the color signals of all color TV systems of the time were encoded in a narrower band than the 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 scanning that is obviated by progressive scan.) Computer-generated text and inserts have to be carefully low-pass filtered to prevent this. The color difference signals in SECAM are 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