PAL

Television encoding systems by nation; countries using the PAL system are shown in blue.

PAL, short for Phase Alternate Line, is an analogue television encoding system used in broadcast television systems in large parts of the world. Other common analogue television systems are SECAM and NTSC. This page primarily discusses the colour encoding system. See the articles on broadcast television systems and analogue television for additional discussion of frame rates, image resolution and audio modulation. For discussion of the 625-line / 25 frame per second television standard, see 576i.

Contents

History

In the 1950s, when the Western European countries were planning to establish colour television, they were faced with the problem that the NTSC standard demonstrated several weaknesses, including colour tone shifting under poor transmission conditions, earning it the comically malign backronym "Never Twice the Same Colour". For these reasons the development of the SECAM and PAL standards began. 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 first unveiled in 1963, with the first broadcasts beginning in the United Kingdom in 1964 and Germany in 1967,[1] 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, historically the first European colour television standard. Thomson 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), television system, and to differentiate from a 525-line/60 Hz (480i) NTSC system. Accordingly, DVDs are labelled 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 the colour transmission used.

Colour encoding

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 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 subcarriers 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 cancelling 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 imperfections of the human eye to do that cancelling; however this resulted in a comblike 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.

Spectrum of a System I television channel with PAL colour.
Oscillogram of composite PAL signal - one frame.
Oscillogram of composite PAL signal - several lines.
Oscillogram of composite PAL signal - two lines.

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.

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 colours may be faulty. The PAL standard automatically removes hue errors by utilising phase alternation of the colour signal (see above), 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 unfavourable. 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 licence 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 supports SMPTE 498.3 while NTSC is compliant with EBU Recommendation 14.

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
Clock frequency 14.8 MHz[2]
Bandwidth 5.0 MHz[3]
Horizontal sync polarity Negative
Total time for each line 64.000 µs[4][5]
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 colourburst of 10±1 cycles is sent. Most rise/fall times are in 250±50 ns range. Amplitude is 100% for white level (white colour on a monochrome receiver), 30% for black, and 0% for sync.[4] The CVBS electrical amplitude is Vpp 1.0 V and impedance of 75 Ω.[6]

The composite video (CVBS) signal used in analogue television 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)[7]
Active video (H) 18.4 ms

(Total vertical sync time 1.6 ms)

As PAL is interlaced, every two lines are summed to make a complete picture frame.

Luminance, Y, is derived from red, green, and blue (R'G'B') signals:[5]

U and V are used to transmit chrominance. Each has a typical bandwidth of 1.3 MHz.

Composite PAL signal = Y + U \sin (\omega t) + V \cos (\omega t) +timing[5] where \omega = 2\pi F_{SC}.

Subcarrier frequency F_{SC} is 4.43361875 MHz (±5 Hz) for PAL-B/D/G/H/I/N.

An interesting comparison can be made with the VGA signal, the most notable differences being the double horizontal sweep time and interlace mode.

PAL broadcast systems

This table illustrates the differences:

PAL B PAL G, H PAL I PAL M PAL D PAL N PAL Nc
Transmission Band VHF UHF UHF/VHF UHF/VHF VHF UHF/VHF UHF/VHF
Lines/Fields 625/50 625/50 625/50 525/60 625/50 625/50 625/50
Video Bandwidth 5.0 MHz 5.0 MHz 5.5 MHz 4.2 MHz 6.0 MHz 5.0 MHz 4.2 MHz
Sound Carrier 5.5 MHz 5.5 MHz 6.0 MHz 4.5 MHz 6.5 MHz 5.5 MHz 4.5 MHz
Channel Bandwidth 7 MHz 8 MHz 8 MHz 6 MHz 8 MHz 6 MHz 6 MHz
Active lines 576 576 582* 480 576 576 576

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, standard I in the UK, Ireland, Hong Kong 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.

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)

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

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 subcarrier frequency of 3.582 MHz. VHS tapes recorded from a PAL-Nc or a PAL-B/G, D/K, H, I broadcast are indistinguishable because the downconverted subcarrier on the tape is the same.

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 subcarrier frequency.

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:

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 disc 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 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 - this has the result of 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 standard FM sound system is utilised. The sound offset in B and G is +5.5 MHz, whereas in L the offset is +6.5 MHz - in layman's terms, PAL-L is PAL-BG with AM sound modulation (hence positive mod). An 8 MHz channel spacing is used in 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. Many more Tvs are coming up with this signal, but it may be overridden by hospitality mode TVs meant to be for business uses. 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).

On RF (i.e. through 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 analogue 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 analogue Zweikanalton and the digital 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 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 4.43MHz - this is known as "PAL 60" (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).

PAL B, G, D, K or I

PAL-M

PAL-N and PAL-Nc

Countries and territories that once used PAL

Country Switched to Switchover completed
 Andorra DVB-T 2007-09-2525 September 2007
 Belgium DVB-T 2010-03-011 March 2010
 Denmark DVB-T 2009-11-011 November 2009
 Estonia DVB-T 2010-07-011 July 2010
 Finland DVB-T 2007-09-011 September 2007
 Latvia DVB-T 2010-06-011 June 2010
 Luxembourg DVB-T 2006-09-011 September 2006
 Netherlands DVB-T 2006-12-1414 December 2006
 Norway DVB-T 2009-12-011 December 2009
 Spain DVB-T 2010-04-033 April 2010
 Sweden DVB-T 2007-10-2929 October 2007
 Switzerland DVB-T 2007-11-2626 November 2007
 United Kingdom DVB-T Still in Process[t 1]
Notes
  1. ↑ See UK TV areas undergone the switch-over [1]

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. ↑ 1/(51.95 µs active videotime / 768 pixels)
  3. ↑ "PGC categories - Countries using PAL standard". http://www.dvd-replica.com/DVD/palnations.php.  090426 dvd-replica.com
  4. ↑ 4.0 4.1 "Horizontal Blanking Interval of 405-, 525-, 625- and 819-Line Standards". http://www.pembers.freeserve.co.uk/World-TV-Standards/HBI.pdf.  090426 pembers.freeserve.co.uk
  5. ↑ 5.0 5.1 5.2 "NTSC, PAL, and SECAM Overview". http://www.deetc.isel.ipl.pt/Analisedesinai/sm/downloads/doc/ch08.pdf.  090426 deetc.isel.ipl.pt page 52
  6. ↑ "empty". http://www.thomsongrassvalley.com/docs/Manuals/cameras/ldk5400/3922-496-46791.s03.v01.pdf.  090426 thomsongrassvalley.com
  7. ↑ "empty". http://www.pembers.freeserve.co.uk/World-TV-Standards/VBI-625-PAL.pdf.  090426 pembers.freeserve.co.uk
  8. ↑ Report 308-2 of the XIIth Pleniary Assembly of the CCIR - Characteristics of Monochrome Television Systems
  9. ↑ "Changes to the terrestrial television systems in Central and East European countries". EBU. https://www.ebu.ch/CMSimages/en/tec_text_i33-1996_tcm6-16532.pdf. Retrieved 18 March 2009. 

External links

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