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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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