Editing SECAM (section)

=== 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.