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UHF home distribution and witchcraft



 
 
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  #11  
Old August 22nd 19, 10:01 PM posted to uk.tech.digital-tv
charles[_2_]
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Posts: 936
Default UHF home distribution and witchcraft

In article , Indy Jess John
wrote:
On 22/08/2019 20:48, Brian Gaff wrote:
I do remember back in the analogue days being surprised that a
connection lead just 5 inches longer than one that misbehaved worked
flawlessly, when there was no discernable difference other than length.


I saw something like that some years ago. The place where I worked at
the time was putting in a communications network. They had bought the
kit and I was doing the monitoring software; the network when live was
supposed to reroute across a mesh design in the event of a link failure.
I was testing the logic of this.


I set the network up in a spare rack and linked it in the connection
arrangement that would emulate the live configuration. When I first
fired it all up, it said there was a link down. All the "wide area"
connections showed the appropriate working lights, so it had to be the
"local" wiring. There were two supposedly identical connecting leads,
and one worked and the other one didn't. The only difference was the
length - a pin-to-pin check showed that both provided an identical
connection on a DC meter, yet one passed data and the other one didn't.
Disconnecting one end of the "dud" one and cutting 8 inches off before
reconnecting the end and moving the unit it served down a shelf so that
the shorter wire reached, brought it back into life.


I never did find out why 8 inches of wire made such a difference, but it
had to be a capacitance or crosstalk alteration.


or cancellation due to reflection from an unterminated end. Change the
cable length and you change the phase of the return signal.

--
from KT24 in Surrey, England
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle
  #12  
Old August 23rd 19, 04:48 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 3,601
Default UHF home distribution and witchcraft

On 22/08/2019 21:35, Indy Jess John wrote:
On 22/08/2019 20:48, Brian Gaff wrote:
I do
remember back in the analogue days being surprised that a connection lead
just 5 inches longer than one that misbehaved worked flawlessly, when
there
was no discernable difference other than length.


If this happens it means that there are standing waves caused by poor
matching. Just changing the length of the cable is a fudge, not a fix.

A 3dB or 6dB attenuator can often help, if the poor matching can't be
corrected.

Bill
  #13  
Old August 23rd 19, 04:54 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 3,601
Default UHF home distribution and witchcraft

On 22/08/2019 21:40, Bob Latham wrote:

Easiest way to answer that please see diagram at:
www.mightyoak.org.uk/diag.jpg


There's certainly plenty of scope there for problems. Twelve items on
the signal path before the final amp! Standing waves, howl round, mains
ripple...

Bill
  #14  
Old August 23rd 19, 07:52 AM posted to uk.tech.digital-tv
[email protected]
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Posts: 122
Default UHF home distribution and witchcraft

Agree with you there, there is a feedback loop where the UHF outputs of the modulators are finding their way back into the digiboxes RF inputs.

I would start with redesigning the system.... When I have time I will see if I can knock up a drawing and upload to Dropbox or similar.

S.
  #15  
Old August 23rd 19, 08:16 AM posted to uk.tech.digital-tv
Bob Latham
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Posts: 747
Default UHF home distribution and witchcraft

The author has marked this message not to be archived. This post will be deleted on September 6, 2019.

In article ,
Bill Wright wrote:
On 22/08/2019 21:40, Bob Latham wrote:


Easiest way to answer that please see diagram at:
www.mightyoak.org.uk/diag.jpg


There's certainly plenty of scope there for problems. Twelve items
on the signal path before the final amp! Standing waves, howl
round, mains ripple...


Trouble is all items are needed, there is nothing there I can remove
without killing the whole point of the system.

Bob.

--
Bob Latham
Stourbridge, West Midlands
  #16  
Old August 23rd 19, 08:25 AM posted to uk.tech.digital-tv
Bob Latham
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Posts: 747
Default UHF home distribution and witchcraft

The author has marked this message not to be archived. This post will be deleted on September 6, 2019.

In article ,
wrote:

Agree with you there, there is a feedback loop where the UHF
outputs of the modulators are finding their way back into the
digiboxes RF inputs.


The only feedback loop possible is in the Humax PVR if you both tune
and select the Humax's modulator, otherwise no loop so why would I do
that. The sky box doesn't "see" digital on the aerial feed.

I would start with redesigning the system.... When I have time I
will see if I can knock up a drawing and upload to Dropbox or
similar.


That would be good of you thanks but I've arrived at this with years
of experimenting to get the facilities we want. Yes, you could feed
the Humax more directly from the aerial so that a feedback isn't
possible but then it couldn't record from the Sky box which is
useful. I can't see how those facilities can be achieved with
anything very different from what I have got.


Bob.

