Telephone line impedance matching

[John Rumm]

I have an analogue PABX which was probably designed with the US market in mind. It all works nicely but for two issues...

Firstly its exchange line impedance probably does not exactly match that provided by our (very very long) phone lines. This can result in some calls being a bit "quiet". Secondly, the poor thing gets confused by the automated BT line tests and rings once in response to a test (it also rings if you unplug the line from the master socket).

Is anyone aware of a gizmo that one can interpose betwixt line and PABX to improve the matching, and hopefully stop it sensing the brief disconnection from the lines during a test?

Ready made gadget or circus diagram would do...

[Owain]

It might be that the PBX isn't providing enough loop current or a high enough line voltage for your extension phones. A different type of extension phone might help - typically if you have problems with an electronic phone but a 746 works okay. Or the BT line isn't providing enough for the PBX.

Secondly, the poor thing gets confused by the

I think BT will turn off the automatic testing if you ask (and if the call centre person knows how to do it)

Owain

[tony sayer]

What type of exchange is it?, a post to uk.telecom sometimes throws up good responses..

[Andy Cap]

I have also asked on the THG. As you suggest, it was probably designed for the US and US phones would improve the performance a bit :-{ If anyone's aware of a suitable converter, I'll let you know.

Andy C

[Peter Parry]

That isn't likely to be an impedance mismatch but rather that the UK and US speech signal levels differ, it has long been a problem with phones purchased in the USA for UK use.

[NT]

hybrid plus amp?

NT

[John Rumm]

Extensions are all fine - internal calls are loud and clear. Its just external calls can sometimes be softer than I would like.

(external via a VoIP line is also ok)

Yup I have had mixed results with their call centres! (having said that, I have moved them from BT now - so I should at least get to speak to a sentient being!)

[The Natural Philosopher]

Correct.

Had a lot of issues with a Panasonic PABX and its phones.

[Dave Plowman (News)]

I thought the basic levels used by telephones were a world-wide standard? The old 0.75v across 600 ohms?

[Andrew Gabriel]

Also, on long lines, traditional BT/GPO phones noticed the lower DC current and increase the gain to automatically compensate for the long lines. I don't know what happens in the US, but this auto gain adjustment for long lines certainly isn't universal - much of Europe doesn't do it, and gain had to be manually set to compensate for line length, which made phones not trivially interchangable between lines.

[Dave Plowman (News)]

A 'traditional' phone which does this? I assume you mean a modern one with some form of amplification?

[Andrew Gabriel]

No, it's a standard feature of old BT dial/bell phones, and that's why they can be moved between lines of any length, without any adjustment (unlike equivalent French telephones, for example).

Various techniques have been used such as thermisters heated by the loop current as part of the variable attenuation circuit, to using the line current to partially saturate a transformer core used to couple the audio. There's no amplifier - just variable attenuation for short lines. On later phones (such as 709 IIRC, but I haven't got one handy to open up), the automatic gain control is on a plug-in card which can in theory be removed and fixed gain card used for extremely long lines, but I never came across this being required.

I assume modern electronic phones do something similar.

[Owain]

Regulator No. 1 consists of a small printed wiring board on which are mounted a Resistor, Bulb, No. 15, Rectifier-element No. 209 and two carbon resistors of 15 and 47 ohms connected as shown on Dgm N 801. These components are soldered directly into the printed wiring pattern which is connected to the telephone circuit by five plug points formed on one end of the board. Three more plug points, which are strapped together, are provided at the other end of the board so that reversal of the regulator board in its jack will remove it electrically from the telephone circuit.

Advantages of a plug-in unit for the regulator are as follows:

(a) It facilitates the locating of faults.

(b) A faulty regulator may be changed, instead of a complete telephone.

(c) Production testing is simplified. The regulator can be tested on its own, as are transmitters, etc., and a simple test to prove correct wiring is all that is then needed for the assembled telephone.

[John Rumm]

In this case however standard UK phones seem to be fine on internal calls and voip calls (where the external "exchange" line is provided locally on a voip router). So I suspect that given a "stronger" line there would be no problem. (the voip router is set to present a line using UK line specifications I believe - it may be tuneable at the command line)

[The Natural Philosopher]

exactly the same here. Vopi great. external lines s**te.

Basically BT push down the signal levels with respect to the receiver. relative to US practice.

[Dave Plowman (News)]

But in the UK trunk lines have long since been amplified to balance any losses. Since STD, if not before.

[Owain]

I found several BT Marquis systems, in different locations, to be almost inaudible

[Andrew Gabriel]

The issue is vastly different lengths (resistances) of local loop lines between longest and shortest, not truck lines.

The link Owain gave talks about the automatic gain control circuit in detail about 80% down the page (calls it Regulation). It also covers a case which I hadn't considered - a phone on a PBX might be loud on internal calls where the PBX drives it over a short line, but quiet on outside calls (where some PBXs would have patched it directly onto the exchange line). Again, the automatic regulation of gain based on the DC current (line resistance) equalises the volume in these two cases.

[Dave Liquorice]

Losses in the local end Dave. 6 miles of twisted pair has rather more attenuation than 600 yards... Easily 10dB maybe as much as 30dB or

40dB.

[The Natural Philosopher]

So 'half as loud' :-) :-)