MP3 Encoding and File Format

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∟MP3, MPEG-1 Audio Layer 3, Encoding Format

∟MP3 Encoding and File Format

This section describes MP3 encoding and file format for audio recording.

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              Unknown
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.