From Retro CDN
Revision as of 14:53, 28 June 2006 by SoNick (Categorization)
SÉCAM (Séquentiel couleur à mémoire, French for "sequential colour with memory") is an analog color television system first used in France. SÉCAM has been invented by a team lead by Henri de France and working at Thomson. It is historically the first European color television standard.
Just as the other color standards adopted for broadcast usage over the world, SÉCAM is a compatible standard, which means that monochrome television receivers predating its introduction are still able to correctly show the programs, although only in black and white. Because of this compatibility requirement, colour standards add a second signal to the basic monochrome signal, and this signal carries the colour information, called chrominance or C in short, while the black and white information is called the luminance (Y in short). Old TV receivers only see the luminance, while colour receivers process both signals.
Another aspect of the compatibility being not using more bandwidth than the monochrome signal alone, the colour signal has to be somehow inserted into the monochrome signal, without disturbing it. This insertion is possible because the spectrum of the monochrome TV signal is not continuous, hence empty space exists, which can be recycled. This lack of continuity results from the discrete nature of the signal, which is divided into frames and lines. Analog colour systems differ by the way in which empty space is used. In all cases, the colour signal is inserted at the end of the spectrum of the monochrome signal.
In order to be able to separate the colour signal from the monochrome one in the receiver, a fixed frequency subcarrier has to be used, this subcarrier being modulated by the colour signal.
The colour space is three dimensional by the nature of the human vision, so after subtracting the luminance, which is carried by the base signal, the colour subcarrier still has to carry a two dimensional signal. Typically the red (R) and the blue (B) information are carried because their signal difference with luminance (R-Y and B-Y) is stronger than that of green (G-Y).
SÉCAM differs from the other colour systems by the way the R-Y and B-Y signals are carried.
First, SÉCAM uses frequency modulation to encode chrominance information on the subcarrier.
Second, instead of transmitting the red and blue information together, it only sends one of them at a time, and uses the information about the other colour from the preceding line. It uses a delay line, an analog memory device, for the purpose of storing one line of colour information. This justifies the "Sequential, With Memory" name.
This means that the vertical colour resolution is halved relative to NTSC. It is however not halved compared to PAL. Although PAL does not eliminate half of vertical colour information during encoding, it combines colour information from adjacent lines at the decoding stage, in order to compensate for colour subcarrier phase errors occurring during the transmission of the Amplitude-Modulated colour subcarrier. This is normally done using a delay line borrowed from SÉCAM (the result is called PAL DL or PAL Delay-Line, sometimes interpreted as DeLuxe), but can be accomplished "visually" in cheap TV sets (PAL standard). Since the FM modulation of SÉCAM's colour subcarrier is insensitive to phase (or amplitude) errors, phase errors do not cause loss of colour saturation in SÉCAM, although they do in PAL. In NTSC such errors cause colour shifts.
The colour difference signals in SÉCAM are actually calculated in the YDbDr colour space, which is a scaled version of the YUV colour space. This encoding is better suitable to the transmission of only one signal at a time.
Because of the FM modulation, SÉCAM is free of the dot crawl problem commonly encountered with the other analog standards and first widely noticed with the Laserdiscs. Dot crawl can be removed from PAL and NTSC-encoded signals using a comb filter. Such filters are usually only included in high-end displays. Dot crawl patterns (animated checkerboard) are easily visible along vertical lines in DVD menus displayed even by expensive (eg. plasma) displays if these displays are connected to a signal source (DVD player) using a composite PAL or NTSC connection rather than for example RGB.
The idea of reducing the vertical colour resolution comes from Henri de France, who observed that colour information is approximately identical for two successive lines. Because colour information was always supposed to be a cheap and backwards-compatible addition to the monochrome signal, it never had the same horizontal resolution as the monochrome information: the colour subcarrier has always a more limited bandwidth than the luminance signal. Therefore, it was not logical to keep the full colour resolution vertically either.
DVD and other digital television formats have perpetuated the implementation of this idea, subsampling colour both horizontally and vertically. Hence, paradoxically, VHS NTSC videos can have a greater vertical colour resolution than DVD.
A similar paradox applies to the vertical resolution in television in general: reducing the bandwidth of the video signal will preserve the vertical resolution, even if the image loses sharpness and is smudged in the horizontal direction. Hence, video could be sharper vertically than horizontally. However, because of the interlacing, vertical resolution is effectively not as great as the number of scan lines. Additionally, transmitting an image with too much vertical detail will cause annoying flicker on television screens, as small details will only appear on a single line, and hence be refreshed at half the frequency. Therefore computer generated text and inserts have to be carefully down-pass filtered to prevent this.
Work on SÉCAM began in 1956. The technology was ready by the end of the fifties, but this was too soon for a wide introduction. Notably, SÉCAM did not work with the 819-line television standard then used by the then sole French TV network. France had to start the conversion by switching over to a 625-line television standard, which happened at the beginning of the sixties with the introduction of a second network.
SÉCAM was inaugurated in France on October 1st, 1967, on la seconde chaîne (the second network), currently called France 2. A group of four men, all dressed in suits, presumably presenters and network officials, were shown standing in a studio. The image was originally black and white and suddenly switched to colour; one of the people said something along the lines of "now you can see us as we really are".
The first colour television sets cost 5000 Francs. Colour TV was not very popular initially; only about 1500 people watched the inaugural program in colour. A year later, only 200,000 sets had been sold of an expected million. This pattern was similar to the earlier slow build-up of colour television popularity in the USA.
