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Posted by Alan Pemberton on 10/11/06 17:01
charles <charles@charleshope.demon.co.uk> wrote:
> The French are terribly pedantic when it comes to the use of words.
> "Sub-Carrier" is only used when there is something being carried, at other
> times it's called the "reference frequency".
However, there's nothing in a SECAM decoder that requires a reference
frequency - it uses bog-standard FM discriminators. The bursts of
subcarrier in the field and/or line blanking intervals that would be
used to synchronise a local reference oscillator in PAL or NTSC are
there merely to identify which lines carry red, and which blue,
information.
In the Russian NIIR ("SECAM IV") standard, alternate lines carry
continuous subcarrier of a fixed phase. This is used in the decoder (in
place of a locally generated reference signal) to demodulate the
quadrature modulated chrominance information on the adjacent line, one
or the other having been passed through a glass ultrasonic delay line as
in PAL D.
Note that the delay line in French SECAM is a very different beast from
the one in PAL. Its timing accuracy only has to be sufficient to line up
the colours on adjacent lines to avoid smearing them, whereas a PAL
delay line needs to deliver the 4.43361875MHz subcarrier in its correct
phase.
[FU set to utb as I filter out crosspostings and only came across this
thread by chance]
--
Alan Pemberton
Sheffield, South Yorkshire, England
To e-mail me directly, please visit
<http://www.pembers.freeserve.co.uk/index.html#Mail-me>
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