Philips Tech. Rev. 42, No. 10/ II /12,361-373, Sept. 1986 361 Half a century of 'electronification' in telephony systems J. F. Brouwer TELEPHONE TRANSMISSION SYSTEMS The vast progress in electronics in countless fields of application has led to striking results. It is certainly Background no mere chance that the thermionic valve (or electron Not long after the invention of the triode by Lee De tube) was first used for wireless telegraphy. The inven- Forest in 1906, electronics entered the world of tele- tor of the diode (Sir J. A. Fleming) and the inventor phony in the form of repeaters. As long ago as of the triode (Lee De Forest) were closely involved in 25 January 1915 the first transcontinental telephone this branch of electrical telecommunication, which circuit came into operation in America: a route some was then in its infancy. 5000 km long with overhead copper wires fixed to Later, the cross-fertilization between electronics poles, incorporating repeaters at three points [11. and telecommunication led to wireless transmission of speech and music in radio communication and radio broadcasting, two related fields that gave rise to great growth and expansion in the electronics industry. This article is mainly concerned with another form of cross-fertilization: between electronics and line- dependent telephony systems. Without electronics it would not have been possible to span the Earth with underground and submarine cables for telephone transmission. Modern digital telephone exchanges controlled by special telephony computers would also be inconceivable without the use of the most ad- vanced electronic components and modules. An interesting aspect here is the long time it has taken to introduce electronics into the most important telephony functions. Although the first electronic telephone repeaters were in use before World War I, it is only in the last fifteen years that electronics has really penetrated into the heart of the telephone ex- change. Because of the great complexity of the switch- ing operations and the reliability already achieved with electromechanical systems, electronics had not previously had much to offer here - and there was also the question of price. In this article transmission systems will be con- sidered first, then switching systems and the review Fig. 1. At first, telephone connections were almost always made concludes with a look at the near future. Much space with overhead lines. Because there were so many wires, the result has intentionally been given over to developments at was not always particularly attractive. The picture shows a street scene in one of the larger cities in the United States in the early days Philips, although the treatment must necessarily be of telephony (about 1880). incomplete in range and detail. [IJ More than 1000 tons of copper wire were used in the link; the number of repeaters was later increased to eight. See M. D. Ir J. F. Brouwer was a Director of Philips' Telecommunicatie Fagen (ed.), A history of engineering and science in the Bell Industrie B. v., Hilversum, and of the former Philips Telecommuni- System; the early years (1875-1925), Bell Telephone Labora- cation and Defence Systems Division, before his retirement. tories 1975, p. 262. 362 J. F. BROUWER Philips Tech. Rev. 42, No. 10/11/12 With the rapid growth in telephone traffic, however; over thesame pair of wires. The only fundamentally it very soon became necessary to use underground correct solution to this problem was the four-wire cables, first for the urban networks and subsequently circuit (fig. 2). Connections of this type were desirable for the other circuits as well. This resulted in less because of their low overall attenuation, but they interference from induction, fewer interruptions and were also expensive, so that the telephone companies less maintenance but - inevitably - also in a lower wanted to use them for more than one telephone call signal strength at the end of the line. Although the at a time. The most appropriate principle for this was intrinsically higher attenuation of telephone cables carrier telephony, a form of frequency-division multi- could be significantly reduced by artificially increasing plexing (FDM); see fig. 3. Such systems were first used their self-inductance - as demonstrated by Pupin in expensive submarine cable links and long overhead and Krarup - it could never achieve such low values routes, then later in both coaxial and multi-wire (sym- as those attainable with the overhead systems using metrical) underground cables as well. 0" ,. ., Q J ----~.B>......____ ,.B>>--------~~ ~L -«~}- _ ~'""'..I---- r Fig.2. a) In two-wire circuits both telephone signals for each telephone call are transmitted over only one pair of wires. The signals from the two directions are only separated, amplified and recombined at repeaters (A), if installed. Separation and recombination take place in 'hybrids' (>-). b) In a four-wire circuit hybrids are only found at the two ends; two wires are available for the signals in both the go and return paths for the entire length of the circuit. (N.B. one continu- ous line in this diagram represents two wires.) wires a few millimetres thick (overhead lines, fig. 1). SSB r-, ~. ~: ................