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SoundExpert explained, Methodology issues
Serge Smirnoff
post Nov 24 2010, 13:27
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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.


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Woodinville
post Nov 26 2010, 08:25
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QUOTE (Serge Smirnoff @ Nov 24 2010, 04:27) *
SE testing methodology is new and questionable, but all assumptions look reasonable and SE ratings promising, at least to me. Time will show.


Seeing as all this will be entirely dependent on the short-term spectrum of both signal and interferer, I wonder how you can develop any "metric" that is not specifically designed for one track, or one short bit of music.

In your example, I see no accounting for spectra, which is a key factor for the human auditory system.

If we're talking "one kind of instrument music" vs. "white noise" we have nothing useful at hand. So what is your point?


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Serge Smirnoff
post Nov 26 2010, 16:25
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QUOTE (Woodinville @ Nov 26 2010, 10:25) *
Seeing as all this will be entirely dependent on the short-term spectrum of both signal and interferer, I wonder how you can develop any "metric" that is not specifically designed for one track, or one short bit of music.

The metric works as long as you can measure Diff.Level (always) and estimate annoyance of diff. signal in some sound excerpt (not always, for long excerpts the term "basic audio quality" could be inapplicable). In short - if listening tests are valid for the excerpt, the metric is valid too.

QUOTE (Woodinville @ Nov 26 2010, 10:25) *
In your example, I see no accounting for spectra, which is a key factor for the human auditory system.

If it is a key factor, the human auditory system will account it during listening tests which are integral part of the metric.


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Woodinville
post Nov 27 2010, 07:17
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QUOTE (Serge Smirnoff @ Nov 26 2010, 07:25) *
QUOTE (Woodinville @ Nov 26 2010, 10:25) *
Seeing as all this will be entirely dependent on the short-term spectrum of both signal and interferer, I wonder how you can develop any "metric" that is not specifically designed for one track, or one short bit of music.

The metric works as long as you can measure Diff.Level (always) and estimate annoyance of diff. signal in some sound excerpt (not always, for long excerpts the term "basic audio quality" could be inapplicable). In short - if listening tests are valid for the excerpt, the metric is valid too.


Um, I don't think so. I can measure a difference level that is exactly the same, i.e. the same exact SNR, and have enormously different percieved quality.

See "13 dB miracle", please.


