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SECAM




SECAM, also written SÉCAM (Séquentiel couleur à mémoire, French for "Sequential Color with Memory"), is an analog color television system first used in France. A team led by Henri de France working at Compagnie Française de Télévision (later bought by Thomson, now Technicolor) invented SECAM. It is, historically, the first European color television standard.


Television color encoding systems. Countries using SECAM are shown in orange.

Technical details


spectrum of a system G (bands IV and V) television channel with PAL color subcarrier.

Just as with the other color standards adopted for broadcast usage over the world, SECAM is a standard which permits existing monochrome television receivers predating its introduction to continue to be operated as monochrome televisions. Because of this compatibility requirement, color standards added a second signal to the basic monochrome signal, which carries the color information. The color information is called chrominance or C for short, while the black and white information is called the luminance or Y for short. Monochrome television receivers only display the luminance, while color receivers process both signals.

Additionally, for compatibility, it is required to use no 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 utilized. 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 sub carrier has to be used, this sub carrier 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 sub carrier 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).

SECAM differs from the other color systems by the way the R-Y and B-Y signals are carried.

First, SECAM uses frequency modulation to encode chrominance information on the sub carrier.

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 color from the preceding line. It uses a delay line, an analog memory device, for storing one line of color information. This justifies the "Sequential, With Memory" name.

Because SECAM transmits only one color at a time, it is free of the color artifacts present in NTSC and PAL resulting from the combined transmission of both signals.

This means that the vertical color resolution is halved relative to NTSC. The later PAL system also displays half the vertical resolution of NTSC (i.e., the same as SECAM). 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 sub carrier phase errors" occurring during the transmission of the Amplitude/Phase-Modulated color sub carrier. This is normally done using a delay line like in SECAM (the result is called PAL D or PAL Delay-Line, sometimes interpreted as DeLuxe), but can be accomplished "visually" in cheap TV sets using PAL-S (PAL simple) decoders. Because the FM modulation of SECAM's color sub carrier is insensitive to phase (or amplitude) errors, phase errors do not cause loss of color saturation in SECAM, although they do in PAL. In NTSC, such errors cause color shifts.

The color difference signals in SECAM are actually calculated in the YDbDr color space, which is a scaled version of the YUV color space. This encoding is better suited to the transmission of only one signal at a time.

FM modulation of the color information allows SECAM to be completely free of the dot crawl problem commonly encountered with the other analog standards. SECAM transmissions are more robust over longer distances than NTSC or PAL. However, owing to their FM nature, the color signal remains present, although at reduced amplitude, even in monochrome portions of the image, thus being subject to stronger cross color even though color crawl of the PAL type doesn't exist.

Though most of the pattern is removed from PAL and NTSC-encoded signals with a comb filter by modern displays, some can still be left in certain parts of the picture. Such parts are usually sharp edges on the picture, sudden color or brightness changes along the picture or certain repeating patterns, such as a checker board on clothing. Dot crawl patterns can be completely removed by connecting the display to the signal source through a cable or signal format different than composite video (yellow RCA cable) or a coaxial cable, such as, S-video. FM SECAM is a continuous spectrum, so unlike PAL and NTSC a digital Comb Filter can't entirely separate SECAM Colour and Luminance

The idea of reducing the vertical color resolution comes from Henri de France, who observed that color information is approximately identical for two successive lines. Because the color information was designed to be a cheap, backwards compatible addition to the monochrome signal, the color signal has a lower bandwidth than the luminance signal, and hence lower horizontal resolution. Fortunately, the human visual system is similar in design: it perceives changes in luminance at a higher resolution than changes in chrominance, so this asymmetry has minimal visual impact. It was therefore also logical to reduce the vertical color resolution.

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. 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. Computer-generated text and inserts have to be carefully low-pass filtered to prevent this.

History

Work on SECAM began in 1956. The technology was ready by the end of the fifties, but this was too soon for a wide introduction. Initially, a version of SECAM for the French 819-line television standard was devised and tested, but not introduced. Following a pan-European agreement to introduce color TV only in 625 lines, France had to start the conversion by switching over to a 625-line television standard, which happened at the beginning of the 1960s with the introduction of a second network.

The first proposed system was called SECAM I in 1961, followed by other studies to improve compatibility and image quality.

These improvements were called SECAM II and SECAM III with the later being presented at the 1965 CCIR General Assembly in Vienna.

Further improvements were SECAM III A followed by SECAM III B, the adopted system for general usage in 1967.

