Although his name is little known today, Alec Harley Reeves CBE was one of the most influential telecommunications engineers of the twentieth century. Born in Redhill, he developed ideas that were first used during the Second World War and later became fundamental to modern telephone and digital communication systems.
Alec Harley Reeves was born in Redhill on 10 March 1902, the son of Edward Reeves and Grace Harley. His father was a respected geographer, astronomer and cartographer who worked as an instructor in surveying and later became Map Curator for the Royal Geographical Society.
The family lived at 9 Ridgway Road, Redhill, before moving to Hillside on Reigate Road and later to Stranraer in Rushworth Road, Reigate. Reeves attended Reigate Grammar School. From there he won a scholarship to the City and Guilds Engineering College, followed by postgraduate study at Imperial College London.
In 1923 Reeves joined the International Western Electric Company, where he worked on improving long-distance telephone communication. In 1927 he moved to their new Paris laboratory to continue his work. 
Reliable telephone calls across the Atlantic were still in their infancy when Reeves began his career because analogue voice signals became increasingly distorted over long distances.
Reeves developed an idea that would become known as Pulse Code Modulation (PCM). Instead of transmitting speech as a continuous analogue signal, he proposed sampling the signal and converting it into a series of numerical values before transmission. He patented the system in 1938.
Unfortunately, the idea was well ahead of the available technology. Electronic equipment of the period was too large and complex for the system to be used commercially, and the patent was not exploited for a number of years.
When the Second World War broke out Reeves escaped from France over the Spanish border and then made his way back to England aboard a cargo ship. He turned his skills to military research at the Telecommunications Research Establishment at Malvern as part of a team of scientists and engineers developing Britain's navigational aids.
His first project was OBOE, a radio navigation and bombing aid developed with engineer Francis Edgar Jones.
The system used radio transmissions from a ground station to an aircraft that were pinged back to the ground station. By measuring the ‘time-of-flight’ of the radio signal you knew exactly how far the aircraft was from the ground station. With two ground stations, you could triangulate precisely the position of the aircraft.
Two ground stations were established – one called Cat, near Walmer in Kent, and the other called Mouse, near Trimingham in East Anglia. The pilot would be guided to the correct distance from the Cat station and then fly on a circular arc around Cat using an on-board signal to know if he weas deviating from the path. When he received a signal from Mouse he knew he was the right distance from both ground stations and therefore precisely over the target.
Pathfinder aircraft were specially equipped with the system to locate targets with a high degree of accuracy (within 50 yards) even in poor visibility. They would release marker flares for the main force of bombers to follow.
The system proved highly effective and entered operational service in December 1942. It was widely used by the RAF's Pathfinder Force, particularly by fast de Havilland Mosquito aircraft marking targets for the main bomber force.
Reeves also contributed to SIGSALY, the first practical secure digital speech communication system developed in 1943.
SIGSALY used a voice encoder to convert an analogue voice signal into a digital one using PCM techniques. The digital signal was encrypted when transmitted and then unencrypted when received. The equipment was large by modern standards, occupying an entire room, and was staffed 24 hours a day so it was always ready for use. It enabled senior Allied leaders, including Winston Churchill and Franklin Roosevelt, to hold confidential conversations over long distances.
The invention of the transistor in 1948 finally made Reeves' earlier work practical to implement for everyday use. During the 1950s and 1960s Pulse Code Modulation began to be adopted in commercial telephone networks and later became the standard method of transmitting digital voice signals. It made possible a very robust transmission system with considerable immunity to noise.
The same principles were subsequently used in many other applications, including satellite communications and the telemetry systems of the Apollo Moon missions. Today PCM remains one of the basic technologies underlying all digital storage and communications such as digital radio, compact disc, digital recording and MP3.
Over the course of his career Reeves was credited with around one hundred patents. He was given the Order of the British Empire (OBE) in 1946 in recognition of his wartime work and was promoted to Commander of the Order of the British Empire (CBE) in 1969. In 1965, the Franklin Institute awarded him the Stuart Ballantine Medal for his pioneering work on PCM and in 1969 the Post Office honoured his P.C.M. work with a special postage stamp.
Alec Harley Reeves died at Harlow on 13 October 1971, aged 69. Although much of his career was spent in government research establishments and overseas, his education began in Redhill and Reigate.
All About Circuits: How Alec Reeves Revolutionized Telecom with Pulse-Code Modulation
https://www.allaboutcircuits.com/news/how-alec-reeves-revolutionized-telecom-pulse-code-modulation/
IEE Review 1990, Kenneth Cattermole
https://digital-library.theiet.org/doi/abs/10.1049/ir%3A19900163?download=true
The Battle of the Beams: The secret science of radar that turned the tide of the Second World War, Tom Whipple, Bantam Press (Transworld Publishers) 2023, ISBN-13: 978-1787634138
Image of Mosquito shooting down Junkers Ju-88 (circa 1942). This spectacular series of six stills was taken from a De Haviland Mosquito NF Mk. II while intercepting a Ju 88 over the Bay of Biscay during bad weather.
Image of SIGSALY Speech Encipherment System exhibit - National Cryptologic Museum (2013)