EPL/T/RPC A tr_99_092=sqi_to_mos.fm Dokansv/Godk - Doc respons/Approved Kontr - Checked File Rev Datum - Date

EPL/T/RP Heikkilä/Nordlund EPL/T/R 99:092

99-05-05

Uppgjord - Prepared Nr - No. Faktaansvarig - Subject responsible

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EPL/T/RP AllEPL/T/RC Tor Björn MindeEPL/TMC Kari KorkalaEPL/T/TX Niclas HugossonEPL/T/TX Roger ByströmEPL/T/MP Robert JonssonERA/T/VC Hans HermanssonERA/LVN/DPC Anders Carlsson

ERA/LVR/PR Saeed Yousefi

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Conversion between SQI and MOS

1 SummaryThe scale chosen for the SQI measure is based on the MNRU stand-ard [1], and expressed in dBQ. The reason for this choice is discussed in [2]. In some cases, however, it might still be preferrable to use a “MOS-like” scale, e.g. for comparison with other five-grade based rat-ing methods [3].

This document describes one possible conversion which can be ap-plied so that the resulting SQI scores are expressed in a scale compa-rable to a MOS scale.

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2 Conversion CurvesFigure 1 below shows the dBQ-to-MOS relation for three different sub-jective tests. The relations can be computed since all three tests in-clude MNRU samples which are also rated. By making a regression over these samples the curves can be found.

The intuitive approach would be to select one of these curves and use it for the conversion. However, such a method will actually produce a biased conversion (the predicted MOS values will be too low). The rea-son for this is described below, together with a correct conversion method.

One of the benefits with the dBQ scale is that it does not saturate, while the MOS scale does saturate, especially in the low-quality region. To be able to exploit this non-saturation feature of the dBQ scale, the rat-ing procedure used when the SQI measure was developed could not be a simple MOS test, for obvious reasons.

Even if another MOS scale; MOSLE (MOS Listening Effort) would be possible to use in the low-quality region, the merging of the two MOS scales would still pose a problem.

The approach chosen for the SQI listenings was to perform compara-tive tests (A-B test) where the listener is to judge which of two samples are the best one. In this way it was possible to compare MNRU sam-ples with recorded real-life samples and thus finding the dBQ score for each recorded sample.

Figure 1 Regression curves from three different MOS tests.

−20 −10 0 10 20 30 40

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2

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4

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dBQ

MO

S

GSM InteroperabilityIs−136 EvaluationFloating Point IS−641

alld

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The effect of this listening method is that at the high-quality region the “goodness” (or the “quality”) of the samples are compared, while at the low-quality region the “badness” (or the “ease of understanding”) are compared. This gives a seamless transition between different percep-tual ways of comparing, enabling the full usage of the dBQ scale.

Since effectively two “scales” (comparable to MOSQ and MOSLE) are used, the relation between dBQ and only one of the scales, MOSQ, will differ from the corresponding curves regressed from a “normal” MOS test (as those shown in figure 1). This is most evident at medium to low quality levels.

By performing a MOS test on real-life recorded material, and compar-ing the results with the corresponding SQI values, a correct curve (fig-ure 2) can be obtained. Note that this is only one possible curve, since the shape of the curve will differ sligthly depending on the setup of the MOS test.

By comparing this curve with the ones in figure 1 the difference in the lower quality regions is obvious. The implication of this is that when an A-B test method is used together with MNRU samples, the listener does accept slightly worse MNRU conditions than during a MOS test (which is expected due to the low-end saturation problem in a MOS test).

Even if the curve above is only one possible example of a conversion, the basic shape of the curve is expected to be approximately the same for other variants of the curve. As with all MOS-like measures, compar-isons of absolute levels between different rating methods should not

Figure 2 Conversion curve between SQI and a MOS-like scale.

−20 −15 −10 −5 0 5 10 15 20 25 30

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SQI

MO

SQ

conversion

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be done. However, by using the conversion curve, relative perform-ance metrics (e.g. correlation towards subjective MOS scores) can be used.

3 Conversion TableThe table below contains the numerical equivalence of the curve in fig-ure 2. By using these values a conversion between SQI and a five-grade MOS-like scale can be obtained.

SQI MOS

-20 1.01

-19 1.02

-18 1.02

-17 1.03

-16 1.03

-15 1.04

-14 1.04

-13 1.05

-12 1.06

-11 1.07

-10 1.09

-9 1.10

-8 1.12

-7 1.15

-6 1.17

-5 1.20

-4 1.24

-3 1.28

-2 1.34

-1 1.39

0 1.46

1 1.54

2 1.62

3 1.72

4 1.82

5 1.94

6 2.06

7 2.20

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4 References[1] Modulated Noise Reference Unit; ITU-T P.810.

[2] TEMS SQI and MOS; Gunnar Heikkilä; OSQAR/AJ-013.

[3] Methods for Subjective Determination of Transmission Quality;ITU-T P.800.

8 2.34

9 2.48

10 2.63

11 2.78

12 2.93

13 3.07

14 3.21

15 3.34

16 3.46

17 3.57

18 3.67

19 3.76

20 3.84

21 3.91

22 3.97

23 4.02

24 4.07

25 4.11

26 4.15

27 4.17

28 4.20

29 4.22

30 4.24

SQI MOS