DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

For details, consult the
Hazeltine Laboratories publication
cited on page 366.
fSC,NTSC = fH,480i 455
2
modulated, and with an analog sound modulator even
perfect silence cannot be guaranteed to generate an FM
carrier of exactly 4.5 MHz. Nonetheless, making the FM
sound carrier average out to 4.5 MHz was thought to
have some value.
Addition of composite color
NTSC and PAL color coding both employ the frequencyinterleaving
technique to achieve compatibility with
monochrome systems. With frequency interleaving, the
color subcarrier frequency is chosen to alternate phase
line by line, so as to minimize the visibility of encoded
color on a monochrome receiver. This line-to-line phase
relationship makes it possible to accurately separate
chroma from luma in an NTSC decoder that incorporates
a comb filter (although a cheaper notch filter can
be used instead).
NTSC color subcarrier
In 1953, the second NTSC decided to choose a color
subcarrier frequency of approximately 3.6 MHz. They
recognized that any nonlinearity in the processing of
the composite color signal with sound – such as limiting
in the intermediate frequency (IF) stages of a receiver –
would result in intermodulation distortion between the
sound subcarrier and the color subcarrier. The difference,
or beat frequency, between the two subcarriers,
about 920 kHz, falls in the luminance bandwidth and
could potentially have been quite visible.
The NTSC recognized that the visibility of this pattern
could be minimized if the beat frequency was lineinterlaced.
Since the color subcarrier is necessarily an
odd multiple of half the line rate, the sound subcarrier
had to be made an integer multiple of the line rate.
The NTSC decided that the color subcarrier should be
exactly 455 ⁄2 times the line rate. Line interlace of the
beat could be achieved by increasing the sound-to-line
rate ratio (previously 285 5 ⁄7) by the fraction 1001 ⁄1000 to
the next integer (286).
Setting broadcast standards was (and is) the responsibility
of the Federal Communications Commission. The
CHAPTER 31 FRAME, FIELD, LINE, AND SAMPLE RATES 373