SoundExpert explained
Reply #36 – 2010-11-29 21:04:30
[...] So, yes, difference signal is used in SE testing. [...] This is the concept. [...] SE testing methodology is new and questionable, Yes, very questionable. I said this 4 years ago and I'm still saying this now.but all assumptions look reasonable and SE ratings Not really. It's not hard to imagine the possibility of signal pairs (main,side) where you can't hear any difference between main and main+side but you can easily hear a difference between main and main+0.5*side. Hint: phase is a bitch. ;-) Your implicit assumption is that both signals are independent . But this is not necessarily the case with perceptual audio coders. Take for example the MPEG4 tool called PNS (perceptual noise substitution). It just replaces some high frequency noise with synthetically generated noise of the same level. This is done by transmitting the noise level only. Obviously, we can use this tool in cases when the main perceptual feature is the energy level and anything else is not important. Then, we have the following properties: Noise level of original matches the noise level of the encoded result, so energy(main) = energy(main+side). Probability theory tells us that main and main+side are orthogonal. This implies a coherence between main and side of 0.7 -- ZERO POINT SEVEN. Hardly independent. This also implies that a 50/50 mix -- main+0.5*side -- would lose 3dB power. You can easily compute this viamain = [1 0]; side = [0 1] - main; 20*log10(norm(main+0.5*side)) (Matlab code) So, by attenuating the sample-by-sample difference we actually amplify the perceived difference (since we lose power) in this case! What does that tell us? It tells us that you overrate sample-by-sample differences. Perceptual audio coders try to retain certain things so it sounds similar and tolerate other losses. And you're focussing on the "other losses" (as well). What you're doing is basically violating some of a perceptual encoder's principles (like keeping energy levels similar no matter how large the sample-by-sample difference will be). By amplifing the difference you could destroy some signal properties the encoder and our HAS cares about much more than you do. Sound perception is not as simple as you want us to believe. Sample-by-Sample differences are not important. And "extrapolating artefacts" this way is nothing but a big waste of time. Even testing with "attenuated artefacts" doesn't tell you anything. Your methodology breaks down because you're assuming that the difference is independent from the original. It is not. Cheers! SG