Phase alternating line
The phase alternating line process [ feɪz ˈɒltəneɪtɪŋ laɪn ], PAL for short , is a process for color transmission in analog television . It was developed with the aim of automatically compensating for disruptive color tone errors that can only be compensated manually and unsatisfactorily in the NTSC process. The method is based on the idea that two successive lines of images are more similar than they are different because images consist of surfaces. The technical trick of transmitting the red color difference signal every second image line with a phase shift of 180 ° to the previous one (hence the name) enables any color error that may occur on the receiver side to be completely eliminated by offsetting the two lines, just a small color saturation error remains. However, an error in color saturation is much more difficult to perceive for humans than a color error. Because 2 image lines are used to obtain color information, the vertical color resolution is reduced by half. However, since the spatial resolution of the human sense of sight for color information is lower than that for brightness information, this disadvantage is accepted.
Until it was replaced by digital television standards, PAL was used primarily in Europe, but also in Australia and many countries in Africa, Asia and South America. For details, see the Distribution section .
Colloquially, the term PAL is often used for the totality of all parameters of the television standard.
The beginnings of television were achromatic . Only the brightness values of the image were transmitted, no colors. In order to be able to continue to use existing black and white television sets after the introduction of color television, the color television systems were developed to be downward compatible . With a black-and-white television, you could also receive color broadcasts with a slightly deteriorated picture quality, and black-and-white broadcasts on a color TV.
PAL was developed by Walter Bruch at Telefunken GmbH in Hanover in the early 1960s, a patent was applied for on December 31, 1962 and first presented to experts from the European Broadcasting Union (EBU) on January 3, 1963 .
When asked why he named the process developed under his leadership the name "PAL", he replied in a similar manner: "A Bruch system would have been difficult to sell."
The color television in the Federal Republic of Germany on the 25th Great German Radio Exhibition in West Berlin by the Vice Chancellor of the Federal Republic of Germany Willy Brandt launched on 25 August 1967 by pressing a red button. In this scene the color was switched on a few seconds too early; the red button was a dummy. At 9.30 a.m., the TV channels ARD and ZDF broadcast the welcome moderation by Edith Grobleben from the Sender Freies Berlin (SFB) in color.
As early as August 5, 1967, Switzerland decided to introduce the PAL color television system, but did not begin broadcasting in color until later.
PALplus was developed in the 1990s as a possible, downwardly compatible successor and intermediate step to digital television, but has not become widespread.
PAL has adopted the basic concepts of signal transmission from the American color transmission system NTSC. Like NTSC, it uses quadrature amplitude modulation for color transmission. As an improvement, the chromaticity fluctuations typical in NTSC transmission no longer occur, but this is bought at the cost of considerable additional expenditure for the switching and (mostly barely) visible fluctuations in the color saturation. However, both systems can lead to cross-color and cross-luminance disturbances , which manifest themselves as annoying colored patterns ( moiré effect ) or as unrest in color transitions. Moiré occurs particularly with fine structures in the image, for example with small-checked shirts. These interferences can be reduced with increased circuit complexity ( comb filter ). In addition, the vertical color resolution deteriorates with PAL compared to NTSC.
The French color television system SECAM is much more different from NTSC than PAL.
Television standards with PAL color transmission
In the PAL color system itself, no line or frame frequency is defined; instead there are different standards . In Germany, a video format with 625 lines per image is usually used, which has an image transmission rate of 25 full images per second. These are transmitted half-image-wise, i. In other words, first a field with 312½ odd lines and then a field with 312½ even lines is transmitted, which results in a field frequency of 50 Hz, the so-called interlace method . This results in a low-flicker picture with a low bandwidth of the television signal. The PAL system transmits the television standards B , G , H , I and N . Some Eastern European countries that have switched their television standard from SECAM D and K to PAL are using PAL D / K, although there are some exceptions in which the countries have switched completely to PAL B / G. In Brazil, PAL is used in conjunction with 525 lines and 29.97 frames per second (System M ) and an almost identical color subcarrier frequency as NTSC. All other countries that use the “M” transmission system use NTSC for color television. In Argentina, Paraguay and Uruguay, PAL is used with the normal 625 lines, but with a color subcarrier frequency almost identical to that for NTSC. This variation of the PAL standard is called PAL-N and PAL-CN.
