The foundations of BT's completely digital network were laid on 11 September 1968 when the then Postmaster General, John Stonehouse, formally opened for live traffic the experimental Empress digital tandem exchange near Earls Court, London with an inaugural call to the Mayor of Hammersmith at a ceremony attended by representatives of the telecommunications industry, universities, the press and senior members of the Post Office.  In his address the PMG emphasised the significance of the exchange as the first example in the world of switching pulse-code-modulation (PCM) signals carrying live traffic.

The possibilities of PCM systems for the transmission of speech had been known for some 30 years.  The idea was originally developed in Paris in 1937 by A H Reeves working for the Western Electric Company and was first patented in France in 1938.  He proposed a transmission system in which voice signals were electronically coded into strings of digital pulses, transmitted in this form, and then turned back into speech at the receiving end.  His ideas were well in advance of his time, for the techniques discovered could not be realised economically until suitable components, particularly transistors, were available.

Technical advances made during the early 1960s enabled several companies to develop an interest in PCM as a solution to the problem of providing multichannel systems on cables designed for speech networks.  The aim was to increase the number of circuits that could be provided by the existing junction cables.  Conventional analogue transmission meant that only 2 conversations could be carried by 2 pairs of wires at one time; PCM transmission increased this substantially to 24 simultaneous conversations by interleaving the groups of pulses corresponding to different callers (Time Division Multiplexing), reducing the need for many new cables.  Other advantages of PCM transmission are that it is free from interference suffered with audio cables, and the use of regenerative repeaters at intervals along the line to amplify the signal allows the  transmission of speech over considerable distances without distortion.  PCM transmission also allows a greater diversity of telecommunications services in addition to telephony, including facsimile and data transmission.

Prior to the opening of Empress exchange, PCM techniques had been in operational use since the previous year in selected areas on junction routes linking different exchanges, but where there was no direct route the existing network included tandem exchanges for switching junctions.   When using PCM transmission this switching was still done by conventional electromechanical means.  The calls were transmitted digitally, but had to be converted to analogue before they could be switched, and then changed back yet again into digital form for transmission over the next PCM system.  The equipment needed for this rather inefficient process was expensive, so that it was uneconomic on routes of less than 10 miles, and the method also detracted from the improved quality of speech which PCM had produced.  PCM had potential to enable much of the growth in demand for circuits to be catered for on the existing system, but the tandem switching was a major disadvantage.

Post Office research scientists were in the forefront of projects to overcome this problem.  A small team began working on the solution early in 1965.  They first designed a model of a digital tandem switching centre to test the feasibility of switching traffic by digital means, and to be a skeleton for a much larger exchange to be used for field trials.  When experiments were complete the model was demonstrated to the press at the Post Office Research Station, Dollis Hill, on 15 February 1968.   In April the experimental exchange was moved for a trial under field conditions to the site of Empress exchange (see below) in West Kensington to switch calls originating from nearby exchanges:  Acorn (01-992), Ealing (01-567) and Shepherd's Bush (01-743).

The particular significance of Empress was that it was the first of its type in the world to switch PCM signals from one group of lines to another in digital form.  The field trial was essential to establish that an integrated PCM transmission and switching system was capable of working fully within the existing network of electromechanical (Strowger and Crossbar) systems and of carrying live traffic.  It had also to provide indications of the technical possibilities and the financial implications of a more widespread use of digital switching.  These objectives were clearly achieved as the system was the origin of a total digital service and the integrated digital network to which British Telecom is now committed.  This first use of advanced computer-like technology with micro-electronic circuits was part of a deliberate policy to exploit  technological innovation to the full and has led directly to the development of the System X family of Digital Switching Systems which is revolutionising today's network and the services offered to British Telecom's customers.

[This article was kindly contributed by the BT Archives and Historical Information Centre]


Feedback from Barry Ellis

I read with interest the history of the Empress PCM Digital Tandem Exchange that was located in Warwick Avenue, West Kensington, London SW5. I worked in that exchange and was a TTA (Trainee Technician Apprentice) reporting to the TO (Steve Farr) who was responsible for the field trial/maintenance. Just a point that needs rectifying the AFN code for Empress was 01-603 and not 01-367 as stated in the script [now corrected above].  

Bit of info on the TXS: "Empress" 01-603 was a 4000 type Director exchange with level 1 on the incoming N1st graded to NOBle 01-602 N1st. All subscribers shared the short holding A digit selector,Director and Local registers (STD). ie Director translations from other exchanges had an extra digit (1) inserted in order to route the call to 602 equipment. For example say the translation routing code from Earls Court 01-373 to Empress 01-603 was 82....the translation for 01-602 would have been 821.  Both exchanges had their own graded C1st but shared the C2nd/C3rd and OGJ's (Outgoing junctions). This was known as an alpha beta exchange.   It was a sad day when the exchange was modernised to a GPT System X Double cluster Processor site. I was actually a Level 2 manager who witnessed the change-over. Several of the staff remaining wore black arm bands that day!  

I do hope that you enjoyed this little bit of info....sorry if I went on a bit!...could even draw the exchange trunking diagram to this day....Can tell you a lot about Chelsea (FLAxman) 01-352...that's where I became Level 1 (AEE) and OIC of the building prior to its demise! Still that's another story.

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