Sequentiel color à mémoire
Séquentiel couleur à mémoire [ sekɑ̃sjɛlkuˈlœːʀ ameˈmwaːʀ ] ( SECAM or SÉCAM [ seˈkam ]) was an analog television standard used primarily in France , Eastern Europe and parts of Africa for color transmission on analog television . It was developed by Henri de France and introduced in 1956. In the German language, the full name can be translated as “color sequence with memory”.
Basic idea
Just like NTSC and PAL , SECAM is a system for black-and-white television-compatible color transmission. Compared to NTSC, the aim of the new system was to improve color rendering under non-ideal reception conditions. Compared to PAL, a different method was found with SECAM: It was not about optimizing the studio recordings, but rather optimizing the transmission from the television transmitter to the receiver.
Similarities with NTSC and PAL
As with NTSC and PAL, the color information required in addition to the brightness signal Y (i.e. the black and white image) is transmitted in the form of two color difference signals D R and D B. The letters stand for the underlying YDbDr color model , which is very similar to the YUV color model used in PAL and NTSC and only differs in the different "stretching factors" of the two color difference signals.
For the basics of color transmission, see also under television signal .
functionality
SECAM used instead of that used in NTSC and PAL quadrature modulation , the frequency modulation for the transmission of the two color difference signals. The advantage is that phase errors in the color difference signals do not lead to any color errors. However, as with quadrature modulation, two signals cannot be accommodated orthogonally and without mutual influencing on just one carrier frequency.
SECAM therefore transmits one of the two color signals D R and D B alternately per line . This signal is additionally delayed by one line in the receiver so that both chrominance signals are still available to the decoder in each line. The vertical resolution reduction of the color signals has hardly any adverse effects for the human eye, since it has a low resolution for color information.
The two color difference signals are initially band-limited to around 1.3 MHz and subjected to a predistortion in order to reduce the interference on the brightness signal . The function of this pre-distortion is slightly different depending on the specific version and was adapted depending on the specific standard. In the SECAM I version, the pre-distortion of the color subcarriers was carried out according to the following complex equation:
After the predistortion, the two color signals were alternately modulated per line on two different carrier frequencies of 4.25 MHz and 4.40625 MHz, D R being frequency modulated with a deviation of 280 kHz and D B with 230 kHz. The modulation index typical for frequency modulation is less than 1, which corresponds to a spectral compression, and is around 0.21 or 0.18. There is no space available for a larger modulation index in the frequency grid of the television transmitters in the context of SECAM and as a result the color signal is sensitive to interference. The frequency range between 3.9 MHz and 4.756 MHz is reserved for the carriers.
Before mixing with the brightness signal Y , there is a further pre-distortion of the modulated color difference signal to be transmitted in the respective line. With this second, likewise complex predistortion, the amplitude of the modulated carrier signal is distorted as a function of the instantaneous deviation of the respective color difference signal. The reason for this is to minimize the effects of the color subcarriers on image content with low brightness and to improve the signal-to-noise ratio in saturated colors. In certain encoders there is also a bandpass before the pre-distortion, in order to prevent components of the frequency modulation from having effects in the luma range. Then the respective modulated color difference signal is added to the brightness signal Y and the sum signal thus formed is emitted.
Details of this more complex second predistortion and images of the envelope of the spectrum formed from it can be found in and. In the television receiver, the predistortion is reversed and the two color difference signals D R and D B are obtained for further signal processing.
Identification signals
There are two different identification signals so that the recipient can assign the lines to the correct colors:
- the line identification (burst)
- the image identification (bottles)
The former is the method used today. To do this, the carrier signal starts before the image information and contains the base carrier frequency (i.e. 4.25 MHz for D B or 4.40625 MHz for D R ). With the image method, an identification signal is inserted in lines 6 to 15 and 313 to 322. In the D B lines, the signal starts at 4.25 MHz and then goes down to 3.9 MHz. For the D R lines, it starts at 4.406 and goes up to 4.756 MHz. Since this signal has to go through the carrier pre-distortion, the amplitude of the signal increases from about 200 mV to about 500 mV. The name “bottle” comes from the shape of the signal when viewed on an oscilloscope . This signal is no longer used today as the lines were intended to be used for the French antiope teletext system. However, the system was no longer in operation until the last bottles were switched off in 2007.
