Thread: Listening tests
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Old 09 August 2005, 11:20 pm   #12
guruboolez's Avatar
Join Date: Aug 2004
Location: Strasbourg (France)
Posts: 13

pieroxy is not completely wrong. The latest 128 kbps include samples supposed to be difficult. Bitrate allocation is therefore logicially superior to 128 kbps (it's not a problem, and it doesn't biased the test).
But one question remain: how will perform MPC on musical parts supposed to be less difficult (bitrate allocation < 128 kbps)? Theoretically, it shouldn't be a problem (it was said: mpc is quality based). In in practice, things are nevertheless different. This is not "crack": in the same test giving MPC on first place, there were only two samples for which bitrate drops to an unusual value (<100 kbps): Debussy.wav and ItCouldBeSweet.wav. And for both samples mpc quality was also unusually lower:

I wouldn't call "bullshit" or "crack" pieroxy questioning about quality for MPC on low complex part. It's really pertinent, and there are samples and tests (debussy for example) which proves that MPC obviously fails on musical part supposed to be easy-to-encode. You can also find the same questioning on, here.

These two samples raise the question about a possible methodological problem with this test. If MPC has problems on "non-difficult" samples [I don't like this concept], and if the test only include "difficult" one, could we still say that samples are representative? Note that it doesn't only apply to MPC, but to all VBR encoders having sudden bitrate+quality diminution at a given VBR setting; LAME MP3 is therefore in the same situation (same problem with Debussy and even higher with ItCouldBeSweet).

From my experience, MPC VBR mode is unstable at mid/low bitrate. Bitrate could suddenly drop and quality will follow the same curve. But with some other samples, the lower bitrate hasn't a big impact on quality. My feeling is that a listening test including samples divided in two equal parts (one for bitrate < 128 kbps and the second one for bitrate >128 kbps) would probably be less favorable to MPC. But how much is something I can't say.
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