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Digital Audio Essentials by Bruce Fries, Marty Fries

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Sound Quality

Because hearing varies from person to person, sound quality is subjective, and traditional measures such as total harmonic distortion (THD) and signal-to-noise ratio are not very useful for rating perceptual encoding schemes. The perceived quality of the sound is more important than any characteristic that can be measured with test equipment. Controlled tests with trained listeners are the best way of measuring the performance of perceptual encoders. For example, during the MPEG-1 development process, three international listening tests were performed using the Centre for Communication Interface Research (CCIR) impairment scale (shown in Table 10-4), used to rate the quality of encoded audio in controlled tests with trained listeners. At 128 kbps, MP3 scored between 3.6 and 3.8. This indicates that listeners detected a difference between the MP3 and the original, but the difference was not annoying. At 240 kbps and above, MP3 scored at the high end of the scale, and most listeners found it difficult to distinguish between the MP3 and the original version.

Table 10-4. The CCIR impairment scale

5.0

Imperceptible (indistinguishable from the original)

4.0

Perceptible (perceptible difference, but not annoying)

3.0

Slightly annoying

2.0

Annoying

1.0

Very annoying

Variables that affect sound quality

The type of encoder, bit-rate, type of music, and sensitivity of the listener’s hearing all affect the sound quality of encoded audio. The quality of commercially available ...

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