MPEG 1 Layer 3 File Structure

Table of Contents

1 - General information

2 - Bit layout

3 - Bitrate index

4 - Sampling frequency index

5 - Mode and Mode Extension

6 - Disclaimer

Please note: Some of the following informations have been derived from a sample c file available from the Fraunhofer Gesellschaft. But most informations have been assembled with the "try & error-method", which means that they seem to be correct, but I cannot guarantee for it.

There are three types of MPEG 1 Layer 3 files: MPEG 1.0 which is used for high sampling frequencies and bit rates, MPEG 2.0 which is for lower ones and MPEG 2.5. This is a propriatory standard of Fraunhofer Gesellschaft and is used only for very low frequencies and bit rates, so you will rarely encounter a file of this type.

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All available information on an mp3 file is stored in its first 4 bytes. The rest of the file contains the actual sound information and is of no interest unless you are planning to write a decoder (then you should know the file format much better than I do).

Bits	Meaning
1-11	always 1
12-13	mp3 version: 00 = MPEG 2.5 (proprietary to Fraunhofer Gesellschaft), 10 = MPEG 2, 11 = MPEG 1 (important for the next infos)
14-15	Mpeg layer (the actual layer is calculated by substracting the number from 4, e.g. 01 means layer 3)
16	CRC: 0 = CRC checksum, 1 = no CRC checksum
17-20	Bitrate index (see below)
21-22	sampling frequency index (see below)
23	Padding (???)
24	Extension (reserved but unused)
25-28	Mode + mode extension (see below)
29	Copyright
30	Original

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Bitrate index

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Sampling frequency index

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Mode and Mode Extension

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Disclaimer

If you find any errors or if you can fill any gaps, please tell me.

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Last change: 98/02/01