SECAM, also written SÉCAM (, Système Électronique Couleur Avec Mémoire, French for electronic colour system with memory), is an analogue colour television system that was used in France, Russia, and some other countries or territories of Europe and Africa. It was one of three major analog colour television standards, the others being PAL and NTSC. Similar to PAL, a SECAM picture is made up of 625 interlaced lines and displayed at a rate of 25 frames per second (except SECAM-M). However, because of how SECAM processes colour information, it is not compatible with the PAL video format standard. SECAM video is composite video; the luminance (luma, monochrome image) and chrominance (chroma, colour applied to the monochrome image) are transmitted together as one signal.
All the countries using SECAM have either converted to Digital Video Broadcasting (DVB), the new pan-European standard for digital television, or are currently in the process of conversion. SECAM remained a major standard into the 2000s.
History
thumb|Chromat 2062, East German–produced dual-standard PAL/SECAM TV set
Invention
Development of SECAM predates PAL, and began in 1956 by a team led by Henri de France working at Compagnie Française de Télévision (later bought by Thomson, now Technicolor). NTSC was considered undesirable in Europe because of its tint problem, requiring an additional control, which SECAM (and PAL) solved.
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 unusual decision to adopt positive video modulation for 819-line French broadcast signals (only the UK's 405-line was similar; widely adopted 525- and 625-line systems used negative video).
The first proposed system was called SECAM I in 1961, followed by other studies to improve compatibility and image quality,
Following a pan-European agreement to introduce colour TV only on 625-line broadcasts, France had to switch to that system, which happened in 1963 with the introduction of "la deuxième chaîne ORTF" France 2, the second national TV network.
Further improvements during 1963 and 1964 to the standard were called SECAM II
Soviet technicians were involved in a separate development of the standard, creating an incompatible variant called NIIR or SECAM IV, which was not deployed. The team was working in Moscow's Telecentrum. The NIIR designation comes from the name of the Nautchno-Issledovatelskiy Institut Radio (NIIR, rus. Научно-Исследовательский Институт Радио), a Soviet research institute involved in the studies. in which a process analogous to gamma correction is used, and Linear NIIR SECAM IV was proposed by France and USSR at the 1966 Oslo CCIR conference
Further improvements were SECAM III A, followed by SECAM III B, In the same year of 1967, CLT of Lebanon became the third television station in the world, after in France and the Soviet Central Television in the Soviet Union, to broadcast in colour utilizing the French SECAM technology.
The first colour television sets cost 5000 French new francs (the change to new franc occurred in 1960; see francs for more details). At that time a pocket book costed roughly 1 NF.
Colour TV was not very popular initially; only about 1500 people watched the inaugural program in colour. A year later in 1968, 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 US.
In March 1969, East Germany decided to adopt SECAM III B. This did not hinder mutual reception in black and 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, such as the RFT Chromat, even included a dual-standard PAL/SECAM decoder as an option.
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.
Other countries, notably the United Kingdom and Italy, briefly experimented with SECAM before opting for PAL. SECAM was adopted by former French and Belgian colonies in Africa, as well as Greece, Cyprus, and Eastern Bloc countries (except for Romania), and some Middle Eastern countries.
European efforts during the 1980–90s toward the creation of a unified analogue standard, resulting in the MAC standards, still used the sequential colour transmission idea of SECAM, with only one of time-compressed U and V components being transmitted on a given line. The D2-MAC standard enjoyed some short real market deployment, particularly in northern European countries. To some extent, this idea is still present in 4:2:0 digital sampling format, which is used by most digital video media available to the public. In this case, however, colour resolution is halved in both horizontal and vertical directions thus yielding a more symmetrical behavior.
Decline
With the fall of communism and following a period when multi-standard TV sets became common in the early 2000s, many Central and Eastern European countries decided to switch to the West German-developed PAL system. Yet SECAM remained in use in Russia, Belarus, and the French-speaking African countries. In the late 2000s, SECAM started a process of being phased out and replaced by DVB.
