spec

[Segher Boessenkool]

Ralph Giles wrote:
> This is meant to identify the 'encoder' so it's a little ambiguous when 
> you're rewriting the file. In as much as this is metadata describing the 
> compressed content itself, I'd leave it alone if you're just editing the 
> tags.

In my peelers etc., which _are_ changing the content, I
leave it alone as well; I add "normal" comments instead.
Much more readable, if you run multiple tools on a stream :-)

>> *) There seems to be no provision for "padding" (to use the ID3v2 word).
>> Since the comment is very near to the beginning of a Vorbis stream, 
>> assuming
>> a file based implementation, the overwhelming majority of the file's 
>> contents
>> will have to be moved when the comment size changes.  Is this correct?
> 
> That's correct.

You can pad the comment packet, with no ill effects.  But you
are not required to pad it.  Or does the spec prohibit padding
altogether?  I would hope not, if only for upward compatibility
issues.

> There is one hitch in that the various codecs have different preambles 
> before the common decode. Vorbis starts with 0x03,'vorbis'; Theora 
> begins 0x81,'theora'; Speex has no preamble at all and begins the packet 
> directly with the vendor length. You'll have to handle this variation 
> somehow.

Aren't all Speex header packets identifiable as such?  That's bad.

<p>Segher

<p>--- >8 ----
List archives:  http://www.xiph.org/archives/
Ogg project homepage: http://www.xiph.org/ogg/
To unsubscribe from this list, send a message to 'vorbis-dev-request at xiph.org'
containing only the word 'unsubscribe' in the body.  No subject is needed.
Unsubscribe messages sent to the list will be ignored/filtered.