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Musical-Applications-of-Microprocessors-2ed-Chamberlin-H-1987

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350 MUSICAL ApPLICATIONS OF MICROPROCESSORS<br />

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(B)<br />

Fig. 11-5. Simple (A) and interlaced (8) raster scanning<br />

The maximum resolution <strong>of</strong> a raster scan display is fairly well defined<br />

by the scanning frequencies used. Since television receivers and TV standard<br />

video monitors are so common, the low-cost raster display maker is essentially<br />

locked into standard TV frequencies, which are 15,750 Hz horizontal<br />

and 60 Hz vertical. For a normal scanning pattern, this yields 262 horizontal<br />

scan lines <strong>of</strong> which roughly 240 are usable (the others are wasted during<br />

vertical retrace). There is no hard limit on horizontal resolution, but there is<br />

little reason to have more points per inch than the vertical axis. With a 4:3<br />

aspect ratio (screen width:height), this gives a horizontal resolution <strong>of</strong> 320<br />

points.<br />

A technique called interlacing effectively doubles the vertical resolution<br />

to 480 lines. It does this by vertically shifting odd-numbered screen scans<br />

one-half the scan line spacing with respect to even-numbered scans. Although<br />

effective for moving pictures viewed at a distance which is typical <strong>of</strong>

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