(5) There is a very objectionable 0.92 MHz (920 kHz) signal generated from the
beat between the sound carrier and the color subcarrier (4.5 MHz - 3.58 MHz = 0.92
MHz).
The beat is much less objectionable if the sound-carrier frequency is a
multiple of the horizontal scan frequency away from the video carrier frequency.
In standard black and white TV, the 286th harmonic of the horizontal line rate,
15750 Hz is 4.5045 MHz.
The line frequency whose 286th harmonic is 4.5 MHz is
15734.26 Hz. F (line) = 4.5 MHz / 286 = 15734.26 (new line frequency for color).
(a) This frequency is within the deviation limit set by the NTSC monochrome
standards.
With the horizontal scan frequency changed, the color subcarrier must
be chosen to interleave.
It must be an odd multiple of one-half the horizontal
scan rate. With 3.6 MHz as an upper limit, it was found that the 455th harmonic of
half the horizontal scan rate becomes:
F (color subcarrier) = 455 x 15734.26 Hz / 2 = 3.58 MHz.
Since there are 525 lines, and using a 2:1 interlace, the new vertical scan
frequency becomes:
F (vertical) = 2 / 525 x 15734.26 Hz / 1 = 59.94 Hz.
(b) Notice that the new scanning frequencies used for color transmission
are slightly below the nominal values used in monochrome receivers. However, the
changes amount to less than the 1 percent tolerance allowed, and the new
frequencies fulfill the requirements for compatibility in monochrome reception.
6. Tolerance of subcarrier frequency.
To maintain close synchronization of the
color receivers, the tolerance for the subcarrier frequency is set at +/- 0.0003
percent, or around +/10 Hz; and the rate of change cannot be more than 1/10th Hz
per second.
The same tolerance applies to the field and line frequencies.
In
actual practice, all of these frequencies are developed from the same source to
minimize variations from established figures.
7. Chrominance sideband transmission.
Since picture detail is a function of the
allowed for chrominance information, this is an important point to consider.
The
highest usable video brightness signal frequency is 4.1 MHz, with the chrominance
subcarrier at 3.58 MHz (fig 1-17). Transmitting symmetrical or double sidebands of
chrominance would mean that color definition would be limited to 0.5 MHz and color
would be absent in all fine detail in the picture.
It is possible however, to
transmit the chrominance sideband vestigially: that is, with one side band
suppressed, the same as the black and white signal.
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