VIT signals

Last updated

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 (Because analogue television is being phased out globally, VIT standards are considered superseded.)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

Contents

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.


Related Research Articles

Analog television original television technology that uses analog signals to transmit video and audio; in an analog television broadcast, the brightness, colors and sound are represented by rapid variations of either the amplitude, frequency or phase of the signal

Analog television or analogue television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by rapid variations of either the amplitude, frequency or phase of the signal.

NTSC analog television system

NTSC, named after the National Television System Committee, is the analog television color system that was used in North America from 1954 and until digital conversion, was used in most of the Americas ; Myanmar; South Korea; Taiwan; Philippines; Japan; and some Pacific island nations and territories.

PAL Colour encoding system for analogue television

Phase Alternating Line (PAL) is a colour encoding system for analogue television used in broadcast television systems in most countries broadcasting at 625-line / 50 field per second (576i). Other common colour encoding systems are NTSC and SECAM.

Composite video analog video transmission

Composite video is an analog video transmission that carries standard definition video typically at 480i or 576i resolution as a single channel. Video information is encoded on one channel, unlike the higher-quality S-video and the even higher-quality component video. In all of these video formats, audio is carried on a separate connection.

Genlock is a common technique where the video output of one source, or a specific reference signal from a signal generator, is used to synchronize other picture sources together. The aim in video applications is to ensure the coincidence of signals in time at a combining or switching point. When video instruments are synchronized in this way, they are said to be generator-locked, or genlocked.

Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance.

Component video video signal that has been split into two or more component channels

Component video is a video signal that has been split into two or more component channels. In popular use, it refers to a type of component analog video (CAV) information that is transmitted or stored as three separate signals. Component video can be contrasted with composite video in which all the video information is combined into a single line level signal that is used in analog television. Like composite, component-video cables do not carry audio and are often paired with audio cables.

SMPTE color bars standard television test pattern

The SMPTE color bars is a trademarked television test pattern used where the NTSC video standard is utilized, including countries in North America. The Society of Motion Picture and Television Engineers (SMPTE) refers to it as Engineering Guideline EG 1-1990. Its components are a known standard. Comparing it as received to the known standard gives video engineers an indication of how an NTSC video signal has been altered by recording or transmission and what adjustments must be made to bring it back to specification. It is also used for setting a television monitor or receiver to reproduce NTSC chrominance and luminance information correctly. It was originally conceived by Norbert D. Larky and David D. Holmes of RCA Laboratories and first published in RCA Licensee Bulletin LB-819 on February 7, 1951. U.S. patent 2,742,525 Color Test Pattern Generator was awarded on April 17, 1956 to Norbert D. Larky and David D. Holmes. Previously categorized by SMPTE as ECR 1-1978, its development was awarded an Engineering Emmy in 2001-2002. An extended version of SMPTE Color Bars signal, developed by the Japanese Association of Radio Industry and Businesses as ARIB STD-B28 and standardized as SMPTE RP 219:2002 was introduced to test HDTV signal with an aspect ratio of 16:9 that can be down converted to a SDTV color bar signal with an aspect ratio of either 4:3 or 16:9. The Color Bar signal is generated with unconventionally slow rise and fall time value to facilitate video level control and monitor color adjustments of HDTV and SDTV equipment. Digital test images generated following the SMPTE RP 219:2002 specifications and adapted to perfectly fit 114 standard and non-standard resolutions for both 16bpp and 8bpp, are freely available in the COLOR dataset of the TESTIMAGES archive.

Broadcast television systems are encoding or formatting standards for the transmission and reception of terrestrial television signals. There were three main analog television systems in use around the world until the late 2010s (expected): NTSC, PAL, and SECAM. Now in digital terrestrial television (DTT), there are four main systems in use around the world: ATSC, DVB, ISDB and DTMB.

Dot crawl

Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video, as in terrestrial broadcast television. It consists of animated checkerboard patterns which appear along horizontal color transitions. It results from intermodulation or crosstalk between chrominance and luminance components of the signal, which are imperfectly multiplexed in the frequency domain.

NABTS, the North American Broadcast Teletext Specification, is a protocol used for encoding NAPLPS-encoded teletext pages, as well as other types of digital data, within the vertical blanking interval (VBI) of an analog video signal. It is standardized under standard EIA-516, and has a rate of 15.6 kbit/s per line of video.

A video signal generator is a type of signal generator which outputs predetermined video and/or television oscillation waveforms, and other signals used in the synchronization of television devices and to stimulate faults in, or aid in parametric measurements of, television and video systems. There are several different types of video signal generators in widespread use. Regardless of the specific type, the output of a video generator will generally contain synchronization signals appropriate for television, including horizontal and vertical sync pulses or sync words. Generators of composite video signals will also include a colorburst signal as part of the output.

Nominal analog blanking or nominal analogue blanking is the outermost part of the overscan of a standard definition digital television image. It consists of a variable and arbitrary gap of black pixels at the left and right sides, which correspond to the end and start of the horizontal blanking interval: the front porch at the right side, and the back porch at the left side. Digital television ordinarily contains 720 pixels per line, but only about 702 (PAL) to 704 (NTSC) of them contain picture signal. The edge may not be clean — some blurring can occur — and the location is arbitrary, since analogue equipment may typically shift the picture sideways in an unexpected amount or direction. Really old analogue equipment can cause the total width of 'active picture' to vary, usually down, perhaps to as few as 680 pixels.

C-MAC Video format

C-MAC is the variant approved by the European Broadcasting Union (EBU) for satellite transmissions. The digital information is modulated using 2-4PSK, a variation of quadrature PSK where only two of the phaser angles (±90°) are used.

Differential gain is a kind of linearity distortion which affects the color saturation in TV broadcasting.

The following outline is provided as an overview of and topical guide to television broadcasting:

Article needs redesign. See Talk.

Color television as introduced in North America in 1954 is best described as being 'colored' television. The system used the existing black and white signal but with the addition of a component intended only for television receivers designed to show color. By careful application this 'colored' signal was ignored by ordinary TV sets and had negligible effect on the appearance of the black and white image. This meant that color programs were viewable on the many existing black and white receivers which fulfilled a requirement for 'compatibility' desired by the television industry. Once the so called 'composite' video signal containing the color component had been generated it could be handled just as if it were a black and white signal, eliminating the need to replace much of the existing TV infrastructure. Colorplexer was the RCA name for the equipment that created this 'composite' color signal from three separate images each created in the primary colors, Red, Green and Blue supplied by a color video camera. This process was by the standards of the day quite complex and demanded accurate control of all the various parameters involved if an acceptable color image was to be achieved. The simplification afforded by this 'head end' approach became evident and contributed to the gradual acceptance of color programming over the following decades.

Pulse-cross

A pulse-cross is the representation of normally invisible portions of an analog video signal on a studio screen for error analysis.