SECAM
Sequential couleur à mémoire [ sekɑsjɛlkulœ ː ʀ amemwa ː ʀ ] (SECAM or SÉCAM [ Sekam ] ) is a common, particularly in France and Eastern Europe analog television standard for color transmission in analogue television. It was developed by Henri de France and introduced in 1956. In the German language can be the full name translate approximately as " Sequential Color with Memory".
- 4.1 Countries, which used in 2012 SECAM 4.1.1 America
- 4.1.2 Europe
- 4.1.3 Asia
- 4.1.4 Africa
- 5.1 MESECAM
- 5.2 SECAM SECAM I to III
- 5.3 SECAM IV - Linear NIR ( NIIR ) NIR color television system
The basic idea
Just like NTSC and PAL SECAM is a system for black and white television -compatible ink transfer. Compared to NTSC the goal of the new system was to improve the color reproduction under non-ideal reception conditions. Compared to PAL for a different method was found with SECAM: It was not about the optimization of the studio recordings, but to optimize the transmission by the TV to the receiver. Unlike PAL SECAM was available at the beginning of the 1960s.
Common with NTSC and PAL
As with NTSC and PAL in addition to the brightness signal Y ( ie the black and white image) needed color information is transmitted in the form of two color difference signals DR and DB. The letters stand for the underlying YDbDr color model, which is very similar to the YUV color model used in PAL and NTSC and differs only by different " stretching factors " of the two color difference signals.
For the basics of color television signal transmission, see also below.
Operation
SECAM used instead of the modulation amplitude used in NTSC and PAL, the frequency modulation for the transmission of the two color difference signals. The advantage is that phase errors of the color difference signals lead to any color errors. However, the two signals can not be accommodated on a single carrier frequency and orthogonal without interference as the quadrature modulation.
SECAM transmits therefore alternately each line contains one of the two color signals DR and DB. In the receiver, the signal is further delayed by one line, so still both chrominance signals are in each row decoder available. The vertical resolution reduction of the color signals to the human eye has little adverse effects, since it has a low resolution for color information.
The two color difference signals are band limited to approximately 1.3 in. MHz and subjected to pre-emphasis to reduce noise on the luminance signal. The function of this pre-emphasis is slightly different depending on the specific version and was adapted depending on the specific standard. In the SECAM I predistortion version of the color subcarrier according to the following complex equation was:
After the pre-distortion, the two color signals are modulated alternately per line in two different carrier frequencies with 4.25 MHz and 4.40625 MHz, where DR is frequency-modulated with a stroke of 280 kHz and 230 kHz with DB. The typical case of frequency modulation the modulation index is less than 1, corresponding to a spectral compression, and is about 0.21 and 0.18. For a larger modulation index is the frequency step of the television stations as part of SECAM no space available and, consequently, the color signal is sensitive to noise. For carriers, the frequency range between 3.9 MHz and 4,756 MHz is reserved.
Before mixing with the luminance signal Y or, a further pre-emphasis of the respective row to be transmitted, the modulated color-difference signal. In this second, likewise complex pre-distortion, the amplitude of the modulated carrier signal is distorted as a function of Momentanhubs of the respective color difference signal. The reason for this is to minimize the effects of the color subcarrier of the image contents with low brightness and to improve the signal -to-noise ratio for saturated colors. Also in some encoders a band pass in front of the pre-distortion to prevent components of the frequency modulation in the impact Lumabereich. Thereafter, the respective modulated color-difference signal is added to the luminance signal Y and the composite signal thus formed is radiated.
Details of these more complex second predistortion and pictures of the envelope of the spectrum formed therefrom can be found in and. In the television receiver predistortions be undone and won the two color difference signals DR and DB for further signal processing.
Identification signals
So that the recipient can assign to the lines of correct color, there are two different identification signals:
- Line identification ( burst)
- The image identification (bottles)
The former is the method used today. For the carrier signal starts before the image information, and includes the base carrier frequency (ie, 4.25 MHz in the DB or 4.40625 MHz in DR). Method in the image, an identification signal is inserted in the lines 6 to 15 and 313 to 322. In the DB line, the signal starts at 4.25 MHz and then goes down to 3.9 MHz. The DR- line it starts at 4,406 and goes up to 4,756 MHz. Since this signal has the Trägervorzerrung, the amplitude of the signal of approximately 200 mV to 500 mV increase. The name " bottle " comes from the shape of the signal, when one considers this to an oscilloscope. This signal is no longer used today because the lines should be used for the French Antiope Teletext system. Until the shutdown of the last bottles in 2007, but the system was no longer in operation.
