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. All the photographs on this page were taken at Avoncroft Museum, Bromsgrove, in their Telecoms display area.
Carrier System WB300 - Nuclear Radiation Leak Warning System
This topic is a place holder as I have little specific details of this system. I welcome any further information and feedback via the website Home Page, linked from the tabs at the top of every page.
It is thought the WB300 was installed in the early U.K. Magnox Nuclear power stations to broadcast warning messages in the event of a radiation leak. According to Wikipedia, eleven power station were commissioned between 1959 and 1971 at Calder Hall, Chapelcross, Berkeley, Bradwell, Hunterston 'A', Hinckley Point 'A', Trawsfynydd, Dungeness 'A', Sizewell 'A', Oldbury and Wylfa. All but Wylfa closed between 1989 and 2012.
The GPO / BT circuit diagrams in the series WB 28300 date from 1961 to 1964, I haven't seen any of these myself but their titles suggest this was a carrier based warning system with both speech and tones, remarkably similar to the WB400, which dates from the same era.
Signalling System WB700 - Propagation of Local Call Timing in the GPO Network
The Receiver 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.
Receiver WB700 with Cover Removed
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 both radial distance and duration. The radial charging rings for STD were designated A (less than 56 Km), B (more than 56 Km) and C (Channel Islands). Call charge rates were also 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 residential or business 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.
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 Control Point Exchange ( CPE ) equipment cabinet. Timing signal pulses generated a sub audible signal of 44Hz for ORD customers and 26Hz for CCB customers. The sub audio signals modulate the 72KHz carrier as it passes through the cabinet from the Carrier Control Point to the distribution network.
Wire Broadcast System WB700 Schematic Diagram
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 'M' relay, MO for Ordinary, MC for Coinbox. These relays 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 caused a rethink in the generation of local call timing pulses. The fall in cost of electronic circuitry and the invention of the integrated circuit made a new solution possible.
Equipment Pulse Generator 2A
A custom designed quartz clock known as an Equipment Pulse Generator 2A was installed in small exchanges to provide the call timing information. Each installation consists of two pulse generator units and a changeover unit to ensure the resilience of the call charging. These were used until 1995 when the last step by step analogue 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 UK area codes 0XXX became 01XXX and all 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 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. This proved a better quality of customers who in the past were forced to have party lines, or Shared Service to use its correct title. The exchange unit can hold 10 slide in units, each combining two customers in one pair of wires.
Exchange: WB900 Shelf
WB900 Line Card Exposed
The first customer's line worked exactly as before except that it was diverted to run through the WB900 line card in the exchange. The second customerís line is wired to a separate connection on the line card. This connected it via a carrier system over the first customerís wires. A 64KHz carrier takes the speech and ringing signals to the second customer. A 40KHz carrier from the second customer carries the speech, off hook and dialling signals towards the exchange. There is complete privacy between the two 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.
- It was not a Wire Broadcast (WB) system as it carried signals in both directions. By definition broadcasting is unidirectional.
- The engineers who had to work with it hated it, as it was fault prone.
- If either customer left their phone off the hook to stop incoming calls the carrier customerís battery went flat.
- Engineers were advised that customers must not be told they shared a line as they paid full line rental and may confuse this with shared service on reduced rental. The highly unpopular shared service which was in abundance in those days offered no privacy between the two customers sharing the same line.
Customers end: WB900 Adapter
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. It converts the carriers to and from the exchange into the usual analogue conditions expected by a normal telephone. Not shown is the battery used to power this adapter.
Alarms By Carrier (A.B.C.) WB1000
The GPO / BT Alarm By Carrier system, was known internally as WB1000, developed in the late seventies, and marketed as a means of extending Burglar and Fire alarms to central monitoring points. It replaced the rather unreliable 'Home Office' alarm panels installed at Police and Fire Stations. The ABC benefits were included: Cost saving by not requiring a separate private wire for the alarm panel type or extra exchange line needed by dial-up alarms. A circuit fault alarm warning in the event of the phone line being cut maliciously or due to a fault, this feature not available to dial up alarms.
A.B.C. was rebranded as 'BT Redcare' in the mid eighties. The high frequency WB1000 carrier system being incompatible with ADSL broadband has been replaced with a system using low frequencies. See http://www.redcare.bt.com/