Delay lines
For SECAM you absolutely need a memory to store the color signal for the duration of a line, while with PAL you can do without it (Simple-PAL). This memory was implemented in older receivers in the form of a delay line in the receiver. Ultrasonic delay lines have been available since the early 1960s. In digital television receivers, which have been widespread since the late 1990s and can usually receive several - including analog - television standards, digital storage devices are mostly used.
Compatibility with black and white images
Due to the frequency modulation used, the color subcarrier is always present with the same intensity in the picture, regardless of the color intensity - in contrast to PAL and NTSC, where it shrinks to amplitude 0 with non-colored picture content, i.e. practically disappears and can therefore no longer be cross-talked. With SECAM, the carrier is therefore regularly switched in phase position according to a pattern, as shown in the table below, in order to suppress interference patterns. Since this suppression does not work with sufficient interference, SECAM is said to have poorer black and white compatibility. In the case of pure black and white broadcasts, the color carrier on GDR television was therefore completely switched off, i.e. a real black and white signal was sent, which was no longer common for West German broadcasters with the PAL television standard (so the color contrast on the receiver had to be set to zero so that you did not see any color noise in black and white broadcasts).
| Field | Line number | Transmitted color difference signal |
Phase position of the color carrier |
|---|---|---|---|
| odd | 1 | D R | 0 ° |
| odd | 2 | D B | 0 ° |
| odd | 3 | D R | 180 ° |
| odd | 4th | D B | 0 ° |
| odd | 5 | D R | 0 ° |
| odd | 6th | D B | 180 ° |
| ... | ... | ... | ... |
| straight | 314 | D B | 180 ° |
| straight | 315 | D R | 0 ° |
| straight | 316 | D B | 180 ° |
| straight | 317 | D R | 180 ° |
| straight | 318 | D B | 0 ° |
| straight | 319 | D R | 180 ° |
| ... | ... | ... | ... |
Crossfade
SECAM-modulated signals ( composite video signal) can not be cross- faded directly because of the frequency modulation, since the frequency modulation represents a non-linear modulation and therefore the addition of two FM signals does not result in a useful signal. Cross-fading is only possible via the detour of demodulation and subsequent cross-fading of the individual components.
For this reason, broadcasters in SECAM countries work in studios with signal formats that can be faded natively, for example PAL, components or with digital video interfaces such as the Serial Digital Interface , and convert the signal to SECAM before it is broadcast. Therefore, after the fall of the Wall, GDR television could easily be converted to PAL.
Other disadvantages
Cross-color disturbances are the most unpleasant with SECAM. They are noticeable as blue and red stripes (“SECAM fire”) that flash out on sharp edges or appear as intensely red colored areas in fine patterns. This can be prevented if the luma signal is limited so that no information can overlap in the carrier signal.
distribution
The development of SECAM in France was politically motivated to protect the domestic appliance industry from imports. In this context, the abbreviation is jokingly interpreted as "Système élégant contre l'Amérique" (German: "Elegant system against America" ). Political reasons also played a role in the introduction of SECAM in the former Eastern Bloc countries. France was in a rapprochement with these states. Furthermore, it wanted to spread its system and sold studio and broadcast technology cheaply. Incidentally, during the phase of preparation and introduction of color television in the 1960s, French President Charles de Gaulle had good contacts with the then Soviet head of state Nikita Khrushchev . De Gaulle was able to win Khrushchev for SECAM, so that all other Eastern Bloc countries also introduced SECAM. The technicians of the GDR television were convinced that the PAL system would be the better, but it was not politically enforceable to introduce PAL in the GDR.
In the GDR they wanted to make western television unattractive by only being able to see it in black and white. This was of course only short-lived, since very soon PAL decoders were self-made and later color television sets were partly equipped with SECAM and PAL decoders ex works. PAL decoders were also built into televisions destined for export to the West.
