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Posted by Stephen on 10/10/06 07:21
"Radium" <glucegen1@excite.com> wrote in message
news:1160431017.628911.56120@b28g2000cwb.googlegroups.com...
> Hi:
>
> What is the color sub carrier frequency in SECAM video?
>
> http://www.high-techproductions.com/pal,ntsc.htm
>
> The above site shows the color sub carrier frequency for NTSC [3.579545
> MHz] and PAL [4.433618 MHz].
There's an excellent and very detailed explanation of the SECAM system here:
http://www.pembers.freeserve.co.uk/World-TV-Standards/Colour-Standards.html#SECAM
The SECAM colour subcarrier frequency is
4.40625 MHz on "Red" lines
4.25000 MHz on "Blue" lines
The precise frequency for PAL is
4.43361875 MHz
All these frequencies arise from the original 625-line variant of the NTSC
colour system (which was never actually used). 625 line NTSC had a colour
subcarrier frequency of 4.4296875 MHz or 283.5 times line frequency. PAL has
to have a "quarter line offset" instead of the "half line offset" in NTSC,
so the NTSC subcarrier frequency was tweaked for PAL and became 283.75 times
line frequency or 4.43359375 MHz. They then decided to add a "25 Hz offset"
to PAL giving the frequency used today of 4.43361875 MHz or 283.7516 times
line frequency. (25 Hz is 0.0016 times line frequency.)
SECAM needs to use an even multiple of line frequency instead of the odd
multiple used by NTSC and PAL, and it has no offset. So the original NTSC
number, 283.5, was tweaked to the nearest even number, 284, giving the
original SECAM colour subcarrier frequency of 4.437500 MHz or 284 times line
frequency (This was the same on both Red and Blue lines). This was further
tweaked to reduce visibility of the subcarrier dot pattern, and we ended up
with 4.40625 MHz or 282 times line frequency on "Red" lines, and 4.25000 MHz
or 272 times line frequency on "Blue" lines.
In SECAM, alternate horizontal scanning lines have only a Red colour
difference signal, while the remaining lines have only Blue colour
difference. The final tweak of the SECAM colour subcarrirer frequency was
to introduce a 10 times line frequency offset between the frequency used on
on "Red" lines and that used on "Blue" lines.
Why 283.5 in the first place? The original mulitple of 283.5 was chosen
because it is one of a small selection of numbers which are convenient to
produce with a few frequency divider circuits using vacuum tubes or valves.
283.5 is 567/2. 567 = 7 x 3 x 3 x 3 x 3 so it required a divide by 7 circuit
and four divide by 3 circuits. Other convenient numbers of this type are:
5 x 3 x 3 x 3 x 3 = 405
7 x 3 x 3 x 3 x 3 = 567
7 x 5 x 5 x 3 = 525
5 x 5 x 5 x 5 = 625
13 x 7 x 5 = 455
13 x 7 x 3 x 3 = 819
The Dutch used a 567 line TV system experimentally sometime in the late
1940's or early 1950's inbetween trying out the British 405 line system, and
finally adopting the later 625 line European standard.
In American NTSC, the colour subcarrier is 227.5 times line frequency =
455/2. 455 = 13 x 7 x 5. This is the same as the number of lines in the
French pre-war TV system, and so is vaguely related to the French 819 line
system which was an upgrade to 455 lines in the ratio of 9 to 5. Both the
American NTSC system and the French 819 line system involve a "divide by 13"
circuit.
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