TV sets with PAL
Newer PAL television receivers can process almost all PAL variants (except PAL-M and PAL-N) and display them correctly. Many of them can also accurately represent SECAM , which is common in Eastern Europe and the Middle East. However, they usually do not work with the variant of the SECAM system used in France; this does not apply to devices of French origin. Many of these newer devices can also easily cope with NTSC-M signals that are generated by video recorders, DVD players or game consoles and fed into the television set via the video socket or the SCART socket (so-called baseband signals). However, problems often arise when it comes to processing NTSC signals that are broadcast by television stations or transmitted via cable networks and that are fed into the television set via the antenna socket (high-frequency modulated signals).
Cinema films are traditionally shot at 24 frames per second, which results in a 4% runtime reduction on PAL devices, since PAL reproduces 25 frames per second. This faster process of the film (technical term: PAL acceleration ) is hardly noticed by people, only the associated sound reproduction , about a semitone higher, can be noticed if, for example, the pieces of music that appear in it are already known from other sources ( CDs etc.).
Like NTSC and SECAM, PAL is based on black and white television . For reasons of compatibility , the color components must be transmitted “hidden” within the black and white luminance signal ( gray value ). Because this is already composed of all three color components, the transmission of two color difference signals for red (RY) and blue (BY) is sufficient. These two signals are formed from the difference between the color and luminance signal (black-and-white signal). The three color signals R, G and B can be generated again from the three signals RY, BY and Y in the receiver. (This is described in the articles YUV and color transfer .) Due to the additive color mixing , all other colors can be combined with the three individual colors red, green and blue, limited by the color space of the color picture tube .
Just like NTSC, PAL uses quadrature amplitude modulation (QAM) for the transmission of the two color difference signals red minus brightness (RY) and blue minus brightness (BY ). Since the carrier is suppressed in QAM , but is required for demodulation, it is regenerated in the receiver by a quartz-controlled subcarrier oscillator. This is synchronized with the transmitter signal by the “ burst ”, an oscillation approx. 10 periods long that is transmitted on the rear porch of the composite signal.
PAL automatically corrects phase errors on the transmission path that lead to incorrect color representation. For this purpose, the RY component of the color signal is phase shifted by 180 ° after each transmitted line (i.e. simply "reversed") and then occurs in the chrominance signal with phase shifts of + 90 ° or −90 ° (see false colors). The information about the current phase position of the RY signal is also transmitted in the burst. At + 90 ° the phase of the burst is + 135 °, at −90 ° it is −135 °. The BY signal always has the phase position 0 °.
Representation of the composite signal with PAL, one line of the picture. The PAL burst is at point 5.
Avoidance of color errors
Phase Alternating Line inverts the phase of the red difference signal from line to line. In the receiver, in contrast to NTSC, hue errors (which in these systems correspond to the frequently occurring electrical phase errors) are automatically compensated for by averaging the color signal of two adjacent lines if the color and the hue error between the two lines are constant, and converted into a low color saturation error . Color saturation errors are much less noticeable to the human eye than color hue errors. This is the decisive advantage of the PAL method over NTSC.
If one imagines the analog quadrature amplitude modulation (QAM) in the phasor diagram (see figure), the color type (the hue) is in the phase (direction) of the respective phasor and the color contrast (the color saturation) is in the length of the phasor. The two color signals RY and BY are shifted by 90 degrees to each other in the transmitter, then modulated onto the color subcarrier by means of QAM and transmitted as one signal. If phase errors occur, these would therefore show up as color errors with a simple demodulation such as with NTSC. With PAL, however, the carrier of the red component (RY) is rotated by 180 degrees in every second line, the blue component (BY) is transmitted without a current phase jump. The name PAL is derived from this principle . During demodulation, this phase shift is compensated accordingly and any phase error (color tone error) that may have occurred is thus averaged over two successive lines.
