I think that TV stations  may also sync their signals so that when propagation 
is
good the interfering signals don't "roll' through one another.  I can remember 
in
the early days of TV here in the states that we would occasionally see such
things --- when they were not synched.  Now you just see a stable ghost of the
'interfering' station in the background. Since TV is AM, there is no appreciable
'capture effect' like on FM and a weak signal can ghost through a marginal local
signal.

Just a supposition, but I think it makes sense.   I suspect it makes even more
sense on your side of the pond where spectrum is even more highly congested.

Rye K9LCJ


Talbot Andrew wrote:

Wolf - are you absolutely sure about the TV timebase frequencies ?   My
understanding is that ZDF sync pulse form a national standard for time
distribution in D Land and should be exactly 15625.000   This is borne
out by German microwave opertors who use ZDF TV sync for high accuracy
frequency standards and I'm told it is an inheritance from Unification
where the DDR originally used this method of national time and frequency
distribution.  Your measurement of 0.19 Hz high suggests a soundcard
sampling error of 12ppm which looks feasible.

I find it hard to believe that any of the TV stations would be out by
more than 30ppm even if a cheap and nasty TTL oscillator module were
used - which WDR and RTL evidently are.  Your measurement of ARD looks
to be within tolerance of your quoted accuracy so the betting is those
are the identical frequency

Andy  G4JNT

> To verify that these differences are not propagation effects,
> I also compared
> some the TV line frequences (using a soundcard at
> 44100samp/sec, mixed down,
> decimated by 81, 64k FFT size, 0.008Hz resolution). All TV
> stations were
> received 'old style' via antenna.
> The TV line synch frequencies found this way (using the same
> "correction
> factor as above):
> ARD:  15625.18 Hz
> ZDF:   15625.19 Hz
> WDR: 15626.18 Hz
> RTL:   15626.20 Hz
> The difference between these signals are still above 1ppm
> (!?), but this may
> be based on propagation effects (It's not the measurement, I
> verified by
> switched the stations and watching the effect on the waterfall).
>
>
> Hope to meet you on the band in regular CW next weekend, even
> if it is our
> 10th  QSO !
>
> 73's Wolf (DL4YHF, DF0WD)
>

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