🚀 go-pugleaf

In article <1992Nov17.161717.2694@acri.fr>, pjhicks@hard03.acri.fr (Peter J. Hicks) writes:
>  Is there anybody out there that knows how to transcode Secam to Pal ?
>  
>  Are there chips available that do this function or is it a matter of building a mass of logic 

Its not logic chips you need, the signals are analogue so you need
analogue conversion circuitry. Usually means decoding it to RGB and
re-encoding it.

I have a catalogue from a UK company which sells SECAM to PAL transcoders,
I can't remember the price but I can check.
> 
>  if  so,  hows about a circuit diagram.
> 
>   Final question  What is the difference between  Secam and Pal ?

When colour TV systems were designed they had to be compatible with
Black&White ones, so they couldn't send the three colour signals (RGB)
separately. Instead several systems were developed to transmit a normal
monochrome picture, with the colour signal squeezed in, in such a way that
a mono TV would ignore it but a colour one could get it.

Since the black & white image (L) is R+G+B, you don't need to send
all three colours, just two is enough. By sending L, R and B the set
can recalculate G as L - (R+B). Monochrome sets use the L signal and
ignore the rest.

NTSC combines the R and B signals into one extra signal which is transmitted
on every line of the picture. That takes fairly complex circuitry to extract
the separate R & B, but it means that all three signals (L, R, B) are always
available so getting R G & B on each line is then fairly easy. Unfortunately
the NTSC colour signal is vulnerable to distortion (green faces) so
Telefunken, in Germany, modified the NTSC signal slightly to reduce that
effect, and the result is PAL.

 
SECAM sends R on one line, B on the next. That simplifies things a bit
since you don't need to separate the R and B signals, but does require
the TV to have a delay line (analogue memory) to store the colour signal from
one line so it can be combined with the other colour on the next line.
(SECAM = SEquential Couleur Avec Memoire)

At the time it was developed (early 60's) delay lines were expensive
and since PAL doesn't need one it was more popular in most countries, except
France (where SECAM was invented).  Nowadays delay lines are really cheap
and even PAL decoders use them to counteract colour changes caused by
signal distortions.


Steve

-- 
Steve McKinty
SUN Microsystems ICNC
38240 Meylan, France
email: smckinty@france.sun.com	   BIX: smckinty