Synchronization of color carriers of different color TV standards having the same line frequency
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United States of America Patent
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-
Dec 1, 1992
Grant Date -
N/A
app pub date -
Mar 6, 1991
filing date -
Mar 26, 1990
priority date (Note) -
Expired
status (Latency Note)
Importance
Abstract
When two color television standards prescribe the same horizontal scanning frequency but different color carrier frequencies, one of them, for example, requiring a frequency providing a one-half line offset to the horizontal scanning frequency (NTSC Standard) and the other requiring a three-quarter line offset (PAL-M Standard), each color carrier frequency being locked to the common line frequency, a modified phase locked loop circuit makes it possible to synchronize the color carrier frequencies to each other. The oscillator at the color carrier frequency of one standard has its output compared, in a phase comparison stage, with the color synchronization signal of the other standard. The output of the phase comparison stage is supplied to a sample-and-hold circuit which is sampled during the presence of that color synchronizing signal in every fourth line. The content of the sample-and-hold circuit is passed through a low-pass filter and then supplied to the control input of the oscillator, thus locking the latter to the color carrier of the other standard.
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