PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system. The articles on broadcast television systems and analogue television further describe frame rates, image resolution and audio modulation. For discussion of the 625-line / 50 field (25 frame) per second television standard.
History
In the 1950s, the Western European countries commenced planning to introduce colour television, and were faced with the problem that the NTSC standard demonstrated several weaknesses, including colour tone shifting under poor transmission conditions. To overcome NTSC's shortcomings, alternative standards were devised, resulting in the development of the PAL and SECAM standards. The goal was to provide a colour TV standard for the European picture frequency of 50 fields per second (50 hertz), and finding a way to eliminate the problems with NTSC.
PAL was developed by Walter Bruch at Telefunken in Germany. The format was unveiled in 1963, with the first broadcasts beginning in the United Kingdom in 1964 and Germany in 1967, though the one BBC channel initially using the broadcast standard only began to broadcast in colour from 1967.
Telefunken was later bought by the French electronics manufacturer Thomson. Thomson also bought the Compagnie Générale de Télévision where Henri de France developed SECAM, the first European Standard for colour television. Thomson, now called Technicolor SA, also owns the RCA brand and licenses it to other companies; Radio Corporation of America, the originator of that brand, created the NTSC colour TV standard before Thomson became involved.
The term PAL is often used informally to refer to a 625-line/50 Hz (576i) television system, and to differentiate from a 525-line/60 Hz (480i) NTSC system. Accordingly, DVDs are labeled as either PAL or NTSC (referring informally to the line count and frame rate) even though technically the discs do not have either PAL or NTSC composite colour. The line count and frame rate are defined as EIA 525/60 or CCIR 625/50. PAL and NTSC are only the method of colour transmission.
Colour encoding
Both the PAL and the NTSC system use a quadrature amplitude modulated subcarrier carrying the chrominance information added to the luminance video signal to form a composite video baseband signal. The frequency of this subcarrier is 4.43361875 MHz for PAL, compared to 3.579545 MHz for NTSC. The SECAM system, on the other hand, uses a frequency modulation scheme on its two line alternate colour subcarrier 4.25000 and 4.40625 MHz.
The name "Phase Alternating Line" describes the way that the phase of part of the colour information on the video signal is reversed with each line, which automatically corrects phase errors in the transmission of the signal by canceling them out, at the expense of vertical frame colour resolution. Lines where the colour phase is reversed compared to NTSC are often called PAL or phase-alternation lines, which justifies one of the expansions of the acronym, while the other lines are called NTSC lines. Early PAL receivers relied on the human eye to do that canceling; however, this resulted in a comb-like effect known as Hanover bars on larger phase errors. Thus, most receivers now use a chrominance delay line, which stores the received colour information on each line of display; an average of the colour information from the previous line and the current line is then used to drive the picture tube. The effect is that phase errors result in saturation changes, which are less objectionable than the equivalent hue changes of NTSC. A minor drawback is that the vertical colour resolution is poorer than the NTSC system's, but since the human eye also has a colour resolution that is much lower than its brightness resolution, this effect is not visible. In any case, NTSC, PAL, and SECAM all have chrominance bandwidth (horizontal colour detail) reduced greatly compared to the luminance signal.
The 4.43361875 MHz frequency of the colour carrier is a result of 283.75 colour clock cycles per line plus a 25 Hz offset to avoid interferences. Since the line frequency (number of lines per second) is 15625 Hz (625 lines x 50 Hz / 2), the colour carrier frequency calculates as follows: 4.43361875 MHz = 283.75 * 15625 Hz + 25 Hz.
The original colour carrier is required by the colour decoder to recreate the colour difference signals. Since the carrier is not transmitted with the video information it has to be generated locally in the receiver. In order that the phase of this locally generated signal can match the transmitted information, a 10 cycle burst of colour subcarrier is added to the video signal shortly after the line sync pulse, but before the picture information, during the so called back porch. This colour burst is not actually in phase with the original colour subcarrier, but leads it by 45 degrees on the odd lines and lags it by 45 degrees on the even lines. This swinging burst enables the colour decoder circuitry to distinguish the phase of the R-Y vector which reverses every line.
PAL vs. NTSC
NTSC receivers have a tint control to perform colour correction manually. If this is not adjusted correctly, the colors may be faulty. The PAL standard automatically cancels hue errors by phase reversal, so a tint control is unnecessary. Chrominance phase errors in the PAL system are cancelled out using a 1H delay line resulting in lower saturation, which is much less noticeable to the eye than NTSC hue errors.
However, the alternation of colour information — Hanover bars — can lead to picture grain on pictures with extreme phase errors even in PAL systems, if decoder circuits are misaligned or use the simplified decoders of early designs (typically to overcome royalty restrictions). In most cases such extreme phase shifts do not occur. This effect will usually be observed when the transmission path is poor, typically in built up areas or where the terrain is unfavorable. The effect is more noticeable on UHF than VHF signals as VHF signals tend to be more robust.
In the early 1970s some Japanese set manufacturers developed decoding systems to avoid paying royalties to Telefunken. The Telefunken license covered any decoding method that relied on the alternating subcarrier phase to reduce phase errors. This included very basic PAL decoders that relied on the human eye to average out the odd/even line phase errors. One solution was to use a 1H delay line to allow decoding of only the odd or even lines. For example, the chrominance on odd lines would be switched directly through to the decoder and also be stored in the delay line. Then, on even lines, the stored odd line would be decoded again. This method effectively converted PAL to NTSC. Such systems suffered hue errors and other problems inherent in NTSC and required the addition of a manual hue control.
