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TELEVISION STANDARDS

Learn about the major TV standards used worldwide: American NTSC, European PAL, and French-Former Soviet Union SECAM. Understand the differences in frame rates, lines, and color encoding principles. Get insights into color encoding for PAL and SECAM systems. Explore how television works and the technical details of transmitting sound and pictures through VHF and UHF ranges.

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TELEVISION STANDARDS

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  1. TELEVISION STANDARDS

  2. PAL , SECAM and NTSC • There are three major TV standards used in the world today. These are the 1. American NTSC (National Television Systems Committee) color television system, 2. European PAL (Phase Alternation Line rate) 3. French-Former Soviet Union SECAM (Sequential Couleur avec Memoire)

  3. The largest difference between the three systems is the vertical lines. NTSC uses 525 lines (interlaced) while both PAL and SECAM use 625 lines. • NTSC frame rates are slightly less than 1/2 the 60 Hz power line frequency, while PAL and SECAM frame rates are exactly 1/2 the 50 Hz power line frequency. Lines a. lines v. resolution aspect h.resolution frame rate • NTSC 525 484 242 4/3 427 29.94 • PAL 625 575 290 4/3 425 25 • SECAM 625 575 290 4/3 465 25

  4. Color Encoding Principles for the PAL • All three systems use the same definition for luminance: • The color encoding principles for the PAL system are the same as those of the NTSC system -- with one minor difference. • In the PAL system, the phase of the R-Y signal is reversed by 180 degrees from line to line. This is to reduce color errors that occur from amplitude and phase distortion of the color modulation sidebands during transmission.

  5. Saying this more mathematically, the chrominance signal for NTSC transmission can be represented in terms of the R-Y and B-Y components as • The PAL signal terms its B-Y component U and its R-Y component V and phase-flips the V component (line by line) as:

  6. Color Encoding Principles for the SECAM • SECAM system differs very strongly from PAL and NTSC • In SECAM the R-Y and B-Y signals are transmitted alternately every line. (The Y signal remains on for each line). Since there is an odd number of lines on any given scan, any line will have R-Y information on the first frame and B-Y on the second.

  7. Furthermore, the R-Y and B-Y information is transmitted on different subcarriers. The B-Y sub-carrier runs at 4.25 MHz and the R-Y subcarrier runs at 4.4 MHz. • In order to synchronize the line switching, alternate R-Y and B-Y sync signals are provided for nine lines during he vertical blanking interval following the equalizing pulses after the vertical sync.

  8. Summary • Television is the radio transmission of sound and pictures in the VHF and UHF ranges. The voice signal from a microphone is frequency-modulated. A camera converts a picture or scene into an electrical signal called the video or luminance Y signal, which amplitude-modulated • Vestigial sideband AM is used to conserve spectrum space. The picture and sound transmitter frequencies are spaced 4.5 MHz apart, with the sound frequency being the higher.

  9. TV cameras use either a vacuum tube imaging device such as a vidicon or a solid-state imaging device such as the charged-coupled device (CCD) to convert a scene into a video signal.

  10. A scene is scanned by the imaging device to break it up into segments that can be transmitted serially. The National Television Standards Committee (NTSC) standards call for scanning the scene in two 262½ line fields, which are interlaced to form a single 525-line picture called a frame. Interlaced scanning reduces flicker. • The field rate is 59.94 Hz, and the frame or picture rate is 29.97 Hz. The horizontal line scan rate is 15,734 Hz or 63.6 s per line.

  11. The color in a scene is captured by three imaging devices, which break a picture down into its three basic colors of red, green, and blue using color light filters. Three-color signals are developed (R, G, B). These are combined in a resistive matrix to form the Y signal and are combined in other ways to form the I and Q signals. • The I and Q signals amplitude-modulate 3.58-MHz subcarriers shifted 90 from one another in balanced modulators producing quadrature DSB suppressed signals that are added to form a carrier composite color signal. This color signal is then used to modulate the AM picture transmitter along with the Y signal. • .

  12. A TV receiver is a standard superheterodyne receiver with separate sections for processing and recovering the sound and picture. The tuner section consists of RF amplifiers, mixers, and a frequency-synthesized local oscillator for channel selection. Digital infrared remote control is used to change channels in the synthesizer via a control microprocessor.

  13. The tuner converts the TV signals to intermediate frequencies of 41.25 MHz for the sound and 45.75 MHz for the picture. These signals are amplified in IF amplifiers. The sound and picture IF signals are placed in a sound detector to form a 4.5-MHz sound IF signal. This is demodulated by a quadrature detector or other FM demodulator to recover the sound. Frequency-multiplexing techniques similar to those used in FM radio are used for stereo TV sound. The picture IF is demodulated by a diode detector or other AM demodulator to recover the Y signal.

  14. .The color signals are demodulated by two balanced modulators fed with 3.58-MHz subcarriers in quadrature. The subcarrier is frequency- and phase-locked to the subcarrier in the transmitter by phase-locking to the color subcarrier burst transmitted on the horizontal blanking pulse.

  15. .To keep the receiver in step with the scanning process at the transmitter, sync pulses are transmitted along with the scanned lines of video. These sync pulses are stripped off the video detector and used to synchronize horizontal and vertical oscillators in the receiver. These oscillators generate deflection currents that sweep the electron beam in the picture tube to reproduce the picture.

  16. .The color picture tube contains three electron guns that generate narrow electron beams aimed at the phosphor coating on the inside of the face of the picture tube. The phosphor is arranged in millions of tiny red, green, and blue color dot triads or stripes in proportion to their intensity and generate light of any color depending upon the amplitude of the red, green, and blue signals. The electron beam is scanned or deflected horizontally and vertically in step with the transmitted video signals. Deflection signals from the internal sweep circuits drive coils in a deflection yoke around the neck of the picture creating magnetic fields that sweep the three electron beams.

  17. The horizontal output stage, which provides horizontal sweep, is also used to operate a flyback transformer that steps up the horizontal sync pulses to a very high voltage. These are rectified and filtered into a 30- to 35-kV voltage to operate the picture tube. The flyback also steps down the horizontal pulses and rectifies and filters them into low-voltage dc supplies that are used to operate most of the circuits in the

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