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Date: Sat, 27 Dec 2003 19:48:37 +0000
From: "Stewart Bryant" <stewart.bryant@virgin.net>
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Subject: Re: LF: Re: 60kHz: JJY and WWWB visible behind MSF
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Alberto di Bene wrote:
> MarkusVester@aol.com wrote:
> 
>> Hi LF group,
>>
>> in the gaps of the MSF timecode on 60kHz, it is possible to observe 
>> signal strength and phase progression of DX time stations JJY (Japan) 
>> and WWVB (Colorado) in Europe. You can find details of the experiment 
>> and a comment on ZDF TV-timebase jitter on
>>
>> http://members.aol.com/df6nm/MSFgaps.htm
> 
> Markus
>   do you (or anybody else) know if the re-broadcasting of ZDF via the 
> Astra satellite keeps
> the timebase accuracy, or some frame buffer is inserted in between, that 
> would of course
> ruin that precison ?   Thanks
> 

I am fairly certain that this is a digital service, in which case there
is going to be an MPEG encoder/decoder in the way, and that does not
have the same constraints on accuracy that an analogue system has.
The timebase in a digital system is carried via a 27MHz clock that is 
reconstructed at the receiver from the packetised transmission system.
Now the number of frames per second will be fundamentally derived from
the incomming source and so should be as good as the analogue system,
but the constraints on jitter are much more relaxed in a digital
system. Furthermore there are going to be issues across programme
boundaries where the accuracy may not be maintained and there is also
the impact of the poorer signal to noise ratio (digital systems may
interpolate missing frames).

The reason that an analogue system needs to be so accurate is that the
interference artifacts are reduced if all the frames are synchronised,
so the frequency re-use plan required hightly accurate clocks. This
does not apply in digital systems where the interfering signal
looks just like noise. In a digital system the requirement on accuracy
is driven by the need to recosntruct the colour-burst from the 27MHz
MPEG system clock.

73

Stewart G3YSX