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Special Measurements
OVERSCANNED DISPLAYS. For various video
mea-
surements,
it
may
be
desirable
to
magnify the video
wave-
form
vertically beyond the limits
of
the screen. Under
these
circumstances, the trigger amplifiers
or
Sync Separator may
be
overloaded, blocking
out
some
sync pulses in the
vicinity
of
strong video transitions,
or
losing sync pulses altogether.
To
avoid overload problems,
use
external sync
or
use
the
other vertical channel
to
supply a constant amplitude signal
to
the Sync Separator while the overscanned observations
are
being made. Note, however,
that
transient-response
aberrations in the main vertical
amplifier
will
be
increased
when the signal
is
driven offscreen, becoming relatively
serious
if
the amplifier
is
driven
to
saturation and
cutoff.
HORZONTAL
SYNC PULSE MEASUREMENTS.
Rise
and fall times and
width
of
horizontal sync pulses may
be
measured while using the Sync Separator
to
determine
whether part
or
all
of
the lines
or
groups
of
lines appear
to
be
abnormal. A bright display
of
all horizontal sync pulses
is
obtained when the A
TRIGGER
COUPLING switch
is
set
to
TV
LINE.
RF INTERFERENCE. Operation in the
vicinity
of
some
FM
and
TV
transmitters may show objectionable amounts
of
rf
signal energy in the display,
even
when coaxial
input
connections
are
used. The front-panel 20 MHz
BW
LIMIT
switch
will
usually eliminate
such
interference
from
the
display,
but
will
not
affect
the signal reaching the Sync
Separator. Where the
rf
interferes
with
Sync Separator
operation, external filters
will
be
required.
Use
of
probes
designed
for
10-30 MHz oscilloscopes
will
provide 6
to
10
dB attenuation in the 50-100 MHz
range
and may
be
beneficial in reducing interference.
IDENTIFYING FIELDS, FRAMES AND
LINES IN 525/60 AND 625/50
TV SYSTEMS
NTSC (CCIR System
M)
Field 1
is
defined
as
the field whose
first
equalizing
pulse
is
one
full
H interval (63.5
J..Ls)
from
the preceding
horizontal sync pulse. The Field
1 picture starts
with
a
full
line
of
video. Field 1 lines
are
numbered 1 through 263,
starting
with
the leading
edge
of
the
first
equalizing pulse.
The
first
regular horizontal sync pulse after the second
equalizing interval
is
the start
of
line 10.
Field 2 starts
with
an
equalizing pulse a half-line interval
from
the preceding horizontal sync pulse. The Field 2
picture starts
with
a
half
line
of
video. Field 2 lines
are
numbered 1 through 262, starting
with
the leading
edge
of
the second equalizing pulse.
After
the second equalizing
interval, the
first
full
line
is
line 9.
@
4658/DM44
Operators
105
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