time length of an mp3 file

Question

What is the simplest way to determine the length (in seconds) of a given mp3 file, without using outside libraries? (python source highly appreciated)

Answer 1

You might count the number of frames in the file. Each frame has a start code, although I can't recollect the exact value of the start code and I don't have MPEG specs laying around. Each frame has a certain length, around 40ms for MPEG1 layer II.

This method works for CBR-files (Constant Bit Rate), how VBR-files work is a completely different story.

From the document below:

For Layer I files us this formula:

FrameLengthInBytes = (12 * BitRate / SampleRate + Padding) * 4

For Layer II & III files use this formula:

FrameLengthInBytes = 144 * BitRate / SampleRate + Padding

Information about MPEG Audio Frame Header