MEASUREMENTS

Short Time Distortion
• DEFINITION
Short time distortions cause amplitude changes, ringing, overshoot
and undershoot in fast rise times and 21 pulses. The affected
signal components range in duration from 0.100 microsecond
to 1.0 microsecond.
For PAL systems, distortions in the short time domain are most often
characterised by measuring K2T or K pu | se /b ar . These measurements
are described in the K Factor section of this booklet (page 44).
Alternatively, the aberrations in a T rise time bar can be described
in terms of the "percent SD" method described in this section.
• PICTURE EFFECTS
Short time distortions produce fuzzy vertical edges. Ringing
can sometimes be interpreted as chrominance information
(cross colour), causing colour artifacts near vertical edges.
Figure 28. A J rise time bar has a 10% to 90% rise
time of nominally 100 nanoseconds.
• TEST SIGNALS
2T pulses are generally specified when K Factor and pulse-to-bar
methods are used to characterise short time distortion. In order
to use the "percent SD" method, however, a signal with a T rise
time white bar is required. As shown in Figure 28, a T rise time
bar has a 10%-to-90% rise time of nominally 100 nanoseconds.
(See Appendix B for a discussion of the time interval T.)
It is very important to make sure you are using a T rise time bar
with the short time distortion graticule. Many common test
signals have 2T rather that T rise times, and 2T bars are not
suitable for this measurement. It should also be noted that T
rise times cannot survive transmission, because they contain
fast components which do not pass through the 5 or 6 MHz
lowpass filter. Short time distortion measurements made on
transmitted signals will therefore evaluate only those components
in approximately the 200 nanosecond to 1 microsecond range.
• MEASUREMENT METHODS
Measurements of the undershoot, overshoot and ringing at the
edge of a T rise bar are not generally quoted directly as a
percent of the transition amplitude, but rather are expressed in
terms of an amplitude-weighting system that yields results in
"percent SD". This weighting is necessary because the amount
of distortion depends not only on the distortion amplitude, but
also on the time the distortion occurs with respect to the transition.
Although results can be calculated from the time and
amplitude of the measured ringing lobes, special graticules,
conversion tables or nomographs are used in practice.
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