VBR can do one of two things, depending on the type of music/sound you are compressing. You can get a smaller file with the same music quality, or better music quality with the same or similar filesize.

If the Diamond Rio does not correctly display the time for VBR files then its implementation of VBR is not correct.

I compress all of my files into VBR, it just makes more sense to lower the bitrate for portions of the music that contains less information, and to increase it for those portions that contains more.

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Andre Miller