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What is supposed to sound "better"?, And questions about double blind tests
Shade[ST]
post Aug 19 2005, 20:19
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Hello,

I have always wondered to what extent VBR is better than CBR encodings -- Say you encode a song with Lame -V 5, and another using generic encoders, for CBR 192 (Xing, Gogo, FhG, Itunes, etc..)
Has anyone done a similar test? Or would my best bet be to ABX it? (or would ABC/HR be better, in this case?)
Also, what is the difference between these two methods, and to what degree does the testing hardware matter ? (should one resample to 48000 Hz on a SBLive! ; dither? -- speakers are ok? or headphones are essential?)

Thank you,
T.
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Rotareneg
post Aug 19 2005, 21:18
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"Better" in the context of lossy encoding means "sounds more like the original."

It doesn't matter if you're listening through blown out 4" speakers piped through a FM adaptor in a car or from a $100,000,000 sound system that audiophiles would give their right arm to listen too, we're just aiming for the minimum audible difference between the original and the compressed audio.

This post has been edited by Rotareneg: Aug 19 2005, 21:19
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guruboolez
post Aug 19 2005, 22:03
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QUOTE (Shade[ST] @ Aug 19 2005, 08:19 PM)
Or would my best bet be to ABX it? (or would ABC/HR be better, in this case?)
Also, what is the difference between these two methods (...)

ABC/HR consists on a blind comparison between one or more encoding and a reference. If you want to compare two or more encodings, this phase is the most informative: you must give a notation, and it implies that you're drawing a hierarchy according to your own preference.

ABX has another purpose: it's here to ensure the validity of whay you've heard and said during the ABC/HR phase.
Imagine that you hear a very tiny difference you can't really explain ("cold sound", "missing bass", etc...), and that you consequently rate this file. Stop the test, open the log file. There are now 2 possibilities:
- the rating concerns the reference -> what you've heard was imaginative (it happens frequently) and you're obliged to admit that both files (encoding and reference) are identical to you.
- you've rate the encoding -> good, but there was one chance on two (50%, or pval = 0.50) to pass the test by luck. In other words, the ABC/HR doesn't prove that you're really able to discern the encoded file.

QUOTE
, and to what degree does the testing hardware matter ? (should one resample to 48000 Hz on a SBLive! ; dither? -- speakers are ok? or headphones are essential?)

It depends on the difficulty of the test. I did my last 96 kbps multiformat listening test on my portable computer (crap audio components) but with a good headphone. But I wouldn't make any test at high bitrate with such material, except maybe for testing pre-echo (much easier and less hardware dependent IMO). It also depends on your ability to catch specific artefacts. Some people have excellent headphones, amp... but are not able to differenciate a basic MP3 encoding from a CD. You can put excellent shoes on your feet: without training you can't expect finish a marathon. But a bad heaphone as well as bad shoes is an handicap for trained people.


P.S. By testing LAME -V5 vs Blade & Cie, your test would rather look as a test between different encoders than a CBR vs VBR comparison.
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