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NTSC

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NTSC is the analog television system in use in Korea, Japan, United States, Canada and certain other places, mostly in the Americas (see map). It is named for the National Television System(s) Committee, the industry-wide standardization body that created it.

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Contents

History

The National Television System Committee was established in 1940 by the Federal Communications Commission (FCC), in the United States (US), 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. This built upon a 1936 recommendation made by the Radio Manufacturers Association (RMA) that used 441 lines. With the advancement of the vestigial sideband technique for broadcasting that increased available bandwidth, there was an opportunity to increase the image resolution. The NTSC compromised between RCA's desire to keep a 441-line standard (their NBC TV network was already using it) and Philco's desire to increase it to between 605 and 800, settling on a 525-line transmission. Other technical standards in the final recommendation were a frame rate (image rate) of 30 frames per second consisting of 2 interlaced fields per frame (2:1 interlacing) at 262 1/2 lines per field or 60 fields per second along with an aspect ratio of 4 by 3, and frequency modulation for the sound signal.

In January 1950 the Committee was reconstituted, this time to decide about color television. In March 1953 it unanimously approved what is now called simply the NTSC color television standard, later defined as RS-170a. The updated standard retained full backwards compatibility with older black and white television sets. Color information was added to the black and white image by adding a color subcarrier of 3.58 MHz to the video signal. Due to certain technical considerations, the addition of the color subcarrier also required a slight reduction of the frame rate from 30 frames per second to 29.97 frames per second.

The FCC had briefly approved a different color television system starting in 1950. It was developed by CBS and was incompatible with black and white broadcasts. That system used a rotating color wheel, reduced the number of scanlines from 525 to 405, and increased the field rate from 60 to 144 (but had an effective frame rate of 24 frames per second). Delay tactics by rival RCA kept the system off the air until mid-1951, and regular broadcasts only lasted a few months before manufacture of CBS-compatible systems was banned by the National Production Authority (NPA). Most of the existing devices were soon destroyed and only two receivers are known to exist today. The CBS system was rescinded by the FCC in 1953 and was replaced later that year by the NTSC color standard, which had been developed with the cooperation of several companies including RCA and Philco. A variant of the CBS system was later used by NASA to broadcast pictures of astronauts from space.

A third "line sequential" system from Color Television Inc. (CTI) was also considered. The CBS and final NTSC systems were called "field sequential" and "dot sequential" systems, respectively.

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. This was replaced later that year by an improved version, the TK-41, which became the standard camera used through much of the 1960s.

The NTSC 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 per second. Each frame consists of 480 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 SECAM 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 SECAM.

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 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 (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 × 5 × 5 × 7 = 525. Similarly, the British 405-line system used 3 × 9 × 9 × 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".

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 integer 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.

Framerate conversion

There is a large difference in framerate between NTSC and film, the latter consisting of 24.0 frames per second whereas NTSC is displayed at approximately 29.97 frames per second. Unlike the two other video formats, PAL and SECAM, this difference cannot be overcome by a simple speed-up. A complex process called "" is needed, which duplicates parts of frames. This induces noticeable judder during slow pans of the camera. See telecine for more details.

For viewing native PAL or SECAM material (such as European television series and some European movies) on NTSC equipment, a standards conversion has to take place. There are basically two ways to accomplish this.

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, though a number include it anyway. 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 - which almost matches PAL and SECAM with 25 frames a second (therefore possible to speedup instead - per second cannot be overcome by a simple small speedup, but needs a more complex process (see Framerate conversion).

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.

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. The United Kingdom 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

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. [link] Suitably-equipped television sets could then employ this data in order to adjust the display to a closer match of the original studio image. The actual VIR signal contains three sections, the first having 70 percent luminance and the same chrominance as the color burst signal, and the other two having 50 percent and 7.5 percent luminance respectively. [link]

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 [link] [link]), test data on lines 17-18, a network source code on line 20 and closed captioning, XDS 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 [link].

Countries and territories that use NTSC

North America

Central America and the Caribbean

South America

Asia

The Pacific

US Territories Other Pacific island nations Historic (used NTSC experimentally before adopting PAL)

Indian Ocean

Middle East

Europe

See also

References

External links

Digital video resolutions
Designation Usage examples Definition (lines) Rate (Hz)
Interlaced (fields) Progressive (frames)
MP@LL LDTV, VCD SIF) 24, 30; 25
Standard; MP@ML SDTV, SVCD, DVD, DV 480 (NTSC, PAL-M); 60; 24, 30;
576 (PAL, SECAM) 50 25
Enhanced EDTV 480; 576 60; 50
High; MP@HL HDTV, HD DVD, BD, HDV 720 24, 25, 30, 50, 60
1080 50, 60 24, 25, 30

This table illustrates total horizontal and vertical detail via box size. It does not accurately reflect the screen shape (aspect ratio) of these formats, which is always stretched or squeezed to 4:3 or 16:9. The table assumes an average vertical detail loss of .75x due to interlace. The actual loss is variable due to content, motion, opinion on acceptable levels of flicker, and possible success of deinterlacing. 1920 × 1080i is not included because all common use of 1080i is filtered to 1440 or less.

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
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