d. Note that an area of either side of the window is disregarded in this
measurement, and that only the enclosed area along the top of the bar is used.
This is true for either horizontal or the vertical rate waveform displays.
4.
Figure 3-18 illustrates the basic T-pulse responses encountered.
a. In Figure 3-18a, an amplitude-response error is apparent without phase-
response error.
Waveform errors this close to the transition do not impair the
signal as much as errors farther away.
In fact, this type of error should be
recognized as that obtained from "phaseless aperture correction" in camera chains.
Thus, there is a "crispening" effect of a single overshoot as compared with the
actual picture impairments such as would result from the remaining waveforms of
Figure 3-18.
b. Figure 3-18b, shows the "skew symmetrical" distortion caused when the delay
increases with increasing frequency.
Figure 3-18c, shows the opposite type of
phase distortion, where the delay decreases with increasing frequency. In a system
with a fairly rapid rolloff that uses phase equalizers to correct the resulting
phase distortion, proper equalizer adjustment is indicated when ringing amplitudes
are equally distributed preceding and following the pulse as shown in Figure 3-18d.
c. The amplitude-frequency and amplitude-phase response at frequencies higher
than about 100 kHz is most evident in the measurement of the sin2 pulse.
Amplitude-phase response at frequencies below 100 kHz is most evident in
measurement of the window signal.
5. Distortions. Distortions at low frequencies produce waveform distortion with a
long time constant, as for example, streaking.
This is most evident in window
measurement.
Distortions at higher frequencies produce waveform distortions with
shorter time constants as, example, smearing, loss of resolution, or edge effects
from bad transient response. This is most evident in sin2 pulse measurement or in
window-signal transitions.
a. High-frequency rolloff results in loss of amplitude.
Loss of amplitude
results in a widening of the pulse, since the area of the pulse represents a
constant DC component.
A slow rolloff within the video band produces a large
reduction in amplitude (and pulse-width increase) with little or no ringing.
A
rapid rolloff close to the top of the band but still within the desired video
bandwidth produces both a reduction in amplitude, and ringing.
A rapid rolloff
just above the video bandwidth concerned results in practically no effect on
amplitude, but does not produce ringing. The shape of the rolloff and whether the
resulting phase shift is leading or lagging is revealed by the distribution of
ringing before and after the pulse.
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