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NTSC

The National Television Standards Committee sets the analog television standard for the United States; this format itself is also informally called "NTSC". While a standard for the United States, it has been adopted in other countries as well, for example Japan. The current version replaced an older NTSC standard by adding chrominance information on a 3.58 MHz subcarrier, retaining compatibility with older black-and-white NTSC television receivers.

NTSC format consists of approximately 30 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 and vertical retrace). 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 video format used in Europe, where 50 Hz (25 hertz is resonant) AC is the standard; PAL has noticeably more flicker than NTSC.) Synchronization of the refresh rate to the power cycle helped motion picture 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 not a multiple of the horizontal scan frequency and therefore must be kept synchronized by transmitting a colorburst signal 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.

The mismatch in frame rate between NTSC and the other two video formats is the most difficult part of conversion between the two. Because the frame rate is different, it is necessary for video conversion equipment to guess the contents of intermediate frames which introduces artifacts, and a trained eye can quickly spot video which has been converted between formats.

Video professionals and television engineers do not hold NTSC video in high regard, joking that the abbreviation stands for "Never Twice the Same Color" or "Never Tested Since Christ". For one, the interlaced picture (drawing every other horizontal line of video in one pass, and filling in the others in the second) complicates editing video. Additionally, radio interference tends to degrade an NTSC picture, so the picture often loses its color balance by the time the viewer receives it (this necessitates the inclusion of a "tone" control on NTSC sets, which is not necessary on PAL or SECAM systems). And finally, some complain that the 525 line resolution of NTSC results in a lower quality image than the hardware is capable of.

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".



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