MP3 Encoding and File Format
What is MP3? MP3 is a short name for MPEG-1 Audio Layer 3, which is a very popular encoding format for audio recording. MP3 was invented by a team of European engineers at Philips, CCETT, IRT and Fraunhofer Society in 1991.
MP3 supports several bit rates: 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 and 320 kbit/s. But 128 kbit/s is most commonly used since it typically offers very good audio quality in a relatively small space. By contrast, uncompressed audio as stored on an audio CD (Compact Disc Digital Audio or CD-DA) has a bit rate of 1,411.2 kbit/s (16 bits/sample * 44100 samples/second * 2 channels / 1000 bits/kilobit).
A MP3 file name has the file extension of .mp3.
A MP3 file contains many MP3 frames with a MP3 header and a MP3 data in each frame:
MP3 Header | MP3 Frame MP3 Data | MP3 Header | MP3 Frame MP3 Data | MP3 Header | MP3 Frame MP3 Data | ...
A MP3 header has the following structure:
Bits Meaning Value Note
1-12 Sync word 111111111111 13 Version 1 1 = MPEG-1 14-15 Layer 01 01 = Layer 3 16 Error protection 1 1 = No 17-20 Bit rate 1000 1000 = 128 kbps 21-22 Frequency 00 00 = 44100 Hz 23 Pad bit 0 0 = Frame is no padded 24 Priv. bit 0 Unkown 25-26 Mode 01 Joint stereo 27-28 Mode extension 00 00 = Intensity and MS off 29 Copyright 0 0 = Not copyrighted 30 Original 0 0 = Not original 31-32 Emphasis 00 00 = None
The original MP3 file format does not allow any places to store information about the MP3 file. So a new standard called ID3v1 was developed in 1996. ID3v1 adds a 128-byte tag space to the end of a MP3 file. The tag space starts with the string TAG, and contains information about the MP3 file like, title, artist, album, comment, etc.
In 1998, ID3v2 was created to add tags at the start of MP3 files. ID3V2 tags uses frames to store pieces of metadata.
