Editing SECAM (section)

== Design ==
[[File:Spectre SECAM NICAM.png|thumb|400px|Spectrum of a SECAM broadcast, with colour (red) and sound (green, yellow) sub-carrier frequencies]]

Just as with the other color 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, color standards added a second signal to the basic monochrome signal, which carries the color information. The color information is called [[chrominance]] or <math>C</math> for short, while the black-and-white information is called the [[Luma (video)|luminance]] or <math>Y</math> for short. Monochrome television receivers only display luminance, while color receivers process both signals. The [[YDbDr]] [[color space]] is used to encode the mentioned <math>Y</math> (luminance) and <math>D_BD_R</math> ([[R-Y|red]] and [[B-Y|blue color difference]] signals that make up chrominance) components.

Additionally, for compatibility, it is required to use no more [[Bandwidth (signal processing)|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 [[Bandwidth (signal processing)|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 color information, at the cost of reducing the possible [[Image resolution|resolution]].

European monochrome standards were not compatible when SECAM was first being considered. France had introduced an [[819 line|819-line system]] that used [[CCIR System E|14&nbsp;MHz of bandwidth]] (System E), much more than the [[CCIR System A|5&nbsp;MHz standard]] used in the UK (System A) or the [[CCIR System M|6&nbsp;MHz]] in the US (System M). The closest thing to a standard in Europe at the time was the [[CCIR System D|8&nbsp;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 lines|625-line]] system started in the 1950s and was first implemented in [[CCIR System I|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 color information that could be carried. In the 8&nbsp;MHz standard, the signal is split into two parts, the video signal, and the audio signal, each with its own [[Carrier wave|carrier frequency]]. For any given channel, one carrier is located 1.25&nbsp;MHz above the [[Broadcast band|channel's listed frequency]] and indicates the location of the luminance portion of the signal. A second carrier is located 6&nbsp;MHz above the luma carrier, indicating the center of the audio signal.

To add color to the signal, SECAM adds another carrier located 4.4336...&nbsp;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 line]]s differ little from their immediate neighbors, both luma and chroma signals are close to being periodic on the [[Horizontal scan rate|horizontal scan]] frequency, and thus their [[Spectral density|power spectra]] tends to be concentrated on multiples of such frequency. The specific color carrier frequency of SECAM results from carefully choosing it so that the [[Harmonic|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 [[color space]] perceived by humans is three-dimensional because of the nature of their [[retina]]s, which include specific detectors for red, green and blue light. So in addition to luminance, which is already carried by the existing monochrome signal, color 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 color, its color difference signals tend to zero and its color-encoded signal converges to its equivalent monochrome signal.

=== Colorimetry ===
SECAM [[colorimetry]] was similar to PAL, as defined by the ITU on REC-BT.470.<ref name="auto">{{cite web |title=Recommendation ITU-R BT.470-6 Conventional Television Systems |url=https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.470-6-199811-S!!PDF-E.pdf |url-status=live |archive-url=https://web.archive.org/web/20220121001941/https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.470-6-199811-S!!PDF-E.pdf |archive-date=21 January 2022 |access-date=21 January 2022}}</ref> Yet the same document indicates<ref name=":0">{{cite web |title=Recommendation ITU-R BT.470-6 Conventional Television Systems, page 16 |url=https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.470-6-199811-S!!PDF-E.pdf#page=16 |url-status=live |archive-url=https://web.archive.org/web/20220125063006/https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.470-6-199811-S!!PDF-E.pdf#page=16 |archive-date=25 January 2022 |access-date=15 February 2022}}</ref> that for existing (at the time of revision, 1998) SECAM sets, the following parameters (similar to the original 1953 color NTSC specification<ref>47 CFR § 73.682 (20) (iv)</ref>) could be allowed:
{| class="wikitable"
|+ SECAM colorimetry<ref name="auto" /><ref name=":0" />
! rowspan="3" | [[Color space]] specified by
! rowspan="3" | Year
! rowspan="3" | [[Standard illuminant#White points of standard illuminants|White point]]
! colspan="6" | [[RGB color model|Primaries]]
! rowspan="3" | Display [[Gamma correction|gamma]] [[Transfer functions in imaging|EOTF]]
|-
! colspan="2" | Red
! colspan="2" | Green
! colspan="2" | Blue
|-
! [[CIE 1931 color space|xʀ]]
! [[CIE 1931 color space|yʀ]]
! [[CIE 1931 color space|xɢ]]
! [[CIE 1931 color space|yɢ]]
! [[CIE 1931 color space|xʙ]]
! [[CIE 1931 color space|yʙ]]
|-
| REC-BT.470<ref>{{cite web |title=ITU-R BT.470-6 - Conventional Television Systems |url=https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.470-6-199811-S!!PDF-E.pdf |url-status=live |archive-url=https://web.archive.org/web/20220121001941/https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.470-6-199811-S!!PDF-E.pdf |archive-date=21 January 2022 |access-date=21 January 2022}}</ref>
| 1970
| [[Standard illuminant#Illuminants B and C|C]]
| 0.67
| 0.33
| 0.21
| 0.71
| 0.14
| 0.08
| 2.8
|-
| [[European Broadcasting Union|EBU]] 3213-E, [[Rec. 601|ITU-R BT.470/601 (B/G)]]
| 1998
| [[Standard illuminant#Illuminants D|D65]]
| 0.64
| 0.33
| 0.29
| 0.6
| 0.15
| 0.06
| 2.8
|}

