In television broadcasting, VIT signals (vertical interval test signals) are a group of test signals inserted in the composite video signal. These signals are used to weight the transmission characteristics of the system between the
test generator and the output of the
demodulator
Demodulation is the process of extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from ...
, where the system includes the
microwave links, or
TVROs as well as the
TV transmitters and the
transposer
In broadcasting, a transposer or translator is a device in or beyond the service area of a radio or television station transmitter that rebroadcasts signals to receivers which can’t properly receive the signals of the transmitter because of a p ...
s. There are both
ATSC and
EBU standards for VIT. (Because analogue television is being phased out globally, VIT standards are considered superseded.)
Blanking in CVS
In a
composite video signal
Composite video, also known as CVBS (composite video baseband signal or color, video, blanking and sync), is an analog video format that combines image information—such as brightness (luminance), color (chrominance), and synchronization, int ...
(CVS) there are two types of blanking:
horizontal and
vertical. Horizontal blanking is between lines and vertical blanking is between fields (half frames). In a poorly tuned
TV receiver the horizontal blanking can be seen at the right or left of the image and the vertical blanking can be seen at the top or bottom of the image. VIT signals are inserted in the vertical blanking.
Vertical blanking
In each field vertical blanking is about 1612 μs in
System B (also G and H; analogue system in most of Europe) and 1333 μs in
System M (analogue TV system in USA). This duration is equal to 25 lines in system B and 21 lines in system M. Although 7.5 lines are used for synchronization of the image, the remaining lines can be used for other purposes.
[For example, communication between the studio and transmitter station.] Two of these lines in each field are reserved for test signals. Since there are two fields in each frame (image), the number of lines reserved for test signals is four per frame.
Test signals
In both systems, line numbers 17 and 18 are assigned for VIT signals in each field. (These line numbers are used just for the first field. For second field, they correspond to line 280 and 281 in system M, and line 330 and 331 in system B.)
Usually the following test signals are used:
*Luminance bar (low frequency tilt)
*2T signal (Overshoot)
*20T signal (differential gain and phase)
*Staircase (luminance linearity)
*Group of sine waves with different frequencies (video characteristics)
*Color carrier superimposed on staircase (differential gain and phase)
*Group of color carriers with different amplitudes (intermodulation of luminance and color)
See also
*
Signal generator
A signal generator is one of a class of Electronics, electronic devices that generates electrical signals with set properties of amplitude, frequency, and wave shape. These generated signals are used as a stimulus for electronic measurements, typ ...
*
TV transmitter
References and notes
{{Analogue TV transmitter topics
Broadcast engineering
Television technology
Broadcast transmitters