SoundExpert explained, Methodology issues Options

[Serge Smirnoff]

I found this thread among SoundExpert referals and was a bit surprised with almost complete misunderstanding of SE testing methodology and particularly how diff signal is used in SE audio quality metrics. Discussion on the topic from 2006 actually seems more meaningful. So I decided to post here some SE basics for reference purposes. I will use a thought experiment which is close to reality though.

Suppose we have two sound signals – the main and the side one. They could be for example a short piano passage and some noise. We can prepare several mixes of them in different proportions:

• equal levels of main and side signals (0dB RMS)
• half level of side signal (-6dB RMS)
• quarter level of side signal (-12dB RMS)
• 1/8 level of side signal (-18dB RMS)
• 1/16 level of side signal (-24dB RMS)

After normalization all mixes have equal levels and we can evaluate perceptibility of the side signal in the mixes. Here at SE we found that this perceptibility is a monotonous function of side signal level and looks like this:

Figure: Side signal perception

(1) In other words, there is a relationship between objectively measured level of side signal and its subjectively estimated perceptibility in the mix. And what is more:

(a) this relationship is well described by 2-nd order curve (assuming levels are in dB)
(b) the relationship holds for any sound signals whether they are correlated or not, the only differences are position and curvature of the curve.

(2) These side stimulus perceptibility curves are the core of SE rating mechanism. Each device under test has its own curve plotted on basis of SE online listening tests.
(3) Side signals are difference signals of devices being tested. Levels of side signals are expressed in dB of Difference level parameter which is exactly equal to RMS level of side signal in our case.
(4) Subjective grades of perceptibility are anchor points of 5-grade impairment scale.
(5) Audio metrics beyond threshold of audibility is determined by extrapolation of that 2-nd order curves. Virtual grades in extrapolated area could be considered as objective quality parameters regarding human auditory peculiarities.

So, yes, difference signal is used in SE testing. We take into account both its level and how human auditory system perceives it together with reference signal. Some difference signals having fairly high levels still remain almost imperceptible against the background of reference signal and vice versa; perceptibility curves reflect this.

This is the concept. Many parts of it still need thorough verification in carefully designed listening tests, which are beyond SE possibilities. All we can do is to analyze collected grades returned by SE visitors. This will be done for sure and yet this can't be a replacement of properly organized listening tests.

SE testing methodology is new and questionable, but all assumptions look reasonable and SE ratings – promising, at least to me. Time will show.

[greynol]

That's a mighty big if.

For years people have requested verification and none has been forthcoming.

This post has been edited by greynol: Nov 28 2010, 20:17

[2Bdecided]

That's a mighty big if.

For years people have requested verification and none has been forthcoming.

I think something like it is justified.

I think it's commonly accepted* that signal detection (e.g. artefact detection in these tests) is a psychometric function - an S-curve, generated by integrating a Gaussian distribution...



X-axis is level, and y axis is chance of detecting the artefact.

If you know the function takes this shape, then it's apparently that you don't need to test at the threshold. You can test at several levels somewhat above threshold, and fit the resulting data to this graph/shape, thus giving you the actual threshold value.

The major problem with this is that, if you are testing only a long way above threshold, then very minor errors in the data will give huge errors in the threshold estimate because the fit to the graph could be wildly wrong.


Now, Sound Expert isn't doing this - it's at least one step away from it, testing at levels where people can hear the artefact all the time, and asking them how bad it sounds.

As you say, we have no proof that a graph of these results can be extrapolated back to find the threshold.


An obvious criticism is that two different kinds of artefacts, 12dB above threshold, might give very different results - i.e. one might be far more annoying than the other. But that's not necessarily a failing - if just means the curve might be steeper for one than the other - which would become apparent with more points on the curve (e.g. 6dB and 18dB, for example) so could be accounted for by the method.


It would be interesting to try to prove/disprove all this. A good starting point might be to take one of the archived listening tests from from HA with known results, and use exactly the same samples on SoundExpert. The results should speak for themselves.

Cheers,
David.