From The TV IV
SÉCAM also known as Séquentiel couleur avec mémoire (French for "sequential color with memory") is an analog color television system first used in France, and was invented by Henri de France in 1956. SÉCAM is historically the first European color television standard. Just as the other color standards adopted for broadcast usage over the world such as PAL and NTSC, 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.
SÉCAM has 625 scanlines and 25 frames per second, and it uses a horizontal frequency of 15.625 kHz, and similar to PAL in these respects. However, it is very different from both PAL and NTSC in that it uses frequency modulation to encode chrominance information on the subcarrier, which is explained in the transmission modulation scheme section below. Unlike PAL or NTSC, analog SÉCAM television cannot easily be edited in its native analog form. Because it uses frequency modulation, SÉCAM is not linear with respect to the input image (this is also what protects it against broadcasting distorsions), so electrically mixing two (synchronized) 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, the demodulated signals mixed, and be remodulated again. Hence, post-production is often done in PAL, or in component formats, with the result encoded or 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.
SÉCAM is totally different when it comes to color encoding. It's based on the information that the color resolution is less than the black/white resolution. So, only half the vertical resolution is needed for color. Instead of transmitting both R-Y and B-Y at the same time, as in NTSC and PAL do, SÉCAM only transmits one color signal at a time.
Because of backwards compatibility requirement, color standards are added as a second signal to the basic monochrome signal, and this signal carries the color information, called chrominance (C), while the black and white information is called the luminance (Y). Old TV receivers only see the luminance, while color receivers process both signals.
Another aspect of the compatibility being not using more bandwidth than the monochrome signal alone, the color 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 color systems differ by the way in which empty space is used. In all cases, the color signal is inserted at the end of the spectrum of the monochrome signal.
In order to be able to separate the color signal from the monochrome one in the receiver, a fixed frequency subcarrier has to be used, this subcarrier being modulated by the color signal.
The color space is three dimensional by the nature of the human vision, so after subtracting the luminance, which is carried by the base signal, the color 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 color systems by the way the R-Y and B-Y signals are carried.
Transmission modulation scheme
SÉCAM uses frequency modulation to encode chrominance information on the subcarrier, this differs from NTSC and PAL. Also, instead of transmitting the red and blue information together, SÉCAM only sends one of them at a time, and uses the information about the other color from the preceding line. It uses a delay line, an analog memory device, for the purpose of storing one line of color information. This justifies the "Sequential, With Memory" name.
Because SÉCAM transmits only one color at a time, it is free of the color artifacts present in NTSC and PAL and resulting from the combined transmission of both signals.
This means that the vertical color resolution is halved relative to NTSC. It is however not halved compared to PAL. Although PAL does not eliminate half of vertical color information during encoding, it combines color information from adjacent lines at the decoding stage, in order to compensate for color subcarrier phase errors occurring during the transmission of the Amplitude-Modulated color 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). Because the FM modulation of SÉCAM's color subcarrier is insensitive to phase (or amplitude) errors, phase errors do not cause loss of color saturation in SÉCAM, although they do in PAL. In NTSC such errors cause color shifts.
Countries and territories which use SÉCAM
Andorra, Belarus, Bulgaria, Czech Republic, France, Estonia, Greece, Hungary, Latvia, Lithuania, Luxembourg, Monaco, Poland, Romania, Slovakia, Serbia, Ukraine
Afghanistan, Armenia, Azerbaijan, Cambodia (Kampuchea), Georgia, Iran, Iraq, Kazakhstan, Kyrgyzstan, Mongolia, Russia, Saudi Arabia, Syria, Tajikistan, Turkmenistan, Uzbekistan
Benin, Burkina Faso, Burundi, Central African Republic, Chad, Congo (Democratic Republic), Congo (People's Republic), Côte d'Ivoire, Djibouti, Egypt, Equatorial Guinea, Gabon, Libya, Madagascar, Mali, Mauritania, Mauritius, Morocco, Niger, Réunion, Rwanda, Senegal, Tunisia, Togo
Saint-Pierre and Miquelon (Territories of France just south of Canadian Province of Newfoundland)
French Guiana, Guadeloupe, Haiti, Martinique
Australia and Oceania
French Polynesia, New Caledonia, Tahiti, Wallis Islands