[Vorbis] Hz vs bitrate?

[Graham Mitchell]

> 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?

The kHz measurement refers to the characteristics of the original audio being 
fed to the encoder, and similarly to the decompressed audio output by the 
decoded.

The bitrate refers to the average number of bits per second used to encode the 
compressed data itself.

An average case would be 44.1 kHz, 16-bit stereo input, which is literally 
1411.2 kbps.  A typical bitrate for compressed audio is 128 kbps, which is 
right about elevenfold compression.  Then the 128 kbps Vorbis file is 
uncompressed during playback to a 44.1 kHz, 16-bit stereo signal.

Put differently, the uncompressed input is 1411.2 kbps, aka 44.1 kHz, 16-bit 
stereo.  The compressed file uses 128k bits per second to store the 'same' 
audio.  And that file decompresses to 1411.2 kbps again.

Vorbis supports inputs as low as 8 kHz, 16-bit mono, and as high as 48.0 kHz, 
24-bit (I think) 5.1.

> Doesn't MP3 support higher bitrates?

Not really.  Note that the bitrates listed are *per channel*, so 128 kbps per 
channel for a stereo file will result in a 256 kbps total bitrate.  And I 
just encoded a random WAV file into Vorbis at quality 10 (the max), and the 
resulting file averaged 470 kbps (235 kbps per channel), so I think that 
maybe the FAQ is in error there.  Or maybe I'm doing something wrong.

Note that quality 10 is definitely overkill; I haven't encountered ANYONE who 
could successfully ABX Vorbis audio at bitrates higher than 256 kbps.

> Pointers for more reading are welcome.

I always love selfishly pointing people to my "Introduction to Compressed 
Audio with Ogg Vorbis", which is a bit dated but still a good intro to the 
major concepts involved:
	http://grahammitchell.com/writings/vorbis_intro.html

-- 
Graham Mitchell - computer science teacher, Leander High School
"Come thou no more for ransom, gentle herald.  They shall have none, I
swear, but these my joints, which if they have as I will leave 'em them,
shall yield them little."                             - Henry the Fifth