(2) If the type of distortion is strictly linear, dimensions a and b are
equal.
If nonlinear distortion is present, these dimensions may differ.
If
reducing the level of the test signal into the system changes the relative
dimensions of a and b, nonlinear distortion is present, and a lower level of test
signal input should be used to check the actual linear distortion.
(3) Using the vertical-rate CRO display of the same signal, the white-going
setup between the bottom of the white signal and blanking serves as an accurate
indicator of the percentage of distortion. This defect is the result of excessive
gain at low frequencies and causes an increase in setup level, in addition to the
streaking effect from the attendant low-frequency phase shift. Such distortion is
usually the result of a defective equalizer on long lines, or overcompensation with
low-frequency compensation controls or tilt controls.
(4) LD resulting in negative streaking (black-after-white) impairs the display
of lettering. The vertical-rate display indicates clearly the loss of setup, which
occurs because this type of phase distortion is the result of insufficient gain at
low frequencies, up to about the tenth harmonic of the nominal linescanning
frequency of 15,750 Hz. In practice, the loss of gain occurs below the first few
harmonics, or approximately 50 kHz.
8. Field-time Waveform Distortion (FD) results in impairment of brightness
reproduction from top to bottom of the picture. Measurement of FD is done across
the top of the window signal viewed at the vertical rate, and by the relationship
of the leading and trailing edge of reference black.
9. Relative Chroma Level (RCL) is a measure of the faithfulness of reproduction of
the saturation of all colors in a color picture. High RCL causes more vivid colors
than intended; low RCL causes colors more pale than intended. Measurement of RCL
is done most readily with the modulated 20T pulse.
10. Relative Chroma Time (RCT) is a measure of relative chroma and luminance delay.
The results of RCT errors is misregistration of all colors with their respective
luminance components.
Delayed RCT places chroma to the right of its luminance
component; advanced RCT places chroma to the left. Measurement of RCT is done with
the modulated 20T pulse.
11. Modulated 20T Pulse. The modulated 20T pulse is the most convenient method of
displaying RCL and RCT.
Figure 3-21 typifies the display when pure amplitude
distortion exists (no phase distortion).
A change in amplitude of the 3.58-MHz
subcarrier results in a cosine-shaped distortion of the base line, and a departure
from reference peak level (top of window signal). When the distortion is linear,
dimensions dl and d2 are equal.
If these dimension are unequal, nonlinear
distortion (differential gain) is present.
In this case, linear distortion
normally can
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