Unlike some other manufacturers, the company where SECAM was invented, Technicolor (known as Thomson until 2010), still sold television sets worldwide under different brands until the company sold its Trademark Licensing operations in 2022; this may be due in part to the legacy of SECAM. Thomson bought the company that developed PAL, Telefunken, and even co-owned the RCA brand – RCA being the creator of NTSC. Thomson also co-authored the ATSC standards which are used for American high-definition television.
Design
thumb|400px|Spectrum of a SECAM broadcast, with colour (red) and sound (green, yellow) sub-carrier frequencies
Just as with the other colour standards adopted for broadcast usage over the world, SECAM is a standard that permits existing monochrome television receivers predating its introduction to continue to be operated as monochrome televisions. Because of this compatibility requirement, colour standards added a second signal to the basic monochrome signal, which carries the colour information. The black-and-white information is called the luminance or <math>Y</math> for short, and the colour information is called chrominance or <math>C</math> for short. Monochrome television receivers only display luminance, while colour receivers process both signals. The YDbDr colour space is used to encode the mentioned <math>Y</math> (luminance) and <math>D_BD_R</math> (red and blue colour difference signals that make up chrominance) components.
Additionally, for compatibility, it is required to use no 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 bandwidth of the monochrome TV signal is generally not fully utilized; the high-frequency portions of the signal, corresponding to fine details in the image, were often not recorded by contemporary video equipment, or not visible on consumer televisions anyway, especially after transmission. This section of the spectrum was thus used to carry colour information, at the cost of reducing the possible resolution.
European monochrome standards were not compatible when SECAM was first being considered. France had introduced an 819-line system that used 14 MHz of bandwidth (System E), much more than the 5 MHz standard used in the UK (System A) or the 6 MHz in the US (System M). The closest thing to a standard in Europe at the time was the 8 MHz 625-line system (System D), which had originated Germany and the Soviet Union and quickly became one of the most used systems. An effort to harmonize European broadcasts on the 625-line system started in the 1950s and was first implemented in Ireland in 1962 (System I).
SECAM thus had the added issue of having to be compatible both with their existing 819-line system as well as their future broadcasts on the 625-line system. As the latter used much less bandwidth, it was this standard that defined the amount of colour information that could be carried. In the 8 MHz standard, the signal is split into two parts, the video signal, and the audio signal, each with its own carrier frequency. For any given channel, one carrier is located 1.25 MHz above the channel's listed frequency and indicates the location of the luminance portion of the signal. A second carrier is located 6 MHz above the luma carrier, indicating the center of the audio signal.
To add colour to the signal, SECAM adds another carrier located 4.4336... MHz above the luma carrier. The chroma signal is centered on this carrier, overlapping the upper part of the luma frequency range. Because the information of most scan lines differ little from their immediate neighbors, both luma and chroma signals are close to being periodic on the horizontal scan frequency, and thus their power spectra tends to be concentrated on multiples of such frequency. The specific colour carrier frequency of SECAM results from carefully choosing it so that the higher-powered harmonics of the modulated chroma and luma signals are apart from each other and from the sound carrier, thereby minimizing crosstalk between the three signals.
The colour space perceived by humans is three-dimensional because of the nature of their retinas, which include specific detectors for red, green and blue light. So in addition to luminance, which is already carried by the existing monochrome signal, colour requires sending two additional signals. The human retina is more sensitive to green light than to red (3:1) or blue (9:1) light. Because of this, the red (<math>R</math>) and blue (<math>B</math>) signals are usually chosen to be sent along luma but with comparably less resolution, to be able to save bandwidth while impacting the perceived image quality the least. (Also, the green signal is on average more closely correlated to luma, making them a poor choice of signal to send separately). To minimize crosstalk with luma and increase compatibility with existing monochrome TV sets, the <math>R</math> and <math>B</math> signals are usually sent as differences from luma (<math>Y</math>): <math>R-Y</math> and <math>B-Y</math>. This way, for an image that contains little colour, its colour difference signals tend to zero and its colour-encoded signal converges to its equivalent monochrome signal.
