[GLLUG] Broadband signal to noise ratio

Chris Bell chrisbell at chrisbell.org.uk
Tue Sep 19 10:03:36 UTC 2017

On Monday, 18 September 2017 08:39:01 BST you wrote:
> I believe that VDSL follows the same/similar scheme as ADSL where there
> are many carriers carrying QAM modulated signals. Looking at my VDSL
> stats (currently running downstream at just under 40 Mbps) its using up
> to BIN (their language for the carrier number)  2600 which works out to
> around 11 MHz. (4.3125×2600).
> Mobile phone signals are up around 900 Mhz and above so I doubt there is
> much interference from them.What I have noticed is that dampness does
> affect the capacity of the line to sync the higher speeds and my VDSL
> sync rate does vary with the weather. I have seen ADSL services show
> this behaviour too. Also there is crosstalk between the pairs in the
> cable bundles in the multicore cables that run from the manhole/phone
> pole back to the green street cabinets, and I believe this also affects
> the performance. Is your iron pipe dry inside? The BT cable's
> capacitance will change if its lying in water especially if there are
> any nicks in the plastic sheath allowing moisture in.
> I'm astonished xDSL broadband works at all over lines originally
> intended for baseband voice service. Aren't you ex-BBC Chris? I recall
> hearing that TV OB services sometimes used specially equalised telephone
> pairs to carry baseband PAL TV (up to 8Mhz) back to the studio where
> other feeds weren't practical. I was surprised at the time that enough
> bandwidth was available in a telephone pair to carry PAL TV.
> :-) Ken

Thanks for the reply.
The old rusty pipe gave some protection against garden tools but ended a few 
feet from the house perhaps 30 years ago, the cable was only changed when the 
termination capacitor in the old external bell box failed, so the original 
line termination and phone had to be replaced. I would have to search for any 
sign of the cable and remains, if any,  of the pipe, knowing that the first few 
feet will be threaded through the roots of an old rose bush.
I am connected via a roadside cabinet on the other side of the A40, which is a 
very busy dual carriageway with three lanes each way. The multicore cable from 
the roadside cabinet is buried, although the duct has been damaged many times 
and has previously suffered from flooding. I suspect this is common anywhere 
near London, yet the government is pressing for Fibre To The Premises. I 
suspect I may be buried first.
As for the use of phone cables to carry video, it was often point to point 
across race courses, so not very far, but we had what was known as a fairy 
fingers box to get the best out of a line, and it was not really suitable for 
use with colour TV, for which we used microwave radio links. More common was 
the use of phantom circuits, using transformer centre taps from two quiet 
pairs to bypass the normal telephone line equalisation, for music quality 
analogue audio. Engineering control lines were often direct plain pairs 
without amplification or equalisation, so difficult to use if you were in the 
north of England trying to speak to London.
The frequency response of ordinary twisted pair telephone cable is considered 
flat over the normal speech bandwidth but drops to a null, then recovers enough 
to be used for data, perhaps sent at different data rates over a number of 
frequency bands.

Chris Bell
Website http://chrisbell.org.uk

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