The process of converting analogue signals to digital had been invented back in 1937 by British scientist Alec Reeves. Known as pulse code modulation (PCM) it involves sampling the strength (amplitude) of the analogue signal at intervals and then encoding the sampled amplitude as a binary number which can then be stored, switched and transmitted. Today PCM is at the heart of all digital phone networks, radio, TV, cameras and music. Back in the 60''s the Post Office had begun using PCM digital lines between exchanges as a way of more efficiently carrying multiple phone circuits on one cable over long distances without the noise and signal degradation problems suffered with the older analogue circuits. These inter-exchange circuits known as 'junctions' and 'trunks' had to be converted back to analogue at the distant end - they were simply a point to point wire - any time routing/switching was needed the signal had to be converted back to analogue, switched through an analogue exchange such as a Strowger or Crossbar electromechanical exchange or maybe one of the new reed relay electronic exchanges and then converted back to digital to send it on its way again.

Empress 1968. The worlds first digital exchange

The obvious next step was to design a fully digital exchange so that phone calls could remain in digital format throughout the network, needing only to return to analogue near the customer. Empress was fully digital and fully solid state and proved to be the first switching of live telephone traffic digitally in the world and its success spurred on the development of a production ready digital system which could be installed nationwide to replace not only the electromechanical Strowger and Crossbar systems but also the relatively new reed relay analogue electronic systems too. This system came to be known as System X

The PUS has evolved greatly over the years, as new computer hardware has become available. In 1987 a System X exchange broke the world record for simultaneous call handling capacity under the watch of independent judges. The control software has also evolved over the years, and indeed largely been re-written in C (originally it was written in CORAL) and the real-time operating system replaced with a more modern and advanced off the shelf system LynxOS - a 'Unix-like' real time operating system from LynuxWorks.

The other major subsystem is the Digital Subscriber Switching Subsystem (DSSS) which is the part to which customers lines are directly connected. The DSSS concentrates up to 4096 lines down into between 2-16 digital links each of which can carry up to 30 calls simultaneously.

The first System X local exchange was opened in Woodbridge Suffolk in 1981 and from 1983 onwards installation progressed across the country.

Now the phone line was about far more than having a conversation between two people, System X introduced new services such as 3 way calling, automatic alarm calls, call barring, caller display and ISDN. ISDN (Integrated Services Digital Network) extended the digital network from the local exchange to the customers premesis using a digital line. Conventional phone lines are still analogue between the customer and exchange, even on digital exchanges This weak link in the chain reduces data throughput (see side box on PCM) and introdces noise. ISDN, by using a digital link end to end, offers the full data rate available over the phone network and crystal clear sound. As described in the side box on PCM, each line is 64kbits/sec. ISDN offered the facility to multiplex together 2 or 30 digital lines as one connection giving a variable data rate of 64kbits upto 2Mbits but with the associated cost of 30 simultaneous phone calls! Although ISDN is popular in the radio world for carrying out remote interviews with crystal clear sound, it never gained widespread popularity as a means of computer data transfer, primarily due to its high cost.

Growth of System X was rapid, in just 4 years 55 exchanges were put in place. Early System X roll-out plans from 1983 show Crossbar and TXE4 remaining in use until 2014! The target was to replace all the large Strowger exchanges by 1992 which was achieved . The last Crossbar exchanges were replaced by 1994. The very last being at Droitwich Worcestershire. TXE2 exchanges and the very last Strowger exchanges were gone by June 1995 leaving just the TXE4 large analogue reed relay system operating in some areas. These too were replaced by March 1998 when the whole BT network become completely digital.