~.' a' After 1920, therefore, an ever-increasing number of L _ low-frequency ('voice-frequency') repeaters were used SSB on both international and long-distance national trunk circuits. Typical distances between repeaters were 100 SSB to 200 km, depending on the diameters of the wires used in the cables. For the time being, however, over- head lines continued to be used for very long circuits. It was soon found that transmission circuits incor- ~COM porating repeaters had to be very carefully dimen- sioned. Excessive gain could lead to oscillation or 'singing' (negative feedback had not yet been in- vented); too many repeaters connected in cascade o 4 8 12 16 2024kHz caused echo effects, and the longer the line the more -f annoying these became. Excessive differences in signal Fig. 3. Carrier telephony is based on frequency-division multiplex- strength in neighbouring pairs of wires led to un- ing; a number of telephone signals are displaced to different posi- acceptable interference between one channel and an- tions on the frequency scale by single-sideband modulation with a suppressed carrier (SSB). They are then combined (COM) into a other (crosstalk). Many of these problems were caused single multiplexed signal. An arbitrarily chosen example is shown here for three signals (a, b and c), each with a bandwidth of 4 kHz, by the use of the traditional two-wire circuits in which which produce a single multiplexed signal (a'b'c') extending from telephone signals were transmitted in two directions 12 to 24 kHz. Philips Tech. Rev. 42, No. 10/11/12 TELEPHONY SYSTEMS 363 The year 1936 was marked by extensive pioneering Table I. Internationally recommended hierarchy of carrier systems activities in this field: the first coaxial routes were laid for telephony. from New York to Philadelphia and from Berlin to Number of Basic frequency Leipzig. In the Netherlands, the Dutch PTT Adminis- channels position JkHz) tration brought its own 12-channel symmetrical cable Group 12 60-108 system into use between Groningen and Leeuwarden. Supergroup 60 312-552 .Extending for 60 km, this route was one of the first Mastergroup 300 812-2044 object lessons for the small Philips Transmission Supermastergroup 900 8516-12388 Group of those days. It was to be almost fifty years before it would receive an order for the longest digital coaxial link in the world (see page 366), which, in- when they were purchased. Such an increase in band- cidentally, made use of a principle now also nearly width can be seen in the symmetrical carrier cable, half a century old: Reeves's pulse-code modulation in which the number of channels was extended to 120 (PCM), dating from 1938. in about 1962. Since not every type of cable had the same trans- mission capacity, and the number of channels required Transmission activities in Philips - the start varied very widely from connection to connection, the After it was set up at the start of the thirties, the need arose for carrier systems with varying channel activities of the Transmission Group at Philips were capacities. This led to international agreements about initially confined to voice-frequency amplifiers and a hierarchy of carrier systems: in a number of suc- Pupin coils. The company offered fertile soil for this: cessive modulator stages twelve channels are first knowledge in the field of thermionic valves was avail- combined or 'multiplexed' (fig. 3) to form a 'group' able (the pentode patent and the negative-feedback with a fixed basic position on the frequency scale. patent), and use could also be made of research Next, five groups are multiplexed to form one super- results in the field of magnetic materials. Even before group, etc. (Tablel). When the highest required level World War 11a 17-channel carrier system was devel- in the hierarchy has been reached, the signal is con- oped for which the first export order was obtained in verted by a final modulation operation from its basic 1938 (from Sydney to Maitland, in Australia). frequency position to the frequency position ulti- After World War 11the transmission activities were mately required on the cable. Through the years, continued at Philips Telecommunicatie Industrie in Philips have developed a virtually complete hierarchy Hilversum. Development and production centres in of analog transmission systems for use on overhead this field also gradually grew up in Philips plants in lines, cables and microwave links. The modular struc- other countries. Now let us look at what has happened ture meant that, for example, the same channel and in Philips since those times, particularly as regards group modulation equipment was used in a 120-chan- analog, digital and optical transmission systems for nel system as in a system for 10 800 channels.