Switching card

This digital switching card was used at the Empress Telephone Exchange on Warwick Road near Earl's Court, today known as the West Kensington Exchange. Empress was opened on the 11 September 1968 by Post-Master General the Rt. Hon John Stonehouse MP who made the inaugural call to the Mayor of Hammersmith, Sir Samuel Salmon. Empress was a trunk exchange: an exchange that routes calls between exchanges. This card was a B switch, part of the outgoing trunk to another exchange. It could handle up to 24 simultaneous calls.

Empress is historically significant because it was the world's first digital exchange and utilized Pulse Code Modulation (PCM) technology. The PCM concept was developed in 1937 by Alec Harley Reeves, an employee of the Western Electric Company working in Paris. PCM worked by sampling analogue signals at specific time intervals and then converting them into binary form for transmission.

A viable method for realising the technology was conceived at the General Post Office's Research Station at Dollis Hill in north-west London. Engineers at Dollis Hill constructed an experimental digital tandem exchange as a prototype for Empress. Analogue transmission was prone to noise interference and only allowed one call to be carried by a pair of wires at a time. Before Empress the digital signal had to be converted back into analogue form before a call could be switched between trunks. Empress allowed calls to be switched between trunks digitally.

This new technology paved the way for faster, more efficient and cheaper telecommunications systems. Digital exchanges could handle a significantly larger volume of calls and produce much better sound quality. Empress was the forerunner of the modern System X and Y digital switching systems in operation today.