Part III: Background and Concepts
26-3 Principles of Gated Measurements
Field
A television image, or frame, is composed of 525 (or 625 lines). When displayed, these are delivered in two
successive fields of 262.5 (or 312.5 lines) interlaced together on a CRT. When
Field
is set to
Entire Frame
, the
line count starts at line one in field one (often referred to as the “odd field”) and ends at 525 (or 625) in field
two (often referred to as the “even field”). When
Field
is set to
Field One
or
Field Two
, the line count begins
at “1” with the first full line in the selected field and ends at count 263 (or 313) for Field One, and 262 (or 312)
for Field Two.
Sync
Analog broadcast or cable television signals are usually amplitude-modulated on an RF carrier. For NTSC
and PAL broadcasts, typically the RF carrier amplitude is maximized at the sync tips of the baseband video
waveform and minimized at the “white” level. This results in a demodulated waveform on the analyzer
where the sync pulses are on top, or positive (
Sync
[Pos]). With SECAM broadcasts, typically the RF carrier
amplitude is minimized at the sync tips of the video waveform and maximized at the “white” level. This
results in a waveform on the analyzer where the sync pulses are at the bottom, or negative (
Sync
[Neg]). A
normal baseband video waveform for all TV standards will have the sync tips on the bottom. When
TV
Source
is set to
Ext Video In
,
Sync
should be set to
Neg
.
Composite color TV video signal is composed by the modulated chrominance, and is added to the
luminance information along with appropriate horizontal and vertical sync signals, blanking signals, and
color burst signals. In a system of 625 scanning lines, there are 25 blanking lines, lines 623~22 in odd field and
line lines 311~335 in even field. These lines are used to transmit blanking signal for the electron beam at the
end of the line scanning and field scanning can come back to the start point of the picture. In a system of
625 scanning lines, every field has 25 lines as the flyback line, so every frame only has 575 effective lines.
When engineers test TV systems, they often use full-field test signals, requiring interruption of TV broadcast.
Obviously, if we want to test the dynamic parameters, we cannot conduct measurements and broadcast
TV programs at the same time. As the TV industry has continued to develop, TV program broadcast hours
continuously increase. TV program service providers hope to monitor TV system performance parameters
and judge whether the broadcast equipment is sufficient or not according to these parameters. They can
modify this equipment according to these parameters. This requirement is very important for TV program
service providers.
Vertical blanking interval is not used to transmit any information. Later, researchers use these free lines to
convey auxiliary signals for increasing the bandwidth utilization ratio. Vertical Interval Test Signal (VITS) can
give important system dynamic performance parameters without influencing the TV program broadcast.
VITS also provides the possibility to automatically monitor, modify and broadcast. Inserting test signals in the
vertical blanking interval is used in real time monitoring, helping to modify and improve TV transmission
quality by at least determining which parameter is below acceptable quality or does not meet mandatory
thresholds prescribed by the FCC. Now, the VITS signal is an indispensable part of international and
domestic TV signals.
Some signals can measure video parameters: in-channel frequency response, luminance nonlinear
distortion, Differential Gain, Differential Phase, Chrominance Luminance Gain Inequality and Chrominance
Luminance Time Delay Inequality. These test signals are combined into some composite test signals
meeting the video transmission line signal and insert vertical blanking interval after the field sync pulse.
These composite signals are called VITS signal and these vertical blanking interval line are called insert test
line.
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