| 摘要 |
The circuit uses a reference frequency Fo of, say, 62500 Hz and does not need an expensive quartz crystal to provide the reference for demodulating the colour subcarrier. A voltage controlled oscillator (1) goes to a multiplier (2) whose other input is the colour subcarrier and whose output is the detected colour difference signal. A fixed reference frequency Fo is divided in stages (4,5,6,7), and the output is added to the VCO (1) output in a gate (8). The gate (8) output is compared with divided components of Fo in a comparator (9) whose output is digitally processed and then, via a DAC (13), the signal is added (3) to the colour difference signal: the adder (3) output controls the VCO (1).
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