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ar. The range of the pulse delay
adjustments is such as to permit
placing the narrow gray bar
close to the beginning of the line
interval to permit detailed examination
with a minimum of demand
on the oscilloscope delay
and expansion circuits.
The shape of the edges of the
narrow (gray) pulse as generated
is of interest, since inherent
symmetry is essential. In addition,
the rise time of the pulse
edges should be such as to permit
it to be passed through the luminance
channel of a typical encoder
without serious effect on its
symmetry. It has been found that
a rise time of 0.2 secs is the
shortest that will meet this requirement,
as shorter rise times
tend to produce in the average
luminance channel an overshoot
of sufficient amplitude to confuse
the desired final symmetry indication.
Fortunately, simple capacitive
loading in the narrow
pulse amplifier of the unibar generator
can produce an output
pulse of adequate symmetry with
this rise time. An oscillogram of
the narrow pulse (the green output
of the unibar generator) is
shown in Fig. 4.
Encoder Application
To check chroma -vs- luminance
delay in an encoder, independent
tests are made of Y vs Q delay,
then I vs Y delay. The delays are
stated in this order because, although
the Y signal is used as a
reference, the Q delay is normally
fixed by the characteristic of the
Q channel filter. Any required
adjustment is accomplished by
changing the lengths of the Y and
I delay cables.
Checks are made of each chroma
signal's delay relative to luminance,
deactivating the circuits of
the encoder that produce the unwanted
chroma signal component.
It has been found that a narrower
gray pulse width is desirable for
checking I vs Y delay than for
checking Y vs Q. The optimum
widths are roughly 1 sec for I
and 2 µsecs for Q.
series of oscillograms are
A
shown in Figs. 5 through 12, to
illustrate typical appearances of
(Continued on page 190)
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189