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NTSC

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.

Table of contents

History

The National Television Systems Committee was established in 1940 by the Federal Communications Commission (FCC) 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 600 and 800, settling on a 525-line transmission.

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. The updated standard retained full backwards compatibility with older black and white television sets.

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 The CBS system was rescinded by the FCC in 1953 and was replaced later that year by the NTSC color standard, which had been developed with the cooperation of several companies including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, Incorporated (CTI) was also considered. The CBS and final NTSC systems were called "field sequential" and "dot sequential" systems, respectively.

The first commercially available color NTSC television camera was the RCA TK-40A, introduced in March 1954. It 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. 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 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 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).

To implement this system, NTSC modulates the chrominance signal with a subcarrier at a frequency of 3.579545 (exactly 315/88) MHz. The subcarrier itself is suppressed in transmission, but is made known by transmitting a sinusoidal reference signal known as colorburst, located on the front porch of each scanline, an otherwise unused period between the horizontal synchronization pulse and the actual start of each video line. The colorburst consists of eight to ten cycles of the unmodulated subcarrier at 180° phase. The modulated chrominance is then added to the video signal in any portion of the scanline displaying color. Once properly decoded, the subcarrier's amplitude and its phase in relation to the colorburst's phase determine color, using a system called YIQ.

The addition of this subcarrier was what necessitated the slight downward adjustment in the refresh rate. When NTSC is modulated over a VHF or 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 color carrier may beat with the sound carrier to produce a dot pattern on the screen. The frame rate was adjusted in such a way that any possibly occurring pattern wouldn't be noticeable.

Another important factor in choosing the new exact frame rate was to make sure that the color signal phase would be shifted exactly 180 degrees for each scanline. There are two reasons why this is important. First, the chroma signal does cause some distortion to older TV sets, especially those that were used at the time of the introduction of color TV and which didn't have notch filters to filter out the chroma information. In addition, early color TV sets (and newer cheap ones) suffer from imperfect luminance and chrominance separation, causing dots to appear near strong-colored edges. These dots are called creepy crawlies or, more commonly, dot crawl. They are particularly visible along vertical lines in the transmitted video, especially when SMPTE color bars are transmitted. The phase shift makes these dots non-stationary and thus reduces their visibility. The second reason to the phase shift is that it makes it possible to use a comb filter, which allows separating chrominance and luminance information with much better fidelity. While an exact 180 degree phase shift per scanline is not an absolute necessity for a comb filter to work, it makes implementation easier and also gives the best potential quality. This is a lesson that was later forgotten when developing the PAL color coding scheme. This probably didn't seem like a big omission at the time, since comb filters didn't become widely available in NTSC television sets before the 1980's (and, because of huge implementation difficulties, high-end PAL 100 Hz TV sets didn't get comb filters before the late 1990s). Nevertheless, the theoretical groundwork that made comb filters possible was there from the beginning.

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. 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 with even the crudest equipment since the dawn of television 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.

Standard

525 is the number of lines in the NTSC television standard. The choice of this number for the number of lines in that standard was not an accident. The reasons for this are apparent upon examination of the technical properties of analog television, as well as the prime factors of 525. 525 when divided by five is 105, which in turn can be divided by five again to obtain 21, which in turn is just three times seven. Using 1940's technology, it was not technically feasible to electronically multiply or divide the frequency of an oscillator by any arbitrary ratio.

So if one started with a 60 Hertz reference oscillator, (such as the power line frequency in the US) and sought to multiply that frequency to a suitable line rate, which in the case of black and white transmission was set at 15750 Hertz, then one would need to have such a means of multiplying or dividing the frequency of an oscillator with a minimum of circuitry. In fact, the field rate for NTSC television has to be multiplied to twice the line rate to obtain a frequency of 31500 Hertz, i.e. for black and white transmission synchronized to power line rate.

One means of doing this is of course to use harmonic generators and tuned circuits, i.e. if using the direct frequency multiplication route. With the conversion of US television to color, beginning in the 1950's the frequencies were changed slightly, so that a five-megahertz oscillator could be used as a reference. In that case, it is possible to use the five-megahertz oscillator, and multiply that frequency by 63 and divide it then by 88 to obtain the 3.579545454 MHz color sub-carrier frequency. That frequency in turn would then be multiplied by 2 and divided by 455 to obtain the new horizontal line rate which is 15.734 kilohertz, and that which thereupon from which is derived the new vertical frequency rate of 59.94 Hertz.

Interestingly enough, when one analyses how we get the 59.94 vertical field rate, one realizes that it is just 60 Hertz multiplied by 1000/1001. Now 1001 in turn has prime factors of 7, 11, and 13, so that when cascading simple flip flop based circuitry it is possible to take a 60 kilohertz reference source and divide it by 1001 exactly to obtain the vertical field rate. It is not a coincidence that the National Bureau of Standards (now the National Institute of Standards and Technology) operates a radio station, WWVB, that broadcasts a time and frequency standard synchronized to an atomic clock on this frequency, that is, 60 kHz.

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.

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.

History of the NTSC signal

  • NTSC I is the original monochromatic 525/60 signal that first became standard in the US & Canada during the late 1940s to early 1960s.
  • 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 seperate prodcution 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 compatibiltiy being achived by signal processing in the digtal 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 ASTC source. All free to air analog satcom transmissions are NTSC III. Video scrambling systems such as VideoCipher can't acheve full NTSC III compability due 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 the color sync subcarrier that exist in NTSC II.

Countries and territories that use NTSC

Green: NTSC. Yellow: PAL. Orange: SECAM. Grey: No information

North America

Central America

South America

Asia

The Pacific

See also

Selective Access

Analog and digital NTSC have several selective access systems that are used for Pay Per View (PPV) and other subscriber related services.

VideoCipher II RS (VCII RS) is the scrambling system that most C-Band premium satellite TV channels use. A VCII satellite receiver is required to decode VC II channels.

  • VideoCipher I
  • VideoCipher II
  • VideoCipher II+
  • VideoCipher II RS (Renewable Security)

DigiCipher is Motorola's proprietary video distribution system. DCII is based upon MPEG-2. A 4DTV satellite receiver is required to decode DCII channels.

  • DigiCipher II (digital, common usage)

Other systems

B-MAC has not been used for DTH applications since Primestar switched to an all-digital delivery system in the mid-1990s.

Viewguard Like VCI, the Viewguard system digitally encrypts video through the use of the cut and rotate method. The audio component of the satellite TV signal also is digitally encrypted.

References








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