DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

horizontal and vertical dimensions – nonsquare
sampling. The degree of inequality was small, just 4%,
but for many applications any departure from equal
spacing imposes a burden. In about 1995, the standard
was adapted to achieve square pixels by choosing
a count of active lines 9 ⁄16 times 1920, that is,
1080 lines. SMPTE, and subsequently ATSC, enshrined
square sampling in the 1920×1080 image array. The
system has about two million pixels per frame; the
exact number is very slightly less than 2 21 , a neat fit
into binary-sized memory components.
NHK planned to operate the 1920×1035 system at
a frame rate of exactly 30 Hz (“30.00 Hz”), and early
1035i equipment operated only at that rate. However,
the discrepancy of about one frame every 16 seconds
between 1035i30.00 and 480i29.97 is a big nuisance
in standards conversion. To ease this problem, and
engineering difficulties associated with digital audio
sample rates, current 1080i HDTV standards accommodate
both 29.97 Hz and 30 Hz frame rates.
While NHK and others were developing 1125/60 interlaced
HDTV, progressive-scan systems having nearly
identical pixel rate were being developed by other
organizations, mainly in the United States. Technology
permitted a rate of about 60 megapixels per second,
whether the scanning was interlace or progressive. With
interlace scanning, 60 Mpx/s at 30 Hz frame rate allows
a two-megapixel image structure. With progressive
scanning, 60 Mpx/s at 60 Hz frame rate allows just one
megapixel. Partitioning one megapixel into a square
lattice yields an image structure of 1280×720; this led
to the 720p family of standards.
Audio rates
Digital audio has two standard sample rates: 48 kHz, for
professional applications, and 44.1 kHz, for consumer
applications. In the standardization of digital audio,
manufacturers decided to adopt two different standards
in order that professional and consumer equipment
could be differentiated! That goal failed miserably,
and now the dichotomy in sample rates is a major
nuisance in video and audio production.
CHAPTER 31 FRAME, FIELD, LINE, AND SAMPLE RATES 379