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<title>LAME MP3 Encoder :: Mid/Side Stereo</title>
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Index page
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About LAME
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Intro to encoding
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CBR
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ABR
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VBR
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M/S Stereo
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Usage of LAME
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Version history
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Coders of LAME
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Contact LAME
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LAME links
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LAME - Mid/Side Stereo
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During years, what is called Joint-stereo has been misunderstood.
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Joint stereo in MP3 is a mechanism to selectively choose between three modes
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of storing stereo information. These three modes are Simple Stereo
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, Mid-Side Stereo, and Intensity-Stereo.
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In Simple Stereo, the encoder analyzes the left and the right channels
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independently and stores the information as-is, without further checking the
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similarities in the signal<sup>1</sup>
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In Mid-Side Stereo, the encoder analyzes the left, right<sup>2
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</sup>, mid (l+r) and side (l-r) channels. It then gives more bits to the
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mid than the side channel (as usually the side channel is less complex) and then
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stores just the mid and side channels into the resulting MP3.
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This way, the mid channel can be encoded as if the frame was bigger, and as such
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have more quality with the same bitrate.
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Note: Mid/side in MP3 is switched frame-by-frame. In AAC, it can be switched
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band by band.
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Intensity-Stereo (not supported in LAME) uses a technique known as joint
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frequency encoding, which is based on the principle of sound localization.
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Human hearing is predominantly less acute at perceiving the direction of certain
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audio frequencies. By exploiting this 'limitation', intensity stereo coding can
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reduce the data rate of an audio stream with little or no perceived change in
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apparent quality.
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It works by merging the upper spectrum into just one channel (thus reducing
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overall differences between channels) and transmiting a little side information
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about how to pan certain frequency regions.
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This type of coding does not perfectly reconstruct the original audio because
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of the loss of information and can cause unwanted artifacts. However, for very
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low bitrates this tool usually provides a gain of perceived quality.
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<sup>3</sup>
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The LAME mid/side switching criterion, and mid/side masking thresholds are
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taken from Johnston and Ferreira, <u>Sum-Difference Stereo Transform
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Coding,</u> Proc. IEEE ICASSP (1992) p 569-571.
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The MPEG AAC standard claims to use mid/side encoding based on this paper.
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This is not the same than dual-mono. Dual-mono should be
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used where the left and right channels of the input file contain two different
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streams, where you should choose one (as in two different languages)
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If one channel has much less noise masking in a certain
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band than the other, it could happen than the noise spread (by mid/side stereo)
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may no longer be masked for that channel. If both channels have the same
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masking, then the noise spread between both channels will be equally well masked.
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To prevent this from happening, there is an analysis done on the left and right
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channel to determine the noise masking thresholds and properly mask the noise.
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Quote from wikipedia
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href="http://en.wikipedia.org/wiki/Joint_stereo">Joint_stereo.
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