5.
Grayscale test signals must be able to convey information at low and high
frequencies over every possible picture value likely to be encountered.
This
picture value is interpreted not only by amplitude, frequency, and phase response
of the system but also on a widely varying duty cycle. Duty cycle in pulse work
simply correlates the pulse duration with the pulse-repetition frequency (PRF):
Duty cycle = pulse duration x PRF
For television, this effect is most appropriately termed "average picture level"
(APL).
The amplitude, frequency, and phase response of the system must be held
within tolerable limits over the gamut of APLs encountered in practice.
6.
Even experienced TV engineers sometimes forget that a 1-volt peak-to-peak
video signal must be transferred through an amplifier capable of handling twice
this range with little degradation (figure 3-4).
Although the DC component is
restored at such points as blanking insertion, sync insertion, gamma-correction
stages, and transmitter modulators, practically all stages in between, as well as
distribution and stabilizing amplifiers, are AC coupled.
a. Waveform monitors such as those in master monitor positions use clamping
circuits to hold blanking level at the reference graticule line.
Some scopes
designed for waveform monitoring allow switchable operation, either clamped or
unclamped.
Even though the monitoring CRO is clamped, "bounces" of a momentary
duration will occur upon drastic scenic changes in APL, and this is normal. Most
scopes for routine testing and servicing do not use DC restorers or clamping
circuits.
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