In the 1990s, Greece and many countries in the former Eastern Bloc switched their television systems from SECAM to PAL.
In West Germany , television sets and video recorders with the SECAM or East reception feature were offered until the fall of the Berlin Wall . Since around 20% of the population of the old FRG and West Berlin's GDR television were able to receive television, this was definitely a selling point. However, SECAM-France and SECAM-Eastern Europe were not fully compatible: Most of the SECAM-capable televisions and video recorders mentioned (except for French models) only managed SECAM-Eastern Europe, but did not work with SECAM-France. The reason is that SECAM was used in France with the television standard L, while in Eastern European countries the standards D / K were used. Among other things, the distance between the image and sound carrier, the video bandwidth and the type of image modulation (positive or negative) are different. It is therefore not a problem for SECAM itself, but rather with the underlying incompatible television standards that transmit the SECAM color signal.
Countries that used SECAM in 2012
America
Europe
France , Russia , Belarus , Moldova , Ukraine
Asia
Armenia , Azerbaijan , Georgia , Kazakhstan , Kyrgyzstan , North Korea , Tajikistan , Uzbekistan
Africa
Morocco , Mauritania , Senegal , Mali , Burkina Faso , Réunion , Niger , Chad , Central African Republic , Republic of the Congo , Democratic Republic of the Congo , Equatorial Guinea , Gabon , Ivory Coast , Togo , Benin , Burundi , Rwanda , Djibouti , Madagascar
variants
MESECAM
MESECAM (“Middle East SECAM”) is a method for recording SECAM signals on modified PAL VHS video recorders. All MESECAM-capable devices therefore always also support PAL. MESECAM originated in the Middle East, where there was a mixed bag of PAL and SECAM states in order to be able to offer standardized devices. It is the usual recording method in the Eastern European and non-European SECAM countries. Most of the VHS recorders sold in Germany as SECAM-compatible are actually only capable of MESECAM. However, this recording format is incompatible with a normal (French) SECAM recording; French VHS recordings use a different recording method for the color signal, which is not compatible with MESECAM. MESECAM devices - as well as PAL devices - therefore only play French SECAM recordings in black and white. The ability to play back recordings from France in color is usually referred to as "SECAM-West" in the trade.
SECAM I to SECAM III
Standardization work on SECAM began in 1956, and a version of SECAM with 819 image lines was tested as part of experimental programs in France, but was never used on a regular basis. Due to uniform regulations in Europe for the use of television with 625 lines, SECAM with 625 lines was put into regular operation in France at the beginning of the 1960s.
The first standard was named SECAM I and was completed in 1961. Further compatible improvements led to SECAM II and SECAM III, which were published in 1965 at a CCIR conference in Vienna. The CCIR is known today as ITU-R.
Further improvements led in 1967 to the standards SECAM III A and SECAM III B. SECAM III B was used in the GDR until it was replaced by PAL when the program ended on December 14th and 15th, 1990. Further details can be found in the article TV in the GDR .
SECAM IV - Linear NIR (NIIR) NIR color television system
SECAM IV is a color television standard developed by the Russian research institute NIIT. Actually two standards have been developed: the non-linear NIR, in which the square root of the color signal is transmitted (in a process analogous to gamma correction) and the linear NIR, in which this process is omitted. The linear version of NIR is called SECAM IV.
Color test transmissions in NIR began in Moscow in 1963 in UHF standard D, before the switch to SECAM III took place at the same time as the launch in France on October 1, 1967. News of the new Soviet color system reached the West in 1966. At that time the BBC was quoted as saying: “It is of interest to note that this proposal appears identical to one made in April 1963 by the BBC engineer Mr. WB Pethers, but which was not pursued because at the time its advantages in relation to the other systems were not attractive enough ”. Pethers' original system was similar to the nonlinear NIR, and he also developed two variants.
