Rgb-composite-converter

Posted on Mar 24, 2013

The incoming RGB inputs are terminated with resistors Rl, R2, and R3 and potentiometers RVl, RV2, and RV3. These provide input impedances of approximately 75 0. The presets should be adjusted to provide a maximum input of 1 V pk-pk into the MC1377. The inputs are ac-coupled into the encoder; the large value capacitor is required for the 60 Hz field component. The Colpitts oscillator for the color burst is formed around pins 17 and 18. About 0.5 V pk-pk should appear on pin 17 and 0.25 V rms into pin 18 with the oscillator components removed.

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The incoming composite sync signal at pin 2 should be negative-going. The device will accept CMOS and TTL directly. If it is necessary to ac-couple the sync, then a pull-up to 8.2 V is required-a regulated 8.2 V is provided on pin 16. From the composite sync input, the MC1377 generates a ramp which it uses to provide the burst gate pulse. The slope of this ramp can be varied by a potentiometer on pin 1. However, a preset value, shown as 43 KO, is usually sufficient. The chrominance filter should be fitted between pins 13 and 10. If the filter is not used, a compensatory potential divider should be fitted (both are shown). We used a prealigned Thko bandpass filter centered on 4.43 MHz. If the chroma filter is~fitted, the delay through it, 400 ns, has to be compensated for by a luminance delay line between pins 6 and 8. This line is shorted out if the filter is not fitted. The composite video output from the IC is buffered to provide a low-impedance drive for a monitor, or it can be applied directly to a UHF modulator commonly used in computers.

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