The Carrier Distribution System
Warning Points in the WB400 Era
The 72kHz carrier generated at the Police Station's CCP was distributed via the telephone network to both the Warning Receivers and Siren Points. Confusingly these were known by two different designations - WB400 and WB600. The fact they were both the same carrier could be proved by plugging an earpiece in the rear of the WB600 receiver to hear the WB400 confidence tick tone.
Verbal Warning
The WB400 Warning points were located in Post Offices, Pubs, Schools, R.O.C posts, Civil Defence, Council Offices, Police and Fire stations. Consisting of a "Receiver Carrier WB400" connected to a telephone line. In peacetime, the receiver was normally switched off, only being turned on for tests. In small communities, the warning point alerted the populous of the impending attack by cranking a hand siren. |
Power Sirens
The WB600 Siren points tended to be located on the top of large factories, schools, water towers and fire stations (where they doubled as a fire siren). Power sirens were used in main towns, but not usually villages. The siren point still retained a Home Office Autowailer, a small box with three buttons that could be used to control the siren locally. More details and photographs may be found on the WB600 Page. |
District Wide Distribution - WB400 Era
In the previous topic we saw that the Carrier Control Point (C.C.P.) for a warning district, generates two identical carriers sent over the X and Y path to the Carrier Control Exchange (C.C.E.). In the diagram these are shown in Red and Blue. To guard against damage to the underground cables, the X and Y paths are kept physically as far apart as possible. At the CCE, the X and Y carriers are amplified and forwarded to other dual fed exchanges, these are shown as Large Towns A&B. The exchange in Town B, also amplifies the X&Y carriers and forwards them to Large Town C.
At the dual fed exchanges including the CCE, a changeover unit monitors the incoming carriers. Should one fail, it selects the other. This switched supply (shown in Green) is amplified and distributed to the Warning Points in the town. If it is necessary to extend the distribution to villages, these are fed from the switched supply.
In the 1960-70 era, many villages only had a single underground cable feeding the exchange, so there was little point in duplicating the carrier feed. In more remote locations, one cable passed through a number of exchanges. In the diagram above, village D is supplied from village C. There were design rules that limited the number of tandem links that could be used. Some villages may have only had one or two feeds to warning points.
Local Distribution to Warning Points
The telephone line going to the warning point was used to convey the WB carrier to the receiver or siren. In the 1960-70 period, lines were often in short supply, waiting lists for phones was not uncommon and two customers may have shared the same pair of wire back to the exchange in what was known as shared service or party line. Sending the carrier over the existing phone line removed the need for an extra pair of wires.
In the WB400 equipment at the exchange, the carrier was amplified and divided into a number of individual supplies for each warning point. A connection was made from the carrier distribution unit to the line towards the customer. To prevent the EXCHange Line Circuit from shunting the carrier, the wiring was altered so an inductor unit was introduced. Inductors block the passage of the higher frequency of the carrier whilst still allowing the direct current and ringing for the telephone through.
At the warning point, the telephone is rewired via a filter unit. This separates the direct current and ringing from the LINE and sends it to the INSTrument connection whilst blocking the carrier, so the telephone (telephone instrument as they were called) does not shunt the carrier. The filter's other connection point has the opposite effect, it allows the carrier to pass and blocks the passage of direct current and ringing, so the RECeiver does not affect the operation of the phone, whilst allow the higher frequency of the carrier to pass.
The drawing below shows a WB Filter Unit 3A, containing a high and low pass network arrangement.
WB1400 Carrier Distribution Differences
The distribution of the WB1400 carrier which was of the same 72KHz frequency as its predecessor was so similar it is only worth highlighting the differences.
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Refering to the district wide distribution diagram above. The X and Y carrier paths only extended as far as the Carrier Control Exchange. Large towns were connected in the same way as small villages. However the loss of carrier at an exchange would raise a maintenance alarm unlike WB400 distribution. The customers line connections at the exchange and customers premises vary slightly from the WB400 diagrams above. The WB1400 receiver being line power is connected to the LINE port on the filter and not the REC port. |
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