joint encoding
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In
audio engineering Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound * Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum * Digital audio, representation of sou ...
, joint encoding refers to a joining of several channels of similar information during
encoding In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...
in order to obtain higher quality, a smaller file size, or both.


Joint stereo

The term joint stereo has become prominent as the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
has allowed for the transfer of relatively low
bit rate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
, acceptable-quality audio with modest Internet access speeds. Joint stereo refers to any number of encoding techniques used for this purpose. Two forms are described here, both of which are implemented in various ways with different
codec A codec is a device or computer program that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder. In electronic communications, an endec is a device that acts as both an encoder and a decoder on a signal or da ...
s, such as
MP3 MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio developed largely by the Fraunhofer Society in Germany, with support from other digital scientists in the United States and elsewhere. Origin ...
,
AAC AAC may refer to: Aviation * Advanced Aircraft, a company from Carlsbad, California * Alaskan Air Command, a radar network * American Aeronautical Corporation, a company from Port Washington, New York * American Aviation, a company from Cleveland, ...
and
Ogg Vorbis Vorbis is a free and open-source software project headed by the Xiph.Org Foundation. The project produces an audio coding format and software reference encoder/decoder (codec) for lossy audio compression. Vorbis is most commonly used in conjun ...
.


Intensity stereo coding

This form of joint stereo uses a technique known as joint frequency encoding, which functions on the principle of
sound localization Sound localization is a listener's ability to identify the location or origin of a detected sound in direction and distance. The sound localization mechanisms of the mammalian auditory system have been extensively studied. The auditory system u ...
. Human hearing is predominantly less acute at perceiving the direction of certain audio frequencies. By exploiting this characteristic, intensity stereo coding can reduce the data rate of an audio stream with little or no perceived change in apparent quality. More specifically, the dominance of ''inter-aural time differences (ITD)'' for sound localization by humans is only present for lower frequencies. That leaves ''inter-aural amplitude differences (IAD)'' as the dominant location indicator for higher frequencies. The idea of ''intensity stereo coding'' is to merge the lower spectrum into just one channel (thus reducing overall differences between channels) and to transmit a little side information about how to pan certain frequency regions to recover the ''IAD'' cues. This type of coding does not perfectly reconstruct the original audio because of the loss of information which results in the simplification of the stereo image and can produce perceptible
compression artifacts A compression artifact (or artefact) is a noticeable distortion of media (including images, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the media's data so that it beco ...
. However, for very low bit rates this type of coding usually yields a gain in perceived quality of the audio. It is supported by many audio compression formats (including
MP3 MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio developed largely by the Fraunhofer Society in Germany, with support from other digital scientists in the United States and elsewhere. Origin ...
,
AAC AAC may refer to: Aviation * Advanced Aircraft, a company from Carlsbad, California * Alaskan Air Command, a radar network * American Aeronautical Corporation, a company from Port Washington, New York * American Aviation, a company from Cleveland, ...
,
Vorbis Vorbis is a free and open-source software project headed by the Xiph.Org Foundation. The project produces an audio coding format and software reference encoder/decoder (codec) for lossy audio compression. Vorbis is most commonly used in conjun ...
and
Opus ''Opus'' (pl. ''opera'') is a Latin word meaning "work". Italian equivalents are ''opera'' (singular) and ''opere'' (pl.). Opus or OPUS may refer to: Arts and entertainment Music * Opus number, (abbr. Op.) specifying order of (usually) publicatio ...
) but not always by every encoder.


