NOTICE: SHUTTING DOWN 4 JAN 2015

On January 4, 2015, I will be shutting down the server that hosts The TV IV website. It has been a very long time since I've been able to put any decent amount of time into the site, and ad revenue is plummeting. I think it is time to shut it down or hand it off to someone who can keep it going properly. If you are interested in taking over the site's code and data, contact administrators at tviv.org. --CygnusTMtalk


NTSC

From The TV IV
Jump to: navigation, search

NTSC is a an analog video television standard, and the type of television signal that is used throughout North America and several other countries throughout the world. Defined by the National Television System Committee in 1953, it is named after this committee, although the acronym has been jokingly referred to as "Never The Same Color" as it is the only analog television format that supports adjusting of the color encoding on the receiving end.

Contents

Technical details

Refresh rate

The NTSC video format 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. 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."

Color Encoding

For backward compatibility with black and white television, NTSC 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. NTSC is encoded in the YUV color space, which provides a mathematical equivalent of red, green and blue. NTSC also includes an audio FM frequency and an MTS signal for stereo.

When NTSC is broadcast over VHF or UHF, a radio frequency carrier is amplitude modulated by the NTSC signal, while an audio signal is transmitted by frequency modulating 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 of occurring patterns 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).

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," 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.

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 over seven decades. The coming of digital television and high-definition television may eventually spell its doom.

Countries and territories that use NTSC

bright green - NTSC, yellow - PAL, orange - SECAM, olive - no information

North America

Canada, Mexico, United States

Central America

Antigua and Barbuda, Aruba, Bahamas, Barbados, Belize, Bermuda, British Virgin Islands, Cayman Islands, Costa Rica, Cuba, Dominica, Dominican Republic, El Salvador, Guatemala, Grenada, Honduras, Jamaica, Leeward Islands, Montserrat, Netherlands Antilles, Nicaragua, Panama, St. Kitts and Nevis, St. Lucia, St. Vincent and the Grenadines, Trinidad and Tobago, U.S. Virgin Islands

South America

Bolivia, Chile, Colombia, Ecuador, Guyana, Peru, Suriname, Venezuela, Brazil[1]

  • Brazil uses a PAL-M system, based on NTSC-M standard but using PAL color encoding, so NTSC televisions would not display in color.

Asia

Japan, Myanmar, Philippines, South Korea, Taiwan, North Korea[1], South Vietnam[2]

  • North Korea uses NTSC only for a propaganda station aimed at South Korea; domestic broadcasts use PAL
  • South Vietnam using NTSC is historic; all of Vietnam now uses PAL

The Pacific

American Samoa, Fiji, Guam, Marshall Islands, Micronesia, Midway Atoll, Northern Mariana Islands, Palau, Samoa

References

  • A standard defining the NTSC system was published by the International Telecommunication Union in 1998 under the title "Recommendation ITU-R BT.470-6, Conventional Television Systems." It isn't publicly available on the Internet, but it can be purchased from the ITU.
  • Ed Reitan (1997). CBS Field Sequential Color System.

External links