.MP3

MPEG-1 Audio Layer III

Introduction

MP3 is the lossy audio coding of the MPEG-1 standard (ISO/IEC 11172-3), later extended to lower sample rates by MPEG-2. Much of the work on it was done at the Fraunhofer Institute in Germany. It shrinks audio to a small fraction of its uncompressed size by removing sound the ear is unlikely to notice.

MP3 became the format of digital music downloads and portable players, and nearly every device and program that plays audio can play it.

Software

Structure

An MP3 file is a run of independent frames, often wrapped in tags. An ID3v2 tag at the start (beginning ID3) holds the title, artist, cover art and other details. An older ID3v1 tag may sit in the last 128 bytes, beginning TAG.

Each frame starts with a 4-byte header: an 11-bit sync word of ones, then the MPEG version, layer, bit rate, sample rate, padding flag and channel mode. The header gives the frame’s length, so a reader can step from frame to frame. An MPEG-1 Layer III frame holds 1152 samples per channel. Variable bit rate files often carry a Xing or VBRI header in the first frame, with the frame count and a seek table.

  1. 0ID3v2 tag optional, title, artist, picturesvaries
  2. …Frames 4-byte header, side information, main datarepeated
  3. endID3v1 tag optional, begins TAG128 bytes
The layout of an MP3 file.