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Posts in this topic
- Serge Smirnoff   SoundExpert explained   Nov 24 2010, 13:27
- - drewfx   What is the justification for the "dashed...   Nov 24 2010, 18:20
|- - Serge Smirnoff   QUOTE (drewfx @ Nov 24 2010, 21:20) What ...   Nov 24 2010, 20:00
||- - drewfx   QUOTE (Serge Smirnoff @ Nov 24 2010, 14:0...   Nov 24 2010, 20:24
||- - Serge Smirnoff   QUOTE (drewfx @ Nov 24 2010, 23:24) Exact...   Nov 24 2010, 21:49
|- - Porcus   QUOTE (drewfx @ Nov 24 2010, 18:20) What ...   Nov 27 2010, 15:49
|- - drewfx   QUOTE (Porcus @ Nov 27 2010, 09:49) QUOTE...   Nov 29 2010, 18:43
|- - greynol   QUOTE (drewfx @ Nov 29 2010, 09:43) And t...   Nov 29 2010, 19:18
|- - Serge Smirnoff   QUOTE (greynol @ Nov 29 2010, 22:18) Some...   Nov 29 2010, 20:21
- - drewfx   Just to be clear - I am not necessarily questionin...   Nov 24 2010, 22:17
|- - Serge Smirnoff   If you want to build human-hearing-oriented audio ...   Nov 25 2010, 00:24
||- - alexeysp   QUOTE (Serge Smirnoff @ Nov 25 2010, 01:2...   Nov 25 2010, 11:35
||- - Serge Smirnoff   QUOTE (alexeysp @ Nov 25 2010, 13:35) ...   Nov 25 2010, 19:33
|- - knutinh   QUOTE (drewfx @ Nov 24 2010, 22:17) I rep...   Nov 25 2010, 19:15
|- - Serge Smirnoff   QUOTE (knutinh @ Nov 25 2010, 21:15) If t...   Nov 25 2010, 19:49
|- - Kees de Visser   In the recently closed thread which the OP referre...   Nov 25 2010, 21:39
- - 2Bdecided   Just to be clear, your graph example shows grades ...   Nov 25 2010, 12:30
|- - Serge Smirnoff   QUOTE (2Bdecided @ Nov 25 2010, 14:30) Ju...   Nov 25 2010, 23:50
- - Woodinville   QUOTE (Serge Smirnoff @ Nov 24 2010, 04:2...   Nov 26 2010, 08:25
|- - Serge Smirnoff   QUOTE (Woodinville @ Nov 26 2010, 10:25) ...   Nov 26 2010, 16:25
|- - Woodinville   QUOTE (Serge Smirnoff @ Nov 26 2010, 07:2...   Nov 27 2010, 07:17
|- - Serge Smirnoff   QUOTE (Woodinville @ Nov 27 2010, 09:17) ...   Nov 27 2010, 08:29
|- - Woodinville   QUOTE (Serge Smirnoff @ Nov 26 2010, 23:2...   Nov 27 2010, 23:05
|- - knutinh   QUOTE (Woodinville @ Nov 27 2010, 23:05) ...   Nov 28 2010, 19:24
- - greynol   That's a mighty big if. For years people have...   Nov 28 2010, 20:14
|- - Kees de Visser   The technique isn't new, according to this AES...   Nov 28 2010, 21:35
||- - Serge Smirnoff   QUOTE (Kees de Visser @ Nov 29 2010, 00:3...   Nov 28 2010, 22:47
|- - 2Bdecided   QUOTE (greynol @ Nov 28 2010, 19:14) That...   Nov 29 2010, 11:49
|- - Porcus   QUOTE (2Bdecided @ Nov 29 2010, 11:49) I ...   Nov 29 2010, 13:00
|- - 2Bdecided   QUOTE (Porcus @ Nov 29 2010, 12:00) QUOTE...   Nov 29 2010, 16:27
|- - Porcus   [Heavily edited] QUOTE (2Bdecided @ Nov 29 2...   Nov 29 2010, 16:47
|- - knutinh   QUOTE (2Bdecided @ Nov 29 2010, 16:27) QU...   Nov 30 2010, 09:53
|- - Porcus   QUOTE (knutinh @ Nov 30 2010, 09:53) Why ...   Nov 30 2010, 11:28
|- - knutinh   QUOTE (Porcus @ Nov 30 2010, 11:28) QUOTE...   Nov 30 2010, 11:34
- - greynol   If we aren't going to consider real-world usag...   Nov 29 2010, 20:27
|- - Serge Smirnoff   QUOTE (greynol @ Nov 29 2010, 23:27) What...   Nov 29 2010, 20:36
