of Microprocessors

348 MUSICAL ApPLICATIONS OF MICROPROCESSORS
127,254
0,127
254, I27
127,0
PROPER TEST PATTERN
ONE SHOT TOO LONG
ONE SHOT TOO SHORT
UNMATCHED RESPONSE
(Y AXIS SLOWER)
UNMATCHED RESPONSE
(X AXIS SLOWER)
..'
.'
.........•......
.............
TEST PATTERN SHOWING
CURVATURE
Fig. 11-4. Adjustment of vector generator
constant velocity in the interest of simplicity. The effect of varying beam
velocity is that short lines will appear to be brighter than long lines and the
portion toward the endpoints may also be brighter than the middle. To the
eye, the effect is not objectionable but it does create difficulty if one wishes to
photograph the screen.
Vector generation is accomplished by using a portion of the step response
of a resonant low-pass filter. Figure 11-3 shows the step response of
the filter used. Note that the filter output rises from its previous value fairly
linearly, overshoots the final value, and then oscillates around it for a time
before settling. Since the time from the input step to the output first crossing
the final value is constant and the curve during this time is reasonably linear,
it is suitable for use as a vector generator. Note that line straightness on the
screen depends on matching the X and Y curves, not on the straightness of the
curves themselves. 51 and 52 have been added to quickly damp out the
oscillation around the final value after the line has been drawn and to allow
fast moving with the beam off. Their effect is to raise the cutofffrequency of