Adding color to a display can be done with a programmable color subcarrier generator like the one in Fig. 2-11. Color information is added to a tv signal by inserting a burst of high-frequency signal on the “back porch” of all the horizontal sync pulses (see Fig. 2-11A). This color “burst” is exactly 8 cycles at 3.579545 MHz. The phase of the color burst, rather than the frequency, is made to vary with the original color information. When the color burst phase is compared to a reference signal with a fixed phase the phase changes are converted back to the original color changes for the display. This is called color encoding and decoding.

Fig. 2-11B shows the actual circuit for a simple color encoder. The important thing to realize about color is that it on the screen is encoded in bits of memory just like characrequires memory, that is, the color for a particular location ters are encoded. That is why in the figure there are three color select inputs to the 4512 1-of-8 decoder. The three color bits are decoded from an ASCII character, and enable one of the eight CMOS delay buffers. These three color inputs are tied indirectly to the computer memory and use the incoming character bytes to enable a particular tap on the delay line set up by the six CMOS inverters. The oscillator generates the required 3.579545-MHz color burst signal and the hex inverters produce the desired delays. Once a phase delay is selected by the 4512, it is passed to the video combiner, where it is added to the raw video and sync signals.

In Fig. 2-12 the circuitry for an 8-color display is shown. Note we are using our NOR gate block graphics/text modification to the character generator ROM. Each ASCII character is converted to a block graphics character and the upper three bits determine the color of that particular character.


(A) 8 cycles of 3.579545 MHz added to the back porch of horizontal sync pulse.


(B) Programmable color subcarrier generator. Fig. 2-11. Adding color to computer display.
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