Subject: MOS

From: "Kevin Davis" <[email-masked]>
Date: Thu, 15 Apr 1999 17:40:47 -0400

>>MOS is Mean Opinion Score. It is a metric which denotes the
>>perceived quality of the voice connection. It has five
>>categories going from Excellent to Unacceptable. This is
>>an important aspect of IP telephony ....

As Tom has pointed out, MOS is a measure of percieved quality.
The ITU-T calls it a "subjective" measure, which is to say
it depends upon human listeners.
In contrast to MOS, there is also an "objective" measure;
see ITU-T P.861 - PSQM (Perceptual Speech Quality Measure).
Both MOS and PSQM both will provide a numerical value
to represent the quality level, there is no single function
which maps one to the other.

  P.861 section 10.1
  "Since the relationship between the MOS and PSQM values
  is not necessarily the same for different languages or even
  for different subjective tests within a language, it is
  difficult to determine a unique function which transforms
  the PSQM value to the estimated MOS value."

The objective measure (PSQM) estimates the subjective measure.
The latter (human perception of quality) is presumably what
everyone really wants to know, but the it is the former which
can be automated and provide higher consistency.

  P.861 section 10.1
  "It is difficult to compare the MOSs obtained in different
  subjective experiments since subjective judgement is
  affected by the experimental settings ...."

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