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, where the system includes the
microwave link
Microwave transmission is the transmission of information by electromagnetic waves with wavelengths in the microwave frequency range of 300MHz to 300GHz(1 m - 1 mm wavelength) of the electromagnetic spectrum. Microwave signals are normally lim ...
s, or
TVRO
Television receive-only (TVRO) is a term used chiefly in North America, South America to refer to the reception of satellite television from FSS-type satellites, generally on C-band analog; free-to-air and unconnected to a commercial DBS prov ...
s as well as the
TV transmitter
A television transmitter is a transmitter that is used for terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized au ...
s 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 (Because analogue television is being phased out globally, VIT standards are considered superseded.)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 (CVS) there are two types of blanking: Horizontal blanking interval, horizontal and Vertical blanking interval, 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 CCIR System B, System B (also G and H; analogue system in most of Europe) and 1333 μs in CCIR System M, 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
*
TV transmitter
A television transmitter is a transmitter that is used for terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized au ...
References and notes
{{Analogue TV transmitter topics
Broadcast engineering
Television technology
Broadcast transmitters