How Alec Reeves Revolutionized Telecom With Pulse-Code Modulation

British engineer Alec Harley Reeves is best known for being the inventor of pulse-code modulation (PCM), a telecommunications technique that made the digital information age possible. 

British engineer Alec Reeves. Image used courtesy of High Fidelity

Reeves dedicated his relatively short life to the telecommunications sector. Born in 1902 in Redhill, Surrey, he earned his electrical engineering degree from the City and Guilds Engineering College, London, in 1918. Over his lifetime, he was awarded nearly 100 patents. He passed away in 1971 from cancer and kept up with research until the very end, with fiber optical systems being one of his last investigations. 

Below is a spotlight on historical engineer Alec Reeves. 

From Telegraphy to Telephones

After his post-graduate studies, Reeves joined the International Electric Company in 1923 to work on transatlantic telephone systems. During this era, telegraphy was one of the primary ways to achieve reliable long-distance communication and was important to global business and politics. However, there was anticipation about how cross-oceanic telephone links could connect people worldwide.

While the IEC did succeed in establishing a transatlantic telephone link between the U.S. and the U.K., the telephone suffered from a feature that telegraphy did not. The analog voice signals would pick up noise and distortion while being transmitted through transatlantic cables, and while repeaters helped propagate the signals, the repeaters would also amplify the noisy distortion on the signal, too. 

The first official transatlantic telephone call in 1927 was made between the presidents of the American Telephone & Telegraph Company and the secretary of the General Post Office of Great Britain, W.S. Gifford and Sir Evelyn P. Murray, respectively. The recording occurred in the U.S., and the audio received in the U.K. suffered from notable distortion. 

The Invention of PCM

It was these analog noise problems on telephone lines that inspired Reeves. In 1938, Reeves submitted a patent titled “Electrical Signaling System,” which described a way to transmit “complex wave forms, for example, speech, which are practically free from any background noise.” 

Ultimately, this patent described pulse-code modulation, which involved taking discrete samples of an analog signal, quantizing it so that each sample was assigned an integer value, and then transmitting the binary of that value. This method was more in line with how telegraphy worked and theoretically would provide the same level of clarity and reliability.

Image used courtesy of the Institution of Engineering and Technology

Unfortunately, the technology of the era did not make PCM commercially viable, and so even with the patent, PCM would go unadopted for a number of years.

World War II Accelerates Comms Techniques 

As with many technological advances, the urgency of war proved to be an opportunity for PCM. Reeves used his skills to support the Allied effort in the war, first with the development of the bomb-aiming system Oboe and then with the secure speech system SIGSALY. 

Oboe was developed in 1942 and used two stations, one called Cat and the other Mouse. Oboe determined the relative distance an aircraft could receive and transmit a response back to the stations. Using time-of-flight, the stations could determine the location of the aircraft as well as the distance from a bombing target. Within the aircraft, the pilot could see the direction of the target. 

Oboe presented several significant improvements for war-time navigation systems: greater accuracy, greater immunity to jamming since signals were being transmitted over VHF (which was more difficult to jam), and robustness to weather conditions.

PCM would shine with the introduction of SIGSALY in 1943. SIGSALY was not an acronym and more or less a non-sensical code name for the project. Secure communication was vitally important for the Allied forces to coordinate. Both Allied and Axis forces employed scientists to work on decrypting each other’s communication, among them Alan Turing. 

SIGSALY used a vocoder (voice encoder) to convert an analog voice signal into a digital one using PCM techniques. The digital signal was encrypted when transmitted and then unencrypted when received. The SIGSALY system was based on vacuum tube technology and consisted of 30 racks of 19” high equipment. 

The SIGSALY system. Image used courtesy of the Crypto Museum

SIGSALY was considered the most secure method of voice communication during this era and was used by Churchill and Roosevelt. SIGSALY was instrumental to coordinate the Normandy invasion, as well as the Yalta conference. 

The Transition of PCM to Civilian Use

The invention of the transistor in 1948 finally presented PCM with the opportunity to move from the domain of specialized military application to everyday use. The first civilian use of PCM was in Bell Labs' telephone systems in the early 1950s. The first commercial telephone use of PCM would follow in 1964.

PCM was also used in the communication system of the Apollo program, which put the first human on the surface of the Moon in 1969. The Apollo spacecraft used PCM to transmit its telemetry to ground systems. 

PCM is foundational to digital communications today. Reeves believed that someday telecommunication systems would allow people to transmit the knowledge in their brains without having to physically relocate their bodies. The tools we use for remote work today trace their foundations to PCM, whether it be video conferencing or voice chat.