Television

The transmission and reproduction of moving pictures and associated sound by electronic means; developed in the late 19th-c and early 20th-c, with the first pictures presented by Baird in 1926. The image of a scene in a TV camera using a vidicon tube is analyzed by scanning along a series of horizontal lines, the variations of brightness along each line being converted into a train of electrical signals for transmission or recording.

Cameras using solid-state sensors such as charge-coupled device arrays read out the image information by sequential interrogation of each pixel. At the receiver the picture is reconstituted on the fluorescent screen of a cathode-ray tube by an electron beam scanning a precisely similar pattern, the brightness of each point depending on the beam intensity controlled by the incoming signal.

The number of scanning lines and the picture frequency vary in different systems, the American standard having 525 lines with 30 pictures per second (pps) and the European 625 lines at 25 pps. In both cases one complete picture, or frame, is scanned in two sets of alternate lines, termed fields,which are interlaced to reduce flicker in the receiver image. As scanning returns from the end of one line to the start of the next, there is a brief period without picture information, horizontal blanking; similarly after each completed field there is the vertical interval, field blanking, while scanning returns from bottom to top.

Synchronizing pulses at precise time intervals are inserted in these periods to ensure correct scanning in the receiver. For terrestrial broadcasting the complete vision signal modulates a radio wave in one of the ultra-high-frequency (UHF) bands between 470 and 890 mHz as a carrier, while the corresponding sound (audio) signal has its own carrier at a slightly higher frequency. High-definition TV systems with more than 1000 scanning lines will be transmitted via satellite using the super-high-frequency (SHF) band, 11.7 to 12.5 gHz.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Television"

Television is a telecommunication system for broadcasting and receiving moving pictures and sound over a distance. The term has come to refer to all aspects of television programming and transmission as well. The first long distance public television broadcast was from Washington, DC to New York City and occurred on April 7, 1927. The image shown was of then Commerce Secretary Herbert Hoover. A semi-mechanical analogue television system was first demonstrated in February 1924 by John Logie Baird and moving pictures by Baird on October 30 1925. A fully electronic system was demonstrated by Philo Taylor Farnsworth in the autumn of 1927. The first analogue service was WGY, Schenectady, New York inaugurated on May 11 1928. CBS's New York City station began broadcasting the first regular seven days a week television schedule in the U. S. on July 21, 1931. The first broadcast included Mayor James J. Walker, Kate Smith, and George Gershwin. The first all-electronic television service was started in Los Angeles, CA by Don Lee Broadcasting. Their start date was December 23, 1931 on W6XAO - later KTSL. Los Angeles was the only major city that avoided the false start with mechanical television.