Soviet technicians were involved in the development of the standard, and even created their own incompatible variant called NIR or SECAM IV, which was not deployed. The team was working in Moscow's Telecentrum under Professor Chmakov's direction. The NIR designation comes from the name of the Nautchno-Issledovatelskiy Institut Radio NIIR research institute involved in the studies. Two standards were developed: Non-linear NIR in which a process analogous to gamma correction is used and Linear NIR or SECAM IV that omits this process.

SECAM was inaugurated in France on October 1, 1967, on la deuxième chaîne (the second channel), now called France 2. A group of four suited men—a presenter and 3 contributors to the system's development, including De France—was shown standing in a studio. Following a count from 10, the originally black and white image switched to color; the presenter then declared "Et voici la couleur !" (fr: And here is color!) In 1967, CLT of Lebanon became the third television station in the world after the Soviet Union and France to broadcast in color, utilizing the French SECAM technology.

The first color television sets cost 5000 Francs. Color TV was not very popular initially; only about 1500 people watched the inaugural program in color. 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 color television popularity in the USA.

SECAM was later adopted by former French and Belgian colonies, Greece, the Soviet Union and Eastern bloc countries (except for Romania, Albania and Yugoslavia which used PAL from the beginning), and Middle Eastern countries. However, with the fall of communism, and following a period when multi-standard TV sets became a commodity, many Eastern European countries decided to switch to PAL.

Other countries, notably the United Kingdom and Italy, briefly experimented with SECAM before opting for PAL.

Development

Some have argued that the primary motivation for the development of SECAM in France was to protect French television equipment manufacturers. However, incompatibility had started with the earlier decision to unusually adopt positive video modulation for French broadcast signals. The earlier systems System A & the 819-line systems were the only other systems to use positive video modulation. In addition, SECAM development predates PAL. NTSC was considered undesirable in Europe because of its tint problem requiring an additional control, which SECAM and PAL solved. The joke was that "SECAM" stood for "Something exceedingly Contrary to the American Method" versus NTSC "Never Twice the Same Color" whilst "Peace At Last" or "Perfection At Last" could only be obtained through the PAL system. (But another version of the joke expands PAL as "People Are Lavender", "Picture Always Lousy", or "Pay Another License"). Nonetheless, SECAM was partly developed for reasons of national pride. Henri de France's personal charisma and ambition may have been a contributing factor. PAL was developed by Telefunken, a German company, and in the post-war De Gaulle era there would have been much political resistance to dropping a French-developed system and adopting a German-developed one instead.[citation needed]

Unlike some other manufacturers, the company where SECAM was invented, Technicolor (known as Thomson until 2010), still sells TV sets worldwide under different brands; this may be due in part to the legacy of SECAM. Thomson bought the company that developed PAL, Telefunken, and today even co-owns the RCA brand —RCA being the creator of NTSC. Thomson also co-authored the ATSC standard which is used for American high-definition TV.

Why SECAM elsewhere?

The adoption of SECAM in Eastern Europe has been attributed to Cold War political machinations. According to this explanation, East German political authorities were well aware of West German television's popularity and adopted SECAM rather than the PAL encoding used in West Germany. This did not hinder mutual reception in black & white, because the underlying TV standards remained essentially the same in both parts of Germany. However, East Germans responded by buying PAL decoders for their SECAM sets. Eventually, the government in East Berlin stopped paying attention to so-called "Republikflucht via Fernsehen", or "defection via television". Later East German–produced TV sets even included a dual standard PAL/SECAM decoder.

Another explanation for the Eastern European adoption of SECAM, led by the Soviet Union, is that the Russians had extremely long distribution lines between broadcasting stations and transmitters. Long co-axial cables or microwave links can cause amplitude and phase variations, which do not affect SECAM signals.

However, PAL and SECAM are just standards for the color sub carrier, 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 color sub carriers are. They differ by AM or FM sound modulation, signal polarization, relative frequencies within the channel, bandwidth, etc. For example, a PAL D/K TV set will be able to receive a SECAM D/K signal (although in black and white), while it will not be able to decode the sound of a PAL B/G signal. So even before SECAM came to Eastern European countries, most viewers (other than those in East Germany and Yugoslavia) could not have received Western programs. This, along with language issues, meant that in most countries monochrome-only reception did not pose a significant problem for the authorities.

