PAL

The Phase Alternating Line method [ feɪz ɒltəneɪʃən laɪn ] PAL, short for, is a color encoding system analog television. It has been developed with the aim of interfering tone errors can be compensated for in the NTSC method, only manually and unsatisfactory to compensate automatically. The method is based on the idea that two consecutive image lines have more similarity than difference, because images of surfaces are made. The technical trick, the red color difference signal to be transmitted every second image line to the previous 180 ° out of phase (therefore the name), makes it possible to completely cancel a possibly occurring hue error at the receiver side by offsetting the two lines, only a small color saturation error remains. An error in the color saturation is for humans but much less visible than a hue error. The fact that each two image lines are used for color information extraction, the vertical color resolution is reduced to half. However, since the spatial resolution of the human visual sense for color information relative to that for luminance information is low, take this drawback into account.

PAL is mainly used in Europe, but also in Australia and many countries in Africa, Asia and South America. For details, see distribution section.

Colloquially, the term PAL is often used for the set of all parameters of the television standard.

History

The beginnings of television were black and white. Only the brightness values ​​of the image has been transferred, no colors. To pre-existing black and white television sets after the introduction of color television continue to use that color television systems were developed backward compatible. With a black and white TV you could in slightly degraded image quality even color radiances also receive black and white on a color television broadcasts.

PAL was developed in the early 1960s by Walter Bruch at Telefunken in Hannover, applied for a patent and demonstrated for the first time in front of experts from the European Broadcasting Union ( EBU) on January 3, 1963.

When asked why he gave the name "PAL", developed under his management methods, he replied something like: " A fracture system would have been hard to sell. "

The color television in the Federal Republic of Germany was launched at the 25th Great German Radio Exhibition in West Berlin by the Vice Chancellor of the Federal Republic of Germany Willy Brandt on August 25, 1967, a press of a red button. In this scene a few seconds the color was switched on too early, since the red button was just a dummy. At 9.30 clock television channels ARD and ZDF broadcast the Welcome Host by Edith Grobleben from Sender Freies Berlin ( SFB) in color.

Already on August 5, 1967, the Switzerland opted for the introduction of the PAL color television system, but began broadcasting in color later.

As a possible downward compatible successors and intermediate step to digital television PAL plus was developed in the 1990s, but has not been widely used.

Comparison

PAL has taken the basic concepts of signal transmission from the American NTSC color broadcast system. It uses, such as NTSC, the quadrature amplitude modulation for the ink transfer. As an improvement to the typical NTSC broadcast Farbartschwankungen no longer occur, but this is possible with considerable additional effort in the circuit and (usually barely) visible variations in color saturation bought. However, it can occur in both systems to cross- color and cross - luminance disorders who express themselves as interfering colored pattern ( moire effect) or as a balance of color transitions. Moiré occurs especially at the fine structures in the image, for example, small-minded shirts. With increased circuit complexity, these disorders can be reduced (comb filter). In addition deteriorates in PAL compared to NTSC, the vertical color resolution.

The French TV color system SECAM differs considerably more from NTSC and PAL.

Television standards PAL color transfer

In the PAL color system itself is not a row or defined frame rate, instead there are different standards. In Germany, usually a video format with 625 lines per frame is used which has a frame rate of 25 frames per second. These are field by field transfer, that is, there is only one field with 312 ½ odd lines and then a field with 312 ½ transferred straight lines, resulting in a field frequency of 50 Hz, the so-called interlacing. This yields at low bandwidth of the television signal, a flimmerarmes image. The PAL system transmits the television standards B, G, H, I and N. Some Eastern European countries that have changed their color system SECAM D and K to PAL, use PAL D / K, where there are some exceptions in which the countries complete B / G have converted to PAL. In Brazil, PAL is used in conjunction with 525 rows and 29.97 frames per second ( System M ) and an almost identical color carrier frequency as NTSC. All other countries that use the transmission system "M", use NTSC color television. In Argentina, Paraguay and Uruguay, PAL is used with the normal 625 lines, but with a color subcarrier frequency, which is identical for NTSC almost with the. This variant of the PAL standard is called PAL-N, and PAL -CN.

TV with PAL

Newer PAL television receivers can process almost all PAL versions (except PAL -M and PAL -N) and play correctly. Many of them can also be error-free display SECAM, which is common in Eastern Europe and the Middle East. However, they usually do not work with the version of the SECAM system, which is used in France; exception of devices of French origin. Many of these newer devices are also fine with NTSC-M signals, which are generated from video recorders, DVD players or game consoles, and are fed via the video connector or the SCART socket on the TV (so-called baseband signals). However, problems often occur, when it comes to the processing of NTSC signals, which are broadcast by television stations, or transmitted via cable networks and which are fed via the antenna socket to the TV ( high frequency modulated signals).