--
Bob Latham
Stourbridge, West Midlands
  #17  
Old August 23rd 19, 09:05 AM posted to uk.tech.digital-tv
Bob Latham
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Posts: 747
Default UHF home distribution and witchcraft

The author has marked this message not to be archived. This post will be deleted on September 6, 2019.

In article ,
Bob Latham wrote:
In article ,
wrote:


Agree with you there, there is a feedback loop where the UHF
outputs of the modulators are finding their way back into the
digiboxes RF inputs.


The only feedback loop possible is in the Humax PVR if you both tune
and select the Humax's modulator, otherwise no loop so why would I do
that. The sky box doesn't "see" digital on the aerial feed.


I would start with redesigning the system.... When I have time I
will see if I can knock up a drawing and upload to Dropbox or
similar.


That would be good of you thanks but I've arrived at this with years
of experimenting to get the facilities we want. Yes, you could feed
the Humax more directly from the aerial so that a feedback isn't
possible but then it couldn't record from the Sky box which is
useful. I can't see how those facilities can be achieved with
anything very different from what I have got.


May I also add a couple of other things...

Yesterday Charles reminded me to remove the modulators from the
system and so I did and proved the modulators are not causing either
of my issues.

I should also have said that there is no obvious UHF loop possible.
The only possible loop is via the humax and HDMI and the 1 second
modulator delay.

I would also add that the signal of all muxes is 100% at the input to
the distribution amp.

Thanks.

Bob.

--
Bob Latham
Stourbridge, West Midlands
  #18  
Old August 23rd 19, 01:52 PM posted to uk.tech.digital-tv
Jim Lesurf[_2_]
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Posts: 4,567
Default UHF home distribution and witchcraft

In article , Bob Latham
wrote:
In article , Bill Wright
wrote:
On 22/08/2019 21:40, Bob Latham wrote:


Easiest way to answer that please see diagram at:
www.mightyoak.org.uk/diag.jpg


There's certainly plenty of scope there for problems. Twelve items on
the signal path before the final amp! Standing waves, howl round,
mains ripple...


Trouble is all items are needed, there is nothing there I can remove
without killing the whole point of the system.


I've not commented thus far because I've no experience with TV systems of
this kind/complexity. However some up-thread comments chime with some of
what I've experienced with some complicated RF systems for other purposes.

There a system we got from someone worked very badly due to a mix of
problems. These tended to create RF 'ground loops', etc. Poor matching,
lack of ground referencing, etc.

So, as per an up-thread comment, it might be that simply adding something
like small-value attenuator at the end of a cable could help if it reduces
a tendency for a mismatch somewhere to cause UHF to get onto the outer side
of the cables and boxes in the system. Something like a 3dB or 6dB pad
might work wonders. But it's impossible to say if, or where best to place
it, without experiment.

As above, I've also wondered if a suitable ferrite 'clamp' block around the
outside of a coax somewhere might help.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics https://www.st-andrews.ac.uk/~www_pa...o/electron.htm
biog http://jcgl.orpheusweb.co.uk/history/ups_and_downs.html
Audio Misc http://www.audiomisc.co.uk/index.html

  #19  
Old August 23rd 19, 04:10 PM posted to uk.tech.digital-tv
Indy Jess John
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Posts: 1,620
Default UHF home distribution and witchcraft

On 23/08/2019 04:48, Bill Wright wrote:
On 22/08/2019 21:35, Indy Jess John wrote:
On 22/08/2019 20:48, Brian Gaff wrote:
I do
remember back in the analogue days being surprised that a connection lead
just 5 inches longer than one that misbehaved worked flawlessly, when
there
was no discernable difference other than length.


If this happens it means that there are standing waves caused by poor
matching. Just changing the length of the cable is a fudge, not a fix.


I was there to test and as necessary correct the management software.
The rack of units and cables was just to emulate the final nationwide
network so that I could get everything proved before the units were
shipped out, and the "Local" cables were just to connect things in the rack.

When the testing was complete, an installation team travelled the
country putting the units in their permanent locations and connecting to
the wiring already installed on sites, and once I had checked that the
live locations behaved in the same way as my local test rig the software
was handed over to the support team. The fudged cables were not part of
the live fit-out, so all I was concerned about is that they should pass
data during my testing, not why the change of length made a difference.

Jim
  #20  
Old August 23rd 19, 05:16 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 3,601
Default UHF home distribution and witchcraft

On 23/08/2019 08:16, Bob Latham wrote:
In article ,
Bill Wright wrote:
On 22/08/2019 21:40, Bob Latham wrote:


Easiest way to answer that please see diagram at:
www.mightyoak.org.uk/diag.jpg


There's certainly plenty of scope there for problems. Twelve items
on the signal path before the final amp! Standing waves, howl
round, mains ripple...


Trouble is all items are needed, there is nothing there I can remove
without killing the whole point of the system.

Bob.

Try to move from daisy chain to star.

Bill
 




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