This is a brief and informal document targeted to those who want to deal with the MPEG format. If you are one of them, you probably already know what is MPEG audio. If not, jump to http://www.mp3.com/ or http://www.layer3.org/ where you will find more details and also more links.

NOTE: You cannot just search the Internet and find the MPEG audio specs. It is copyrighted and you will have to pay quite a bit to get the Paper. That's why I made this. Informations I got are gathered from the internet, and mostly originate from sources I found available. Despite my custom to always specify the sources, I am not able to do it this time. Sorry, I did not maintain the list. :(

This is not decoding specs, it just informs you how to read the MPEG headers and the MPEG TAG. MPEG Version 1, 2 and 2.5 and Layer I, II and III are supported, the MP3 TAG (MP3v1 and MP3v1.1) also.. Those of you who use Delphi may find my MPGTools Delphi unit useful, it is where I implemented this stuff.

MPEG Audio Frame Header

An MPEG audio file is separated in smaller parts called frames. Each frame is independent. It has its own header and audio informations. There is no file header. Therefore, you can cut any part of MPEG file and play it correctly.

When you want to read info about an MPEG file, it is usually enough to find the first frame, read its header and assume that the other frames are the same (which may not be always the case).

The frame header is constituated by the very first four bytes (32bits) in a frame. The first eleven bits of a frame header are always set and they are called "frame sync". Therefore, you can search through the file for the first occurence of eleven bits set (meaning that you have to find a byte with a value of 255, and followed by a byte with its three most significant bits set). Then you read the whole header and check if the values are correct. You will see in next table the exact meaning of each bits in the header, and which values may be checked for validity. Each value that is specified as reserved, invalid, bad, or not allowed should indicate an invalid header.

Frames may have a CRC check, but it's pretty rare. The CRC is 16 bits long and, if it exists, it follows the frame header. After the CRC comes the audio data. You may calculate the length of the frame and use it if you need to read other headers too or just want to calculate the CRC of the frame, to compare it with the one you read from the file. This is actually a very good method to check the MPEG header validity.

Here is "graphical" presentation of the header content. The letters are used to indicate the different fields. In the table, you can see the details about the content of each field.

AAAAAAAA AAABBCCD EEEEFFGH IIJJKLMM

MPEG Audio Tag MP3v1

The TAG is used to describe the MPEG Audio file. It contains information about artist, title, album, publishing year and genre. There is some extra space for comments. It is exactly 128 bytes long and is located at very end of the audio data. You can get it by reading the last 128 bytes of the MPEG audio file.

AAABBBBB BBBBBBBB BBBBBBBB BBBBBBBB
BCCCCCCC CCCCCCCC CCCCCCCC CCCCCCCD
DDDDDDDD DDDDDDDD DDDDDDDD DDDDDEEE
EFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFG

MPEG Audio Tag MP3v2

This is new proposed TAG format which is different than MP3v1 and MP3v1.1. Complete tech specs for it may be found at http://www.id3.com/.