Conversion

Films are traditionally shot at 24 frames per second, this results on PAL equipment run time reduction by 4% since PAL reproduces 25 images per second. This faster flow of the film ( technical term: PAL acceleration) is hardly perceived by people, only the accompanying, about a semitone higher sound to stand out when you music, for example, is occurring already know from other sources (CDs, etc.).

Technology

PAL builds as NTSC and SECAM on the black and white television. For reasons of compatibility, the color components must be "hidden " within the S / W luminance ( gray value) are transferred. Because it is already composed of all the three color components, from the transfer of the two color difference signals, red ( RY ), and blue (BY) is sufficient. These two signals are formed from the difference of color and luminance ( black and white signal). In the receiver, the three color signals R, G and B can be produced back out of the three signals, RY, BY and Y. (This article describe the YUV and color transfer. ) The additive color mixing can with the three individual red, green and blue, all other colors are assembled, limited by the color space of the color picture tube.

As well as NTSC PAL used for the transmission of the two color difference signals, red minus luminance (RY ), and blue minus luminance (BY), the quadrature amplitude modulation (QAM). Since, in the QAM, the carrier is suppressed, it is however required for demodulation, it is re-generated by a crystal controlled oscillator in the receiver sub-carrier. This is synchronized by the " burst ," of about 10 cycles long vibration which is transmitted to the back porch of the composite video signal to the transmitter signal.

PAL automatically corrects phase errors during transmission, leading to a false color representation. For this purpose, the RY component of the color signal after each transmitted line by 180 ° out of phase (ie simply " reversed " ) and then enters the chrominance signal with phase shifts of 90 ° or -90 ° (see false colors). The information about which phase of the RY signal just is transmitted in the burst. At 90 °, the phase of the burst 135 °, -90 ° corresponding to -135 °. The BY signal always has the phase angle 0 °.

Representation of the composite video signal in PAL, an image line. The PAL burst is located at point 5

Avoid the chromatic aberration

Phase Alternating Line inverts the phase of the red difference signal from line to line. In the receiver, as opposed to NTSC color errors (which in these systems corresponding to the common electrical phase errors) by averaging the chrominance signal of two adjacent lines is automatically compensated when the color and the hue error between the two lines is constant, and is converted into a low color saturation errors. Saturation errors fall to the human eye to substantially less than hue error. This is the key advantage of the PAL process compared to NTSC.

If you imagine the analog quadrature amplitude modulation ( QAM) in the vector diagram (see Figure ), as inserted by the respective pointers in the phase (direction) of the chromaticity ( hue ), in the length of the pointer, the color contrast ( color saturation ). The two color signals RY and BY are thereby shifted from each other by 90 degrees in the transmitter, then modulated onto the color sub- carrier by the QAM and transmitted as a signal. If phase error would, therefore, show this as a color error on a simple demodulation as in NTSC. PAL in every second line of the carrier of the red component (RY ) However, it is rotated 180 degrees, the blue component ( BY) is transmitted without ongoing phase jump. This principle is also the name of PAL derived. In demodulating this phase rotation is compensated for in accordance with and thus averaged away a possible occurrence of phase errors (color error ) over two successive rows.

Here, it is assumed that to line the color information changes only slightly from row to be concealed and the color error to also line changes from line little.

About these conditions, the hue error first order in a color saturation error of 2nd order is converted, the more difficult to perceive by the eye and therefore is negligible.

As for decoding the PAL signal in each case the information of the current and the previous line are needed by running the incoming PAL signal in the receiver a delay line with a term of almost the length of a television line ( 63.943 microseconds ) for storage. Output is then in each case, an average value between the straight incoming and stored from the previous picture line signal.

The disadvantage, however, is that while the color information by half a line down move, which is particularly unpleasant striking in multiple copied video cassettes, as each copy operation a further shift occurs.

Modern ( digital ) PAL decoders operate much more complex:

  • There are previous and following lines offset to better separate brightness and color signal ( 2D Comb filter).
  • There are offsets the preceding and following images in order to better separate brightness and color signal ( 3D comb filter).
  • There is no averaging of lines used for color correction, but calculated on the basis of statistical variables a correction value for the color signal.

Frequency of the color carrier

Either the NTSC color subcarrier frequency

To understand the PAL color carrier frequency choice the easier choice will be explained first for NTSC:

The color carrier frequency was placed so that the induced them interference moire (especially at the existing black and white receivers ) is as unobtrusive as possible and at the same time fine-textured brightness information (fine plaid shirts and the like in the picture) cause as little interference color images. At the same time, however, the audio signal may not be disturbed.

To do this:

  • Chosen as high a frequency that must be still far enough away from the audio signal ( 4.5 MHz) away
  • The number of color subcarrier cycles per line placed so that between the superposed points of adjacent rows, the phase of the color carrier is rotated by 180 ° ( other than the phase of rotating of the final color signal for PAL).

