[Vorbis] Hz vs bitrate?

[Ian Malone]

James wrote:

> the Vorbis FAQ says:
> "mid to high quality (8kHz-48.0kHz, 16+ bit, polyphonic) audio and 
> music at fixed and variable bitrates from 16 to 128 kbps/channel."
>
> What is the difference between Hz and bitrate?
> Doesn't MP3 support higher bitrates?
>
> Pointers for more reading are welcome.
>
The sampling rate (quoted in Hz) is the rate of sound samples; a sound 
wave is a continuous
signal, but computers can only record points (single numbers), so you 
sample those at
some frequency to get a discrete signal. (In playback through a 
soundcard it gets converted
back to a continuous signal, which is why the 'stepped' signal picture 
that Creative use to
promote 24/192 equipment is a bit misleading)
<http://en.wikipedia.org/wiki/Sampling_rate>
<http://www.saecollege.de/reference_material/pages/Recorders.htm>

In raw sound files (such as wavs) the data rate is directly coupled to 
bit rate, because they
simply record the signal value at each sample: 2channels * 16 bits per 
channel * 44.1kHz
gives 1411.2kbps for CD audio. Lossless codecs like FLAC use compression 
to reduce
the bitrate while retaining the unmodified sample values (so the signal 
is the same as a raw
format records). I have a FLAC file here that averages 857kbps, there is 
an argument to
be made that this represents the amount of information in the recording.

Lossy codecs like Ogg and MP3 attempt to throw away parts of the sound 
that your ears
can't detect (eg, your hearing has very low frequency resolution at high 
frequencies). They
then do straightforward compression on the result. The amount of 
information they retain
determines the bitrate of the final file. I can't find the quote you 
give in the current version
of the FAQ, but a Q10 encode of the same file gives a 468kbps bitrate 
(therefore 234kbps
per channel). I don't know how high mp3 bitrates can go (I think they 
may suffer from
tighter technical restrictions than Ogg/Vorbis - but I seem to remember 
seeing upwards of
200kbps ie 100kbps per channel).

The following are relevant reading for Vorbis as a lossy codec.
<http://www.vorbis.com/faq/#lossy>
<http://en.wikipedia.org/wiki/Vorbis>

Finally, the technical background for most lossy codecs:
<http://en.wikipedia.org/wiki/Modified_discrete_cosine_transform>
(http://www.xiph.org/ also has some detail on the MDCT, probably under 
the documentation
link)
Fraunhoffer page, just don't pay attention when it claims mp3 is CD 
quality at 128kbps:
<http://www.iis.fraunhofer.de/amm/techinf/layer3/>

I also seem to remember discussion (on this list?) about 192kbps in 
Ogg/Vorbis (although I
have Opinions on the usefulness of that).

-- 
imalone ♘