(3) The type of display convenient for one of the tests associated with this
signal occurs when all the pulses and the window are in a single line. The scope
must be double triggered; that is, it must be triggered from successive sync
pulses. The two consecutive-line signal (fig 3-12) eliminates the need for double
triggering, since a repetitive sweep automatically provides the double-triggered
display.
However, the two-consecutive-line signal has the disadvantage of being
subject to error from frequency distortion because of the large difference in APL
between the two separate lines (window on one line and pulses on the other).
2. Use of the pulse-window signal in practice involves, a special graticule to
indicate certain K-factors, particularly for routine testing to provide a quick
observation to go no/go quality.
3. In an attempt to correlate test-signal measurement with an actual degree of
picture impairment, the K-factor is used.
The K-factor is basically defined in
terms of a standard picture distortion which is a single echo spaced in time 8T or
more from the main transition. For example, if the peak amplitude of this single
echo is 4 percent of the original transition amplitude, the K-factor is 4 percent.
a. In Figure 3-14, "A" signal transition with a sine squared shape occurs at t =
0. At a point spaced at +8T, a certain amplitude of "ring", or echo, exists. Let
us arbitrarily assume that this amplitude is 4 percent of the original amplitude,
so B = 4 percent. Waveform distortion A (fig 3-14) much closer to the transition
is larger, but its effect, as judged by an average observer, is only equal to the
picture impairment caused by echo B.
Thus, although echo A may be 16 percent of
the original amplitude, echo B, of only 4 percent amplitude, results in the same
degree of picture impairment.
We may construct a graticule mask which defines
limits within which a waveform must fit if it is to have a K-factor equal to or
less than the limits specified.
Figure 3-14.
Basic qualities involved in explaining K-factor
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