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alt preset extreme -m s ??
post Jan 10 2002, 23:35
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just wanted to make sure this command line was ok. no conflict or anything? I assume i could use -m s with preset standard as well?

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post Jan 11 2002, 06:08
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This was asked before many times, mostly by people who have been 'brainwashed' by some audio ripping community that joint stereo is a bad thing and simple stereo is always better.

While this may be true for a few popular encoders which have screwed up Joint Stereo implementation (FHg, Xing, etc), this is not true for LAME. Phase information is actually preserved in LAME (allowing for Dolby Surround information to be preserved). Used properly (i.e. all the --alt-presets), you should not encounter problems with Joint Stereo, and even if you do, the problems caused by Joint Stereo will not be as bad as the problems caused by the lack of bandwidth if you don't use Joint Stereo.

The second mistake people make is to assume that for VBR, it doesn't matter as long as they don't mind the bitrate increase. However they fail to forget that there are maximum frame sizes in MP3s. Sure, you can use simple stereo and increase a 128kbps frame to 256kbps frame without any problem. But you are talking about high quality VBR presets which average 192kbps and 256kbps. Especially for extreme, most of the frames used will be 224kbps, 256kbps and 320kbps frames.

Now you see the problem? You can use at most 320kbps frames, so in standard and even more so in extreme, using simple stereo, you will end up packing less data into a frame and this will cause problems to occur.

For a better understanding about Joint Stereo vs Stereo, let me refer to a discussion I had with Dibrom a couple of months back when he was tuning --alt-preset normal (an experiment which was eventually abandoned). At that time he was toying with the idea of forcing all short blocks to be 320kbps frames as a method to improve tracks with impulses and preechos (fatboy, castanets etc).

I then suggested to Dibrom to revert to normal joint stereo (the preset defaulted --nssafejoint) to increase the bandwidth for the 320kbps shortblocks (preecho can be reduced by allocating more bits to encode more coefficients of the transform). Dibrom discovered that this did not help. The problems caused by the joint stereo mode actually worsened the pre-echo and impulses.

Dibrom then tried something different, forcing stereo mode for the 320kbps shortblock. This also worsened the pre-echo and impulses. Not surprising because there's less bits available to encode the coefficients. I asked him to try experimenting with --ns-msfix values, but I don't think he actually went on to try that out.

So as you can see, joint stereo is a good thing because it increases bandwidth to encode the coefficients of the transforms. However there is also the chance of introducing artifacts and problems itself it not used properly. The --nssafejoint switch will cause the encoder to use as much JS frames as possible while making it safe against the artifacts. There is therefore no need to use simple stereo at all. JS with --nssafejoint will give you both the safety you need from JS artifacts and the additional bandwidth from using JS (but not as much bandwidth as without the --nssafejoint).

For lower bitrate encoding, of course, you would probably not want to use --nssafejoint because at that rate, the problems introduced by JS will be less than the problems of lower bandwidth. This is illustrated by looking at the --alt-preset ABR settings (Refer to abr_switch_map[] in parse.c in cvs). JS is used normally up to 160kbps. After that, --nssafejoint is introduced with a --nsmsfix modifier of 1.7 at 192kbps and 1.25 at 224kbps. At 256kbps, the full --nssafejoint is used.

For more details about the custom modifications Dibrom made to --nssafejoint and --nsmsfix check out this thread http://www.hydrogenaudio.org/forums/showth...?s=&postid=4727
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