(no subject)

[Jonathan Ingram]

(this is an edited version of a post on http://www.hydrogenaudio.org - I
thought some people might be interested that dodn't read that board)

Previously I created some graphs which show how average bitrate changes as you
vary quality, for various standard test samples. You can find these graphs at

http://www.maths.warwick.ac.uk/~jingram/ogg/

(there's a readme there that explains in more detail).

This time I thought I'd look at how bitrate changes over time, within a Vorbis
file. 
I've written a little tool which takes the output from ogg123, hits it with a
rock, and generates nice graphs of bitrate-vs-time for Vorbis audio files. It's
nothing complicated, but it's possible that some of you may like to play with
it.
 
I've generated two example graphs - they both involve Vorbis encodes of
'Radiohead - Everything In Its Right Place' from the album 'Kid A'. Both were
generated with a datapoint smoothing value of 10 (read the code if you want to
know what his means :)

1)
http://www.maths.warwick.ac.uk/~jingram/ogg/bitrate-time/qualityvsmanaged.png

This demonstrates quite well the differences between quality mode and managed
bitrate mode. The track was encoded three times:
 1) with -q 5, which averaged 181kpbs
 2) with -b 160, which averaged 160kpbs
 3) with -b 160 -M 160, which averaged 152kpbs.
 As you can see, -M enforces the bitrate limit very strictly, -b less so.

2)
http://www.maths.warwick.ac.uk/~jingram/ogg/bitrate-time/varyingquality.png
This shows how bitrate increases throughout the track as you increase the
quality setting.
 
As with my previous graphs, there are postscript versions, which will be of
higher quality for those of you on OSes that understand them . These, along
with the Python script I used to generate the graphs, can be found in the
obvious place:
http://www.maths.warwick.ac.uk/~jingram/ogg/bitrate-time/
 

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