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About Vorbis bitrate modes
post Dec 18 2001, 19:03
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One day I started thinking about OGG Vorbis bitrates considering all contradictory sources I've heard about this matter. Because desired bitrate in Vorbis is specified with a -b [bitrate] switch most people mostly familiar with MP3 think it's an ABR setting and keep telling everyone how Vorbis needs VBR modes. However, ones proficient with Vorbis say that these particular settings only trigger a certain constraint-free VBR mode which "happens" (according to different psymodels etc.) to produce bitrates somewhere near the desired bitrate.

And there's one more statement from Dibrom that intrigues me:

[referring to Vorbis] And as for -b256, you can't compare that to MPC standard because you are forcing a higher bitrate in Vorbis on purpose.

So with -b [bitrate] am I really forcing Vorbis to certain bitrate? That would imply the usage of some kind of ABR system instead of psymodel-based VBR.

I tested this "forcing" thing with two extreme scenarios; a sine wave and white noise encoded with -b 256 setting in order to evaluate Vorbis' flexibility:

f:temp>oggenc -b 256 440HzSine.wav
Opening with wav module: WAV file reader
Encoding "440HzSine.wav" to
"440HzSine.ogg" at bitrate 256 kbps
[ 98,0%] [ 0m00s remaining] /

Done encoding file "440HzSine.ogg"

File length: 0m 10,0s
Elapsed time: 0m 04,0s
Rate: 2,5000
Average bitrate: 24,8 kb/s

f:temp>oggenc -b 256 WhiteNoise.wav
Opening with wav module: WAV file reader
Encoding "WhiteNoise.wav" to
"WhiteNoise.ogg" at bitrate 256 kbps
[ 98,0%] [ 0m00s remaining] /

Done encoding file "WhiteNoise.ogg"

File length: 0m 10,0s
Elapsed time: 0m 05,0s
Rate: 2,0000
Average bitrate: 389,0 kb/s

So, according to this quick and dirty test, Vorbis seems to flex extremely well based on the complexity of signal implying a psymodel-based VBR is indeed used. I also tried inserting sines and white noise in a music sample and detected similar behaviour (although analyzed only with WinAmp's bitrate view). So, if Vorbis would be based on a constrained ABR mode we would see much higher bitrate (~256) in the sine wave sample and much lower bitrate in the white noise sample.

Although I'm not completely certain about switches "forcing" Vorbis to certain bitrate I would dare to say according to experience that they won't. I wish someone more proficient would enlighten me since this issue is widely discussed and and even more widely misunderstood. And I also would like to know about theoretical minimum/maximum bitrates attainable with Vorbis?

Well, the bright side of this consideration can be seen as a newsflash for people wishing true quality-based VBR modes for Vorbis; Think -b 350 as -insane, -b 256 as -xtreme and -b 192 as -standard and there! Quality-based VBR in Vorbis biggrin.gif

As a philosophical standpoint I can't tell which is better, adjective or number. MPC's switches could easily be renamed to -b 175, -b 200 and -b 300 because MPC is particulary stable when it comes to bitrates. But then we would get people wishing "a true quality based VBR" for MPC biggrin.gif


The encoding test wasn't used to measure quality, it was only a demonstration of Vorbis' flexibility. And: I also wished a quality based true VBR for Vorbis prior to this quick experiment.
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