PAL and NTSC have slightly divergent color spaces, but the colour decoder differences here are ignored.
PAL vs. SECAM
SECAM is an earlier attempt at compatible colour television which also tries to resolve the NTSC hue problem. It does so by applying a different method to colour transmission, namely alternate transmission of the U and V vectors and frequency modulation, while PAL attempts to improve on the NTSC method.
SECAM transmissions are more robust over longer distances than NTSC or PAL. However, owing to their FM nature, the colour signal remains present, although at reduced amplitude, even in monochrome portions of the image, thus being subject to stronger cross colour. Like PAL, a SECAM receiver needs a delay line.
PAL signal details
For PAL-B/G the signal has these characteristics.
Parameter | Value |
Pixel Clock frequency (digital sources with 704 or 720 active Pixel/Line) | 13.5 MHz |
Bandwidth | 5 MHz |
Horizontal sync polarity | Negative |
Total time for each line | 64.000 µs |
Front porch (A) | 1.65+0.4 −0.1 µs |
Sync pulse length (B) | 4.7±0.20 µs |
Back porch (C) | 5.7±0.20 µs |
Active video (D) | 51.95+0.4 −0.1 µs |
(Total horizontal sync time 12.05 µs)
After 0.9 µs a 2.25±0.23 µs color burst of 10±1 cycles is sent. Most rise/fall times are in 250±50 ns range. Amplitude is 100% for white level, 30% for black, and 0% for sync. The CVBS electrical amplitude is Vpp 1.0 V and impedance of 75 Ω.
The composite video (CVBS) signal used in systems M and N before combination with a sound carrier and modulation onto an RF carrier.
The vertical timings are:
Parameter | Value |
Vertical lines | 313 (625 total) |
Vertical lines visible | 288 (576 total) |
Vertical sync polarity | Negative (burst) |
Vertical frequency | 50 Hz |
Sync pulse length (F) | 0.576 ms (burst) |
Active video (H) | 18.4 ms |
As PAL is interlaced, every two fields are summed to make a complete picture frame.
Luminance, Y, is derived from red, green, and blue (R'G'B') signals:
Y = 0.299R' + 0.587G' + 0.114B'
U and V are used to transmit chrominance. Each has a typical bandwidth of 1.3 MHz.
- U = 0.492(B' − Y)
- V = 0.877(R' − Y)
Composite PAL signal = Y + Usin(ωt) + Vcos(ωt) + timing where
ω = 2πFSC.
Subcarrier frequency FSC is 4.43361875 MHz (±5 Hz) for PAL-B/D/G/H/I/N.
PAL broadcast systems
This table illustrates the differences:
|
* System I has never been used on VHF in the UK.
** The UK's adoption of 582 active lines has no significant impact on either non system I receivers or non system I source material as the extra lines are not within the normal display area and do not contain anything in the other standards anyway. All Digital TV broadcasts and digital recordings (e.g. DVDs) conform to the 576 active line standards.
** The UK's adoption of 582 active lines has no significant impact on either non system I receivers or non system I source material as the extra lines are not within the normal display area and do not contain anything in the other standards anyway. All Digital TV broadcasts and digital recordings (e.g. DVDs) conform to the 576 active line standards.
PAL-B/G/D/K/I
The majority of countries using PAL have television standards with 625 lines and 25 frames per second, differences concern the audio carrier frequency and channel bandwidths. Standards B/G are used in most of Western Europe, Australia and New Zealand, standard I in the UK, Ireland, Hong Kong, South Africa and Macau, standards D/K in most of Central and Eastern Europe and Standard D in mainland China. Most analogue CCTV cameras are Standard D.
Systems B and G are similar. System B is used for 7 MHz-wide channels on VHF, while System G is used for 8 MHz-wide channels on UHF (and Australia uses System B on UHF). Similarly, Systems D and K are similar except for the bands they use: System D is only used on VHF, while System K is only used on UHF. Although System I is used on both bands, it has only been used on UHF in the United Kingdom due to 405-line TV services on VHF operating until the 1980s.
PAL-M (Brazil)
In Brazil, PAL is used in conjunction with the 525 line, 29.97 frame/s system M, using (very nearly) the NTSC colour subcarrier frequency. Exact colour subcarrier frequency of PAL-M is 3.575611 MHz. Almost all other countries using system M use NTSC.
The PAL colour system (either baseband or with any RF system, with the normal 4.43 MHz subcarrier unlike PAL-M) can also be applied to an NTSC-like 525-line (480i) picture to form what is often known as "PAL-60" (sometimes "PAL-60/525", "Quasi-PAL" or "Pseudo PAL"). PAL-M (a broadcast standard) however should not be confused with "PAL-60".
PAL-N (Argentina, Paraguay, Uruguay)
In Argentina, Paraguay and Uruguay the PAL-N variant is used. It employs the 625 line/50 field per second waveform of PAL-B/G, D/K, H, I, but on a 6MHz channel with a chrominance subcarrier frequency of 3.582 MHz very similar to NTSC.
VHS tapes recorded from a PAL-N or a PAL-B/G, D/K, H, I broadcast are indistinguishable because the down converted subcarrier on the tape is the same. A VHS recorded off TV (or released) in Europe will play in colour on any PAL-N VCR and PAL-N TV in Argentina, Paraguay, and Uruguay. Likewise, any tape recorded in Argentina or Uruguay off a PAL-N TV broadcast, can be sent to anyone in European countries that use PAL (and Australia/New Zealand, etc.) and it will display in colour. This will also play back successfully in Russia and other SECAM countries, as the USSR mandated PAL compatibility in 1985 - this has proved to be very convenient for video collectors.
