[time-nuts] Digital TV signals

Ulrich Bangert df6jb at ulrich-bangert.de
Wed Apr 16 12:11:30 EDT 2008


maybe that your TV derived frequency standard IS already obsolete
ALTHOUGH you may still receive analogue transmissions!

In Germany the ZDF was known to have a rubidium standard in their
transmitting central in Mainz that was phase locked to the atomic clock
ensemble at the PTB (the German equivalent to NIST) in Braunschweig. All
typical frequencies of the video signal such as line and frame frequency
were directly derived from that rubidium standard. The ZDF (in contrast
to private television stations and also in contrast to the other
nationwide television provider ARD) completely owned their nationwide
system of relay stations and transmitters, so that the ZDF could
guarantee that the signal generated in their central was distributed
along known ways over the whole of Germany. So (with a certain phase
delay of course) every television receiver in Germany could receive a
PTB traceable video signal.

Kits, that would enable a quartz oscillator to phase lock to the video
signal were cheap and pretty common in Germany. When I bought such a kit
some 4 years ago to compare it with my other equipment it was a matter
of only a few hours to find that the video signals that I receive at my
home were off by abt. 7.0E-11 against my GPSDO.

With that knowledge I contacted the PTB to ask how this could be. I
received the answer that 

a) Since many years not a single person/organisation/company had shown
the slightest interest into standard frequency distribution by

b) The ZDF, while still providing analogue signals to the end user, had
internally already exchanged their relay system to a digital one that -
due to compressing/decompressing and other stuff - would no more
guarantee for a fixed phase relation betweeen the central and the end


c) That the ZDF due to a) and b) had decided to still use their rubidium
standard in their central but to cut the expensive direct communication
lines to Braunschweig over that they would get their standard signal.

So, what I had measured was the rubidium's offset against NIST after
some years of free running operation. While the whole story was a
striking experience for me it also showed that there must have been a
formidable number of owners of TV based frequency standards that had
simply not seen how bad their standard really was.

Best regards

Ulrich Bangert 

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von david brown
> Gesendet: Mittwoch, 16. April 2008 18:25
> An: time-nuts at febo.com
> Betreff: [time-nuts] Digital TV signals
>  From a complete amateur, is there any useful timing info to 
> be gained 
> from the newer format of digital tv transmission(Australia) 
> to replace 
> that available from current analogue transmissions.? My recently 
> repaired TV derived frequency standard looks to  be becoming 
> obsolete! 
> David
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