NTSC
NTSC (National Television System Committee) is the analog television system used in most of the Americas, Japan, South Korea, Taiwan, the Philippines, Burma, and some Pacific island nations and territories (see map). NTSC is also the name of the U.S. standardization body that adopted the NTSC broadcast standard. The first black-and-white NTSC standard for broadcast was developed in 1941 and had no provision for color transmissions. In 1953 a second standard was issued, which allowed color broadcasting to be compatible with the existing stock of black-and-white receivers, while maintaining the broadcast channel bandwidth already in use. NTSC was the first widely adopted broadcast color system. After over a half-century of use, the vast majority of over-the-air NTSC transmissions in the United States will be replaced with ATSC in June 2009 and by August 31, 2011 in Canada.
The National Television System Committee was established in 1940 by the United States Federal Communications Commission (FCC) to resolve the conflicts that arose between companies over the introduction of a nationwide analog television system in the United States. In March 1941, the committee issued a technical standard for black-and-white television that built upon a 1936 recommendation made by the Radio Manufacturers Association (RMA). Technical advancements of the vestigial sideband technique allowed for the opportunity to increase the image resolution. The NTSC selected 525 scan lines as a compromise between RCA's 441–scan line standard (already being used by RCA's NBC TV network) and Philco's desire to increase the number of scan lines to between 605 and 800. The standard recommended a frame rate of 30 frames (images) per second, consisting of two interlaced fields per frame at 262.5 lines per field and 60 fields per second. Other standards in the final recommendation were an aspect ratio of 4:3, and frequency modulation (FM) for the sound signal (which was quite new at the time).
In January 1950, the Committee was reconstituted to standardize color television. In December 1953, it unanimously approved what is now called the NTSC color television standard (later defined as RS-170a). The "compatible color" standard retained full backward compatibility with existing black-and-white television sets. Color information was added to the black-and-white image by adding a color subcarrier of 4.5 × 455/572 MHz (approximately 3.58 MHz) to the video signal. To reduce interference between the chrominance signal and FM sound carrier required a slight reduction of the frame rate from 30 frames per second to 30/1.001 (very close to 29.97) frames per second, and changing the line frequency from 15,750 Hz to 15,734.26 Hz.
The FCC had briefly approved a different color television standard, starting in October 1950, which was developed by CBS. However, this standard was incompatible with black-and-white broadcasts. It used a rotating color wheel, reduced the number of scan lines from 525 to 405, and increased the field rate from 60 to 144 (but had an effective frame rate of only 24 frames a second). Legal action by rival RCA kept commercial use of the system off the air until June 1951, and regular broadcasts only lasted a few months before manufacture of all color television sets was banned by the Office of Defense Mobilization (ODM) in October, ostensibly due to the Korean War. CBS rescinded its system in March 1953, and the FCC replaced it on December 17, 1953 with the NTSC color standard, which was cooperatively developed by several companies (including RCA and Philco). The first publicly announced network TV broadcast of a program using the NTSC "compatible color" system was an episode of NBC's Kukla, Fran and Ollie on August 30, 1953, although it was viewable in color only at the network's headquarters. The first nationwide view of NTSC color came on the following January 1 with the coast-to-coast broadcast of the Tournament of Roses Parade, viewable on prototype color receivers at special presentations across the country.
The first color NTSC television camera was the RCA TK-40, used for experimental broadcasts in 1953; an improved version, the TK-40A, introduced in March 1954, was the first commercially available color TV camera. Later that year, the improved TK-41 became the standard camera used throughout much of the 1960s.
The NTSC standard has been adopted by other countries, including most of the Americas and Japan. With the advent of digital television, analog broadcasts are being phased out. Most U. S. NTSC broadcasters are mandated by the FCC to shut down their analog transmitters in 2009. Low-power stations, Class A stations and translators are not immediately affected. An analog cut-off date for those stations is to be determined.
NTSC color encoding is used with the M format, which consists of 29.97 interlaced frames of video per second. Each frame consists of a total of 525 scanlines, of which 486 make up the visible raster. The remainder (the vertical blanking interval) are used for synchronization and vertical retrace, and can contain other data such as closed captioning and vertical interval timecode (VITC). In the complete raster (ignoring half-lines), the even-numbered or 'lower" scanlines (lines 21 to 263 in the video signal) are drawn in the first field, and the odd-numbered or "upper" (signal lines 283 to 525) are drawn in the second field, to yield a flicker-free image at the field refresh frequency of approximately 59.94 Hertz (actually 60 Hz/1.001). For comparison, PAL uses 625 lines (576 visible), and so has a higher vertical resolution, but a lower temporal resolution of 25 frames or 50 fields per second.
The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60 Hz frequency of alternating current power used in the United States. Matching the field refresh rate to the power source avoided interference which produces rolling bars on the screen. Synchronization of the refresh rate to the power incidentally helped kinescope cameras record early live television broadcasts, as it was very simple to synchronize a film camera to capture one frame of video on each film frame by using the alternating current frequency as a shutter trigger.
The figure of 525 lines was chosen as a consequence of the limitations of the vacuum-tube-based technologies of the day. In early TV systems, a master voltage-controlled oscillator was run at twice the horizontal line frequency, and this frequency was divided down by the number of lines used (in this case 525) to give the field frequency (60 Hz in this case). This frequency was then compared with the 60 Hz power-line frequency and any discrepancy corrected by adjusting the frequency of the master oscillator.
The only practical method of frequency division available at the time was the use of multivibrators, which could only divide by small numbers. For interlaced scanning an odd number of lines per frame was required in order to make the vertical retrace distance identical for the odd and even fields; an extra odd line means that the same distance is covered in retracing from the final odd line to the first even line as from the final even line to the first odd line, so simplifying the retrace circuitry. This meant that a chain of multivibrators was needed, each of which had to divide by a small, odd number. (Note that an odd number is never integrally divisible by any even number). The closest practical sequence to 500 was 3 × 5 × 5 × 7 = 525. Similarly, 625-line PAL and SECAM uses 5 × 5 × 5 × 5. The British 405-line system used 3 × 3 × 3 × 3 × 5, the French 819-line system used 3 × 3 × 7 × 13. Although other values were theoretically possible, all of them involved division by unacceptably large numbers, which produced reliability problems. Modern systems derive all their frequencies from the color subcarrier frequency.
In the color system the refresh frequency was shifted slightly downward to 59.94 Hz to eliminate stationary dot patterns in the color carrier, as explained below in "Color encoding".