- In the diagram, BY is plotted horizontally and the RY color signal alternating by 180 degrees per line vertically
- Phasor diagram: line n, black: original phasor, blue: phasor with phase error
- Vector diagram: line n + 1, phase position of the received signal rotated by approx. 90 degrees
- Vector diagram: Position of the pointers in line n + 1 after mirroring on the horizontal axis
- Vector diagram: black: vectorial addition of the two original pointers, blue: addition of the two phased pointers
It is assumed that the color information changes only slightly from line to line and the color error to be covered also changes little from line to line.
When these conditions are met, the 1st order hue error is converted into a 2nd order color saturation error which is much more difficult to perceive by the eye and is therefore negligible.
Since the information of the current as well as the previous line is required to decode the PAL signal, the incoming PAL signal runs through a delay line in the receiver with a transit time just about the length of a television line (63.943 μs) for storage. A mean value between the signal just arriving and the signal stored from the previous image line is then output.
The disadvantage, however, is that the color information is shifted down by half a line, which is particularly unpleasant in the case of video cassettes copied several times, since a further shift occurs with each copying process.
Modern (digital) PAL decoders work much more complex:
- The previous and following lines are calculated in order to better separate the brightness and color signal (2D comb filter ).
- The previous and following images are calculated in order to better separate the brightness and color signal (3D comb filter ).
- No averaging of lines is used for color correction, but a correction value for the color signal is calculated on the basis of statistical values.
Frequency of the color carrier
Choice of NTSC color subcarrier frequency
To understand the PAL color subcarrier frequency selection, the simpler choice with NTSC is explained first:
The color subcarrier frequency was set in such a way that the interference moiré caused by it (especially on the existing black-and-white receivers) is as inconspicuous as possible and, at the same time, finely structured brightness information (finely checked shirts in the picture, etc.) causes as little disruptive color images as possible. At the same time, however, the audio signal must not be disturbed.
- as high a frequency as possible has been chosen, but it must be far enough away from the audio signal (4.5 MHz)
- the number of color carrier oscillations per line so that the phase of the color carrier is rotated by 180 ° between the superimposed points of adjacent lines (unlike the phase rotation of the finished color signal in PAL).
This then results in 4.5 MHz / 286 * 227.5 periods = 3.57954545 MHz for the color carrier in NTSC color modulation. About 1.3 MHz of the lower sideband and 0.4 MHz of the upper sideband thereof are transmitted. Due to the nature of the color signal, certain frequencies occur much more strongly in these sidebands than others; In the receiver, it is sufficient to “fish out” these frequencies from the black-and-white image in order to achieve the cleanest possible separation of brightness and color information.
Selection of the PAL color subcarrier frequency
The color subcarrier frequency was set in such a way that the interference moiré caused by it is as inconspicuous as possible and, at the same time, finely structured brightness information causes as few interfering color images as possible.
- as high a frequency as possible, but far enough away from the audio signal (5.5 MHz).
- the number of color carrier oscillations per line so that after two lines the phase of the color carrier is rotated by 180 °. Every two lines because neighboring lines look different due to the 180 ° PAL phase switching, which is why, in contrast to NTSC, the diagonal grid is not built between neighboring lines, but between lines with a distance of two. Overall, the phase position is repeated every four lines (quarter line offset).
- the color carrier is further increased by 25 Hz so that the interference raster alternates between the fields. This is necessary because the line number 625 - unlike the NTSC line number 525 - results in a remainder of 1 when divided by 8, which creates a slowly moving interference pattern that is more noticeable than a fast moving one, as is the case with a remainder of 3 arises. This correction is therefore not used for PAL-M, i.e. PAL with 525 lines. Most DVD players , game consoles and digital satellite receivers also dispense with this correction, since it can only be generated in digital technology with relatively complex - and therefore expensive - components. The devices mentioned are hardly reproduced on black and white televisions anyway, and the interference is less noticeable on color devices anyway.