People in Uruguay, Argentina and Paraguay usually own TV sets that also display NTSC-M, in addition to PAL-N. Direct TV also conveniently broadcasts in NTSC-M for North, Central and South America. Most DVD players sold in Argentina, Uruguay and Paraguay also play PAL discs - however, this is usually output in the European variant (colour subcarrier frequency 4.433618 MHz), so people who own a TV set which only works in PAL-N (plus NTSC-M in most cases) will have to watch those PAL DVD imports in black and white as the colour subcarrier frequency in the TV set is the PAL-N variation, 3.582056 MHz.
In the case that a VHS or DVD player works in PAL (and not in PAL-N) and the TV set works in PAL-N (and not in PAL), there are two options:
- images can be seen in black and white, or
- an inexpensive transcoder (PAL -> PAL-N) can be purchased in order to see the colors
Some DVD players (usually lesser known brands) include an internal transcoder and the signal can be output in NTSC-M, with some video quality loss due to the system's conversion from a 625/50 PAL DVD to the NTSC-M 525/60 output format. A few DVD players sold in Argentina, Uruguay and Paraguay also allow a signal output of NTSC-M, PAL, or PAL-N. In that case, a PAL disc (imported from Europe) can be played back on a PAL-N TV because there are no field/line conversions, quality is generally excellent.
Extended features of the PAL specification, such as Teletext, are implemented quite differently in PAL-N. PAL-N supports a modified 608 closed captioning format that is designed to ease compatibility with NTSC originated content carried on line 18, and a modified Teletext format that can occupy several lines.
PAL-L
The PAL L (Phase Alternating Line with L-sound system) standard uses the same video system as PAL-B/G/H (625 lines, 50 Hz field rate, 15.625 kHz line rate), but with 6 MHz video bandwidth rather than 5.5 MHz. This requires the audio subcarrier to be moved to 6.5 MHz. An 8 MHz channel spacing is used for PAL-L.
System A
The BBC tested their pre-war 405 line monochrome system with all three colour standards including PAL, before the decision was made to abandon 405 and transmit colour on 625/System I only.
PAL interoperability
The PAL colour system is usually used with a video format that has 625 lines per frame (576 visible lines, the rest being used for other information such as sync data and captioning) and a refresh rate of 50 interlaced fields per second (i.e. 25 full frames per second), such systems being B, G, H, I, and N (see broadcast television systems for the technical details of each format).
This ensures video interoperability. However as some of these standards (B/G/H, I and D/K) use different sound carriers (5.5MHz, 6.0MHz 6.5MHz respectively), it may result in a video image without audio when viewing a signal broadcast over the air or cable. Some countries in Eastern Europe which formerly used SECAM with systems D and K have switched to PAL while leaving other aspects of their video system the same, resulting in the different sound carrier. Instead, other European countries have changed completely from SECAM-D/K to PAL-B/G.
The PAL-N system has a different sound carrier, and also a different colour subcarrier, and decoding on incompatible PAL systems results in a black and white image without sound. The PAL-M system has a different sound carrier and a different colour subcarrier, and does not use 625 lines or 50 frames/second. This would result in no video or audio at all when viewing a European signal.
Multisystem PAL support and "PAL 60"
Recently manufactured PAL television receivers can typically decode all of these systems except, in some cases, PAL-M and PAL-N. Many of receivers can also receive Eastern European and Middle Eastern SECAM, though rarely French-broadcast SECAM (because France uses the unique positive video modulation) unless they are manufactured for the French market. They will correctly display plain CVBS or S-video SECAM signals. Many can also accept baseband NTSC-M, such as from a VCR or game console, and RF modulated NTSC with a PAL standard audio subcarrier (i.e. from a modulator), though not usually broadcast NTSC (as its 4.5 MHz audio subcarrier is not supported). Many sets also support NTSC with a 4.43 MHz subcarrier.
Many 1990s onwards VCR players sold in Europe can play back NTSC tapes/discs. When operating in this mode most of them do not output a true (625/25) PAL signal, but rather a hybrid consisting of the original NTSC line standard (525/30), but with colour converted to PAL 4.43 MHz - this is known as "PAL 60" (also "quasi-PAL" or "pseudo PAL") with "60" standing for 60 Hz (for 525/30), instead of 50 Hz (for 625/25). Some video game consoles also output a signal in this mode. Most newer television sets can display such a signal correctly, but some will only do so (if at all) in black and white and/or with flickering/foldover at the bottom of the picture, or picture rolling (however, many old TV sets can display the picture properly by means of adjusting the V-Hold and V-Height knobs — assuming they have them). Some TV tuner cards or video capture cards will support this mode (although software/driver modification can be required and the manufacturers' specs may be unclear). A "PAL 60" signal is similar to an NTSC (525/30) signal, but with the usual PAL chrominance subcarrier at 4.43 MHz (instead of 3.58 as with NTSC and South American PAL variants) and with the PAL-specific phase alternation of the red colour difference signal between the lines.
Most European DVD players output a true NTSC-M signal when playing NTSC discs, which many modern European TV sets can resolve.
Countries and territories using PAL
Over 120 countries and territories use or once used the terrestrial PAL system. Many of these are currently converting terrestrial PAL to DVB-T (PAL still often used by cable TV or in conjunction with a digital standard, such as DVB-C).NTSC
NTSC, named for the National Television System Committee (NTSC), is the analog television system that is used in most of North America, most of South America (except Brazil, Argentina, Uruguay, and French Guiana), Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories.
Most countries using the NTSC standard, as well as those using other analog television standards, are switching to newer digital television standards, of which at least four different ones are in use around the world. North America, parts of Central America, and South Korea are adopting the ATSC standards, while other countries are adopting or have adopted other standards.
The first NTSC standard was developed in 1941 and had no provision for color television. In 1953 a second modified version of the NTSC standard was adopted, which allowed color television broadcasting compatible with the existing stock of black-and-white receivers. NTSC was the first widely adopted broadcast color system. After nearly 70 years of use, the vast majority of over-the-air NTSC transmissions in the United States was replaced with digital ATSC on June 12, 2009, and will be by August 31, 2011 in Canada and most other NTSC markets. Despite the shift to digital broadcasting, standard definition television in these countries continues to follow the NTSC standard in terms of frame rate and number of lines of resolution. In the United States a small number of short-range local and TV relay stations continue to broadcast NTSC, as the FCC allows. NTSC baseband video signals are also still often used in video playback (typically of recordings from existing libraries using existing equipment) and in CCTV and surveillance video systems.
History
The National Television System Committee was established in 1940 by the United States Federal Communications Commission (FCC) to resolve the conflicts that arose between companies over the introduction of a nationwide analog television system in the United States. In March 1941, the committee issued a technical standard for black-and-white television that built upon a 1936 recommendation made by the Radio Manufacturers Association (RMA). Technical advancements of the vestigial sideband technique allowed for the opportunity to increase the image resolution. The NTSC selected 525 scan lines as a compromise between RCA's 441-scan line standard (already being used by RCA's NBC TV network) and Philco's and DuMont's desire to increase the number of scan lines to between 605 and 800. The standard recommended a frame rate of 30 frames (images) per second, consisting of two interlaced fields per frame at 262.5 lines per field and 60 fields per second. Other standards in the final recommendation were an aspect ratio of 4:3, and frequency modulation (FM) for the sound signal.
In January 1950, the Committee was reconstituted to standardize color television. In December 1953, it unanimously approved what is now called the NTSC color television standard (later defined as RS-170a). The "compatible color" standard retained full backward compatibility with existing black-and-white television sets. Color information was added to the black-and-white image by adding a color subcarrier of 4.5 × 455/572 = 315/88 MHz (approximately 3.58 MHz) to the video signal. To reduce the visibility of interference between the chrominance signal and FM sound carrier required a slight reduction of the frame rate from 30 frames per second to 30/1.001 (approximately 29.97) frames per second, and changing the line frequency from 15,750 Hz to 15,750/1.001 Hz (approximately 15,734.26 Hz).
The FCC had briefly approved a different color television standard, starting in October 1950, which was developed by CBS. However, this standard was incompatible with black-and-white broadcasts. It used a rotating color wheel, reduced the number of scan lines from 525 to 405, and increased the field rate from 60 to 144, but had an effective frame rate of only 24 frames per second. Legal action by rival RCA kept commercial use of the system off the air until June 1951, and regular broadcasts only lasted a few months before manufacture of all color television sets was banned by the Office of Defense Mobilization (ODM) in October, ostensibly due to the Korean War. CBS rescinded its system in March 1953, and the FCC replaced it on December 17, 1953 with the NTSC color standard, which was cooperatively developed by several companies, including RCA and Philco.The first publicly announced network television broadcast of a program using the NTSC "compatible color" system was an episode of NBC's Kukla, Fran and Ollie on August 30, 1953, although it was viewable in color only at the network's headquarters. The first nationwide view of NTSC color came on the following January 1 with the coast-to-coast broadcast of the Tournament of Roses Parade, viewable on prototype color receivers at special presentations across the country.
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 television camera. Later that year, the improved TK-41 became the standard camera used throughout much of the 1960s.
The NTSC standard has been adopted by other countries, including most of the Americas and Japan. With the advent of digital television, analog broadcasts are being phased out. Most U.S. NTSC broadcasters were required by the FCC to shut down their analog transmitters in 2009. Low-power stations, Class A stations and translators were not immediately affected. An analog cut-off date for those stations was not set.
Technical details:
Lines and refresh rate
NTSC color encoding is used with the system M television signal, which consists of 29.97 interlaced frames of video per second, or the nearly identical system J in Japan. Each frame consists of a total of 525 scanlines, of which 486 make up the visible raster. The remainder (the vertical blanking interval) are used for synchronization and vertical retrace. This blanking interval was originally designed to simply blank the receiver's CRT to allow for the simple analog circuits and slow vertical retrace of early TV receivers. However, some of these lines now can contain other data such as closed captioning and vertical interval time code (VITC). In the complete raster (ignoring half-lines), the even-numbered or 'lower" scanlines (Every other line that would be even if counted in the video signal, e.g. {2,4,6,…,524}) are drawn in the first field, and the odd-numbered or "upper" (Every other line that would be odd if counted in the video signal, e.g. {1,3,5,….525}) are drawn in the second field, to yield a flicker-free image at the field refresh frequency of approximately 59.94 Hertz (actually 60 Hz/1.001). For comparison, 576i systems such as PAL-B/G and SECAM uses 625 lines (576 visible), and so have a higher vertical resolution, but a lower temporal resolution of 25 frames or 50 fields per second.
The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60 Hz frequency of alternating current power used in the United States. Matching the field refresh rate to the power source avoided intermodulation, which produces rolling bars on the screen. When color was later added to the system, the refresh frequency was shifted slightly downward to 59.94 Hz to eliminate stationary dot patterns in the difference frequency between the sound and color carriers, as explained below in “olor encoding” Synchronization of the refresh rate to the power incidentally 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 to set the speed of the synchronous AC motor-drive camera. By the time the frame rate changed to 29.97 Hz for color, it was nearly as easy to trigger the camera shutter from the video signal itself.
The actual 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 (60 Hz in this case). This frequency was then compared with the 60 Hz power-line frequency and any discrepancy corrected by adjusting the frequency of the master oscillator. For interlaced scanning, an odd number of lines per frame was required in order to make the vertical retrace distance identical for the odd and even fields, which meant the master oscillator frequency had to be divided down by an odd number. At the time, the only practical method of frequency division was the use of a chain of vacuum tube multivibrators, the overall division ratio being the mathematical product of the division ratios of the chain. Since all the factors of an odd number also have to be odd numbers, it follows that all the dividers in the chain also had to divide by odd numbers, and these had to be relatively small due the problems of thermal drift with vacuum tube devices. The closest practical sequence to 500 that meets these criteria was 3 × 5 × 5 × 7 = 525. (For the same reason, 625-line PAL-B/G and SECAM uses 5 × 5 × 5 × 5, the old British 405-line system used 3 × 3 × 3 × 3 × 5, the French 819-line system used 3 × 3 × 7 × 13 etc.).
Colorimetry
The original 1953 color NTSC specification, still part of the United States Code of Federal Regulations, defined the colorimetric values of the system as follows:
Original NTSC colorimetry (1953) | CIE 1931 x | CIE 1931 y |
Primary red | 0.67 | 0.33 |
Primary green | 0.21 | 0.71 |
Primary blue | 0.14 | 0.08 |
White point (CIE Standard illuminant C) | 0.31 | 0.316 |
Early color television receivers, such as the RCA CT-100, were faithful to this specification, having a larger gamut than most of today's monitors. Their low-efficiency phosphors however were dark and long-persistent, leaving trails after moving objects. Starting in the late 1950s, picture tube phosphors would sacrifice saturation for increased brightness; this deviation from the standard both at the receiver and broadcaster ends was the source of considerable color variation.
Color correction in studio monitors and home receivers
To ensure more uniform color reproduction, receivers started to incorporate color correction circuits that converted the received signal — encoded for the colorimetric values listed above — into signals encoded for the phosphors actually used within the receiver.Since such color correction can not be performed accurately on the nonlinear (gamma-corrected) signals transmitted, the adjustment can only be approximated, introducing both hue and luminance errors for highly saturated colors.
Similarly at the broadcaster stage, in 1968-69 the Conrac Corp., working with RCA, defined a set of controlled phosphors for use in broadcast color picture video monitors.This specification survives today as the SMPTE "C" phosphor specification:
SMPTE "C" colorimetry | CIE 1931 x | CIE 1931 y |
Primary red | 0.63 | 0.34 |
Primary green | 0.31 | 0.595 |
Primary blue | 0.155 | 0.07 |
White point (CIE illuminant D65) | 0.3127 | 0.329 |
As with home receivers, it was further recommended that studio monitors incorporate similar color correction circuits so that broadcasters would transmit pictures encoded for the original 1953 colorimetric values, in accordance with FCC standards.
In 1987, the Society of Motion Picture and Television Engineers (SMPTE) Committee on Television Technology, Working Group on Studio Monitor Colorimetry, adopted the SMPTE C (Conrac) phosphors for general use in Recommended Practice 145, prompting many manufacturers to modify their camera designs to directly encode for SMPTE "C" colorimetry without color correction., as approved in SMPTE standard 170M, "Composite Analog Video Signal — NTSC for Studio Applications" (1994). As a consequence, the ATSC digital television standard states that for 480i signals, SMPTE "C" colorimetry should be assumed unless colorimetric data is included in the transport stream.
Variations
Japanese NTSC uses the same colorimetric values for red, blue, and green, but employs a different white point of CIE Illuminant D93 (x=0.285, y=0.293). Both the PAL and SECAM systems used the original 1953 NTSC colorimetry as well until 1970; unlike NTSC, however, the European Broadcasting Union (EBU) eschewed color correction in receivers and studio monitors that year and instead explicitly called for all equipment to directly encode signals for the "EBU" colorimetric values, further improving the color fidelity of those systems.
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 filtering out the chrominance. If it were not removed, the picture would be covered with dots (a result of chroma being interpreted as luminance). All black-and-white TVs sold in the US after the introduction of color broadcasting in 1953 were designed to filter chroma out, but the early B&W sets did not do this and chroma dots would show up in the picture.
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) QAM. These two signals are each amplitude modulated and then added together. The carrier is suppressed. Mathematically, the result can be viewed as a single sine wave with varying phase relative to a reference and varying amplitude. The phase represents the instantaneous color hue captured by a TV camera, and the amplitude represents the instantaneous color saturation.
For a TV to recover hue information from the I/Q phase, it must have a zero phase reference to replace the suppressed carrier. It also needs a reference for amplitude to recover the saturation information. 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 the end of the blanking pulse.) The color burst consists of a minimum of eight cycles of the unmodulated (fixed phase and amplitude) color subcarrier. The TV receiver has a "local oscillator", which it synchronizes to the color bursts and then uses as a reference for decoding the chrominance. By comparing the reference signal derived from color burst to the chrominance signal's amplitude and phase at a particular point in the raster scan, the device determines what chrominance to display at that point. Combining that with the amplitude of the luminance signal, the receiver calculates what color to make the point, i.e. the point at the instantaneous position of the continuously scanning beam. Note that analog TV is discrete in the vertical dimension (there are distinct lines) but continuous in the horizontal dimension (every point blends into the next with no boundaries), hence there are no pixels in analog TV. In CRT televisions, the NTSC signal is turned into RGB, which is then used to control the electron guns. Digital TV sets receiving analog signals instead convert the picture into discrete pixels. This process of discretization necessarily degrades the picture information somewhat, though with small enough pixels the effect may be imperceptible. Digital sets include all sets with a matrix of discrete pixels built into the display device, such as LCD, plasma, and DLP screens, but not CRTs, which do not have fixed pixels. This should not be confused with digital (ATSC) television signals, which are a form of MPEG video, but which still have to be converted into a format the TV can use.
