Mean Opinion Score

You can test voice quality in two ways: subjectively and objectively. Humans perform subjective voice testing, whereas computers—which are less likely to be "fooled" by compression schemes that can "trick" the human ear—perform objective voice testing.

Codecs are developed and tuned based on subjective measurements of voice quality. Standard objective quality measurements, such as total harmonic distortion and signal-to-noise ratios, do not correlate well to a human's perception of voice quality, which in the end is usually the goal of most voice compression techniques.

A common subjective benchmark for quantifying the performance of the speech codec is the mean opinion score (MOS). MOS tests are given to a group of listeners. Because voice quality and sound in general are subjective to listeners, it is important to get a wide range of listeners and sample material when conducting a MOS test. The listeners give each sample of speech material a rating of 1 (bad) to 5 (excellent). The scores are then averaged to get the mean opinion score.

MOS testing also is used to compare how well a particular codec works under varying circumstances, including differing background noise levels, multiple encodes and decodes, and so on. You can then use this data to compare against other codecs.

MOS scoring for several ITU-T codecs is listed in Table 8-2. This table shows the relationship between several low-bit rate coders and standard PCM.

Table 8-2. ITU-T Codec MOS Scoring

Compression Method

Bit Rate (Kbps)

Sample Size (ms)

MOS Score

G.711 PCM

64

0.125

4.1

G.726 ADPCM

32

0.125

3.85

G.728 Low Delay Code Excited Linear Predictive (LD-CELP)

15

0.625

3.61

G.729 Conjugate Structure Algebraic Code Excited Linear Predictive (CS-ACELP)

8

10

3.92

G.729a CS-ACELP

8

10

3.7

G.723.1 MP-MLQ

6.3

30

3.9

G.723.1 ACELP

5.3

30

3.65

Source: Cisco Labs

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