This then results in 15625 Hz * 283.75 periods + 25 Hz = 4.43361875 MHz for the color carrier in PAL color modulation. About 1.3 MHz of the lower sideband and 0.65 MHz of the upper sideband thereof are transmitted. The color subcarrier frequency is usually generated in the receiving device by a quartz oscillator that is post-synchronized by the television transmitter. This oscillator is matched in frequency and phase to the oscillator at the transmitter by the burst. This means that a stable, highly constant reference frequency is available in every television set.
The frequency used is partly also used for baseband transmission of NTSC and is then called NTSC-4.43. This method is often confused with PAL-60, but differs in that the color subcarrier does not change its phase position. Most newer PAL televisions can also display a PAL-60 without any problems, which is why it is used, for example, to play NTSC DVDs on a PAL television set. The interference suppression of the color carrier (carrier frequency is 281.78 times the line frequency, which is no longer half-integer) is no longer optimal.
In contrast to SECAM, PAL does not in principle need to average neighboring lines during decoding. You can also decode each line independently. The correction of hue errors still works properly for minor errors, the averaging is easily taken over by the human eye for minor hue errors (as is often encountered today with cable television and other more phase-stable transmission methods). The vertical resolution is not reduced (compared to the variant with line averaging). Device manufacturers can bypass PAL licenses in this way. With color transmission from PAL via Y / C ( Hosiden connection , S-Video ), i.e. with separate brightness and color signals, a larger color bandwidth is also possible, as there is no longer any limitation to 1.3 MHz bandwidth. However, hardly any use is made of this.
The current latest version of the standard, the PAL system defined (and also the NTSC system), was established in 1998 by the International Telecommunication Union ( International Telecommunications Union - published and has the title "Recommendation ITU-R BT.470-6 ITU) "Conventional Television Systems".
Joking mistranslation of the abbreviation PAL
Based on the false translations Never The Same Color ("Never the same color") and Never Tested Since Christ (" Never Tested Since Christ ") for the American color standard NTSC, the Americans returned the favor with the equally wrong ones , joke translations Pay the Additional Luxury ("Pay the additional luxury") and Pay Another License ("Pay another license") for the European color standard PAL.
These related to the greater circuit complexity and the resulting higher price of PAL color televisions at the beginning of the age of color television. Europe struck back with the breakdowns Peace At Last ("Finally Peace") and Perfection At Last ("Finally Perfection") for PAL, which again referred to the poor quality of the NTSC standard.
The term PAL in digital formats
Everything that has been described so far relates to the term PAL in analog signal transmission, for example in analog television and video recorders. In digital formats, such as digital television , newer game consoles or on a DVD , the color coding, which is compatible with the analog input socket of the target television, is only generated in the player; it is not stored on the medium itself. The color information, regardless of whether it is digital PAL / SECAM or digital NTSC, is always encoded using the digital color model YCbCr .
There is no longer any difference between PAL and SECAM on a digital medium - a PAL DVD player generates an analog PAL video signal from a "PAL DVD", a SECAM DVD player an analog SECAM video signal from the same DVD. This is only used for control via FBAS / Composite Video / RCA or S-Video / YC / Hosiden connection. On RGB / SCART or YPbPr - Component video connector or via digital interfaces ( DVI , HDMI ) does not change in YUV (analog PAL), YDbDr (analog SECAM) or YIQ (obsolete, formerly used for analog NTSC) more instead.
If digital signal processing or storage of the analog video signal takes place (e.g. with more modern analog televisions), there is already a digital representation of the PAL color coding. The analog signal is sampled at four times the color subcarrier frequency . The scanning takes place synchronously with the color carrier. The color difference signal is obtained by adding and subtracting closely related samples . This method is particularly used internally in video processing equipment. Digital television sets often work here with 7 or 8-bit accurate sampling ( analog-to-digital conversion ), better devices use up to 10 bits. Early digital video recorders (e.g. D2) also used this method.
