19 Tell me more about sound quality. How good is MPEG audio compression? And how do you assess that?
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
19 Tell me more about sound quality. How good is MPEG audio compression? And how do you assess that?
Today, there is no alternative to expensive listening tests.
During the ISO-MPEG-1 process, 3 international listening tests
have been performed, with a lot of trained listeners,
supervised by Swedish Radio. They took place in 7.90, 3.91
and 11.91. Another international listening test was
performed by CCIR, now ITU-R, in 92.
All these tests used the "triple stimulus, hidden reference"
method and the so-called CCIR impairment scale to assess the
audio quality.
The listening sequence is "ABC", with A = original, BC = pair
of original / coded signal with random sequence, and the
listener has to evaluate both B and C with a number
between 1.0 and 5.0. The meaning of these values is:
5.0 = transparent (this should be the original signal)
4.0 = perceptible, but not annoying (first differences
noticable)
3.0 = slightly annoying
2.0 = annoying
1.0 = very annoying
With perceptual codecs (like MPEG audio), all traditional
parameters (like SNR, THD+N, bandwidth) are especially
useless.
Fraunhofer-IIS (among others) works on objective quality
assessment tools, like the NMR meter (Noise-to-Mask-Ratio),
too. If you need more informations about NMR, please
contact nmr@iis.fhg.de
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