SÉCAM was later adopted by former French and Belgian colonies, Eastern European countries, the former Soviet Union and Middle Eastern countries. However, with the fall of communism, and following a period when multi-standard TV sets became a commodity, a lot of Eastern European countries decided to switch to PAL.
Why SÉCAM in France?
Many have argued that the primary motivation for the development of SÉCAM in France was to protect French television equipment manufacturers. However, incompatibility had started with the earlier decision to uniquely adopt positive video modulation for French broadcast signals. Also, SÉCAM development predates PAL; and because of frame rate differences (50 versus 60 Hz) and the requirement for compatibility with monochrome TV receivers, it was not possible for Europeans to adopt NTSC. SÉCAM and PAL addressed the chroma phase problem, whereas NTSC required the tint control on U.S. sets.
Nonetheless, SÉCAM was partly developed for reasons of national pride . Henri de France's personal charisma and ambition may have been a contributing factor.
Unlike some other manufacturers, the company where SÉCAM was invented, Thomson, still sells TV sets worldwide under different brands; this may be due in part to the legacy of SÉCAM. Thomson bought the company which developed PAL, Telefunken, and today even co-owns the RCA brand —RCA being the creator of NTSC. Thomson also co-authored the current American high-definition TV standard ATSC.
Why SÉCAM elsewhere?
The adoption of SÉCAM in Eastern Europe has been attributed to Cold War political machinations: it has been claimed that its use made it impossible for most Eastern Europeans to view television broadcast from outside the Iron Curtain using PAL.
However, remember that PAL and SÉCAM are just standards for the colour subcarrier, used in conjunction with older standards for the base monochrome signals. The names for these monochrome standards are letters, such as M, B/G, D/K, and L. See CCIR, OIRT and FCC (the standardization bodies).
These signals are much more important to compatibility than the colour subcarriers. They differ by AM or FM modulation, signal polarisation, relative frequencies within the channel, bandwidth, etc. For example, a PAL D/K TV set will be able to receive a SÉCAM D/K signal (although in black and white), while it will not be able to receive a PAL B/G signal at all. So even before SÉCAM came to Eastern European countries, most viewers could not have received Western programs —and colour TV sets were not exactly widespread in the Communist bloc anyway, so the B/W-only reception wasn't actually much of a problem.
Another, speculative political theory is that PAL was originally German, while SÉCAM came from a country which had better political relations with Eastern Europe after the war.
There are three varieties of SÉCAM:
- French SÉCAM, used in France and its former colonies
- MESÉCAM, used in the Middle East
- SÉCAM D/K, used in the Commonwealth of Independent States and Eastern Europe (this is simply SÉCAM used with the D and K monochrome TV transmission standards).
Around 1983-1984 a new colour identification standard has been introduced in order to make more space available inside the signal for adding teletext information (originally according to the Antiope standard). Identification bursts have been made per-line (like in PAL) rather than per-picture. Older SÉCAM TV sets might not be able to display colour for today's broadcasts.
Problems with the standard
Unlike PAL or NTSC, analog SÉCAM television cannot easily be edited in its native analog form. This is because of the FM modulation SÉCAM is not linear with respect to the input image, so that electrically mixing two SÉCAM signals does not yield a valid SÉCAM signal, unlike with analog PAL or NTSC. For this reason, to mix two SÉCAM signals, they must be demodulated, have the mix applied to the demodulated signals, and be remodulated again. Hence, post-production is often done in PAL, or in component formats, with the result transcoded into SÉCAM at the point of transmission. Reducing the costs of running television stations is one reason for some countries' recent switchovers to PAL.
TVs currently sold in SÉCAM countries support both SÉCAM and PAL, and more recently baseband NTSC as well (though not usually broadcast NTSC). Although the older analog camcorders (VHS, VHS-C and 8 mm) were produced in SÉCAM versions, none of the Hi-band models were (S-VHS, S-VHS-C and Hi-8). There are no SÉCAM Digital camcorders or DVD players. However, this is of dwindling importance: since 1980 most European domestic video equipment uses SCART connectors, allowing the transmission of RGB signals between devices. This eliminates the legacy of PAL, SÉCAM and NTSC colour subcarrier standards.
In general, modern professional equipment is now all-digital, and uses component-based digital interconnects such as CCIR 601 to eliminate the need for any analog processing prior to the final modulation of the analog signal for broadcast. However, large installed bases of analog professional equipment still exist, particularly in third world countries.
Countries which use or have used SÉCAM
Afghanistan, Andorra, Armenia, Azerbaijan, Belarus, Benin, Bulgaria, Burkina Faso, Burundi, Cambodia (Kampuchea), Central African Republic, Chad, Congo (People's Republic), Côte d'Ivoire, Czech Republic, Djibouti, East Germany, Egypt, Equatorial Guinea, Estonia, France, French Guiana, French Polynesia, Gabon, Georgia, Greece, Guadeloupe, Hungary, Iran, Iraq, Kazakhstan, North Korea, Kyrgyzstan, Latvia, Libya, Lithuania, Luxembourg, Madagascar, Mali, Martinique, Mauritania, Mauritius, Monaco, Mongolia, Morocco, New Caledonia, Niger, North Korea, Poland, Réunion, Romania, Russia, Rwanda, Saint-Pierre and Miquelon, Saudi Arabia, Senegal, Serbia, Slovakia, Syria, Tahiti, Tajikistan, Togo, Tunisia, Turkmenistan, Ukraine, Uzbekistan, Vietnam, Wallis Island, Zaire.
Some SÉCAM countries are in the process of switching to PAL and are broadcasting in both SÉCAM and PAL formats. The list does not contain certain countries known to have totally switched to PAL.