DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

60 1000
525×
2 1001
315
88
3 579545
⎛ ⎞
⎜ Hz⎟
× ×
⎝ ⎠
= MHz
≈ . MHz
60
525×
2
⎛
⎜
⎝
315
= MHz
22
≈ 14. 318181 MHz
455
2
⎞ 1000
Hz⎟
× × 455× 2
⎠ 1001
FCC could have allowed the sound subcarrier rest
frequency to be increased by the fraction 1001 ⁄1000 –
that is, increased it by 4.5 kHz to about 4.5045 MHz.
Had the FCC made this choice, the color subcarrier in
NTSC would have been exactly 3.583125 MHz; the
original 525/60 line and field rates would have been
unchanged; we would have retained exactly 60 frames
per second – and NTSC would have no dropframes!
Since sound is frequency modulated, the sound carrier
was never crystal-stable at the subcarrier frequency
anyway – not even during absolute silence – and the
tolerance of the rest frequency was already reasonably
large (±1 kHz). The deviation of the sound subcarrier
was – and remains – 25 kHz, so a change of 4.5 kHz
could easily have been accommodated by the intercarrier
sound systems of the day.
However, the FCC refused to alter the sound subcarrier.
Instead, the color/sound constraint was met by
reducing both the line rate and field rate by the fraction
1001 ⁄1000, to about 15.734 kHz and 59.94 Hz.
Color subcarrier was established as 3.5795 _ 4 _ 5 MHz.
What was denoted 525/60 scanning became
525/59.94, though unfortunately the 525/60 notation
is still used loosely to refer to 525/59.94.
The factors of 1001 are 7, 11, and 13. This numerical
relationship was known in ancient times: The book
1001 Arabian Nights is based on it. The numbers 7, 11,
and 13 are considered to be very unlucky. Unfortunately
the field rate of 60 ⁄1.001, about 59.94 Hz, means
that 60 fields consume slightly more than one second:
Counting 30 fields per second does not agree with
clock time. Dropframe timecode was invented to alleviate
this difficulty; see Timecode, on page 381.
NTSC sync generators generally use a master oscillator
of 14.3181 _ 8 _ MHz. This clock is divided by 4 to obtain
color subcarrier, and simultaneously divided by 7 to
obtain a precursor of line rate. Prior to the emergence
of framestore synchronizers in the 1980s, each major
broadcast network in the United States had an atomic
clock to provide 5 MHz, followed by a rate multiplier of
63 ⁄22 to derive its master 14.318 _ 1 _ 8 MHz clock.
374 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES