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Revision as of 09:06, 30 June 2008

For other uses, see PAL (disambiguation).
Television encoding systems by nation. Countries using the PAL system are shown in blue. (20th century)

PAL, short for Phase Alternating Line, is a colour encoding system used in broadcast television systems in large parts of the world. Other common analogue television systems are SECAM and NTSC. This page discusses the colour encoding system only. See Broadcast television systems and analogue television for discussion of frame rates, image resolution and audio modulation. For discussion of the 625-line 25 frame/s television standard, see 576i.

History of the PAL standard

In the 1950s, when the Western European countries were planning to establish colour television, they were faced with the problem that the already existing American NTSC standard wouldn't fit the 50 Hz AC frequency of the European power grids. In addition to that NTSC demonstrated several weaknesses, including colour tone shifting under less-than-ideal transmission conditions. For these reasons the development of the SECAM and PAL standards began. The goal was to provide a colour TV standard with a picture frequency of 50 fields per second (50 hertz), and sporting a better colour picture than NTSC.

PAL was developed by Walter Bruch at Telefunken in Germany. The format was first unveiled in 1963, with the first broadcasts beginning in the United Kingdom and Germany in 1967.[1]

Telefunken was later bought by the French electronics manufacturer Thomson. Thomson also bought the Compagnie Générale de Télévision where Henri de France developed SECAM, historically the first European colour television standard. Thomson nowadays also co-owns the RCA brand for consumer electronics products, which 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, principally European) television system, and to differentiate from a 525-line/60 Hz (480i, principally North American/Central American/Japanese) NTSC system. Accordingly, DVDs are labelled as either PAL or NTSC (referring informally to the line count and frame rate) even though technically the European discs do not have PAL composite colour. This usage may lead readers to believe that PAL defines image resolution, even though it doesn't. The PAL colour system can be used in conjunction with any resolution and frame rate, and various such combinations exist. NTSC, by contrast does define the video line and frame format.

Technical details

The basics of PAL and the NTSC system are very similar; a quadrature amplitude modulated subcarrier carrying the chrominance information is added to the luminance video signal to form a composite video baseband signal. The frequency of this subcarrier is approximately 4.43 MHz for PAL, compared to approximately 3.58 MHz for NTSC. The SECAM system, on the other hand, uses a frequency modulation scheme on its colour subcarrier.

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 cancelling them out. 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 imperfections of the human eye to do that cancelling; however this resulted in a comb-like effect 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.

For a 1:1 pixel aspect (square pixels) on a 50 Hz interlaced PAL signal the pixel rate should be 14.75 MHz.

spectrum of a system G (bands IV and V) television channel with PAL OR SECAM colour).

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 is 15625 Hz, the colour carrier frequency calculates as follows: 4.43361875 MHz = 283.75 * 15625 Hz + 25 Hz.

  • CVBS is an initialism, but it doesn't stand for "composite video baseband signal", CVBS actually stands for (C)hroma, (V)ideo, (B)urst, and (S)ync; which are the four basic components of a composite video signal. That's why it's called "composite".

PAL vs. NTSC

NTSC receivers have a tint control to perform colour correction manually. If this isn't adjusted correctly, the colours may be faulty. The PAL standard automatically removes hue errors by utilizing phase alternation of the colour signal (see technical details), so a tint control is unnecessary. Chrominance phase errors in the PAL system are cancelled out using a 1H delay line resulting 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 mis-aligned or use the simplified decoders of early designs (to overcome royalty restrictions). Usually 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 unfavourable. The effect is more noticeable on UHF signals than VHF as VHF signals tend to be more robust.

A PAL decoder can be seen as a pair of NTSC decoders:

  • PAL can be decoded with two NTSC decoders.
  • By switching between the two NTSC decoders every other line it is possible to decode PAL without a phase delay line or two phase-locked loop (PLL) circuits.
  • This works because one decoder receives a colour sub carrier with negated phase in relation to the other decoder. It then negates the phase of that sub carrier when decoding. This leads to smaller phase errors being cancelled out. However a delay line PAL decoder gives superior performance. Some Japanese TVs originally used the dual NTSC method to avoid paying royalty to Telefunken.
  • PAL and NTSC have slightly divergent colour spaces, but the colour decoder differences here are ignored.
  • PAL supports SMPTE 498.3 while NTSC is compliant with EBU Recommendation 14.
  • The issue of frame rates and colour sub carriers is ignored in this technical explanation. These technical details play no direct role (except as subsystems and physical parameters) to the decoding of the signal.

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 long distances than NTSC or PAL. However due 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.

Broadcast systems PAL has been used with

PAL B/G/D/K/I

The majority of countries using PAL have television standards with 625 lines and 25 frames, differences concern the audio carrier frequency and channel bandwidths. Standards B/G are used in most of Western Europe, standard I in the UK and Ireland, standards D/K in most of Eastern Europe.

