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Aspiring Contributor
jsavage
Posts: 6
Registered: ‎21-02-2010
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DTT Signal Strength - Technical Question

We are really pleased with our Vision Box but have recently moved to a poor signal area and are still troubleshooting the TV signal.  I now have a high gain aerial in place and am now optimising the cabeling to reduce losses which I can see by the effect of changes on the Signal Strength displayed for each multiplex on the the DTT Signal Strength Page  (Silver coloured Vision STB).

 

Unfortunately the signal strength is displayed without units.

 

Does anyone know (or has anyone worked out) what the numbers relate to?  And more importantly is this measurement linear or a logarithmic measurement (dB)?  

 

Many thanks,

 

James

Aspiring Expert
AlunH
Posts: 201
Registered: ‎27-01-2010
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Re: DTT Signal Strength - Technical Question

I can't answer you question I'm afraid, however, I remember from when I did a bit of user testing on the box, that for the Signal strength you want around the 50 to 70 mark, I suspect that as you edge towards the 100 mark you can start to get issues with "too much" signal and then require some sort of attenuation.  Pay attention to the signal quality figures... that really is a case of getting it as close to 100 as possible, although even on 50 you should see very little break up.

 

Also, without teaching you to suck eggs... if your redoing the wiring, recheck every connection, I spent ages going back over my wiring to find one strand of coax shielding touching something it shouldn't, which was causing interference!  Flipping annoying!

 

Cheers
Alun

"Can a leopard change it's shorts?" - T Prattchet
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stuartrogerson
Posts: 4,358
Registered: ‎27-01-2010
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Re: DTT Signal Strength - Technical Question

http://www.megalithia.com/elect/aerialsite/dttpoorman.html
http://www.digitalspy.co.uk/forums/showthread.php?t=588367

These links will answer some of your question.
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PheeragHfre
Posts: 806
Registered: ‎01-02-2010
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Re: DTT Signal Strength - Technical Question

 


stuartrogerson wrote:
http://www.megalithia.com/elect/aerialsite/dttpoorman.html
http://www.digitalspy.co.uk/forums/showthread.php?t=588367

These links will answer some of your question.

 

 

If you are reasonably adept at using a soldering iron this link will also help you prevent a possible poor contact on the inner coaxial connector.

 

http://www.megalithia.com/elect/bellinglee/index.html

 

This is more important in low signal areas or when there is a fair distance between Aerial & TV etc.

 

 

"I have this awful feeling someone is watching every move I make (one of my pet hates is router location tagging)." Marvin (A paranoid Android)
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jsavage
Posts: 6
Registered: ‎21-02-2010
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Re: DTT Signal Strength - Technical Question

Whilst I do appreciate that Signal Quality is the most important measure I would still like a definative answer on this question about the Signal Strength Indicator. However, on researching this subject using the links provided (thanks all) it apears likely that as I suspected the signal strength is measured as a logarithmic value.  Inserting a coaxial 'y' splitter also reduced the signal strength by about 4 on the scale which is broadly what attenuation one would expect when inserting this.  There is also a suggestion at one of the sites that manufacturers may not think it is in their interests to produce a calibrated signal strength indicator.  Shame on them!

 

I have ordered some attenuators so may end up calibrating my own system but in the meantime if anyone has an better information about the specification of the Signal Strength measurement within the Vision Box then please let me know.

 

In the meantime, whilst following the links provided I found myself on http://www.aerialsandtv.com which has a good explanation of why Digital TV Aerials dont exist and some facinating practical research on siting of tv aerials.

 

Finally, anyone looking for attenuators at a good price may be interested in BestStuff.co.uk  I have no affiliation except that I have just ordered mine from them and they were significantly cheaper than Maplin and other sources I had found.

 

All the best,

 

James

Aspiring Expert
Somerled
Posts: 172
Registered: ‎29-01-2010

Re: DTT Signal Strength - Technical Question


jsavage wrote:

Whilst I do appreciate that Signal Quality is the most important measure I would still like a definative answer on this question about the Signal Strength Indicator. However, on researching this subject using the links provided (thanks all) it apears likely that as I suspected the signal strength is measured as a logarithmic value.  Inserting a coaxial 'y' splitter also reduced the signal strength by about 4 on the scale which is broadly what attenuation one would expect when inserting this.  There is also a suggestion at one of the sites that manufacturers may not think it is in their interests to produce a calibrated signal strength indicator.  Shame on them!

 


I'd be VERY surprised if signal strength ISN'T presented logarithmically. Getting detailed information from the manufacturers on the calibration will be all but impossible. Calibrating a signal strength in absolute terms (microvolts in 75 ohms) is going to add significantly to the cost of the box, so I don't think you will find this on the spec of any consumer grade device.
Just to underline what another poster has said - the inner connection of a Belling-Lee coaxial plug MUST be soldered. If not - in a short time, you are going to have lots of intermittent signal degradations. 100% guaranteed.  Also very worthwhile, if you're replacing an aerial yourself, is to fill up the connector box of the aerial with some sealant to prevent the coaxial cable "Breathing-in" moisture, and slowly corroding. Araldite will do the job just fine.
Aspiring Contributor
jsavage
Posts: 6
Registered: ‎21-02-2010
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Re: DTT Signal Strength - Technical Question

Tnx but I have new cable and connectors - have also soldered them already tnx
Aspiring Contributor
jsavage
Posts: 6
Registered: ‎21-02-2010
0

Re: DTT Signal Strength - Technical Question

Somerled,

You suggest that it doesn't matter whether it is calibrated or not.  I beleive it does.  Too few products today come with sufficient technical information.  My VW doesn't even give people enough info to change a lightbulb any more.  It  wouldn't need to be calibrated to high degree of accuracy to be useful but some guidelines of what signal strength represents the minimum acceptable signal level and a value for the maximum which would effectively calibrate the scale would certainly be useful and should not significantly add to the cost.

 

I have almost always soldered my Belling Lee connectors and I have heard firm recomendations on the need for Silicone Grease and waterproofing of F plugs but have never heard of filling a connector box with Araldite.  Whilst I have used this technique for waterproofing home made HF dipole centre connectors I am not sure that is such a good idea for TV connectors but it would certainly be permanent.

 

James

Moderator
DavidM
Posts: 3,931
Registered: ‎27-01-2010

Re: DTT Signal Strength - Technical Question

Hi guys,

 

The details for the silver Vbox client 1.6.25077.xxx are in the table below.

 

Silver VBox Signal strength metering

 

RF level

 

dBm

dBuV

Vbox Reported Value

-73.8

35

43

-68.8

40

56

-63.8

45

68

-58.8

50

73

-53.8

55

80

-48.8

60

100

-43.8

65

100

-38.8

70

100

-33.8

75

100

-28.8

80

100

 

Colour Key

 

 

Reception possible, but subject to interference in poor weather.

 

Reception should be OK, but outside the recommended DTG level

 

Recommended DTG signal strength range.

 

Reception possible, but signal is high / most likely amplified and may be distorted.

 

 

Cheers

 

David

 

 

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Aspiring Contributor
jsavage
Posts: 6
Registered: ‎21-02-2010
0

Re: DTT Signal Strength - Technical Question

[ Edited ]

Excellent.  That is pretty much what I was asking for.  Interesting to find out that it doesn't go over 100.

 

Thank you that David.