Delay lines
For SECAM you absolutely need a memory to store the color signal for the duration of a line, while one can do it for PAL (Simple - PAL). This memory is realized in older receivers in the form of a delay line in the receiver. Ultrasonic delay lines stood since the early 1960s are available. In since the end of 1990 spread digital TV receivers, which may often receive multiple, and analog TV standards, digital storage, however, are usually used.
Compatibility black and white images with
The color subcarrier is independent of the color intensity is always present due to the frequency modulation used with the same intensity in the image - unlike PAL and NTSC, where he shrinks when not colored image content on amplitude 0, practically disappears and therefore can no longer speak. For SECAM, the support is so regularly in the phase position in a pattern as shown in the following table, switched to suppress interference patterns. Since this suppression does not work sufficiently low interference, SECAM is said to have a poorer black and white compatibility. In plain black- and-white broadcasts of the color subcarrier in the GDR television was turned itself off completely, so sent a true black -white signal, which was long no longer common in West German stations from the TV standard PAL therefore (you had the color contrast on the receiver to zero provide, so you saw no color noise in black and white broadcasts).
Fade
SECAM - modulated signals (composite video signal ) can crossfading not directly due to the frequency modulation, since the frequency modulation is a nonlinear modulation and therefore the addition of two FM signals gives no sense usable signal. Possible the transition is only indirectly via the demodulation and subsequent transition of the individual components.
For this reason, working broadcasters in SECAM countries in the studio with signal formats which are native crossfaded, such as PAL, component or digital video interfaces such as Serial Digital Interface, and convert the signal just before the broadcast in SECAM order. Therefore you could also East German television after the fall easily convert to PAL.
Other disadvantages
Cross-color interference among the most unpleasant in SECAM. They manifest themselves as blue and red stripes ( " SECAM fire " ), the revealing flashes of sharp edges or appear as intense red color surfaces with fine patterns, in appearance. This can be prevented if the luma signal is limited, so that no information can be overlapped in the carrier signal. As SECAM was invented, it did not matter because the picture tubes showed no information about 3 MHz at this time.
Dissemination
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 " (Eng. " Elegant system against America "). With the introduction of SECAM in the former Eastern Bloc countries also political reasons have played a role. France was in an approach to these states. In addition, it wanted to achieve a distribution of his system and has studio and broadcasting equipment delivered favorable. In addition, the French President Charles de Gaulle had good contacts with the then Soviet leader Nikita Khrushchev during the phase of preparation and introduction of color television in the 1960s. De Gaulle was able to win Khrushchev for SECAM, so that all other Eastern Bloc countries SECAM introduced. The technicians of the GDR television were indeed convinced that the PAL system would be better, but it was not politically feasible, PAL introduce in the GDR.
In the GDR was desired to make Western television unattractive by could see only black and white. That was certainly short-lived, because very soon PAL decoder originated in DIY and later color television sets were already partially equipped from the factory with SECAM and PAL decoders. PAL decoder were also therefore built into television sets of the GDR, because some of these devices were exported for hard currency in the West. It would have been too expensive to produce separate instrument versions for export and the domestic market.
In the 1990s featured Greece and many countries of the former Eastern Bloc their television systems from SECAM to PAL.
One must note that SECAM SECAM France and Eastern Europe are not fully compatible: Many SECAM -enabled televisions and video recorders (except French models ) cope with SECAM -East Europe, but will not work with SECAM France. The reason is that SECAM is used in France with the television standard L, while the standards D / K are used in Eastern European countries. In this case, inter alia, the distance between image and sound, the video bandwidth and the type of image modulation are (positive or negative ) is different. It is thus not really a problem of SECAM, but the incompatible S / W television standards, which transmit the SECAM color signal.
Countries, which used in 2012 SECAM
America
French Guiana
Europe
France, Russia, Belarus, Moldova, Ukraine
Asia
Armenia, Azerbaijan, Kazakhstan, Kyrgyzstan, North Korea, Tajikistan, Uzbekistan
Africa
Morocco, Mauritania, Senegal, Mali, Burkina Faso, Reunion, Niger, Chad, Central African Republic, Republic of 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 of recording SECAM signals on modified PAL -VHS VCRs. Therefore all MESECAM -enabled devices support always PAL. MESECAM originated in the Middle East, where there was a colorful jumble of PAL and SECAM countries in order to provide uniform devices. It is the usual recording method in the Eastern European and non-European SECAM countries. The most sold as a SECAM -compatible in Germany VHS recorder actually dominate only MESECAM. This recording format is but incompatible with a normal ( French ) SECAM recording; French VHS recordings use a different method for recording the color signal which is not compatible with MESECAM. MESECAM devices - as well as PAL devices - therefore play French SECAM recordings from only black and white. The ability to color Play recordings from France is usually referred to in the trade as " SECAM -West".
