MP3 - VBR - Frame length in time

[Ivo Woltring]

Dear Pythoneers,

I have this problem with the time calculations of an VBR (variable bit rate)
encoded MP3.
I want to make a daisy writer for blind people. To do this I have to know
exactly what the length in time of an mp3 is. With CBR encoded files I have
no real problems (at least with version 1 and 2), but with VBR encoded I get
into trouble.
I noticed that players like WinAMP and Windows Media player have this
problem too.
I don't mind to have to read the whole mp3 to calculate, because performance
is not a real issue with my app.

Can anyone help me? I am really interested in technical information on VBR
and MP3.
Can anybody tell me the length in time of the different bitrates in all the
versions of mp3 and all the layers.

Tooling or links also really welcome. My own googling has helped me some but
on the subject of VBR I get stuck.

Thanks a lot,
Ivo.