OggSquish Vorbis encoding format documentation
Last update to this document: July 15, 1999
Last update to Vorbis documentation: July 19, 1999
As of writing, not all the below document
links are live. They will be populated as we complete the
Ogg Vorbis is a general purpose compressed audio format
for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music
at moderate fixed and variable bitrates (40-80 kb/s/channel). This
places Vorbis in the same class as audio representations including
MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.
Vorbis is the first of a planned family of OggSquish multimedia coding
formats being developed as part of Xiphophorus's OggSquish multimedia
project. See http://www.xiph.org/
for more information.
Vorbis technical documents
A Vorbis encoder takes in overlapping (but contiguous) short-time
segments of audio data. The encoder analyzes the content of the audio
to determine an optimal compact representation; this phase of encoding
is known as analysis. For each short-time block of sound,
the encoder then packs an efficient representation of the signal, as
determined by analysis, into a raw packet much smaller than the size
required by the original signal; this phase is coding.
Lastly, in a streaming environment, the raw packets are then
structured into a continuous stream of octets; this last phase is
streaming. Note that the stream of octets is referred to both
as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the
stream of octets is a physical representation of a true logical
A Vorbis decoder performs a mirror image process of extracting the
original sequence of raw packets from an Ogg stream (stream
decomposition), reconstructing the signal representation from the
raw data in the packet (decoding) and them reconstituting an
audio signal from the decoded representation (synthesis).
The Programming with libvorbis
documents discuss use of the reference Vorbis codec library
(libvorbis) produced by Xiphophorus.
The data representations and algorithms necessary at each step to
encode and decode Ogg Vorbis bitstreams are described by the below
documents in sufficient detail to construct a complete Vorbis codec.
Note that at the time of writing, Vorbis is still in a 'Request For
Comments' stage of development; despite being in advanced stages of
development, input from the multimedia community is welcome.
Vorbis analysis and synthesis
Analysis begins by seperating an input audio stream into individual,
overlapping short-time segments of audio data. These segments are
then transformed into an alternate representation, seeking to
represent the original signal in a more efficient form that codes into
a smaller number of bytes. The analysis and transformation stage is
the most complex element of producing a Vorbis bitstream.
The corresponding synthesis step in the decoder is simpler; there is
no analysis to perform, merely a mechanical, deterministic
reconstruction of the original audio data from the transform-domain
- Vorbis packet structure: Describes the basic analysis components necessary to produce Vorbis packets and the structure of the packet itself.
- Temporal envelope shaping and blocksize: Use of temporal envelope shaping and variable blocksize to minimize time-domain energy leakage during wide dynamic range and spectral energy swings. Also discusses time-related principles of psychoacoustics.
- Time domain segmentation and MDCT transform: Division of time domain data into individual overlapped, windowed short-time vectors and transformation using the MDCT
- The resolution floor: Use of frequency doamin psychoacoustics, and the MDCT-domain noise, masking and resolution floors
- MDCT-domain fine structure: Production, quantization and massaging of MDCT-spectrum fine structure
Vorbis coding and decoding
Coding and decoding converts the transform-domain representation of
the original audio produced by analysis to and from a bitwise packed
raw data packet. Coding and decoding consist of two logically
orthogonal concepts, back-end coding and bitpacking.
Back-end coding uses a probability model to represent the raw numbers
of the audio representation in as few physical bits as possible;
familiar examples of back-end coding include Huffman coding and Vector
Bitpacking arranges the variable sized words of the back-end
coding into a vector of octets without wasting space. The octets
produced by coding a single short-time audio segment is one raw Vorbis
Vorbis streaming and stream decomposition
Vorbis packets contain the raw, bitwise-compressed representation of a
snippet of audio. These packets contain no structure and cannot be
strung together directly into a stream; for streamed transmission and
storage, Vorbis packets are encoded into an Ogg bitstream.
- OggSquish bitstream overview: High-level
description of OggSquish logical bitstreams, how logical bitstreams
(of mixed media types) can be combined into physical bitstreams, and
restrictions on logical-to-physical mapping. Note that this document is
not specific only to Ogg Vorbis.
- OggSquish logical bitstream and framing
spec: Low level, complete specification of OggSquish logical
bitstream pages. Note that this document is not specific only to Ogg
- Vorbis bitstream mapping:
Specifically describes mapping Vorbis data into an
OggSquish physical bitstream.
OggSquish is a Xiphophorus effort to
protect essential tenets of Internet multimedia from corporate
hostage-taking; Open Source is the net's greatest tool to keep
everyone honest. See About
Xiphophorus for details.
Ogg Vorbis is the first OggSquish audio CODEC. Anyone may
freely use and distribute the OggSquish and Vorbis specification,
whether in a private, public or corporate capacity. However,
Xiphophorus and the Ogg project (xiph.org) reserve the right to set
the Ogg/Vorbis specification and certify specification compliance.
Xiphophorus's Vorbis software CODEC implementation (libvorbis and the
vorbis encode/decode/playback utility) are distributed under the GNU
Public License. This does not restrict third parties from
distributing independent implementations of Vorbis software under
OggSquish, Vorbis, Xiphophorus and their logos are trademarks (tm) of
Xiphophorus. These pages are
copyright (C) 1994-1999 Xiphophorus. All rights reserved.