NTSC
(National TV Standards Committee) The committee that developed the television standards for the U.S, which are also used in Canada, Japan, South Korea and several Central and South American countries. Both the committee and the standard are called "NTSC."
Frames and Resolution
Refresh rate
The NTSC format—or more correctly the M format; see broadcast television systems—consists of 29.97 interlaced frames of video per second. Each frame consists of 486 lines 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 SÉCAM video formats used in Europe, where 50 Hz alternating current is the standard; flicker is more likely to be noticed when using these standards (However, modern PAL TV sets use 100 Hz refresh rate to eliminate flicker, effectively displaying one frame twice). Interlacing the picture does complicate editing video, but this is true of all interlaced video formats, including PAL and SÉCAM.
The NTSC refresh frequency was originally exactly 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 synchronize a film camera to capture one frame of video on each film cell 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 (60Hz in this case). This frequency was then compared with the 60Hz power line frequency and any frequency discrepancy corrected by controlling 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, and so a chain of multivibrators was needed, each of which had to divide by a small, odd number. The closest practical sequence to 500 was 3 x 5 x 5 x 7 = 525. Similarly, the British 405-line system used 3 x 9 x 5. Although other values were theoretically possible, all of them involved division by unacceptably large numbers like 13 or 17, which produced reliability problems. Modern systems derive all their frequencies from the color subcarrier frequency (see below).
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".
The mismatch in frame rate between NTSC and the other two video formats, PAL and SÉCAM, is the most difficult part of 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 slightly trained eye can quickly spot video that has been converted between formats. (See also stutter frame.)
Another problem is that there are a lot of possible timings behind a NTSC signal (much more than behind a PAL signal). A NTSC signal can be actually a 60i signal, it can be a 30p signal after a 2:2 pullup, it can be a 24p signal after a 3:2 pullup, a bobbed PAL signal after a 3:2 pullup, to mention some legal examples. A lot of versions due to mastering errors of DVDs follow. For further information see http://www.hometheaterhifi.com/volume_7_4/dvd-benchmark-part-5-progressive-10-2000.html .
Color encoding
For backward compatibility with black and white television, NTSC uses a luminance-chrominance encoding system invented in 1938 by Georges Valensi. Luminance (derived mathematically from the composite color signal) takes the place of the original monochrome signal. Chrominance carries color information. This allows black and white receivers to display NTSC signals simply by ignoring the chrominance. In NTSC, chrominance is encoded using two 3.579545 MHz signals that are 90 degrees out of phase, known as I (intermodulation) and Q (quadrature). The phase relationship of the I and Q signals with the 3.579545 MHz subcarrier corresponds to the instantaneous color hue captured by a TV camera; its amplitude corresponds to the color saturation (purity) of the original signal.
For a TV or a display to recover color information from the varying phase and amplitude signals just described, a constant phase reference 3.579545 MHz signal is needed. A short sample of this reference signal is included in the NTSC signal as color burst, located on the back porch of each horizontal line, the time between the end of the horizontal synchronization pulse and of the blanking pulse on each line. The color burst consists of a minimum of eight cycles of the unmodulated (fixed phase and amplitude) color subcarrier. By comparing the reference signal derived from color burst to the color signal's amplitude and phase, color hue and saturation information are recovered.
When NTSC is broadcast, a radio frequency carrier is amplitude modulated by the NTSC signal just described, while an audio signal is transmitted by frequency modulating a carrier 4.5 MHz higher. If the signal is affected by non-linear distortion, the 3.58 MHz color carrier may beat with the sound carrier to produce a dot pattern on the screen. The original 60 Hz field rate was adjusted down by the factor of 1000/1001, to 59.94059... fields per second, so that the resulting pattern would be less noticeable.
Another important factor in choosing a new field rate (59.94 Hz) was to reduce interference between the chrominance signal and the audio carrier. The chrominance signal is an n + 0.5 multiple (exact 227.5) of the line frequency to minimize interferences between the luminance carrier and the chrominance carrier. The audio carrier is an integral multiple (286.0) of the line frequency to minimize interferences with the chrominance signal, which is a n+0.5 multiple of the line frequency. Because the audio frequency was defined by the former black white standard (4.5 MHz) and the exact audio carrier was much more critical than the exact field rate, the field rate was moved from 60.00 Hz to 4,500,000 Hz / 286 / 262.5 = 15750 Hz / 1.001 / 262.5 = 15734.26573... / 262.5 = Hz = 59.94005... Hz = 60 Hz / 1.001 Hz.