In the digital sector, PAL denotes, detached from the meaning of the acronym, all image formats with an image resolution of 576 visible lines per frame (possibly also 288) at 25 frames per second; the horizontal resolution varies. Today's designations (according to EBU) are 576i / 25 when using the interlace method , 576p (sf) / 25 for full images (it is always counted in "images", not in "fields"). From a technical point of view, 576p is always "psf" (progressive segmented frame), the simplicity is called 576p (there is no actual progressive coding here, as for example with 720p).
The counterpart to "PAL" is " NTSC ", which on digital media has a resolution of 480 (or 486) lines per frame at either 29.97 or 30, or (for feature films) 23.976 or 24 frames per second means, whereby the color information is also stored in YCbCr -coded on the medium . Almost all PAL DVD players, however, can generate a PAL-like signal called PAL-60 from NTSC media , which almost all newer PAL television sets can easily cope with.
The horizontal resolutions refer to the PAL system in its digitized representation, which works with pixels as it is e.g. B. in the ITU-R BT 601 standard is specified. There a digital line consists of non- square pixels.
In digital format, a 4: 3 picture corresponds to a resolution of 702 × 576 in the PAL system, although 720 × 576 pictures are typically saved. (see article CCIR 601 for the creation of 702 pixels)
See also pixel aspect ratio .
After converting to square pixels (e.g. on a PC) the following results are proportionally correct:
- when scaling the full 720 pixels
- when scaling the middle 702 pixels
In many media this is often incorrectly explained and passed on, and many software packages also calculate incorrectly here. For example, Adobe After Effects and Photoshop only calculate correctly from version CS4; in earlier versions, calculations were made with the common but incorrect pixel aspect ratio (PAR).
For non-square pixels is the aspect ratio (Aspect Ratio or AR) of the image (eg. 4: 3) is not identical to the ratio of the horizontal to the vertical pixel number (eg. 11: 9 at 704 × 576). Therefore, in addition to the number of pixels, either the aspect ratio of the pixels or that of the overall image must be specified. It must therefore be clear whether it is the pixel aspect ratio (PAR) or the image aspect ratio (DAR) . Only then can a distortion-free image be displayed.
- 720 × 576 (PAR 12 ÷ 11 normal, 16 ÷ 11 with anamorphic): CCIR 601 , DVDs , digital cameras: usually only 702 of the 720 pixels are displayed. The 720 pixels correspond to (sampling frequency is 13.5 MHz) 53.33 µs. In television sets, however, a maximum of 52 microseconds are used for the image display, which corresponds approximately to the middle 702 pixels displayed.
- 704 × 576 (PAR 12 ÷ 11 normal, 16 ÷ 11 with anamorphic): Digital Video Broadcasting: like 720 × 576, but no overscan is encoded.
- 544 × 576 (PAR 24 ÷ 17 normal, 32 ÷ 17 with anamorphic): e.g. B. via DVB to save costs by reducing the bandwidth required.
- 480 × 576 (PAR 24 ÷ 15 normal, 32 ÷ 15 with anamorphic): e.g. B. to be found in SVCDs .
- 352 × 288 (PAR 12 ÷ 11 normal, 16 ÷ 11 with anamorphic): e.g. B. on video CD .
- Andreas Fickers: "Politique de la grandeur" versus "Made in Germany". Political cultural history of technology using the example of the PAL-SECAM controversy (= Paris historical studies , Volume 78). Oldenbourg, Munich 2007, ISBN 978-3-486-58178-2 (Dissertation RWTH Aachen 2002, 436 pages).
- Patent application DE1252731 .
Concise dictionary of electrical telecommunications , 1970
- Volume 1: pp. 482–485 (television, 3.B PAL)
- Volume 2: pp. 1236–1237 (PAL receiver)
- Recommendation ITU-R BT.470-6, Conventional Television Systems (PDF)
- Representation of the different speeds of PAL, NTSC and cinema films
- Comparison of the technical differences between PAL B / G and NTSC M
- Exact breakdown of all standards + Pal / NTSC video time converter
- 50 years of PAL color television - a funeral speech for a birthday