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 (Because analogue television is being phased out globally, VIT standards are considered superseded.)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 and

vertical Vertical is a geometric term of location which may refer to: * Vertical direction, the direction aligned with the direction of the force of gravity, up or down * Vertical (angles), a pair of angles opposite each other, formed by two intersecting s ...

. Horizontal blanking is between lines and vertical blanking is between fields (half frames). In a poorly tuned

TV receiver A television set or television receiver, more commonly called the television, TV, TV set, telly, tele, or tube, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasts, or using ...

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)

References and notes

{{Analogue TV transmitter topics
Broadcast engineering Television technology Broadcast transmitters

Blanking in CVS

Vertical blanking

Test signals

See also

References and notes