About Encoded Audio; Bitrates; Variable Bitrate - Nokia N91 User Manual

quality audio at bitrates as low as 48 kb/s. Classical music
and music with nuances usually require a higher bitrate
than basic pop music.
For speech or other sources where the quality of the sound
is less important than saving storage space, you can use
bitrates between 8 and 64 kb/s. At low bitrates WMA
or AAC generally gives better results than MP3.
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Variable bitrate

Your device also supports variable bitrate (VBR). Variable
bitrate means that the bit rate of the encoding varies with
the complexity of the encoded material. When using VBR,
the encoding is optimized to preserve a constant audio
quality throughout the track, rather than losing quality
for complex music sections as with constant bitrate
(CBR) encoding.