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 3.579545 MHz color carrier may beat with the sound carrier to produce a dot pattern on the screen. To make the resulting pattern less noticeable, designers adjusted the original 60 Hz field rate down by a factor of 1.001 (0.1%), to approximately 59.94 fields per second. This adjustment ensures that the sums and differences of the sound carrier and the color subcarrier and their multiples (i.e., the intermodulation products of the two carriers) are not exact multiples of the frame rate, which is the necessary condition for the dots to remain stationary on the screen, making them most noticeable.
The 59.94 rate is derived from the following calculations. 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. (Another way this is often stated is that the color subcarrier frequency is an odd multiple of half the line frequency.) They then chose to make the audio subcarrier frequency an integer multiple of the line frequency to minimize visible (intermodulation) 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. Raising the audio subcarrier frequency would prevent existing (black and white) receivers from properly tuning in the audio signal. 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 variation in the line frequency. So the engineers chose the line frequency to be changed for the color standard. In the black-and-white standard, the ratio of audio subcarrier frequency to line frequency is 4.5 MHz / 15,750 = 285.71. In the color standard, this becomes rounded to the integer 286, which means the color standard's line rate is 4.5 MHz / 286 = approximately 15,734 lines per second. Maintaining the same number of scan lines per field (and frame), the lower line rate must yield a lower field rate. Dividing (4,500,000 / 286) lines per second by 262.5 lines per field gives approximately 59.94 fields per second.
Transmission modulation scheme
An NTSC television channel as transmitted occupies a total bandwidth of 6 MHz. 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 most AM signals, 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 1.25 MHz 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 a suppressed carrier. The audio signal is frequency-modulated, like the audio signals broadcast by FM radio stations in the 88–108 MHz band, but with a ±25kHz maximum frequency swing, as opposed to ±75kHz as is used on the FM band. The main audio carrier is 4.5 MHz above the video carrier, making it 250 kHz below the top of the channel. Sometimes a channel may contain an MTS signal, which offers more than one audio signal by adding one or two subcarrier on the audio signal, each synchronized to a multiple of the line frequency. This is normally the case when stereo audio and/or second audio program signals are used. The same extensions are used in ATSC, where the ATSC digital carrier is broadcast at 1.31 MHz above the lower bound of the channel.
The Cvbs (Composite vertical blanking signal) (sometimes called "setup") is a voltage offset between the "black" and "blanking" levels. Cvbs is unique to NTSC. Cvbs has the advantage of making NTSC video more easily separated from its primary sync signals.
Framerate conversion
There is a large difference in frame rate between film, which runs at 24.0 frames per second, and the NTSC standard, which runs at approximately 29.97 frames per second.
Unlike the 576i video formats, this difference cannot be overcome by a simple speed-up.
A complex process called "3:2 pull down" is used. One film frame is transmitted for three video fields (1½ video frame times), and the next frame is transmitted for two video fields (one video frame time). Two film frames are therefore transmitted in five video fields, for an average of 2½ video fields per film frame. The average frame rate is thus 60 / 2.5 = 24 frame/s, so the average film speed is exactly what it should be. There are drawbacks, however. Still-framing on playback can display a video frame with fields from two different film frames, so any motion between the frames will appear as a rapid back-and-forth flicker. There can also be noticeable jitter/"stutter" during slow camera pans (telecine judder).
To avoid 3:2 pulldown, film shot specifically for NTSC television is often taken at 30 frame/s.
For viewing native 576i 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:
- The framerate can be slowed from 25 to 23.976 frames per second (a slowdown of about 4%) to subsequently apply 3:2 pulldown.
- Interpolation of the contents of adjacent frames in order to produce new intermediate frames; unless highly sophisticated motion-sensing computer algorithms are applied, this introduces artifacts, and even the most modestly trained of eyes can quickly spot video that has been converted between formats.
Modulation for analog satellite transmission
Because satellite power is severely limited, analog video transmission through satellites differs from terrestrial TV transmission. AM is a linear modulation method, so a given demodulated signal-to-noise ratio (SNR) requires an equally high received RF SNR. The SNR of studio quality video is over 50 dB, so AM would require prohibitively high powers and/or large antennas.
Wideband FM is used instead to trade RF bandwidth for reduced power. Increasing the channel bandwidth from 6 to 36 MHz allows a RF SNR of only 10 dB or less. The wider noise bandwidth reduces this 40 dB power saving by 36 MHz / 6 MHz = 8 dB for a substantial net reduction of 32 dB.
Sound is on a FM subcarrier as in terrestrial transmission, but frequencies above 4.5 MHz are used to reduce aural/visual interference. 6.8, 5.8 and 6.2 MHz are commonly used. Stereo can be multiplex or discrete, and unrelated audio and data signals may be placed on additional subcarrier.
