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NTSC or the National Television System(s) Committee was an industry-wide standardization
body created in 1940 by the FCC
in order to resolve conflicts which existed at the time between companies regarding the introduction of a nation-wide analog television system for the United States. It issued a black-and-white standard in March 1941. In
January 1950 the committee was re-created, this time to decide about color television, and
in March 1953 unanimously approved what is now simply called the NTSC color television standard. The update retained
full compatibility with older black-and-white television sets.
The standard has been adopted by other countries as well, for example Japan.
Technical details
The NTSC format 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). NTSC interlaces its
scanlines, drawing odd-numbered scanlines in odd-numbered fields and even-numbered scanlines in even-numbered fields, which gives
a nearly flicker-free image at approximately 59.94 hertz (nominally 60 Hz / 1.001)
refresh frequency, which is close to the nominal 60 Hz alternating current power used in the United States. (Compare this to the 50 Hz refresh rate of the 625-line PAL and SECAM video formats used in Europe, where 50 Hz
AC is the standard: the European standards have noticeably
more flicker than NTSC.) Synchronization of the refresh rate to the
power cycle helped film cameras record early live television broadcasts, as it was very
simple to sync a film projector to capture a frame of video to a film cell using the frequency of the alternating current. Also,
it was preferable to match the screen refresh rate to the power source so
as to avoid wave interference that would produce rolling bars on the screen.
For color, NTSC includes a chrominance subcarrier, whose frequency is 3.579545 (exactly 315/88) MHz. This subcarrier is maintained by transmitting a
reference signal known as colorburst on the front porch (an otherwise unused period after the horizontal sync pulse and before the line of video starts).
The colorburst consists of 8-10 cycles of the unmodulated subcarrier at 180°
phase. When color was first added to an NTSC signal, the horizontal scanning rate had to be adjusted to introduce a rational
harmonic relation between it and the color subcarrier frequency, which in turn would reduce dot interference on monochrome sets,
as well as interference with the audio signal due to heterodyne.
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 frame rate is different, it is necessary for video conversion
equipment to interpolate between the contents of intermediate frames which introduces artifacts, and a trained eye can quickly spot video which has been converted between formats.
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 colour subcarrier, as indicated 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 can contain an MTS signal, which is simply more than one audio
signal. This is normally the case when stereo audio and/or SAP signals are used.
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.
While the interlaced picture (drawing every other horizontal line of video in one pass, and filling in the others in the
second) does complicate editing video, this is the case with all interlaced formats (including PAL and SECAM).
Countries that use NTSC
North America
South America
Asia
The Pacific
See also
External reference
- A standard that defines the NTSC system was published by the International Telecommunications Union in 1998 and has the title "Recommendation
ITU-R BT.470-6, Conventional Television Systems".
External links
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