Colourimetry
SECAM colourimetry was similar to PAL, as defined by the ITU on REC-BT.470. Yet the same document indicates that for existing (at the time of revision, 1998) SECAM sets, the following parameters (similar to the original 1953 colour NTSC specification) could be allowed:
{| class="wikitable"
|+ SECAM colourimetry
| style="background:#" | 0.3127
| style="background:#" | 0.3290
| style="background:#" | 6500 (D65)
| style="background-color: color(rec2020 );" | 0.64
| style="background-color: color(rec2020 );" | 0.33
| style="background-color: color(rec2020 );" | 0.29
| style="background-color: color(rec2020 );" | 0.60
| style="background-color: color(rec2020 );" | 0.15
| style="background-color: color(rec2020 );" | 0.06
| rowspan="2" | 2.8
|-
| ITU-R BT.470-6 (provisional) Laos, and Vietnam (Hanoi and other northern cities).
MESECAM (home recording)
MESECAM is a method of recording SECAM colour signals onto VHS or Betamax video tape. It should not be mistaken for a broadcast standard.
"Native" SECAM recording (marketing term: "SECAM-West") was devised for machines sold for the French (and adjacent countries) market. At a later stage, countries where both PAL and SECAM signals were available developed a cheap method of converting PAL video machines to record SECAM signals, using only the PAL recording circuitry. 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 used SECAM, including the Middle East and all countries in Eastern Europe.
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 colour is lost. Most VHS machines advertised as "SECAM capable" outside France can be expected to be of the MESECAM variety only.
Technical details
On VHS tapes, the luminance signal is recorded FM-encoded (on VHS with reduced bandwidth, on S-VHS with full bandwidth) but the PAL or NTSC chrominance signal is too sensitive to small changes in frequency caused by inevitable small variations in tape speed to be recorded directly. Instead, it is first shifted down to the lower frequency of 630 kHz, and the complex nature of the PAL or NTSC sub-carrier means that the down conversion must be done via heterodyning to ensure that information is not lost.
The SECAM sub-carriers, which consist of two simple FM signals at 4.41 MHz and 4.25 MHz, do not need this (actually simple) processing. The VHS specification for "native" SECAM recording specifies that they be divided by 4 on recording to give sub carriers of approximately 1.1 MHz and 1.06 MHz, and multiplied by 4 on playback. A true dual-standard PAL and SECAM video recorder therefore requires two colour 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-downconverter approach for both PAL and SECAM, simplifying VCR design.
Many PAL VHS recorders have had their analog tuner modified in French-speaking western Switzerland (Switzerland used the PAL-B/G standard while the bordering France used SECAM-L). The original tuner in those PAL recorders allows only PAL-B/G reception. The Swiss importers added a circuit with a specific IC for the French SECAM-L standard, making the tuner multi-standard and allowing the VCR to record SECAM broadcasts in MESECAM. A stamp mentioning "PAL+SECAM" was added to these machines.
Video recorders like Panasonic NV-W1E (AG-W1-P for professional), AG-W3, NV-J700AM, Aiwa HV-MX100, HV-MX1U, Samsung SV-4000W and SV-7000W feature a digital standard conversion circuitry.
Adoption
A legacy list of SECAM users in 1998 is available on Recommendation ITU-R BT.470-6 - Conventional Television Systems, Appendix 1 to Annex 1, and the list before many OIRT countries migrated to PAL can be found at CCIR Report 624-3 Characteristics of television systems, Annex I.
Below is an updated list of nations that currently authorize the use of the SECAM standard for television broadcasting. It is subject to ongoing changes as nations move to PAL and DVB-T. These migrations are listed separately.
{| class="wikitable"
! SECAM users
|-
| style="width:50%;" |
; Africa
- (Migrated in late 1980s)