A triangular 60 Hz energy dispersal waveform is added to the composite baseband signal (video plus audio and data subcarrier) before modulation. This limits the satellite downlink power spectral density in case the video signal is lost. Otherwise the satellite might transmit all of its power on a single frequency, interfering with terrestrial microwave links in the same frequency band.
In half transponder mode, the frequency deviation of the composite baseband signal is reduced to 18 MHz to allow another signal in the other half of the 36 MHz transponder. This reduces the FM benefit somewhat, and the recovered SNRs are further reduced because the combined signal power must be "backed off" to avoid intermodulation distortion in the satellite transponder. A single FM signal is constant amplitude, so it can saturate a transponder without distortion.
Field order
An NTSC "frame" consists of an "even" field followed by an "odd" field. As far as the reception of an analog signal is concerned, this is purely a matter of convention and, it makes no difference. It's rather like the broken lines running down the middle of a road, it doesn't matter whether it is a line/space pair or a space/line pair, the effect to a driver is exactly the same.
The introduction of digital television formats has changed things somewhat. Most digital TV formats, including the popular DVD format, record NTSC originated video with the even field first in the recorded frame (the development of DVD took place in regions that traditionally utilize NTSC). However, this frame sequence has migrated through to the so-called PAL format (actually a technically incorrect description) of digital video with the result that the even field is often recorded first in the frame (the European 625 line system is specified as odd frame first). This is no longer a matter of convention because a frame of digital video is a distinct entity on the recorded medium. This means that when reproducing many non NTSC based digital formats (including DVD) it is necessary to reverse the field order otherwise an unacceptable shuddering "comb" effect occurs on moving objects as they are shown ahead in one field and then jump back in the next.
This has also become a hazard where non NTSC progressive video is transcoded to interlaced and vice versa. Systems that recover progressive frames or transcode video should ensure that the "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. This can often be observed in PC based video playing utilities if an inappropriate choice of de-interlacing algorithm is made.
Comparative quality
Reception problems can degrade an NTSC picture by changing the phase of the color signal (actually differential phase distortion), so the color balance of the picture will be altered unless a compensation is made in the receiver. The vacuum-tube electronics used in televisions through the 1960s led to various technical problems. Among other things, the color burst phase would often drift when channels were changed, which is why NTSC televisions were equipped with a tint control. PAL and SECAM televisions had no need of one, and although it is still found on NTSC TVs, color drifting generally ceased to be a problem once solid-state electronics were adopted in the 1970s. When compared to PAL in particular, NTSC color accuracy and consistency is sometimes considered inferior, leading to video professionals and television engineers jokingly referring to NTSC as Never The Same Color, Never Twice the Same Color, or No True Skin Colors, while for the more expensive PAL system it was necessary to Pay for Additional Luxury. PAL has also been referred to as Peace At Last in the color war. This mostly applied to vacuum tube-based TVs, however, and solid state sets have less of a difference in quality between NTSC and PAL. This color phase, "tint", or "hue" control allows for anyone skilled in the art to easily calibrate a monitor with SMPTE color bars, even with a set that has drifted in its color representation, allowing the proper colors to be displayed. Older PAL television sets did not come with a user accessible "hue" control (it was set at the factory), which contributed to its reputation for reproducible colors.
The use of NTSC coded color in S-Video systems completely eliminates the phase distortions. As a consequence, the use of NTSC color encoding gives the highest resolution picture quality (on the horizontal axis & frame rate) of the three color systems when used with this scheme. (The NTSC resolution on the vertical axis is lower than the European standards, 525 lines against 625) However, it uses too much bandwidth for over-the-air transmission. Some home computers in the 1980s generated S-video, but only for specially designed monitors as no TV at the time supported it. In 1987, a standardized 4-pin DIN plug was introduced for S-video input with the introduction of S-VHS players, which were the first device produced to use the 4-pin plugs. However, S-VHS never became very popular as the picture quality was not significantly better than that of standard VCRs and only high-end TVs supported S-video. Video game consoles in the 1990s began offering S-video output as well, but it was not until high-definition appeared in the 2000s that it became standard on most TVs.With the advent of DVD players in the 1990s, component video also began appearing. This provides separate lines for the luminance, red shift, and blue shift. Thus, component produces near-RGB quality video. It also allows 480p progressive-scan video due to the greater bandwidth offered. Like S-video, component inputs first appeared on high-end TVs and became standard with high-definition sets.
The mismatch between NTSC's 30 frames per second and film's 24 frames is overcome by a process that capitalizes on the field rate of the interlaced NTSC signal, thus avoiding the film playback speedup used for 576i systems at 25 frames per second (which causes the accompanying audio to increase in pitch slightly, sometimes rectified with the use of a pitch shifter) at the price of some jerkiness in the video. See Framerate conversion above.
Variants
1.SC-M
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.
2.SC-J
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. Since the difference is quite small, a slight turn of the brightness knob is all that is required to correctly show 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 channel encoding on NTSC-J differs slightly from NTSC-M. In particular, the Japanese VHF band runs from channels 1-12 while the American VHF band uses channels 2-13.
3.AL-M (Brazil)
The Brazilian PAL-M system, introduced in 1972, uses the same lines/field as NTSC (525/60), and almost the same broadcast bandwidth and scan frequency (15.750 vs. 15.734 kHz). Prior to the introduction of color, Brazil broadcast in standard black-and-white NTSC. As a result, PAL-M signals are near identical to North American NTSC signals, except for the encoding of the colour subcarrier (3.575611 MHz for PAL-M and 3.579545 MHz for NTSC). As a consequence of these close specs, PAL-M will display in monochrome with sound on NTSC sets and vice versa.
