Miscellaneous Wire Broadcast Systems

A brief summary of Wire Broadcast systems employed by the Post Office Telecommunications / British Telecom in the period before 1990 but not used for Civil Defence purposes.

Signalling System WB700


The WB700 had nothing to do with Civil Defence itself but carried call charging information to small exchanges. It utilised the Civil Defence WB400, 72KHz carrier for this purpose, which avoided the expense of a separate network of lines conveying the charging information.

Prior to the introduction of Subscriber Trunk Dialling (STD) telephone customers could only dial local calls themselves. Calls to places further a field were connected via the operator. Local calls were charged between one and four metered units depending on radial distance irrespective of the call duration.

The introduction of STD in UK during the early 1960's brought a new charging regime. Local calls were charged by duration only. STD calls were charged by radial distance rings and duration. Call charges were affected by the time of day, day of the week and type of customer. Coinbox customers or public telephone kiosks paid a higher rate than Ordinary customers.

To enable small exchanges to charge for local calls by duration it was decided to generate the timing signals in the main exchange and distribute this out to smaller units thereby avoiding the expense of sophisticated timing equipment in every small exchange. The Signalling System WB700 was devised as a means to distribute Ordinary (ORD) and Coin Collection Box (CCB) call timing signals.


Wire Broadcast System WB700

At the main exchange, accurate timing signals were created by the multi phase pulse supply. These were fed to the WB700 oscillator and modulator contained within the Carrier Control Exchange (CCE) equipment cabinet. Timing pulses generated a sub audio signal 44Hz for ORD customers and 26Hz for CCB customers. The sub audio signals modulated the 72KHz carrier as it passed through the cabinet from the Carrier Control Point (CCP) to the distribution network.

At the dependent exchanges a Receiver Carrier WB700 connected to one feed from the WB400 distribution detected the two sub audio tones and operated the appropriate relay to extend the timing signal to the local call timing devices in the telephone exchange switch block.


In the early 1980's the planned replacement of the WB400 meant a rethink in the generation of call timing pulses. The fall in cost of electronic circuitry and the invention of the integrated circuit made a new solution possible. A custom designed quartz clock known as an Equipment Pulse Generator 2A was installed in the small exchanges to provide the call timing information. These were used until 1995 when the last step by step exchanges were replaced with a modern digital network. The digital network had to be fully in place for PhoneDay on 15th April 1995, when the area codes 0XXX became 01XXX and the mobile network numbers changed to start with 07 plus 9 more digits.


Piped Music Distribution WB800

The WB800 was designed to distribute 'Piped Music' to offices and shopping centres. Marketed under the name 'Planned Music' I think this was conceived as a money spinner but it never appeared to take off. Dedicated audio circuits sent out music from a central source to exchanges that had a customer(s) subscribing to the service. A 144 kHz carrier generated in the exchange was amplitude modulated with the audio and distributed via the normal telephone line to the subscribers premises. Filters like those used by the Civil Defence network were used to separate the carrier from the phone. The carrier was demodulated at the customer's premises and distributed to speakers.


Pair Gain Equipment WB900

WB900 Adapter

WB900 had nothing to do with the Cold War but was a system for connecting two customers to one pair of wires to overcome shortages in the underground cable network. The second customerís line was connected via a carrier system on the first customerís wires. A 64KHz carrier took the speech and ringing signals to the second customer. A 40KHz carrier from the second customer carried the speech, off hook and dialling signals towards the exchange. There was complete privacy between the customers who could both use their phone at the same time. The battery in the remote carrier unit in the second customerís house was trickle charged over the line.

Everything about this system was wrong.

Here is a picture of the internal printed wiring board and inside cover of a "Subscribers Unit WB900, Adapter No.1A" a small white box approximately 80mm square.