NTSC - History, Technical details, Comparative quality, Variants of NTSC, Evolution of the NTSC signal

Abbreviation of National Television Systems Commission, responsible for the coding system for colour television introduced in the USA in 1954, and since then generally adopted throughout the Americas and Japan for all 525-line 60 Hz transmission. The two colour difference signals are 90° out of phase and combined to form the chrominance signal. The colour of the final picture is critically dependent on the correct phase relation being maintained throughout broadcast transmission, and the receiver requires a hue control.

NTSC is the analog television system in use in Canada, Japan, South Korea, the Philippines, the United States, and some other countries, mostly in the Americas (see map).

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. 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 262½ lines per field or 60 fields per second along with an aspect ratio of 4 by 3, and frequency modulation for the sound signal. 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. 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). The CBS system was rescinded by the FCC in 1953 and was replaced later that year by the NTSC color standard, which had been developed with the cooperation of several companies including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 third "line sequential" system from Color Television Inc. 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;

The NTSC standard has since been adopted by several other countries, for example most of the Americas and Japan.

Technical details

Lines and refresh rate

The NTSC format is used with the M format (see broadcast television systems), which consists of 29.97 interlaced frames of video per second. 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 produces a far more stable picture than native NTSC and PAL had, effectively displaying one frame twice.

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. 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).

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 an NTSC signal (much more than behind a PAL signal). An 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.

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. 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 (in-phase) and Q (quadrature). Viewed this way, that phase tells the instantaneous color hue captured by a TV camera, and that amplitude tells the color saturation (purity) of the original signal. So the NTSC signal includes a short sample of this reference signal, known as the 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. By comparing the reference signal derived from color burst to the chrominance signal's amplitude and phase at a particular point in the scan, the device knows what chrominance to assign to the pixel then being displayed.

When a transmitter broadcasts an NTSC signal, it amplitude modulates a radio frequency carrier with the NTSC signal just described, while it frequency modulates a carrier 4.5 MHz higher with the audio signal. If non-linear distortion happens to the broadcast signal, the 3.58 MHz color carrier may beat with the sound carrier to produce a dot pattern on the screen. Designers chose to make the chrominance subcarrier frequency an n + 0.5 multiple of the line frequency to minimize interference between the luminance signal and the chrominance signal. They then chose to make the audio subcarrier frequency an integer multiple of the line frequency to minimize interference between the audio signal and the chrominance signal. The original black and white standard, with its 15750 Hz line frequency and 4.5 MHz audio subcarrier, does not meet these requirements, so designers had either to raise the audio subcarrier frequency or lower the line frequency. Lowering the line frequency is comparatively innocuous, because the horizontal and vertical synchronization information in the NTSC signal allows a receiver to tolerate a substantial amount of slop in the line frequency. In the color standard, this becomes rounded to the integer 286, which means the color standard's line rate is 4.5 MHz / 286 ~= 15734 lines per second.

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 highest 25 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. Sometimes a channel may contain an MTS signal, which is simply more than one audio signal.

One odd thing about NTSC is the Cvbs (Composite vertical blanking signal) is something called "setup." Cvbs has one defect: it makes NTSC more easily separated from its primary sync signals, but Cvbs has a smaller dynamic range when compared with PAL or SECAM.

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.

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. (See also stutter frame)

Use with Progressive Sources

When NTSC is used to transmit content which was originally composed of 29.97 progressive full frames per second, the even field of the frame is transmitted first. Systems which recover progressive frames or transcode video should ensure that this 'Field Order' is obeyed, otherwise the recovered frame will consist of a field from one frame and a field from an adjacent frame, resulting in 'comb' interlacing artifacts.

Comparative quality

Video professionals and television engineers jokingly referred to NTSC as "Never The Same Color," "Never Twice the Same Color," or "Never Tested Since Christ". Reception problems can degrade an NTSC picture by changing the phase of the color signal, so the color balance of the picture will be altered unless a compensation is made in the receiver. This necessitates the inclusion of a tint control on NTSC sets, which is not necessary on PAL or SECAM systems.

However, the mismatch between NTSC's 30 frames per second and film's 24 frames is well overcome by an ingenious process that capitalizes on the field rate of the interlaced NTSC signal, thus avoiding the film playback speedup that is used for PAL and SECAM at 25 frames per second (which results in audio distortion).

There is no question the NTSC system reflects the technology of its originating era, but its compatibility and flexibility has been the key to its longevity over seven decades.

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 625 NTSC system (the BBC proposed 625 NTSC and ITV 405 Pal), but the proposal was eventually scrapped in favor of PAL-I. During the late 1950's some experimental Colour broadcasts were made in the UK using the 405 line system with NTSC. The subcarrier frequency was 2.6578125 MHz (525/2 times line frequency) With an "I" signal bandwidth of 500 KHz and a "Q" signal bandwidth of 300 KHz The 625 line varient used a subcarrier frequency of 4.4296875 MHz (567/2 times line frequency) With an "I" signal bandwidth of 1600 KHz and a "Q" signal bandwidth of 800 KHz

Only Japan's variant "NTSC-J" is 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.

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 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.

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. 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; 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. 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.

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 ), 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 .

Countries and territories that use NTSC

North America

Canada Mexico United States

Central America and the Caribbean

Antigua and Barbuda Aruba Bahamas Barbados Belize Bermuda British Virgin Islands Cayman Islands Costa Rica Cuba Dominica Dominican Republic El Salvador Guatemala Grenada Haiti Honduras Jamaica Leeward Islands Montserrat Netherlands Antilles Nicaragua Panama Puerto Rico 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 Argentina (stub) Brazil (uses PAL-M system, based on NTSC-M standard but using PAL color encoding - most films on DVD discs and VHS are NTSC, so DVD players and VCRs are NTSC-compatible)

Asia

Japan Philippines South Korea Taiwan North Korea (Propaganda station aimed at South Korea; US aid funded NTSC adoption) Tonga (US aid funded NTSC adoption)

Historic (used NTSC experimentally before adopting PAL)

Fiji (Historic;