SECAM SECAM I to III
The standardization work on SECAM began in 1956, and a version of SECAM with 819 image lines was tested in the context of experimental programs in France, but never regularly used. Due to uniform rules in Europe to use TV with 625 lines, in the early 1960s was taken in France SECAM starting with 625 TV lines in regular operation.
The first standard was called SECAM I and was completed in 1961. Other compatible improvements led to SECAM II and SECAM III that were released in 1965 on a CCIR - conference in Vienna. The CCIR is now called ITU -R.
Further improvements led in 1967 to the standards SECAM III A and III B. SECAM SECAM III B was in the GDR until its replacement by PAL with the program closing of 14 used on 15 December 1990. More details can be found in the article on GDR television.
SECAM IV - Linear NIR ( NIIR ) NIR color television system
SECAM IV is developed by the Russian research institute NIIT color television standard. Actually two standards have been developed: The nonlinear NIR, where the square root of the chrominance signal is transmitted ( in a process similar to the gamma correction ), and the linear NIR, wherein this process is omitted. A SECAM IV, as described below, is referred to the linear version of the NIR.
Color test transmissions in NIR began in 1963 in Moscow in the UHF Standard D, before a switch to SECAM III at the same time took place with the launch in France on October 1, 1967. The news about 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 is identical appears with a made by the BBC engineer Mr. WB Pethers in April 1963, but was not pursued because the time its benefits in terms of the other systems were not attractive enough. " The original system of Pethers was the non-linear NIR similar, and he also developed two variants.
Tests with NIR were performed by the ITA in the UK with a strong lobby for its introduction into Europe before the nations in the PAL - SECAM polarized division. Although derived from NTSC to SECAM IV differs from both the PAL as well as from the SECAM system: It uses a "third way" to avoid color errors.
In a line of a PAL -like quadrature amplitude modulated signal with suppressed carrier is transmitted and in the next line a same signal, but with a constant phase location as a reference. Both the line with the color content, as well as the subsequent row with the reference carrier through the same transmission paths and therefore the demodulated signal is free of phase errors. A similar idea is used in the VCR systems.
The higher frequency chrominance signal is converted into a lower frequency range and recorded together with a reference signal. During playback, this is used as a reference for the recovery of the chrominance signals BFO. Since both signals through the same tape impairments, the chrominance signal appears jitter. SECAM IV / NIR linear did not have two shortcomings which the other systems (NTSC, PAL and SECAM III) and the result from the use of the transmitted reference signal in its wideband form, in contrast to the usual locally generated reference beams:
First, any noise that is present on both inputs, demodulated because both the chrominance signal and the reference signal of the adjacent row are set to a Ringdemodulator, thereby forming a DC voltage component at the output. Depending on the frequency of the interference signal obtained either an overall color or a color pattern.
Secondly, as an effect of a reduced chroma amplitude after demodulation of the chrominance signal exist, resulting in de-saturation of the colors, and is particularly noticeable on colored skin.
As the color subcarrier frequency is used as the IV PAL standard 4,433,618.75 Hz at 625/50 The SECAM color signals as follows are constructed on the transmitter side: RY and BY 1.14 2.03 a reduction factor. This base -band color -difference signals have a bandwidth of > 1.5 MHz. Thereafter, the color difference signals are modulated onto a carrier. In addition, a DC component is added with 10% of the maximum value. As usual with SECAM, color identification switch 40 microseconds long color carrier is located in the vertical by a Synchronaustastlücke in the receiver synchronized. The chrominance signal itself is recovered by multiplying the line B with the preceding ( in a standard, as well as for PAL glass delay line ) stored in line A. The signal of the line B is used as a reference oscillator for the line A signal that contains the Chrominanzinhalte. Therefore, a separate color subcarrier reference oscillator in the receiver is not required. By the inserted DC component is always a reference frequency available. This should be a 10 - to 20 -fold have greater amplitude than the signal to be demodulated line A at the modulator input.
Other SECAM interpretations
The respective disadvantages of television standards with special image transfer errors led to alternative interpretations of abbreviations. Because of the above-mentioned " SECAM fire" SECAM was jokingly with " system -even Crueler ( than the) American Method" ( An even crueler system (as the ) American Method ) translated. This is alluded to a reinterpretation of the American NTSC system whose color errors ( the same color never ) gave rise to the interpretation of " Never The Same Color".