M/S stereo coding

M/S stereo coding transforms the left and right channels into a mid channel and a side channel. The mid channel is the sum of the left and right channels, or M = L+R. The side channel is the difference of the left and right channels, or S = L-R. Unlike intensity stereo coding, M/S coding is a special case of
transform coding Transform coding is a type of data compression for "natural" data like audio signals or photographic images. The transformation is typically lossless (perfectly reversible) on its own but is used to enable better (more targeted) quantization, whi ...
, and retains the audio perfectly without introducing artifacts. Lossless codecs such as
FLAC FLAC (; Free Lossless Audio Codec) is an audio coding format for lossless compression of digital audio, developed by the Xiph.Org Foundation, and is also the name of the free software project producing the FLAC tools, the reference software p ...
or
Monkey's Audio Monkey's Audio is an algorithm and file format for lossless audio data compression. Lossless data compression does not discard data during the process of encoding, unlike lossy compression methods such as Advanced Audio Coding, MP3, Vorbis, an ...
use M/S stereo coding because of this characteristic. To reconstruct the original signal, the channels are either added L = \frac or subtracted R = \frac. This form of coding is also sometimes known as matrix stereo and is used in many different forms of audio processing and recording equipment. It is not limited to digital systems and can even be created with passive audio
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s or analog
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
s. One example of the use of M/S stereo is in FM stereo broadcasting, where L+R modulates the
carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...
and L-R modulates a
subcarrier A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broa ...
. This enables backwards compatibility with mono equipment, which will only require the mid channel. Another example of M/S stereo is the stereophonic microgroove record. Lateral motions of a stylus represent the sum of two channels and the vertical motion represents the difference between the channels. A similar form of joining multiple channels is seen in the
ambisonics Ambisonics is a ''full-sphere'' surround sound format: in addition to the horizontal plane, it covers sound sources above and below the listener. Unlike some other multichannel surround formats, its transmission channels do not carry speaker si ...
implementation of Opus 1.3. A matrix may be used to mix the spherical harmonic channels together, reducing redundancy.


Joint frequency encoding

Joint frequency encoding is an
encoding In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...
technique used in
audio data compression In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
to reduce the data rate. The idea is to merge a given frequency range of multiple sound channels together so that the resulting encoding will preserve the sound information of that range not as a bundle of separate channels but as one homogeneous data stream. This will destroy the original channel separation permanently, as the information cannot be accurately reconstructed, but will greatly lessen the amount of required storage space. Only some forms of joint stereo use the joint frequency encoding technique, such as intensity stereo coding.


Implementations

When used within the MP3 compression process, joint stereo normally employs multiple techniques, and can switch between them for each MPEG frame. Typically, a modern encoder's joint stereo mode uses M/S stereo for some frames and L/R stereo for others, whichever method yields the best result. Encoders use different algorithms to determine when to switch and how much space to allocate to each channel; quality can suffer if the switching is too frequent or if the side channel doesn't get enough bits. With some encoding software, it is possible to force the use of M/S stereo for all frames, mimicking the joint stereo mode of some early encoders like
Xing XING is a Hamburg-based career-oriented social networking site, operated by New Work SE (until mid-2019 ''XING SE''). The site is primarily focused on the German-speaking market, alongside XING Spain, and competes with the American platform Li ...
. Within the
LAME Lame or LAME may refer to: Music * "Lame" (song) by Unwritten Law * ''Lame'' (album) by Iame People * Ibrahim Lame (born 1953), Nigerian educator and politician * Jennifer Lame (), American film editor * Quintín Lame (1880–1967), Colombian ...
encoder, this is known as forced joint stereo. As with MP3,
Ogg Vorbis Vorbis is a free and open-source software project headed by the Xiph.Org Foundation. The project produces an audio coding format and software reference encoder/decoder (codec) for lossy audio compression. Vorbis is most commonly used in conjun ...
stereo files can employ either L/R stereo or joint stereo. When using joint stereo, both M/S stereo and intensity stereo methods may be used. As opposed to MP3 where M/S stereo (when used) is applied before quantization, an Ogg Vorbis encoder applies M/S stereo to samples in the frequency domain after quantization, making application of M/S stereo a lossless step. After this step, any frequency area can be converted to intensity stereo by removing the corresponding part of the M/S signal's side channel. Ogg Vorbis' floor function will take care of the required left-right panning.
Opus ''Opus'' (pl. ''opera'') is a Latin word meaning "work". Italian equivalents are ''opera'' (singular) and ''opere'' (pl.). Opus or OPUS may refer to: Arts and entertainment Music * Opus number, (abbr. Op.) specifying order of (usually) publicatio ...
similarly has support for all three options in the CELT layer; the SILK layer is M/S-only., §§ 4.2.1, 4.3


Notes


References

{{reflist


External links

*Jürgen Herre, Fraunhofer IIS
''From Joint Stereo to Spatial Audio Coding - Recent Progress and Standardization''.
October 2004, Paper 157, DAFx'04 7th International Conference of Digital Audio Effects. Audio engineering