04-01-2013, 12:30 PM
TELEVISION STANDARD AND COMMUNICATION SYSTEM USED IN DOORDARSHAN AT DOORDARSHAN KENDRA PATNA
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INTRODUCTION
DOORDARSHAN (Hindi: दूरदर्शन; literally Distant Show) is an Indian public service broadcaster, a division of Prasar Bharati. It is one of the largest broadcasting organizations in India in terms of the infrastructure of studios and transmitters. Recently, it has also started Digital Terrestrial Transmitters. On September 15, 2009, Doordarshan celebrated its 50th anniversary. The DD provides television, radio, online and mobile services throughout metropolitan and regional India, as well as overseas through the Indian Network and Radio India
Doordarshan had a modest beginning with the experimental telecast starting in Delhi on 15 September 1959 with a small transmitter and a makeshift studio. The regular daily transmission started in 1965 as a part of All India Radio. The television service was extended to Bombay (now Mumbai) and Amritsar in 1972. Up until 1975, only seven Indian cities had a television service and Doordarshan remained the sole provider of television in India. Television services were separated from radio in April 1 1976 Each office of All India Radio and Doordarshan were placed under the management of two separate Director Generals in New Delhi. Finally, in 1982, Doordarshan as a National Broadcaster came into existence.
National telecasts were introduced in 1982. In the same year, colour TV was introduced in the Indian market with the live telecast of the Independence Day speech by then prime minister Indira Gandhi on 15 August 1982, followed by the 1982 Asian Games which were held in Delhi. Now more than 90 percent of the Indian population can receive Doordarshan (DD National) programmes through a network of nearly 1,400 terrestrial transmitters. There are about 46 Doordarshan studios producing TV programmes today
Presently, Doordarshan operates 21 channels – two All India channels - DD National and DD News, 11 Regional language Satellite Channels (RLSC), four State Networks (SN), an International channel, a Sports Channel DD Sports and two channels Rajya Sabha TV & DD-Lok Sabha for live broadcast of parliamentary proceedings.
On DD National (DD-1), Regional programs and Local Programmers are carried on time-sharing basis. DD News channel, launched on 3 November 2003, which replaced the DD Metro (DD-2) Entertainment channel, provides 24-Hour news service.
TELEVISION STANDARDS
There are three main television standards used throughout the world.
NTSC - National Television Standards Committee
Developed in the US and first used in 1954, NTSC is the oldest existing broadcast standard. It consists of 525 horizontal lines of display and 60 vertical lines. Only one type exists, known as NTSC M. It is sometimes irreverently referred to as "Never Twice the Same Color."
SECAM - Système Électronique pour Couleur avec Mèmoire.
Developed in France and first used in 1967. It uses a 625-line vertical, 50-line horizontal display. Different types use different video bandwidth and audio carrier specifications. Types B and D are usually used for VHF. Types G, H, and K are used for UHF. Types I, N, M, K1 and L are used for both VHF and UHF. These different types are generally not compatible with one another. SECAM is sometimes irreverently referred to as "Something Essentially Contrary to the American Method" or "SEcond Color Always Magenta."
PAL - Phase Alternating Line
Developed in Germany and first used in 1967. A variant of NTSC, PAL uses a 625/50-line display. Different types use different video bandwidth and audio carrier specifications. Common types are B, G, and H. Less common types include D, I, K, N, and M. These different types are generally not compatible with one another. Proponents of PAL irreverently call it "Perfection At Last," while critics of its enormous circuit complexity call it "Pay A Lot" or "Picture Always Lousy.
COLOUR ENCODING:-
Both the PAL and the NTSC system use a quadrature amplitude modulated subcarrier carrying the chrominance information added to the luminance video signal to form a composite videobaseband signal. The frequency of this subcarrier is 4.43361875 MHz for PAL, compared to 3.579545 MHz for NTSC. The SECAM system, on the other hand, uses a frequency modulation scheme on its two line alternate colour subcarriers 4.25000 and 4.40625 MHz.
The name "Phase Alternating Line" describes the way that the phase of part of the colour information on the video signal is reversed with each line, which automatically corrects phase errors in the transmission of the signal by cancelling them out, at the expense of vertical frame colour resolution. Lines where the colour phase is reversed compared to NTSC are often called PAL or phase-alternation lines, which justifies one of the expansions of the acronym, while the other lines are called NTSC lines. Early PAL receivers relied on the human eye to do that cancelling; however, this resulted in a comb-like effect known as Hanover bars on larger phase errors. Thus, most receivers now use a chrominance delay line, which stores the received colour information on each line of display; an average of the colour information from the previous line and the current line is then used to drive the picture tube. The effect is that phase errors result in saturation changes, which are less objectionable than the equivalent hue changes of NTSC.
TELEVISION PRINCIPLES AND SCANNING
The earliest mechanical television systems used spinning disks with patterns of holes punched into the disc to "scan" an image. A similar disk reconstructed the image at the receiver. Synchronization of the receiver disc rotation was handled through sync pulses broadcast with the image information. However these mechanical systems were slow, the images were dim and flickered severely, and the image resolution very low. Camera systems used similar spinning discs and required intensely bright illumination of the subject for the light detector to work.
Analog television did not really begin as an industry until the development of the cathode-ray tube (CRT), which uses a steered electron beam to "write" lines of electrons across a phosphor coated surface. The electron beam could be swept across the screen much faster than any mechanical disc system, allowing for more closely spaced scan lines and much higher image resolution, while slow-fade phosphors removed image flicker effects. Also far less maintenance was required of an all-electronic system compared to a spinning disc system.
Displaying an image
A cathode-ray tube (CRT) television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a raster. At the end of each line the beam returns to the start of the next line; at the end of the last line it returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the luminance of that point. Acolor television system is identical except that an additional signal known as chrominance controls the color of the spot.
Receiving signals:-
The television system for each country will specify a number of television channels within the UHF or VHF frequency ranges. A channel actually consists of two signals: the picture information is transmitted using amplitude modulation on one frequency, and the sound is transmitted with frequency modulation at a frequency at a fixed offset (typically 4.5 to 6 MHz) from the picture signal.
The channel frequencies chosen represent a compromise between allowing enough bandwidth for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called vestigial sideband is used to reduce the channel spacing, which would be at least twice the video bandwidth if pure AM was used.
Signal reception is invariably done via a superheterodyne receiver: the first stage is a tuner which selects a television channel and frequency-shifts it to a fixed intermediate frequency (IF). The signal amplifier (from the microvolt range to fractions of a volt) performs amplification to the IF stages.
Extracting the sound:-
At this point the IF signal consists of a video carrier wave at one frequency and the sound carrier at a fixed offset. A demodulator recovers the video signal and sound as an FM signal at the offset frequency (this is known as intercarrier sound).
The FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker. Until the advent of the NICAM and MTS systems, television sound transmissions were invariably monophonic.
COLOUR COMPOSITE VIDEO
SIGNAL
The video carrier is demodulated to give a composite video signal; this contains luminance, chrominance and synchronization signals;[5] this is identical to the video signal format used by analog video devices such as VCRs or CCTV cameras. Note that the RF signal modulation is inverted compared to the conventional AM: the minimum video signal level corresponds to maximum carrier amplitude, and vice versa. The carrier is never shut off altogether; this is to ensure that intercarrier sound demodulation can still occur.