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These three modes are <a href="#ms">Simple Stereo 42 </a>, <a href="#mj">Mid-Side Stereo</a>, and <a href="#mi">Intensity-Stereo.</a> 43 </p> 44 <p> 45 In <a name="ms">Simple Stereo</a>, the encoder analyzes the left and the right channels 46 independently and stores the information as-is, without further checking the 47 similarities in the signal<a href="#note1"><sup>1</sup></a> 48 </p> 49 <p> 50 In <a name="mj">Mid-Side Stereo</a>, the encoder analyzes the left, right<a href="#note2"><sup>2 51 </sup></a>, mid (l+r) and side (l-r) channels. It then gives more bits to the 52 mid than the side channel (as usually the side channel is less complex) and then 53 stores just the mid and side channels into the resulting MP3.<br/> 54 This way, the mid channel can be encoded as if the frame was bigger, and as such 55 have more quality with the same bitrate.<br/> 56 Note: Mid/side in MP3 is switched frame-by-frame. In AAC, it can be switched 57 band by band. 58 </p> 59 <p> 60 <a name="mi">Intensity-Stereo</a> (not supported in LAME) uses a technique known as joint 61 frequency encoding, which is based on the principle of sound localization.<br/> 62 Human hearing is predominantly less acute at perceiving the direction of certain 63 audio frequencies. By exploiting this 'limitation', intensity stereo coding can 64 reduce the data rate of an audio stream with little or no perceived change in 65 apparent quality.<br/> 66 It works by merging the upper spectrum into just one channel (thus reducing 67 overall differences between channels) and transmiting a little side information 68 about how to pan certain frequency regions.<br/> 69 This type of coding does not perfectly reconstruct the original audio because 70 of the loss of information and can cause unwanted artifacts. However, for very 71 low bitrates this tool usually provides a gain of perceived quality. 72 <a href="#note3"><sup>3</sup></a> 73 </p> 74 <p> 75 The LAME mid/side switching criterion, and mid/side masking thresholds are 76 taken from <cite>Johnston and Ferreira, <u>Sum-Difference Stereo Transform 77 Coding,</u> Proc. IEEE ICASSP (1992) p 569-571.</cite> 78 </p> 79 <p> 80 The MPEG AAC standard claims to use mid/side encoding based on this paper. 81 </p> 82 <div id="notes"> 83 <ol> 84 <li><a name="note1"></a>This is not the same than dual-mono. Dual-mono should be 85 used where the left and right channels of the input file contain two different 86 streams, where you should choose one (as in two different languages) 87 </li> 88 <li><a name="note2"></a>If one channel has much less noise masking in a certain 89 band than the other, it could happen than the noise spread (by mid/side stereo) 90 may no longer be masked for that channel. If both channels have the same 91 masking, then the noise spread between both channels will be equally well masked. 92 <br/> 93 To prevent this from happening, there is an analysis done on the left and right 94 channel to determine the noise masking thresholds and properly mask the noise. 95 </li> 96 <li><a name="note3"></a>Quote from wikipedia <a 97 href="http://en.wikipedia.org/wiki/Joint_stereo">Joint_stereo</a>.</li> 98 </ol> 99 </div> 100</div> 101<div id="footer"> 102 103<a href="http://sourceforge.net/projects/lame"><img src="images/sflogo.gif" 104 alt="Get LAME (Lame Aint an MP3 Encoder) at SourceForge.net. 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