HANDEL
The carrier system was sub divided into two separate but mutual systems, WB600 was used to control the Air Raid sirens and WB400 was the means by which speech and warning signals (tones) were sent to the various carrier receivers at warning points. The system used a 72Khz carrier, permanently sent down the line, this was then modulated with an AM (amplitude modulated) speech and signal tones, for reception and demodulation at the carrier receiver.

RECEIVER CARRIER WB400 A

The same carrier could also be modulated with two tones (G and S) to control the switching of the sirens. The most familiar part of the system, at the carrier control point (Police Station) was the unit with 3 handsets known as the 'Operating unit WB401A' the two grey '700' type handsets were connected to the duplicated link to the 'TIM' distribution system, the link was duplicated for back up reasons, and were known as the 'X' and 'Y' paths. The black Bakelite handset was connected to the nearest ROC group headquarters for onward distribution of 'fallout warnings' to warning points. Picking up either of the '700' type handsets in a 'non war' situation would have resulted in the normal 'TIM' time signal being heard

An attack warning from RAF Strike Command would be preceded by two tones (P -2400hz and Q - 2600hz ) which signaled the WB401A to receive a message by sounding an alarm on the unit. The 'war' officer at the Police station would then lift one of the 'X' and 'Y' path handsets (either one would do) to receive a spoken attack warning, the system had no facility to 'talk back' to RAF Strike Command .

Mounted below the WB 401A was an 'Operating Unit 400A'– normally on a table – this was the method by which the officer passed on the warning of attack/fallout to the warning point carrier receivers, there was no direct link between RAF Strike command and the 'public' side of the system, all messages being relayed via the Police Control Points.

The ROC posts however had direct links to their respective Group controls on dedicated Emergency Circuit Private wires but this was an entirely different network. The War Officer upon receiving the message from RAF Strike Command would operate one of three switches on the WB400A 'Warning','Speak' and 'Call', Warning and Call sent a series of pulsed tones to the warning points which were received on the 'Carrier Receiver 400 A' to alert the warning officers, then operating the Speak key allowed the spoken warning to be re broadcast to the now alerted Warning Officers. Warning Officers would then pass on the respective warnings using hand sirens and maroons (sometimes even whistles and rattles) to the general public.

Between the handsets on the WB401A was a control unit used for setting off the Power Sirens. The unit 'Control WB600A' generated the two tones used to modulate the carrier ( S – 1500 Hz and G – 2160 Hz) and switch the siren controlling equipment at remote locations. To initialize the Siren circuit tone 'G' was sent for 10 seconds pulsed 0.4s on and 0.4s off . The 'attack warning' was tone 'S' for 4's on and 4's off and an 'all clear' was signaled by 60 seconds of 'S' continuous. In the photograph the button nearest the two grey 700 handsets, is 'all clear' the centre button is 'stop' and nearest the Bakelite handset is 'attack warning' near to the centre handset can be seen a lock switch, the key to which was held in a 'break glass in emergency' container.

WB600 A RECEIVER

In the apparatus room of the Police Station the tones were generated by oscillators in a 'Unit signaling WB600A' this unit had a duplicate 'Unit Control 600A' as a backup for setting off the sirens. Also in this room would have been a 'Unit equipment carrier WB404A' which generated the tones and controlled

the speech link to the WB400 carrier receivers at warning points. Both the above items were mounted in heavy grey steel cabinets 6 feet high by 2 feet wide. The 72KHz carrier was also generated here. At the remote siren location was a 'Receiver Carrier WB600A' picture of which is shown above.

The line from the Police Station was again connected (via the local exchange) to the remote siren site using an unsuspecting 'normal' subscribers line into a standard GPO street cabinet nearest the siren, a filter WB600 then split the line, between the customers line to their premises and the line feeding the siren control unit.

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