Then the resulting 4.5 MHz / 286 * 227.5 periods = 3.57954545 MHz for the chrominance subcarrier in the NTSC color modulation. Are transmitted which about 1.3 MHz of the lower sideband and 0.4 MHz of the upper sideband. By the nature of the color signal while certain frequencies occur in these side bands substantially more than others; in the receiver, it is sufficient these frequencies from the black and white picture again " fish out " in order to achieve a clean separation as possible of brightness and color information.

Either the PAL color subcarrier frequency

The color carrier frequency was placed so that the interference caused by them moiré is as inconspicuous as possible and at the same time cause fine structured brightness information as little disturbing color images.

To do this:

  • Chosen as high a frequency of sound (5.5 MHz) is removed, however, go far enough.
  • The number of color subcarrier cycles per line set so that after two lines of the chrominance subcarrier, the phase is rotated by 180 °. Every two lines because adjacent rows appear different by 180 ° phase of the PAL circuit, so the diagonal grid is not between adjacent row, but between lines at a distance of two is established, in contrast to NTSC. Overall, the phase angle thus repeated every four lines ( district line offset ).
  • The ink sheet is still further increased to 25 Hz, so that the Störraster between fields alternated. This is necessary because the number of lines 625 - unlike the NTSC line number 525 - in the division by 8 is a radical of 1 results, creating a slowly migrating interference pattern is produced which is more prominent than a fast- migrating, as with a remainder of 3 arises. For PAL -M, PAL ie 525 lines, this correction is therefore not used. Also, most DVD players, game consoles and digital satellite receiver without this correction, since in digital technology with relatively elaborate - can be produced components - and therefore expensive. The aforementioned devices are already barely played on black and white TVs, and on color devices the disturbances are anyway less visible.

This then results 15625 Hz * 283.75 Time 25 Hz = 4.43361875 MHz for the color carrier at the PAL color modulation. Are transmitted which about 1.3 MHz of the lower sideband and 0.65 MHz of the upper sideband. The color carrier frequency is usually generated by a crystal oscillator in the resynchronised by the TV receiving device. This oscillator is adjusted by means of the burst in the frequency and phase of the oscillator in the transmitter. This is in every TV a stable, highly constant reference frequency.

The frequency used is partially used for base band transmission of NTSC and is then called NTSC 4.43 as well. This procedure is often confused with the PAL 60, but differs in that the color sub-carrier does not change its phase. A PAL 60 can also view the most recent PAL TVs easily, which is why it is used, for example, to play NTSC DVDs on a PAL TV. The Suppression of the color carrier ( carrier frequency is 281.78 times the line frequency, which is not half-integral ), however, is no longer optimal.

Demodulation

Averaging of adjacent lines in the decoding is necessary for PAL, SECAM as opposed to, in principle, no. One can also decode independently for each line. The correction of errors hue works with minor flaws still neat, the averaging is with low color error ( as they are nowadays often found through cable television and other phase firmer transmission methods) easily by the human eye taken. The vertical resolution is reduced (compared to the variant with line averaging ) does not. Device manufacturers can work around this way the PAL licenses. In the PAL color transfer via the Y / C ( Hosidenverbindung, S-Video ), that is, with separate luminance and chrominance signal, a greater ribbon width is possible because there is no limitation to 1.3 MHz bandwidth more. Of this, however, little use is made ​​.

Credentials

The current latest version of the standard, which defines the PAL system (and also the NTSC system ) was developed by the International Telecommunication Union (International Telecommunications Union - ITU) published in 1998 and has the title " Recommendation ITU -R BT.470 -6, Conventional Television Systems ". This publication is not publicly available on the Internet, but can be purchased from the ITU.

Dissemination

Illustrative joke mistranslation of the abbreviation PAL

Based on the minted in Europe mistranslations Never The Same Color ( " Never the same color " ) and Never Tested Since Christ ( "For Christ's birth never tested " ) for the American color system NTSC, the Americans retaliated with the equally false, jocular translations Pay the additional luxury ( "Pay the extra luxury ") and Pay another License ( "Pay another license ") for the European PAL color standard.

These related to the greater circuit complexity and therefore the higher price of the PAL color television at the beginning of the color television era. Europe hit with the breakdowns Peace At Last ( " Peace at Last " ) and Perfection At Last ( "Finally perfection" ) for PAL back, which again referred to the poor quality of the NTSC standard.

For PAL automatic color correction is integrated, also was flickering, reduced by shams with the fields 50 Hertz. NTSC is a video transmission in a simpler form, the direct transfer eg from a DVD player to a TV is not a problem, but has in the original analog transmission, especially Terrestrial, significant color variations.

