Page:Gametronics Proceedings.djvu/139

 - . In designing TV games for standard television receivers one should be aware of the signal processing encountered and the corresponding limitations this places on color resolution. Color in a TV display is produced with 3 primary colors, red, green, and blue. So that color and monochrome TV can be compatible a defining relation between the luminance or brightness of each spot in the scene exists:

Y is the "black & white" portion of the signal while C ( for chrominance ) contains information about hue and saturation of the color in the form of a phase and amplitude modulated subcarrier at 3.58 MHz. Figure 4a shows this chroma vector.

The 2 axis of the vector plane are called (R–Y) and (B–Y). With Y given these 2 vectors can be used to algebraicly solve for the third color green. A reference at 180 degrees (called burst) defines the phase for synchronous demodulation in the TV set of the 3.58 MHz signal.

Figure 4b shows a close-up of about 75 kHz of video frequency spectrum. The energy lobes of Y and C are shown. Since each sample of a TV line is scanned at 14 kHz rates the sidebands extend upward in frequency at integer multiples of Fh, the horizontal scan frequency.

In order to minimize interference between chroma information the subcarrier is chosen to be an odd half multiple of the line frequency, causing its sidebands to lie interleaved at integer half multiples of the line frequency. In order to properly separate the two signal components a true comb filter approach must be used; however, virtually no home TV set employs this technique. The result is a less than perfect separation of color and monchrome information from the composite video signal, and a limiting resolution on the color information.

Figure 4d shows a typical TV receiver technique for separating Y and C. Basically, a bandpass filter formed of a resonant parallel tank circuit centered at 3.58 Mhz steers the dominant chroma sidebands to the chroma demodulators to produce R-Y and B-Y signals. The Y signal proceeds through a delay line to compensate for timing shifts through the bandpass. It may also be notch filtered to remove or reduce 3.58 Mhz components.

Unfortunately, the video spectrum extends only about 0.6 Mhz above the chroma frequency so the bandwidth filter must cut off at 4.2 Mhz, severely limiting the actual color resolution. Any attempts to pass fast chroma pulses through the circuit will meet with varying results as the damping of the tank resonance varies from set to set.