B&K Precision 1249B - Manual - Manual (Page 30)

30
APPLICATIONS (CONT.)
SYNC
The NTSC color bars and staircase pattern include NTSC sync
pulses, the same type as those produced by a broadcast station.
The sync amplitude of 40 IEEE units is often the reference
against which the remainder of the luminance signal is compared.
Circuits can be checked for sync clipping by observing the
staircase pattern on an oscilloscope and checking whether the
sync pulse amplitude remains at 0.53 compared to the
75% white
step (grey) which is a reference of 1.0. AGC circuits, which
respond to sync pulse amplitude can be checked by using the
COMPOSITE V I D E O output and varying the level. The
overall amplitude of the video can be varied but the sync pulse
amplitude is a constant percentage of the total video amplitude.
The precise timing of the sync pulses allows proper adjustment
of servo circuits which control tape speed in VCR's and switching
circuits which should switch between video heads to allow a
continuous transition from field to field.
30Hz OUTPUT
Due to their electro-mechanical nature, servos are a common
problem with VCR's. Using standard troubleshooting techniques
and tools, isolating a servo problem can be very difficult if not
impossible. Using the 30Hz output of the NTSC Generator
however, should allow relatively simple isolation of the trouble
spot.
VCR's utilize closed loop feedback circuits to control the heads.
By injecting the 30Hz signal at different points in the closed loop,
the faulty circuit should be relatively easy to isolate. For example,
if you were to inject the 30Hz signal from the Generator into the
feedback path and no change in servo operation was noted, the
problem would be with the circuit that receives the feedback
signal. Conversely, if you noticed a change in the servo
operation, the problem would be in the circuit that sends the
feedback signal.
CCTV APPLICATIONS
Closed circuit television systems do not usually include a
built-in NTSC color reference signal. The NTSC Generator can
be used to supply such a reference. All equipment in the system
can then be adjusted to the same reference while connected into
or removed from the system.
Virtually all equipment used in CCTV systems is designed
around certain standard signal conditions at the input and output
terminals. This allows equipment to the compatible, without
modification, when inter-connected as a system. Among the
standard signal conditions are positive polarity video signal
(negative sync pulses). 1 volt peak-to-peak signal amplitude
75
Ω impedance and unbalanced line (one side grounded). The
NTSC Generator has these characteristics which simplifies set-
up for testing, troubleshooting and adjustment.
CATV APPLICATIONS
Cable television systems also use the standard signal
conditions specified in the "CCTV Applications" paragraph at
input and output terminals. The channel 3 or 4 rf output of the
unit can be used throughout a CATV network to test, adjust, or
troubleshoot amplifiers, cables, and any other equipment.