The relationship between bit rate and picture quality is a major issue
of interest for digital broadcasting services, To evaluate coding per
formance of the MPEG-2 video standard, subjective assessment tests wer
e conducted for both Main Profile and Simple Profile, as well as NTSC.
Since picture quality in digital coding depends largely on such pictu
re characteristics as spatial detail and motion, estimation of the pic
ture quality of TV program material requires statistical analysis. Thi
s paper defines ''criticality,'' a quantitative measure of difficulty
for digital coding, to analyze the picture quality of TV programs stat
istically. Equipment has been developed to measure criticality and to
study a large number of TV program materials over a period of one week
, Based on the statistical evaluation of picture degradation in real T
V programs, the authors have concluded that approximately 9 Mbits/sec
is an appropriate bit rate for digital broadcasting to maintain pictur
e quality comparable to that of studio NTSC and to provide excellent q
uality in more than 99.9% of the broadcasting hours.