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 with suppressed carrier. 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 SÉCAM systems. Some complain that the 525 line resolution of NTSC results in a lower quality image than the hardware is capable of. Additionally, the large mismatch between NTSC's 30 frames per second and cinema's 24 frames per second cannot be overcome by a simple small speedup during telecine of cinematic movies for display on NTSC equipment; unlike PAL a more complex process called "3:2 pulldown" is needed, which duplicates parts of frames. This induces noticeable judder during slow pans of the camera. See telecine for more details.
There is no question the NTSC system reflects the limitations and technology of a bygone era; indeed, its compatibility has been the key to its longevity and ubiquity over seven decades. The coming of digital television and high-definition television may 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 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 (at 0 IRE), as they are in PAL, while in American NTSC, black level is slightly higher (7.5 IRE) 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.
The Brazilian PAL-M system uses the same broadcast bandwidth, frame rate, and number of lines as NTSC, but using PAL encoding. It is therefore NTSC-compatible in sources such as video cassettes and DVDs, but its color picture cannot be received on a standard NTSC television set.
Evolution of the NTSC signal
* NTSC I is the original monochromatic 525/60 signal that first became standard in the U.S. in 1941 and later in Canada.
* NTSC II is the color system with some but not all aspects of the signal rigorously defined. NTSC II has a minor change in its temporal structure, becoming a 525/59.94 system. From this point 525/60 [RGB] becomes a separate production standard that interoperates with NTSC via a 1000/1001 drop frame solution.
* NTSC III came about due to digital television routing during the 1980s; all aspects of NTSC III are rigidly mathematically defined.
The current state of NTSC III
The North American analog transmission chain is strictly NTSC III now. Many NTSC II devices feed into existing transmission chains, with NTSC III compatibility being achieved by signal processing in the digital domain.
Typical terrestrial TV transmitters or cable company distribution units send out NTSC III signals, especially if the originating signal comes from a TVRO or ATSC source. All free-to-air analog satcom transmissions are NTSC III. Video scrambling systems such as VideoCipher cannot achieve full NTSC III compatibility due to end-to-end analog processing issues.
There are no known compatibility problems between NTSC II and NTSC III. Older NTSC II sets should handle NTSC III signals without any problems, even with respect to minor frequency variances of the color sync subcarrier that exist in NTSC II.
Vertical Interval Reference
The standard NTSC video image contains some lines (lines 1-21 of each field) which are not visible; all are beyond the edge of the viewable image, but only lines 1-9 are used for the vertical-sync and equalizing pulses. The remaining lines were deliberately blanked in the original NTSC specification to provide time for the electron beam in CRT-based screens to return to the top of the display.
VIR (or Vertical Interval Reference), widely adopted in the 1980s, attempts to correct some of the color problems with NTSC video by adding studio-inserted reference data for luminance and chrominance levels on line 19. [1] Suitably-equipped television sets could then employ this data in order to adjust the display to a closer match of the original studio image.
A less-used successor to VIR, GCR, also added ghost (multipath interference) removal capabilities.
The remaining vertical blanking interval lines are typically used for datacasting or ancillary data such as video editing timestamps (vertical interval timecodes or SMPTE timecodes on lines 12-14 [2] [3]), test data on lines 17-18, a network source code on line 20 and closed captioning and V-chip data on line 21. Early teletext applications also used vertical blanking interval lines 14-18 and 20, but teletext over NTSC was never widely adopted by viewers [4].
Administered by the FCC, NTSC broadcasts 60 half frames per second, which is known as 60 "fields" per second in TV jargon (59.94 fields per second to be exact). NTSC uses 525 lines of resolution: the first 480 lines in each frame are the image, and the last 45 are the "vertical blanking interval" (VBI), which was designed to give the electron gun time to reposition itself from the bottom of the last frame to the top of the next. See interlaced and raster scan.
Color and Audio
NTSC is encoded in the YUV color space, which provides a mathematical equivalent of red, green and blue. It also includes an audio FM frequency and an MTS signal for stereo. See YUV, YIQ, 4fSC, vertical blanking interval, aspect ratio, DTV, PAL and SECAM.
Monochrome to Composite
In 1940 and 1941, the NTSC met to develop the monochrome TV standard, and commercial broadcasting began in the U.S. for black and white TVs on July 1, 1941. The Committee met again from 1950 to 1953 and added a subcarrier frequency to the black and white signal in order to transmit color in a composite signal (see composite video). Color TV began in the U.S. on January 1, 1954.
Before totally electronic TV cameras and receivers were built, electromechanical "scanning disc" systems produced the first TV images. See video/TV history for an overview.