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Posted by Roderick Stewart on 10/10/06 10:41
On Tue, 10 Oct 2006 08:21:41 +0100, "Stephen"
<stephen@junkmail.sptv.demon.co.uk> wrote:
>The SECAM colour subcarrier frequency is
>4.40625 MHz on "Red" lines
>4.25000 MHz on "Blue" lines
[...]
>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.
It's worth mentioning also that since the SECAM subcarriers are
frequency-modulated, they will only have the frequencies specified
above on those portions of the picture that have no colour
information. For the rest of the picture, the frequencies will vary
above and below.
The SECAM encoder includes a "cloche" (bell-shaped) filter to control
the amplitude of the subcarriers so that they have minimum (but not
zero) amplitude when they are unmodulated, i.e. for monochrome where
it can be reasoned that subcarrier patterning would be most annoying.
Filtering the RGB outputs of the decoders must be tricky, as the
subcarriers can vary over a range of frequencies. You can use a notch
filter in PAL and NTSC, and only see spurious components outside its
range on transitions between coloured areas, but with SECAM, the more
brightly coloured areas will have subcarrier of various frequencies
(depending on colour) all over them. To remove this, a bandpass filter
must be used, which will have a worse effect on luminance.
Rod.
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