VoIP: Voice over Internet Protocol ◾ 313
Subjective Quality Assessment or Opinion Rating
e most widely used subjective quality assessment method is opinion rating,
which is defined in ITU-T Recommendation P.800. e performance of the system
under test is rated directly (absolute category rating, ACR) or relative to the subjec-
tive quality of a reference system (degradation category rating, DCR). e ITU-T
opinion scale used in an ACR test is the most frequently used, with the following
scoring patterns: excellent (5), good (4), fair (3), poor (2), and bad (1).
In a DCR test, the subjects are instructed to rate the conditions according
to the following five-point degradation category scale: degradation is inaudible
(5