SECAM varieties

See also: Broadcast television systems#ITU identification scheme

L, B/G, D/K, H (broadcast)

There are five varieties of SECAM:

  1. French SECAM (SECAM-L)
    French SECAM (SECAM-L) is used only in France, Luxembourg (only RTL9 on CH 21 from Dudelange) and Tele Monte-Carlo Transmitters in the south of France
  2. SECAM-B/G
    SECAM-B/G is/was used in parts of the Middle East, former East Germany and Greece
  3. SECAM D/K
    SECAM D/K is used in the Commonwealth of Independent States and parts of Eastern Europe (this is simply SECAM used with the D and K monochrome TV transmission standards) although most Eastern European countries have now migrated to other systems.
  4. SECAM-H
    Around 1983–1984 a new color identification standard ("Line SECAM or SECAM-H") 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. Very old SECAM TV sets might not be able to display color for today's broadcasts. Although any sets manufactured after the mid-1970s should be able to receive either variant.
  5. SECAM-K
    France also introduced the SECAM standard to its dependencies. However, the SECAM standard used in France's overseas possessions (as well as African countries that were once ruled by France) was slightly different from the SECAM used in Metropolitan France. The SECAM standard used in Metropolitan France used the SECAM-L and a variant of the channel information for VHF channels 2-10. French overseas possessions and many French-speaking African countries use the SECAM-K standard and a mutually incompatible variant of the channel information for VHF channels 4-9 (not channels 2-10).

MESECAM (home recording)

MESECAM is a method of recording SECAM color signals onto VHS video tape. It should not be mistaken as a broadcast standard.

"Native" SECAM recording was originally devised for machines sold for the French market. At a later stage, countries where both PAL and SECAM signals were available (notably the Middle East, hence the Acronym "Middle East SECAM"), developed a cheap method of converting PAL VHS machines to record SECAM signals also using the PAL circuitry. A tape produced by this method is not compatible with "native" SECAM tapes as produced by VCRs in the French market. It will play in black and white only, the color is lost. So the world is left with two different incompatible standards for recording SECAM on VHS.

Although being a workaround, MESECAM is much more widespread than "native" SECAM. It has been the only method of recording SECAM signals to VHS in almost all countries that ever used SECAM, including as mentioned the Middle East and all countries in Eastern Europe. "Native" SECAM recording (marketing term: "SECAM-West") is only used in France and adjacent countries. Most VHS machines advertised as "SECAM capable" outside of France can be expected to be of the MESECAM variety only.

Technical background

The luminance signal is recorded in its original form (albeit with some reduction of bandwidth) but the PAL chrominance signal of about 4.4 MHz is too sensitive to small changes in frequency caused by inevitable small variations in tape speed to be recorded directly (NTSC suffers from the same problem). Instead, it is first down converted to the lower frequency of 630 kHz, and the complex nature of the PAL sub carrier means that the down conversion must be done via heterodyning to ensure that information is not lost.

The SECAM sub carrier, being a simple FM signal, does not need such complex processing. The VHS specification requires that it be simply divided by 4 on recording to give a sub carrier of approximately 1.1 MHz, and multiplied by 4 again on playback. A true dual-standard PAL and SECAM video recorder therefore requires two color processing circuits, adding to complexity and expense. Since some countries in the Middle East use PAL and others use SECAM, the region has adopted a shortcut, and uses the PAL mixer-down converter approach for both PAL and SECAM. This works well and simplifies VCR design.

It is interesting to note that it is often possible to record SECAM video on an unmodified PAL VCR, thus creating MESECAM tapes, which can be played back in color through another PAL VCR into a SECAM TV. Basic PAL VCRs work better for this, ones that are more sophisticated detect the SECAM signal as "not-PAL" and refuse to record it in color. Most PAL VHS recorders, with MESECAM, has been analog tuner modified in western-Switzerland (Suisse Romande). Original tuner is only CCIR B/G reception. Swiss importers added a little artisanal circuit, with specific IC, for France L norm, tuner became multistandard, but VCR record French broadcasts, on air, in MESECAM, the tapes are played in black and white on SECAM players, and SECAM tapes played B/W in these modified tuner VCR. A specific stamp was added on the deck "PAL+SECAM".

Problems with the standard

Unlike PAL or NTSC, analog SECAM television cannot easily be edited in its native analog form. Because it uses frequency modulation, SECAM is not linear with respect to the input image (this is also what protects it against signal distortion), so electrically mixing two (synchronized) SECAM signals does not yield a valid SECAM signal, unlike with analog PAL or NTSC. For this reason, to mix two SECAM signals, they must be demodulated, the demodulated signals mixed, and are remodulated again. Hence, post-production is often done in PAL, or in component formats, with the result encoded or transcoded into SECAM at the point of transmission. Reducing the costs of running television stations is one reason for some countries' recent switchovers to PAL.