- - greynol   Breaking masking by amplifying a difference signal...   Nov 29 2010, 20:45
|- - Serge Smirnoff   QUOTE (greynol @ Nov 29 2010, 23:45) Brea...   Nov 29 2010, 21:19
|- - Kees de Visser   QUOTE (greynol @ Nov 29 2010, 21:45) Brea...   Nov 29 2010, 23:21
|- - greynol   QUOTE (Kees de Visser @ Nov 29 2010, 14:2...   Nov 30 2010, 08:19
- - greynol   How so?   Nov 29 2010, 21:31
|- - Serge Smirnoff   QUOTE (greynol @ Nov 30 2010, 00:31) How ...   Nov 29 2010, 22:10
- - SebastianG   QUOTE (Serge Smirnoff @ Nov 24 2010, 13:2...   Nov 29 2010, 22:04
- - Woodinville   Using a difference signal as a signal-detection te...   Nov 29 2010, 22:14
|- - Porcus   QUOTE (Woodinville @ Nov 29 2010, 22:14) ...   Nov 29 2010, 23:00
||- - Woodinville   QUOTE (Porcus @ Nov 29 2010, 14:00) QUOTE...   Nov 30 2010, 00:26
|- - Serge Smirnoff   QUOTE (Woodinville @ Nov 30 2010, 01:14) ...   Nov 30 2010, 09:20
- - Serge Smirnoff   QUOTE (SebastianG @ Nov 30 2010, 01:04) I...   Nov 30 2010, 09:09
|- - 2Bdecided   QUOTE (Serge Smirnoff @ Nov 30 2010, 08:0...   Nov 30 2010, 16:24
|- - Serge Smirnoff   QUOTE (2Bdecided @ Nov 30 2010, 19:24) Ho...   Nov 30 2010, 17:38
|- - Woodinville   QUOTE (Serge Smirnoff @ Nov 30 2010, 08:3...   Dec 1 2010, 03:11
|- - Serge Smirnoff   QUOTE (Woodinville @ Dec 1 2010, 06:11) Q...   Dec 1 2010, 09:17
|- - Woodinville   QUOTE (Serge Smirnoff @ Dec 1 2010, 00:17...   Dec 1 2010, 22:03
|- - Kees de Visser   QUOTE (Woodinville @ Dec 1 2010, 23:03) T...   Dec 1 2010, 23:47
||- - Woodinville   QUOTE (Kees de Visser @ Dec 1 2010, 14:47...   Dec 1 2010, 23:55
||- - greynol   QUOTE (Woodinville @ Dec 1 2010, 14:55) s...   Dec 2 2010, 06:47
||- - Serge Smirnoff   QUOTE (Woodinville @ Dec 2 2010, 02:55) T...   Dec 2 2010, 08:53
||- - Kees de Visser   QUOTE (Woodinville @ Dec 2 2010, 00:55) T...   Dec 2 2010, 09:35
||- - greynol   QUOTE (Kees de Visser @ Dec 2 2010, 00:35...   Dec 2 2010, 10:34
||- - 2Bdecided   QUOTE (Kees de Visser @ Dec 2 2010, 08:35...   Dec 2 2010, 11:25
|||- - Kees de Visser   QUOTE (2Bdecided @ Dec 2 2010, 12:25) Com...   Dec 2 2010, 13:09
||||- - 2Bdecided   QUOTE (Kees de Visser @ Dec 2 2010, 12:09...   Dec 2 2010, 16:04
|||||- - Kees de Visser   QUOTE (2Bdecided @ Dec 2 2010, 17:04) QUO...   Dec 2 2010, 17:52
|||||- - Serge Smirnoff   QUOTE (2Bdecided @ Dec 2 2010, 19:04) Now...   Dec 2 2010, 19:24
||||- - greynol   QUOTE (Kees de Visser @ Dec 2 2010, 04:09...   Dec 2 2010, 19:15
|||- - Serge Smirnoff   QUOTE (2Bdecided @ Dec 2 2010, 14:25) Com...   Dec 2 2010, 13:10
||- - Woodinville   QUOTE (Kees de Visser @ Dec 2 2010, 00:35...   Dec 3 2010, 00:32
|- - Serge Smirnoff   QUOTE (Woodinville @ Dec 2 2010, 01:03) S...   Dec 2 2010, 09:01
- - Porcus   Joking aside: I'd be surprised if MPEG didn...   Nov 30 2010, 12:03
- - 2Bdecided   I can see how this could work for a simple low pas...   Dec 1 2010, 16:26
- - Serge Smirnoff   QUOTE (2Bdecided @ Dec 1 2010, 19:26) Wit...   Dec 2 2010, 09:41
- - 2Bdecided   QUOTE (Serge Smirnoff @ Dec 2 2010, 08:41...   Dec 2 2010, 11:32
- - Serge Smirnoff   QUOTE (2Bdecided @ Dec 2 2010, 14:32) If ...   Dec 2 2010, 12:18


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