[Vorbis] Sample rates higher than 48k? 24-bit depth?

[Paul Ellis]

Giuliano Pochini wrote:

> I don't know if libvorbis is tuned for 96KHz, but why do you
> want to use a lossy codec for high quality data ?  Encoding
> will throw away higher frequencies anyway, so you can down-
> sample it to 44K before encoding without losing anything.

I ran some test encodes.  All 24-bit 96kHz input.  Encoded with
OggDropXPd built from libVorbis 1.1.

At Q4 Vorbis does cutoff some after 20kHz, however there is still
information about 20kHz.
At Q8 it notches out 20kHz-30kHz for the most part, but above 30kHz
seems unaffected.
At Q10 SoundForge shows that it is full spectrum from 20-48000Hz.
However Q10's nominal bitrate of 500kbit/sec was exceed by 50%.  The
file was 740kbit/sec.

Some interesting findings.  At Q4 the 96kHz file was only 200KB larger
than the downsampled 44kHz file.  Also, even though 740kbit/sec seems
like a lot, the source file was 235MB and the Vorbis file was only
28MB.  About 8.5:1 compression.  Granted it is lossy, but I'd bet a
96kHz Q10 Vorbis file sounds better than a 44kHz wav of the same source
which would be more than 4 times larger.

Does anyone know how the input bit-depth really affects Vorbis?  I know
internal it is all like 48-bit floating point or something to that
effect (Monty mentioned it earlier in this thread).  Would you really
attain a higher quality using a 24-bit source instead of a 16-bit source
though?  If there was a quality difference would it only be noticeable
at 24-bit?

Paul