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Subject: Re: Quick VBR calc question
From: Linus Nielsen Feltzing (
Date: 2003-02-13

Scott Bronson wrote:
> How does Rockbox calculate the average bit rate of the MP3
> so quickly? Within 1/2 second of hitting play, what looks
> to be a fairly accurate VBR appears on the WPS.

We use the information in the Xing VBR header, a special MP3 frame
created by the MP3 encoder. That header tells us how many frames are in
the file, then we can calculate the average bit rate.

> My desktop machine either says that it's a 64kbit file (since
> most songs start with silence and it doen't update the display
> after the first frame) or takes two seconds or so to scan over
> the song.
> Where's the magic? :) Thanks,

I guess your desktop player may display the current bit rate rather than
the whole-file average. Or it may not take advantage of the VBR header.

Rockbox will think it's a CBR file if the file lacks a VBR header. It
will not scan the file, since it would take too long.


Page was last modified "Jan 10 2012" The Rockbox Crew