Contents[hide] |
[edit] 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 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, the first European Standard for colour television. 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 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.
[edit] 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 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 human eye to do that cancelling; 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 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.
[edit] 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 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 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 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 colour spaces, but the colour decoder differences here are ignored.
[edit] 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.
[edit] 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[2] |
| Horizontal sync polarity | Negative |
| Total time for each line | 64.000 µs[3][4] |
| 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 |
After 0.9 µs a 2.25±0.23 µs colorburst 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.[3] The CVBS electrical amplitude is Vpp 1.0 V and impedance of 75 Ω.[5]
| 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)[6] |
| 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:[4]
- Y = 0.299R' + 0.587G' + 0.114B'
- U = 0.492(B' − Y)
- V = 0.877(R' − Y)
Subcarrier frequency FSC is 4.43361875 MHz (±5 Hz) for PAL-B/D/G/H/I/N.
[edit] PAL broadcast systems
This table illustrates the differences:| PAL B | PAL G, H | PAL I | PAL D/K | PAL M | PAL N | |
|---|---|---|---|---|---|---|
| Transmission Band | VHF | UHF | UHF/VHF* | VHF/UHF | VHF/UHF | VHF/UHF |
| Fields | 50 | 50 | 50 | 50 | 60 | 50 |
| Lines | 625 | 625 | 625 | 625 | 525 | 625 |
| Active lines | 576 | 576 | 582** | 576 | 480 | 576 |
| Channel Bandwidth | 7 MHz | 8 MHz | 8 MHz | 8 MHz | 6 MHz | 6 MHz |
| Video Bandwidth | 5.0 MHz | 5.0 MHz | 5.5 MHz | 6.0 MHz | 4.2 MHz | 4.2 MHz |
| Colour Subcarrier | 4.43361875 MHz | 4.43361875 MHz | 4.43361875 MHz | 4.43361875 MHz | 3.575611 MHz | 3.58205625 MHz |
| Sound Carrier | 5.5 MHz | 5.5 MHz | 6.0 MHz | 6.5 MHz | 4.5 MHz | 4.5 MHz |
- System I has never been used in VHF.
- The UK's adoption of 582 active lines[7] 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 standard.
[edit] 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.
[edit] PAL-M (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", "Quasi-PAL" or "Pseudo PAL"). PAL-M (a broadcast standard) however should not be confused with "PAL-60" (a video playback system — see below).
[edit] 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 downconverted 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 colours
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.
[edit] 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.[edit] 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.[edit] 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.[8]
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.
[edit] 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.
Tiada ulasan:
Catat Ulasan