New 128 kbit/s listening test at SoundExpert
Reply #35 – 2006-04-23 02:18:01
This is obviously a codec bug, AAC @320 kbps has enough SNR for the spectral bands, to be far better (less noisy) than a conservative JND. Is it the only bug of the coder or there are some other less obvious ones? It seems that it doesn’t matter for the end user what is it, a bug or a feature that produces artifacts. He needs a measurement scheme to discover all kind of artifacts – either obvious or less audible.I don't think it does - if you have a clearly transparent signal, say AAC @320 kbps - and you amplify the artifacts, what you get is actually distortion of the real judgment - because you amplified something that is inaudible before. There is a problem with the expression “real judgment”. What do you mean by this - PC speakers, headphones, audio installation in dedicated room or may be some midi stereo system with equalizer and “wide stereo” turned on? You probably noticed that post processing usually “amplifies something that was inaudible before”. And if artifacts are too close to the threshold of audibility they will be discovered easier in some of those cases. Thus it’s useful to know how much the quality margin is. Codec A, overcodes by linear scalefactor attenuation Codec B, overcodes by employing some equal-loudness noise-shaping filter prior to attenuation Both signals are perceptually indistinguishable from the original. Now, your tool performs artificial noise amplification, if I am not mistaken? What will happen - codec B will have noise pattern "equalized" to match equal-loudness curve, and it would, of course - sound better to the human ears than artificially noise-amplified Codec A How fair is that? Both would be clearly transparent - but your method would clearly favor Codec B. The method will definitely reveal that Codec B has better quality margin. I suppose, that is what noise-shaping is for.I do agree it's possible to construct some sound signal that is a pathological case for any given transform codec, but what does that mean? Not really anything relevant, I fear. Initially it was my proposal to compare results of two listening tests – with and without artifacts amplification. Not necessarily @320 kbit/s, let it be @256 or even @192 kbit/s. The first one to be performed by ordinary listeners and the other one – by experts in perfect listening environment (in order to reveal artifacts).But what kind of amplification? In what circumstances? Ivan made a specific point regarding two pratical coder approaches, but the problem is really that more generally the above assumption IMHO doesn't hold because it trivializes the amplification. If you could accurately model this, then it would probably vastly improve existing coders smile.gif It’s just a measurement scheme which combines objective distortion measurement with listening test. It can’t show the way how to make difference signal less audible. So, something with a low SNR could sound as good as something with a high SNR. By amplyfing the error you make the first sample sound pretty bad while the second samples won't change much due to the high SNR. Not necessarily so. It depends on specific SN ratios and spectral structures of those noises. Indeed, artifact amplification is only an element of measurement scheme. Another important element is psychometric curve which shows relationship between level of difference signal and perceived quality (some of them are in the papers). Usually audibility of correctly shaped noise increases more slowly than of not shaped one. So in your case it depends.Your intention is clear (make artifacts more audible/testing simpler). But you need to make the following sure to keep the results meaningful: Consider a and b to be the output of two contenders. This amplification step maps a to a' and b to b'. a sounds better than b <=> a' sounds better than b' Yes, this statement is correct (mechanism is shown above). The early works on “coding margin” even proposed to measure this margin as amplification level of difference signal necessary to the latter became “just noticeable”.