Deviser S7200 User Manual page 104

Part III: Background and Concepts
8-3.1 CCN, C/CSO and C/CTB Gated Measurements
CCN, C/CSO and C/CTB can all perform gated measurements, but C/CTB gated measurements are slightly
different from C/N's and C/CSO's: C/CTB interference falls right at the video carrier frequency. First, we shall
discuss CCN and C/CSO gated measurement.
CCN gated measurements must measure video signal carrier peak levels and corrected noise levels (over
a specified bandwidth) on a quiet line during the vertical blanking interval. These two results are then used
to calculate CCN.
A C/CSO gated measurement also needs to measure video signal carrier peak level and second order
beat on a quiet line during the vertical blanking interval, and then uses the results to calculate C/CSO.
C/CTB gated measurements differ from CCN and C/CSO, because the third order beat coherent distortion
is located directly at the video carrier frequency. By definition, this signal is covered by the video carrier. In
order to measure the third order beat, you must shut off the video signal carrier. Using traditional methods,
you would need to shut off the modulator, let the video carrier level disappear, and then measure the third
order beat right at the video carrier frequency. In gated measurement mode, however, it is suggested that
you use a quiet line inserter, which gates off at the necessary time (at the specified field).
The S7200 first measures the video carrier peak level during the sync pulse. Then, while the quiet line inserter
removes a specified line at the same time in every frame, the instrument uses the gated function to
measure third order beat. The instrument uses two results to calculate the C/CTB. The effects of removing a
quite line during a specific vertical blanking interval are the same as if you shut off the modulator, but at a
very brief, specific time.
The figure below illustrates how a quiet line inserter should be installed behind each modulator to be tested.
The advantage of using a quiet line inserter is that, prior to inserting a quiet line, it removes the line to be
used for insertion of the quiet line first. Using this method, C/CTB measurements are almost undetectable by
the end user customer: a low-level audible sound is emitted, but this is a better solution than removing
modulation, which renders TV picture and sound inoperable for the duration of the test.
When performing a C/CTB measurement with a quiet line inserter (remembering to remove the line first) by
removing/inserting only one of the 2 video fields, the audible distortion coming out of the speaker of your
television is almost indistinguishable. (If both fields are used, the pitch of the audible distortion is higher and
more noticeable.)
CTB quit line removing equipment connection
Quiet line inserter in this example: TVMS4200.
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