Channel coupling: Difference between revisions

Revision as of 02:04, 8 August 2005

Channel coupling is a method used to reduce bitrate demand by coding the stereo channel information more efficiently. there are several channel coupling methods. In mp3 the general term is joint stereo.

MP3 can use 2 different channel coupling methods: mid/side-coding and intensity stereo. Mid/Side-coding calculates a "mid"-channel by addition of left and right channel (l+r)/2 and a "side"-channel (l-r)/2. With more mono-like signals one can use less bitrate]] to encode the side-channel, so that the overall bitrate will be less than encoding the left and right channel. Intensity stereo destroys phase information, so it's used only at low bitrates.

Correctly implemented MP3 mid/side stereo (like in LAME) does very little or no damage to the stereo-image and increases compression efficiency either by reducing size or increasing overall quality.

Ogg Vorbis can use a mixture between 3 different types of channel coupling methods: phase stereo, point stereo, and lossless. These are all based upon square polar mapping which is beneficial when the correlation between the left and right channels are strong (this can also be extended to multichannel coupling as well). Phase stereo is the "least agressive means" of quantization. It effects only "diffuse imaging" or (angle) information, due to the fact that the ear is least sensitive to in-phase or out-of-phase noise above 4 kHz. Phase stereo is not curerently implemented in reference encoder due to complexity, but will be re-added again later on. Point stereo is a "more agressive means" of quantization. It eleminates the possibility of an out-of-phase signal entirely as to only have an effect of "point imaging" or (magnitude) information, hence the amplitude. It is currently used in the reference encoder on all modes below a -q 6. Lossless coupling just does polar mapping/channel interleaving using the residue vectors and has a bit-for-bit identical output. Note: also that phase stereo should not be compared to intensity stereo in MP3 coding

Ogg Vorbis uses lossless/point stereo coupling below (-q 6). lossless channel coupling is used for high bitrates entirely (-q 6 and up). This can be adjusted via an advanced-encode switch, but is not done for simplicit sake.

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 Correctly implemented MP3 mid/side stereo (like in [[LAME]]) does very little or no damage to the stereo-image and increases compression efficiency either by reducing size or increasing overall quality.
-[[Ogg Vorbis]] can use a mixture between 3 different types of channel coupling methods: phase stereo, [[point stereo]], and lossless. These are all based upon '''square polar mapping''' which is beneficial when the correlation between the left and right channels are strong (this can also be extended to multichannel coupling as well).  Phase stereo is the "least agressive means" of quantization. It effects only "diffuse imaging" or ''(angle)'' information, due to the fact that the ear is least sensitive to [http://en.wikipedia.org/wiki/In_phase in-phase] or [http://en.wikipedia.org/wiki/Out_of_phase out-of-phase] noise above 4 kHz.  Phase stereo is '''not''' curerently implemented in reference encoder due to complexity, but will be re-added again later on. [[Point stereo]] is a "more agressive means" of quantization. It eleminates the possibility of an out-of-phase signal ''entirely'' as to only have an effect of "point imaging" or ''(magnitude)'' information, hence the amplitude.  It is currently used in the reference encoder on all modes below a -q 5. Lossless coupling just does polar mapping/channel interleaving using the residue vectors and has a bit-for-bit identical output. ''Note: also that phase stereo should not be compared to intensity stereo in MP3 coding''
+[[Ogg Vorbis]] can use a mixture between 3 different types of channel coupling methods: phase stereo, [[point stereo]], and lossless. These are all based upon '''square polar mapping''' which is beneficial when the correlation between the left and right channels are strong (this can also be extended to multichannel coupling as well).  Phase stereo is the "least agressive means" of quantization. It effects only "diffuse imaging" or ''(angle)'' information, due to the fact that the ear is least sensitive to [http://en.wikipedia.org/wiki/In_phase in-phase] or [http://en.wikipedia.org/wiki/Out_of_phase out-of-phase] noise above 4 kHz.  Phase stereo is '''not''' curerently implemented in reference encoder due to complexity, but will be re-added again later on. [[Point stereo]] is a "more agressive means" of quantization. It eleminates the possibility of an out-of-phase signal ''entirely'' as to only have an effect of "point imaging" or ''(magnitude)'' information, hence the amplitude.  It is currently used in the reference encoder on all modes below a -q 6. Lossless coupling just does polar mapping/channel interleaving using the residue vectors and has a bit-for-bit identical output. ''Note: also that phase stereo should not be compared to intensity stereo in MP3 coding''
 Ogg Vorbis uses lossless/point stereo coupling below (''-q 6''). lossless channel coupling is used for high bitrates entirely (''-q 6 and up''). This can be adjusted via an advanced-encode switch, but is not done for simplicit sake.