- PAL-M (PAL=Phase Alternating Line) specs are:
- Transmission Band UHF/VHF,
- Frame Rate 29.97
- Lines/Field 525/60
- Horizontal Freq. 15.750 kHz
- Vertical Freq. 60 Hz
- Color Sub Carrier 3.575611 MHz
- Video Bandwidth 4.2 MHz
- Sound Carrier Frequency 4.5 MHz
- Channel Bandwidth 6 MHz
- NTSC (National Television System Committee) specs are:
- Transmission Band UHF/VHF
- Lines/Field 525/60
- Horizontal Frequency 15.734 kHz
- Vertical Frequency 60 Hz
- Color Subcarrier Frequency 3.579545 MHz
- Video Bandwidth 4.2 MHz
- Sound Carrier Frequency 4.5 MHz
4.AL-N
This is used in Paraguay, Uruguay and Argentina. This is very similar to PAL-M (used in Brazil).
The similarities of NTSC-M and NTSC-N can be seen on the ITU identification scheme table, which is reproduced here:
World television systems | |||||||||
System | Lines | Frame rate | Channel b/w | Visual b/w | Sound offset | Vestigial sideband | Vision mod | Sound mod | Notes |
M | 525 | 30 | 6 | 4.2 | 4.5 | 0.75 | Neg. | FM | Most of the Americas and Caribbean, South Korea, Taiwan, Philippines (all NTSC-M) and Brazil (PAL-M). |
N | 625 | 25 | 6 | 4.2 | 4.5 | 0.75 | Neg. | FM | Argentina, Paraguay, Uruguay (all PAL-N). Greater number of lines results in higher quality. |
As it is shown, aside from the number of lines and frames per second, the systems are identical. NTSC-N/PAL-N are compatible with sources such as game consoles, VHS/Betamax VCRs, and DVD players. However, they are not compatible with broadband broadcasts (which are received over an antenna), though some newer sets come with baseband NTSC 3.58 support (NTSC 3.58 being the frequency for color modulation in NTSC: 3.58 MHz).
5.SC 4.43
In what can be considered an opposite of PAL-60, NTSC 4.43 is a pseudo color system that transmits NTSC encoding (525/29.97) with a color subcarrier of 4.43 MHz instead of 3.58 MHz. The resulting output is only viewable by TVs that support the resulting pseudo-system (usually multi-standard TVs). Using a native NTSC TV to decode the signal yields no color, while using a PAL TV to decode the system yields erratic colors (observed to be lacking red and flickering randomly). The format is apparently limited to few early laserdisc players and some game consoles sold in markets where the PAL system is used.
The NTSC 4.43 system, while not a broadcast format, appears most often as a playback function of PAL cassette format VCRs, beginning with the Sony 3/4" U-Matic format and then following onto Betamax and VHS format machines. As Hollywood has the claim of providing the most cassette software (movies and television series) for VCRs for the world's viewers, and as not all cassette releases were made available in PAL formats, a means of playing NTSC format cassettes was highly desired.
Multi-standard video monitors were already in use in Europe to accommodate broadcast sources in PAL, SECAM, and NTSC video formats. The heterodyne color-under process of U-Matic, Betamax & VHS lent itself to minor modification of VCR players to accommodate NTSC format cassettes. The color-under format of VHS uses a 629 kHz subcarrier while U-Matic & Betamax use a 688 kHz subcarrier to carry an amplitude modulated chroma signal for both NTSC and PAL formats. Since the VCR was ready to play the color portion of the NTSC recording using PAL color mode, the PAL scanner and capstan speeds had to be adjusted from PAL's 50 Hz field rate to NTSC's 59.94 Hz field rate, and faster linear tape speed.
The changes to the PAL VCR are minor thanks to the existing VCR recording formats. The output of the VCR when playing an NTSC cassette in NTSC 4.43 mode is 525 lines/29.97 frames per second with PAL compatible heterodyned color. The multi-standard receiver is already set to support the NTSC H & V frequencies; it just needs to do so while receiving PAL color.
The existence of those multi-standard receivers was probably part of the drive for region coding of DVDs. As the color signals are component on disc for all display formats, almost no changes would be required for PAL DVD players to play NTSC (525/29.97) discs as long as the display was frame-rate compatible.
6. SC-movie
NTSC with a frame rate of 23.976 frame/s is described in the NTSC-movie standard.
Canada/U.S. video game region
Sometimes NTSC-US or NTSC-U/C is used to describe the video gaming region of North America (the U/C refers to U.S. + Canada), as regional lockout usually restricts games released within a region to that region.
Vertical interval reference
The standard NTSC video image contains some lines (lines 1–21 of each field) that are not visible (this is known as the Vertical Blanking Interval, or VBI); 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.Suitably equipped television sets could then employ these 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.
A less-used successor to VIR, GCR, also added ghost (multipath interference) removal capabilities.
The remaining vertical blanking interval lines are typically used for data casting or ancillary data such as video editing timestamps (vertical interval time codes 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.
Many stations transmit TV Guide On Screen (TVGOS) data for an electronic program guide on VBI lines. The primary station in a market will broadcast 4 lines of data, and backup stations will broadcast 1 line. In most markets the PBS station is the primary host. TVGOS data can occupy any line from 10-25, but in practice its limited to 11-18, 20 and line 22. Line 22 is only used for 2 broadcast, DirecTV and CFPL-TV.
TiVo data is also transmitted on some commercials and program advertisements so customers can auto record the program being advertised, and is also used in weekly half-hour paid programs on Ion Television and the Discovery Channel which highlight TiVo promotions and advertisers.
