NTSC

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NTSC is the analog television system in use in the United States and many other countries, including most of the Americas and some parts of East Asia. It is named for the National Television System(s) Committee, the industry-wide standardization body that created it.

History

The National Television Systems Committee was established in 1940 by the Federal Communications Commission to resolve the conflicts which had arisen between companies over the introduction of a nationwide analog television system in the U.S. The committee in March 1941 issued a technical standard for black and white television. In January 1950 the committee was reconstituted, this time to decide about color television, and in March 1953 it unanimously approved what is now called simply the NTSC color television standard. The updated standard retained full backwards compatibility with older black and white television sets.

The standard has since been adopted by many other countries, for example most of the Americas and Japan.

Technical details

Refresh rate

The NTSC format—or more correctly the M format; see broadcast television systems—consists of 29.97 interlaced frames of video a second, each consisting of 480 lines of vertical resolution out of a total of 525 (the rest are used for sync, vertical retrace, and other data such as captioning). The NTSC system interlaces its scanlines, drawing odd-numbered scanlines in odd-numbered fields and even-numbered scanlines in even-numbered fields, yielding a nearly flicker-free image at its approximately 59.94 hertz (nominally 60 Hz / 1.001) refresh frequency. This compares favorably to the 50 Hz refresh rate of the 625-line PAL and SECAM video formats used in Europe, where 50 Hz alternating current is the standard; the European standards have noticeably more flicker than NTSC. Interlacing the picture does complicate editing video, but this is true of all interlaced video formats, including PAL and SECAM.

The NTSC refresh frequency was originally an even 60 Hz in the black and white system, chosen because it matched the nominal 60 Hz frequency of alternating current power used in the United States. It was preferable to match the screen refresh rate to the power source to avoid wave interference that would produce rolling bars on the screen. Synchronization of the refresh rate to the power cycle also helped kinescope cameras record early live television broadcasts, as it was very simple to syncronize a film camera to capture one frame of video on each film cell by using the alternating current frequency as a shutter trigger. In the color system the refresh frequency was shifted slightly downward to 59.94 Hz.

The mismatch in frame rate between NTSC and the other two video formats, PAL and SECAM, is the most difficult part of television video format conversion. Because the NTSC frame rate is higher, it is necessary for video conversion equipment converting to NTSC to interpolate the contents of adjacent frames in order to produce new intermediate frames; this introduces artifacts, and a trained eye can quickly spot video that has been converted between formats. (See also stutter frame.)

Color encoding

For backward compatibility with black and white television, NTSC—in this area the terminology NTSC is technically correct—uses a luminance-chrominance encoding system invented in 1938 by Georges Valensi. Luminance is essentially the original monochrome signal, while chrominance carries color information. This allows black and white receivers to display NTSC signals simply by ignoring the chrominance information. In NTSC, chrominance is encoded as two quadrature signals: I (in-phase) and Q (quadrature). The resulting system is called YIQ.

To implement this system, NTSC includes a chrominance subcarrier at a frequency of 3.579545 (exactly 315/88) MHz. This subcarrier is maintained by transmitting for each scanline a sinusoidal reference signal known as colorburst, located on the front porch, an otherwise unused period after the horizontal sync pulse and before the line of video starts. The colorburst consists of eight to ten cycles of the unmodulated subcarrier at 180° phase. Additional subcarrier is then added to the video signal in any portion of the scanline displaying color, with the subcarrier's amplitude determining color saturation and its phase in relation to the colorburst's phase determining hue.

The addition of this subcarrier was what necessitated the slight downward adjustment in the refresh rate. When NTSC is modulated over a UHF carrier it has a sound signal transmitted on a carrier 4.5 MHz higher. If the signal is affected by non-linear distortion, which can happen in many receivers, the 3.58 MHz colour carrier will beat with the sound carrier to produce a stationary dot pattern on the screen. When the frame rate is adjusted the dot pattern is no longer stationary and becomes much less noticeable to the viewer.

Transmission modulation scheme

An NTSC television channel as transmitted occupies a total bandwidth of 6 MHz. A guard band, which does not carry any signals, occupies the lowest 250 kHz of the channel to avoid interference between the video signal of one channel and the audio signals of the next channel down. The actual video signal, which is amplitude-modulated, is transmitted between 500 kHz and 5.45 MHz above the lower bound of the channel. The video carrier is 1.25 MHz above the lower bound of the channel. Like any modulated signal, the video carrier generates two sidebands, one above the carrier and one below. The sidebands are each 4.2 MHz wide. The entire upper sideband is transmitted, but only 750 kHz of the lower sideband, known as a vestigial sideband, is transmitted. The color subcarrier, as noted above, is 3.579545 MHz above the video carrier, and is quadrature-amplitude-modulated. The highest 250 kHz of each channel contains the audio signal, which is frequency-modulated, making it compatible with the audio signals broadcast by FM radio stations in the 88-108 MHz band. The main audio carrier is 4.5 MHz above the video carrier. Sometimes a channel may contain an MTS signal, which is simply more than one audio signal. This is normally the case when stereo audio and/or second audio program signals are used.

Quality problems

Video professionals and television engineers do not hold NTSC video in high regard, joking that the abbreviation stands for "Never The Same Color", "Never Twice the Same Color", or "Never Tested Since Christ." Cabling problems tend to degrade an NTSC picture (by changing the phase of the color signal), so the picture often loses its color balance by the time the viewer receives it. This necessitates the inclusion of a tint control on NTSC sets, which is not necessary on PAL or SECAM systems. Some complain that the 525 line resolution of NTSC results in a lower quality image than the hardware is capable of.

There is no question the NTSC system reflects the limitations and technology of a bygone era; indeed, its compatibility with even the crudest equipment since the dawn of television has been the key to its longevity and ubiquity over five decades. The coming of digital television and high definition television may indeed spell its doom. There is, however, no way to predict just how many more years its characteristic notched trace may continue to flicker across television station waveform monitors and its basic but effective scheme continue to beam into living rooms over much of the globe.

Variants of NTSC

Unlike PAL, with its many and varied underlying broadcast television systems in use throughout the world, NTSC color encoding is invariably used with broadcast system M, giving NTSC-M. Britain once contemplated introducing a 405-line NTSC-A system on top of its old black-and-white television system, but the proposal was eventually scrapped in favor of the incompatible PAL-I. Only Japan's variant "NTSC-J" is very slightly different: in Japan, black level and blanking level of the signal are identical, as they are in PAL, while in American NTSC, black level is slightly higher than blanking level. Since the difference is quite small, a slight turn of the brightness knob is all that is required to enjoy the "other" variant of NTSC on any set as it is supposed to be; most watchers might not even notice the difference in the first place.

Countries that use NTSC

North America

South America

Asia

The Pacific

See also

External references

  • A standard defining the NTSC system was published by the International Telecommunications Union in 1998 under the title "Recommendation ITU-R BT.470-6, Conventional Television Systems." It isn't publicly available on the Internet, but it can be purchased from the ITU.
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