secam : jeune admirateur cherche historiens et connaisseurs

[Marc de Montréal]

Bonjour à tous,

Né trop récemment et hors France pour être un contemporain du secam, je n'en connais que la théorie et un dénigrement systématique.

Que leurs auteurs passent leur chemin : je voudrais parler aux historiens, intimes et passionnés du secam.

Puissent-ils m'apprendre, entre autres, les choix adoptés ou abandonnés lors de ses périodes d'expérimentation puis de sa mise en exploitation.

Au plaisir de vous lire !

Marc

[jeanphi73]

http://people.csail.mit.edu/tbuehler/vi ... dards.html


http://www.pembers.freeserve.co.uk/Worl ... html#SECAM
Intéressant le papillotement 12,50 Hertz sur les transitions colorées horizontales propres à ce standard

[mutantape]

j'avoue que je serais mois aussi interessé par une telle historique. Peut etre une autobiographie de Henri de France (attention il y a des homonymes) ?

En particulier, j'aimerais connaitre les elements experimentaux qui ont forcé le choix de la modulation de frequence.

Sur ce point, je n'ai jusquà present trouvé que le texte suivant :

SECAM (SEQuentiel A Memoire ("memory sequential"))
In 1957, Henri de France at the french company CSF (Compagnie Generale de Telegraphe Sans Fil) their first version of SECAM as a european standard. In SECAM, one exploits the eye's reduced color resolution also in the vertical direction. The Q and I signals of NTSC aren't used;
instead R-Y and B-Y are used as modulating signals, and are transmitted with the same bandwdth.
They're not transmitted simultaneously though, as in NTSC and PAL, but one after another:
one line contains R-Y information and the next line B-Y information, etc. A delay line of 64
usec (one "line time") in the receiver makes both signals available simultaneously when the
final colour picture is about to be formed.

Henri de France's original suggestion was based on using AM to modulate the colour subcarrier with R-Y and B-Y in sequence. This made SECAM insensitive to phase errors but not to amplitude errors. Since amplitude errors are more common than phase errors, and since this combined with
AM and sequential transmission would make amplitude errors cause hue shifts, this was really no improvement over NTSC. This, plus some other deficiencies, was corrected and the SECAM now in use is SECAM III or "SECAM optimalice", where frequency modulation (FM) is used to modulate
the subcarrier, and R-Y and B-Y use different deviations and somewhat different subcarrier frequencies. No color burst is needed as a frequency reference, and there is also always a color subcarrier present in the video signal, even if there is no color in the image.
By choosing slightly different color subcarrier frequencies for the R-Y and B-Y lines, the visible interference in the luminance signal will be greatly reduced.

Color signal Bandwidth Nominal subcarrier frequency Frequency shift

D(B) = -1.9(B-Y) 1.8 MHz 272 * line_freq = 4.250000 MHz +- 280 kHz
D(R) = +1.5(R-Y) 1.8 MHz 282 * line_Freq = 4.406250 MHz +- 230 kHz

This is the French SECAM system for 625-lines which 1967 became the standard form colour TV braodcasts in Fance and a few other countries. There were two other varieties of SECAM too: one variety now used in Eastern Europe. Another variety was specified for the old French 819-line TV broadcast system, but it was never widely used.