Most TVs currently sold in SECAM countries support both SECAM and PAL, and more recently composite video NTSC as well (though not usually broadcast NTSC, that is, they cannot accept a broadcast signal from an antenna). Although the older analog camcorders (VHS, VHS-C) were produced in SECAM versions, none of the 8 mm or Hi-band models (S-VHS, S-VHS-C, and Hi-8) recorded it directly. Camcorders and VCRs of these standards sold in SECAM countries are internally PAL. They use an internal SECAM to PAL converter for recording of broadcast TV transmitted in SECAM. The result could be converted back to SECAM in some models; most people buying such expensive equipment would have a multistandard TV set and as such would not need a conversion. Digital camcorders or DVD players (with the exception of some early models) do not accept or output a SECAM analog signal. However, this is of dwindling importance: since 1980 most European domestic video equipment uses French-originated SCART connectors, allowing the transmission of RGB signals between devices. This eliminates the legacy of PAL, SECAM, and NTSC color sub carrier 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. In most cases all processing within the TV-station is PAL and on the output line a PAL to SECAM trancoder is used before feeding the transmitter. This is because switchers and effect mixers can easily handle PAL (or NTSC) but the SECAM-signal can't be mixed in the same way due to the frequency modulation of the color information.

Countries and territories that use SECAM

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This is a list of nations that currently authorize the use of the SECAM standard for television broadcasting. Nations that have moved to PAL or DVB-T are listed separately.

Africa

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  •  Benin
  •  Burkina Faso
  •  Burundi
  •  Central African Republic
  •  Chad
  • Republic of the Congo Republic of Congo
  • Democratic Republic of the Congo Democratic Republic of Congo
  •  Côte d'Ivoire
  •  Djibouti
  •  Equatorial Guinea
  •  Gabon
  •  Libya (Simulcast in PAL-B/G)
  •  Madagascar
  •  Mali
  •  Mauritania
  •  Mauritius
  •  Morocco
  •  Niger
  •  Rwanda
  •  Senegal
  •  Togo
  •  Tunisia

Americas

  • France Saint-Pierre and Miquelon
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Asia-Pacific

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  •  Afghanistan (Simulcast in PAL-B)
  •  Cambodia (Kampuchea) (Simulcast in PAL-B/G)
  •  French Polynesia
  •  Iran (Simulcast in PAL-B/G)
  •  Kazakhstan
  •  Kyrgyzstan
  •  Laos (Simulcast in PAL-B)
  • France New Caledonia
  •  North Korea (Simulcast in PAL-D/K)
  •  Saudi Arabia (Simulcast in PAL-B)
  •  Syria (Simulcast in PAL-G)
  •  Tajikistan
  •  Turkmenistan
  •  Uzbekistan
  •  Vietnam (Simulcast in NTSC-M)
  • France Wallis and Futuna

Europe

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  •  Andorra (migrated to DVB in 2007)
  •  Armenia (Simulcast in PAL-B)
  •  Azerbaijan
  •  Belarus
  •  France (SECAM broadcast to be abandoned by 2011, simulcast in IPTV/ADSL and DVB-T)
  •  Luxembourg (migrated to DVB in 2006)
  •  Monaco (Simulcast in PAL-G)
  •  Russia (Limited simulcast in DVB-T since 2010, SECAM to be abandoned by 2015)

Migration from SECAM to PAL

Some former SECAM countries are in the process of migrating to PAL or have already finished doing so.

Europe

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  •  Azerbaijan (migrated in 2001)
  •  Bulgaria (migrated 1994–1996)
  •  Czech Republic (migrated 1992–1994)
  •  Estonia (migrated 1992–1999)
  •  East Germany (switchover on December 14, 1990 because of German reunification)
  •  Georgia (migrated in 2000s (decade))
  •  Greece (migrated in c. 1992)
  •  Hungary (migrated 1995–1996)
  •  Latvia (migrated 1997–1999)
  •  Lithuania (migrated 1997–1999)
  •  Poland (migrated 1993–1995)
  •  Slovakia (migrated 1992–1994)
  •  Ukraine

Asia

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  •  Iraq (switched to PAL-B in 2005)
  •  Mongolia (migrated 1983–1987)

Africa

  •  Egypt (for a few years before was simulcast. Ceased in 1992 for PAL-B/G)

The Czech Republic, Slovakia, Hungary and the Baltic countries also changed their underlying sound carrier standard from D/K to B/G which is used in most of Western Europe, to facilitate use of imported broadcast equipment. This required viewers to purchase multistandard receivers though. The other countries mentioned kept their existing standards (B/G in the cases of East Germany and Greece, D/K for the rest).