VIT signals

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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 [1] the transmission characteristics of the system between the test generator and the output of the demodulator, where the system includes the microwave links, or TVROs as well as the TV transmitters and the transposers. There are both ATSC and EBU standards for VIT. (Because analogue television is being phased out globally, VIT standards are considered superseded.)

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 physical obstruction. A translator receives the signals of the transmitter and rebroadcasts the signals to the area of poor reception. Sometimes the translator is also called a relay transmitter, rebroadcast transmitter or transposer. Since translators are used to cover a small shadowed area, their output powers are usually lower than that of the radio or television station transmitters feeding them.


Blanking in CVS

In a composite video signal (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.

Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly across the screen. Once the beam of the monitor has reached the edge of the screen, the beam is switched off, and the deflection circuit voltages are returned to the values they had for the other edge of the screen; this would have the effect of retracing the screen in the opposite direction, so the beam is turned off during this time. This part of the line display process is the Horizontal Blank.

In a raster graphics display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final line of a frame or field and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the VBI, the incoming data stream is not displayed on the screen. In raster cathode ray tube displays, the beam is blanked to avoid displaying the retrace line; see raster scan for details. The signal source, such as a television broadcast, does not supply image information during the blanking period.

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. [2] Although 7.5 lines are used for synchronization of the image, the remaining lines can be used for other purposes. [3] 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.

CCIR System B was the 625-line analog broadcast television system which at its peak was the system used in most countries. It is being replaced across Western Europe, part of Asia and Africa by digital broadcasting.

CCIR System M

CCIR System M, sometimes called 525 line, is the analog broadcast television system used in the United States since July 1, 1941, and also in most of the Americas and Caribbean, South Korea, and Taiwan. Japan uses System J, which is nearly identical to System M. The systems were given their letter designations in the ITU identification scheme adopted in Stockholm in 1961. Both System M and System J display 525 lines of video at 30 frames per second using 6 MHz spacing between channel numbers, and is used for both VHF and UHF channels.

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:

See also

References and notes

  1. Weighting is a crude measurement. Complete aligning of the transmitter needs professional measuring equipment.
  2. Bernard Grob-Charles E. Herndon: Basic Television and Video systems, Glencoe-McGraw Hill, 1999, p 216.
  3. For example, communication between the studio and transmitter station.

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