Digital PAL

Everything so far has been described refers to the concept of PAL in the analog world, so for example, analog channels and VCR. In the digital world, such as Digital TV, newer game consoles or on a DVD, the color coding that is compatible with the input jack of the target TV, produced only in the player and is not stored on the medium itself. On this, the color information is, however, encoded regardless of whether digital PAL / SECAM or NTSC digital, always by means of the digital YCbCr color model.

Between PAL and SECAM is on a digital medium, no difference - a PAL DVD player generated from a "PAL - DVD" an analogue PAL video signal, a SECAM DVD players from the same DVD analog SECAM video signal. And only when driven by CVBS / Composite Video / RCA or S-Video/YC/Hosiden-Anschluss. On the RGB / SCART or YPbPr component video connection or via digital interfaces (DVI, HDMI ) does not change in YUV ( analog PAL), YDbDr ( analog SECAM) or YIQ (obsolete, formerly used for analog NTSC) instead of more.

In addition to the above-mentioned formation of the digital PAL there is yet another form. In this case, the analog signal at four times the color subcarrier frequency is scanned. The sampling is done synchronously to burst. By addition and subtraction of related samples to obtain the color difference signal. This method is especially used internally in video processing devices. Digital televisions work here often with a 7 - or 8 -bit accurate sampling ( analog - to-digital conversion ), use better equipment up to 10 bits. Early digital video recorder ( for example D2) also used this method.

Resolution

PAL referred to in the digital domain, in isolation from the meaning of the acronym, all image formats with an image resolution of 576 visible lines per frame (possibly 288 ) at 25 frames per second; the horizontal resolution varies. Today's designations ( EBU ) are using the interlacing 576i/25 in frames 576p ( sf ) / 25 ( it is in any case counted in " Images", not in " fields "). Technically, 576p is always " psf " (progressive segmented frame ) simplicity we speak of 576p ( there is no actual Progressive coding here, as for example in 720p).

The counterpart to "PAL" is here again "NTSC", the digital media on a resolution of 480 (or 486) lines per frame at either 29.97 or 30, or ( for movies) 23.976 or 24 frames per second means, wherein the color information is stored on the media is also encoded YCbCr. However, almost all PAL DVD players can generate from NTSC to PAL -60 media called PAL - similar signal to cope easily with almost any newer PAL televisions.

The horizontal resolutions relating to the PAL system in its digitized representation, which works with pixels as defined eg in ITU -R BT 601 standard. There is a digital line from non- square pixels.

Digital corresponds PAL system 4:3 picture a resolution of 702 × 576 although typically 720 × 576 images are stored. ( see Article CCIR 601 to the emergence of 702 pixels)

If the desired output medium with ( idealized ) square pixels works (eg to a PC monitor connected ), this must be taken into account by the aspect ratio is ideally converted.

Pixel Format

See also pixel aspect ratio.

Square pixels

After converting to square pixels (eg the PC) results in proportionally correct:

  • In scaling the full 720 pixels 788 × 576: 4:3
  • 1050 x 576: 16:9 aspect ratio
  • 768 × 576: 4:3
  • 1024 × 576: aspect ratio 16:9

In many media, this is often explained wrong and passed, many software packages expect wrong here. For example, expects Adobe After Effects and Photoshop only from version CS4 correctly in earlier versions was the common, but incorrect pixel aspect ratio ( PAR) calculated.

Non- square pixels

For non - square pixel 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 count (eg 11:9 at 704 × 576 ). Therefore, either the aspect ratio of the pixels or the entire image must be specified in addition to the pure number of pixels in addition; so it must be clear whether it is the pixel aspect ratio (PAR ) or the screen aspect ratio ( DAR). Only then a distortion-free image can be displayed.

Common are:

  • 720 × 576 (pixel AR 12 ÷ 11 normal, 16 ÷ 11 at anamorphic ): CCIR 601, DVDs, digital cameras: Below are usually only 702 of the 720 pixels. The 720 pixels correspond to ( sampling frequency is 13.5 MHz) 53.33 microseconds, but can be used for TVs up to 52 microseconds for image representation, which corresponds to about the middle 702 pixels shown.
  • 704 × 576 (pixel AR 12 ÷ 11 normal, 16 ÷ 11 at anamorphic ): Digital Video Broadcasting: as 720 × 576, it is, however, no overscan with coded.
  • 544 × 576 (pixel AR 24 ÷ 17 normal, 32 ÷ 17 at anamorphic ): eg DVB for cost savings due to lower bandwidth requirements.
  • 480 × 576 (pixel AR 24 ÷ 15 normal, 32 ÷ 15 at anamorphic ): eg encountered in SVCDs.
  • 352 × 288 (pixel AR 12 ÷ 11 normal, 16 ÷ 11 at anamorphic ): eg on Video CD.

Appendix

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