.OGG

Ogg Vorbis audio

Introduction

An .ogg file is usually Vorbis audio inside an Ogg container. Both come from the Xiph.Org Foundation and are open and royalty-free. Vorbis is a lossy codec in the same class as MP3 and AAC, and it is common in games and on Linux.

Ogg is a general container, specified in RFC 3533, so an .ogg file can also hold Opus, FLAC or Speex audio, or Theora video. The extension .oga is recommended for Ogg audio, and Opus files usually use .opus.

Software

Structure

An Ogg file is a sequence of pages. Each page starts with the capture pattern OggS, followed by a version byte, header flags (continued packet, first page, last page), a 64-bit granule position, the stream’s serial number, a page sequence number, a CRC-32 checksum and a segment table. The segment table gives the lengths of the pieces that make up the page’s packets.

A Vorbis stream begins with three header packets: identification (channels and sample rate), comment (tags such as TITLE and ARTIST) and setup (the codebooks). The audio packets follow. For Vorbis, the granule position counts sample frames, which lets players seek.

  1. 0OggS page identification headervaries
  2. …OggS pages comment and setup headersvaries
  3. …OggS pages audio packets, last page flaggedrepeated
    1. +0Page header OggS, flags, granule, serial, sequence, CRC27 bytes
    2. +27Segment table one length byte per segmentvaries
    3. …Segments the packet datavaries
An Ogg Vorbis file as a sequence of pages.