> I've also listened to the ftb_samp example. I agree it's an
> excellent test, as the degradation at 128kpbs is quite noticeable.
> Again, to my ears it sounds like a lot of the problem is
> frame-to-frame amplitude variation. The sounds are a lot more complex,
> though, so it's a little harder for me to pick out what's going on.
> Incidentally, if you want to hear a joke, listen to this track at
> 128kbps with the FhG 2.72 encoder; it does a terrible job (much worse
> than lame 3.50).
>
> I suspect that what makes this track particularly difficult is
> their use of chorusing effects. Intuitively, this should make sense -
> chorusing basically takes narrow frequency peaks in the source stream
> and adds new ones closeby in the output stream. And this is, in fact,
> exactly what the MP3 psychoacoustic theory says you can't hear well
> :). A chorusing unit will also introduce subtle periodic variations. A
> lot of what I've heard from the not-even-lame coders sounds like
> beating between the frame rate and the periodicity of the chorus unit.
>
> I haven't done any serious work with audio compression, but I have
> with image compression, and there are (I think) some interesting
> analogies. JPEG, like MP3, is based on breaking the source signal into
> blocks (576/192 samples for MP3, 8x8 pixel blocks for JPEG), doing a
> DCT, quantizing, and Huffman coding. They are, I think, almost
> cousins. There are differences, of course; JPEG doesn't do the subband
> thing, and its DCT is 2D rather than 1D. However, even the "stereo" is
> in some way analogous: JPEG encodes a three-channel signal (RGB) by
> splitting into a "mid" (Y, intensity) and "side" (Cr and Cb, red and
> blue chromaticities) signal, and encoding each separately.
>
> Perhaps not surprisingly, then, even the artifacts are analogous.
> It's well known that JPEG performs very poorly on detailed edges near
> a smooth (or even white, in the case of most documents) background.
> Remind you of pre-echo? Also, at high compression ratios, you can
> easily see the seams at the edge of each 8x8 block.
>
> Smarter JPEG encoders (like the "optimize" mode in the IJG coder,
> which is free software and almost certainly the best coder out there)
> explicitly do things to reduce the block-to-block seam artifacts.
> Perhaps both camps have something to learn from each other.
This kind of enhancements is described in "A Real-Time PC-Based High Quality
MPEG Layer II Codec", wich can be found on mp3tech. As my domain name is
still unreachable, you can use www.cockpit.be/freeflight/mp3tech/entry
Regards,
Gabriel Bouvigne - France
[EMAIL PROTECTED]
icq: 12138873
MP3' Tech: www.mp3tech.org
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