Hi, I've been using the Vorbis codec for a while and recently I got the
source code and had a look at the spec (great job on the doc BTW). I don't
have any former experience with audio compression, but I do know what a DCT does
and I totally grok Huffman coding (so I guess I'm not that much of a newbie,
am I?).
Anyway I just had some questions about how VQ works on the encoding side. (I
realize the spec doesn't say anything about the process of encoding, I'm
just asking about how the reference encoder (the only encoder?) works.) My
understanding of vector quantization is that vectors of length n can be
interpreted as points in n-dimensional space, and to quantize them you carefully
choose a set of code vectors to minimize the average distance from each code
vector to each vector in the group of input vectors it approximates. Then you
make a codebook of all the code vectors and encode each input vector by the
closest codeword to it.
I had thought this all occurred at encoding time, and the code vectors would be
specifically chosen for each audio file, but after looking closer at the source
it seems like there are static codebooks that are set at compile time. This
confuses me because I thought one of the strengths of Vorbis was its
flexibility, how it wasn't confined to one probability model or whatnot.
Also, if the codebooks are set at compile-time then why are they in the header
of every Ogg Vorbis file? Is it just to ensure compatibility with future
encoders that use different VQ techniques?
Also I have no idea what most of the code in the /vq subdirectory does, and I am
confused about these mysterious ".vqh" and ".vqd" files they
talk about.
The reason I ask about VQ encoding (besides the simple reason that I don't
understand it yet) is that I think the obvious way to implement the long-awaited
feature of bitrate peeling is to reencode the vectors using a smaller and
coarser set of codewords. Is this just stupid or am I on to something here?
P.S. If this email appears as a few annoyingly long lines, please tell me and I
can have my mail client insert newlines.
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