The assumed [[Gamma correction|display gamma]] was also defined as 2.8.<ref name="auto"/>

[[Luma (video)|Luma]] (<math>E'{\scriptstyle\text{Y}}</math>) is derived from red, green, and blue (<math>E'{\scriptstyle\text{R}}, E'{\scriptstyle\text{G}}, E'{\scriptstyle\text{B}}</math>) gamma pre-corrected primary signals:<ref name="auto"/>
* <math>E'{\scriptstyle\text{Y}}= 0.299E'{\scriptstyle\text{R}} + 0.587E'{\scriptstyle\text{G}} + 0.114E'{\scriptstyle\text{B}}</math>

<math>D'{\scriptstyle\text{R}}</math> and <math>D'{\scriptstyle\text{B}}</math> are red and blue color difference signals, used to calculate chrominance:<ref name="auto"/> 
* <math>D'{\scriptstyle\text{R}}=-1.902(E'{\scriptstyle\text{R}}-E'{\scriptstyle\text{Y}})</math>
* <math>D'{\scriptstyle\text{B}} = +1.505(E'{\scriptstyle\text{B}}-E'{\scriptstyle\text{Y}})</math>

=== Comparison to PAL and NTSC ===
SECAM differs significantly from the other color systems by the way the color difference signals are carried. In [[NTSC]] and [[PAL]], each line carries color difference signals encoded using [[quadrature amplitude modulation]] (QAM). To demodulate such a signal, knowledge of the [[Phase (waves)|phase]] of the [[Carrier wave|carrier signal]] is needed. This information is sent along the video signal at the start of every scan line in the form of a short burst of the color carrier itself, called a "[[colorburst]]". A phase error during QAM demodulation produces crosstalk between the color difference signals. On NTSC this creates [[Hue]] and [[Colorfulness|Saturation]] errors, manually corrected for with a "tint" control on the receiving TV set; while PAL only suffers from Saturation errors. SECAM is free of this problem.

SECAM uses [[frequency modulation]] (FM) to encode chrominance information on the color carrier, which does not require knowledge of the carrier phase to demodulate. However, the simple FM scheme used allows the transmission of only one signal, not the two required for color. To address this, SECAM broadcasts <math>R-Y</math> and <math>B-Y</math> separately on alternating [[scan line]]s. To produce full color, the color information on one scan line is briefly stored in an [[analog delay line]] adjusted so the signal exits the delay at the precise start of the next line. This allows the television to combine the <math>R-Y</math> signal transmitted on one line with the <math>B-Y</math> on the next and thereby produce a [[Gamut|full color gamut]] on every line. Because SECAM transmits only one chrominance component at a time, it is free of the color artifacts ("[[dot crawl]]") present in NTSC and PAL that result from the combined transmission of color difference signals.

This means that the vertical color resolution of a field is halved compared to NTSC. However, the color signals of all color TV systems of the time were encoded in a narrower band than their luma signals, so color information had lower horizontal resolution compared to luma in all systems. This matches the human retina, which has higher luminance resolution than color resolution. On SECAM, the loss of vertical color resolution makes the color resolution closer to uniform in both axes and has little visual effect. 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 interlaced television screens, as small details will only appear on a single line (in one of the two interlaced fields), and hence be refreshed at half the frequency. (This is a consequence of [[Interlaced video|interlaced scanning]] that is obviated by [[progressive scan]].) Computer-generated text and inserts have to be carefully [[low-pass filter]]ed to prevent this.

The color difference signals in SECAM are calculated in the [[YDbDr]] color space, which is a scaled version of the [[Y′UV|YUV]] color space. This encoding is better suited to the transmission of only one signal at a time. [[Frequency modulation|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 does not exist. Though most of the pattern is removed from PAL and NTSC-encoded signals with a [[comb filter]] (designed to segregate the two signals where the luma spectrum may overlap into the spectral space used by the chroma) 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. FM SECAM is a [[Spectrum (physical sciences)|continuous spectrum]], so unlike PAL and NTSC even a perfect digital comb filter could not entirely separate SECAM colour and luminance signals.

=== Disadvantages ===
{{Unreferenced|section|date=May 2024}}

Unlike PAL or NTSC, analog SECAM programming 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' 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 [[Broadcasting|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 video format|8 mm]] or Hi-band models ([[S-VHS]], [[VHS-C|S-VHS-C]], and [[8 mm video format#Hi8|Hi-8]]) recorded it directly. Camcorders and VCRs of these standards sold in SECAM countries are internally PAL. 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 color model|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 [[Rec. 601|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.