7-MHz channels are used in VHF (B, D) and 8-MHz channels in UHF (G, K, I), although Australia used 7-MHz channels in UHF and Ireland uses 8-MHz channels in VHF.

PAL-M standard (Brazil)

Main article: PAL-M (television)

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" or "Pseudo PAL"). PAL-M (a broadcast standard) however should not be confused with "PAL-60" (a video playback system — see below).

PAL-Nc

In Argentina, the PAL-Nc (combination N) variant is used. It employs the 625 line/50 field per second waveform of PAL-B/G,D/K,H,I but with a chrominance sub-carrier frequency of 3.582 MHz. VHS tapes recorded from a PAL-Nc or a PAL-B/G,D/K,H,I broadcast are indistinguishable.

PAL-N

In Paraguay and Uruguay, PAL is used with the standard 625 line/50 fields per second system, but again with (very nearly) the NTSC colour

  • PAL-N should not be viewed as wildly incompatible versions of the PAL system, only the choice of colour subcarrier is different.
  • 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 has been very convenient for video collectors in the past.

People in Uruguay, Argentina and Paraguay usually own TV sets that also display NTSC-M, in addition to PAL-N of course. Direct TV broadcasts in NTSC-M for North, Central and South America so this is very convenient too. 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 that 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. 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 because of the systems conversion from a 625/50 PAL DVD disc to the output in NTSC-M 525/60. A few DVD players sold in Argentina and Uruguay allow to output the signal in 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 is no fields/lines conversion, quality is 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 occur several lines.

PAL L

The PAL L (Phase Alternating Line with L-sound system) standard uses the System "PAL" video standard, which is the same as PAL B/G/H (625 lines, 50 Hz field rate, 15.625 kHz line rate) except that it uses 6 MHz video bandwidth rather than 5.5 MHz, lifting the audio subcarrier to 6.5 MHz. When System L is used with SECAM, the audio carrier is amplitude modulated, but when used with PAL, the more usual FM sound system is usually used. The sound offset in B and G is +5.5 whereas in L its +6.5. In layman's language, PAL-L is PAL-BG with positive and AM sound modulation. An 8 MHz channel spacing is used with PAL L.

PAL L is used on some hotel internal distribution systems, as well as other public display and plant television systems. It is not used by any national TV networks. One example of a TV with PAL-L support is Thomson 24WK25. This signal has been defined by Sandipan Bhattacharjee, India.

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.

All PAL systems interoperable except PAL-M (525/60)

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 as systems B, G, H, I, and N (see broadcast television systems for the technical details of each format).

  • 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.
  • However, some European countries have changed completely from SECAM-D/K to PAL-B/G.[citation needed]

On RF (i.e. via a Modulator or TV Aerial) the difference between I, D/H and B/G is audio. These use different audio subcarriers, so with mismatch on Modulator Settings or an imported TV there will be perfectly normal Colour Video, but possibly no audio. Some TVs and VHS tuners have multiple filters in parallel or switched for the 6 MHz, 5.5 MHz, 6.5 MHz or 4.5 MHz sound carriers. Nicam is an additional 6.5 MHz offset carrier carrying stereo digitally, on 6.0 MHz PAL I systems. Germany particularly uses two separate FM sound carriers on PAL B/G. (Stereo FM Radio uses a mono signal with a DSBSC L-R audio centred on 38 kHz with a 19 kHz pilot to aid decoding. Hence the German Zweiton and Nicam both give better performance than FM Radio).

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 them 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 made 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, though not usually broadcast NTSC. Many sets also support NTSC with a 4.43 MHz subcarrier.

Many newer Video Cassette recorders and DVD players sold in Europe can play back NTSC tapes/discs. When operating in this mode most of them don't output a true (625/25) PAL signal but rather a hybrid of PAL and NTSC known as "PAL 60" (or "pseudo PAL") with "60" standing for 60 Hz, instead of 50 Hz. 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 (it can be noted, however, that many analogue-era TV sets can receive the picture by means of adjusting the V-Hold and V-Height knobs — assuming they have them). Very few TV tuner cards or video capture cards will support this mode (a small number can, although software/driver modification is usually required and the manufacturers specs are usually unclear). A "PAL 60" signal is similar to an NTSC (525/30) signal but with a PAL chrominance subcarrier at 4.43 MHz (instead of 3.58) and with the PAL-specific phase alternation of the red colour difference signal between the lines.

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 via cable TV).

PAL B, G, D, K or I

PAL-M

  •  Brazil (simulcast in ISDB-T will start in December 2007 until 2016)
  •  Laos (also uses SECAM)

PAL-N and PAL-NC

Countries and territories that once used PAL

[2]

See also

References

  1. ^ The standard that defines the PAL system was published by the International Telecommunications Union in 1998 and has the title Recommendation ITU-R BT.470-6, Conventional Television Systems
  2. ^ Changes to terrestrial television systems in Central and Eastern European countries

External links

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