On Aug 4, 2010, at 6:46 PM, Doug McNutt wrote:
He was tuning the frequency of the horizontal oscillator. It has to match the rate at which the transmission is sent, about 15 kHz was standard NTSC television. There is also a vertical oscillator which was once 30 Hz.
Pre-NTSC, the horizontal sweep was 15,750 Hz and the vertical sweep was 60 Hz, interlaced 2:1, thereby giving a vertical frame rate of 30 f/sec.
Everything was divided-down from a master 31,500 Hz source, which also happens to be the frequency of the so-called "equalizing pulses" within the vertical interval, five or six cycles of which surround the actual vertical synch pulse.
NTSC introduces the concept of a "color burst", which is 3579545 Hz, precisely.
This is used to multiplex the three primary colors into an I and a Q channel, in "quadrature". The other channel is Luminance (Y), and it is arranged that the bulk of the information is transmitted as Luminance, which can be recovered by a monochrome TV using conventional techniques which ignore I and Q. However, a color TV has additional circuitry which enables it to accept Y, I and Q and to output R, G and B to the "shadow mask", or equivalent CRT.
As the horizontal and vertical sweep rates MUST be divided-down from the 3579545 Hz "burst" in order to eliminate "moire" and other image defects, the resultant vertical frame rate is 29.97 f/sec.
29.97 Hz is close enough to 30 Hz to pass without any significant issues, just as the horizontal sweep frequency is close enough to 15750 Hz to pass without significant issues.
B&W transmissions have been using the NTSC frequencies for quite a few decades, perhaps five, as it became an imperative in the 1960s to be able to seamlessly intermix color and monochrome transmissions, using a switching technique invented by Sarkes-Tarzian Inc, the justifiably famous "vertical interval switching" technique.
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