Tests of NIR were carried out by the ITA in the UK - with a strong lobby for its introduction in Europe - before the states polarized in the PAL-SECAM division. Although derived from NTSC, SECAM IV differs from both the PAL and SECAM systems: It uses a “third way” to avoid hue errors.
In one line a PAL-like quadrature amplitude modulated signal with suppressed carrier is transmitted and in the next line an identical signal, but with a constant phase position as reference. Both the line with the color content and the following line with the reference carrier run through the same transmission paths and therefore the demodulated signal is free from phase errors. A similar idea is used in the video recorder systems.
The higher-frequency chrominance signal is converted into a lower-frequency range and recorded together with a reference signal. During playback, this reference is used as a BFO to recover the chrominance signals. Since both signals are subject to the same impairment of tape movement, the chrominance signal appears jitter-free . SECAM IV / Linear NIR has two deficiencies which the other systems (NTSC, PAL and SECAM III) do not have and which arise from the use of the transmitted reference signal in its broadband form in contrast to the usual locally generated reference carriers:
Firstly, every interference signal that is present on both inputs is demodulated because both the chrominance signal and the reference signal of the adjacent line are placed on a ring demodulator, and thus forms a DC voltage component at the output. Depending on the frequency of the interfering signal, this results in either an overall coloration or a colored pattern.
Secondly, the effect of chroma noise is a reduced amplitude after the demodulation of the chrominance signal, which leads to a desaturation of the colors and is particularly noticeable in the case of facial colors.
As with the PAL standard, 4,433,618.75 Hz is used as the color subcarrier frequency for 625/50 SECAM IV. The color signals are built up on the transmitter side as follows: RY with 1.14 and BY with 2.03 as a reduction factor. These baseband color difference signals have a bandwidth of> 1.5 MHz. The color difference signals are then modulated onto a carrier. In addition, a DC voltage component with 10% of the maximum value is added. As is usual with SECAM, the color identification switch in the receiver is synchronized by a 40 µs long color carrier located in the vertical synchronous blanking interval. The chrominance signal itself is recovered by multiplying line B with the previous stored line A (in a glass delay line as is common with PAL). The line B signal serves as a reference oscillator for the line A signal, which contains the chrominance content. A separate color subcarrier reference oscillator is therefore not required in the receiver. A reference frequency is always present due to the inserted DC voltage component. This should have an amplitude 10 to 20 times greater than the line signal A to be demodulated at the modulator input.
Other SECAM interpretations
In addition to the joking interpretation of "Système élégant contre l'Amérique" (German: "Elegant system against America", see above under Distribution ), the respective disadvantages of television standards with special image transmission errors also led to other alternative interpretations of the abbreviations. Because of the above-mentioned "SECAM fire" SECAM was jokingly with " S ystem E ven C rueler (than the) A merican M ethod" ( An even crueler system (than) the American method ) or S even e xtra C olours AM inute ( seven additional colors per minute ). This alludes to a reinterpretation of the American NTSC system, whose color errors gave rise to the interpretation “ N ever T he S ame C olor” ( Never the same color ).
literature
- Keith Jack: Video Demystified, A Handbook for the Digital Engineer . 3. Edition. LLH Technology Publishing, 2001, ISBN 1-878707-56-6 (English).
- Hermann Kenter: Sound and television transmission technology and technology of lead-bound BK systems . tape 10 . Decker's Verlag, Heidelberg 1988, ISBN 3-7685-2787-5 .
- 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).
Web links
Individual evidence
- ↑ World Analogue Television Standards and Waveforms ( Memento of the original from February 21, 2014 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Keith Jack: Video Demystified. LLH Technology Publishing, 3rd Edition, 2001, ISBN 1-878707-56-6 , pp. 287-290.
- ^ Changes to terrestrial television systems in Central and East European countries
- ^ SECAM (1956) German TV Museum 1 Wiesbaden. Retrieved July 25, 2020.
- ↑ Andreas Fickers received a Friedrich Wilhelm Prize from the Rheinisch-Westfälische Technische Hochschule Aachen 2006 for this study ( online at perspectivia.net )
