During the Second World War, Brisbane played a vital role in one of the most extraordinary technological achievements of the 20th century: secure global voice communication. At the heart of this breakthrough was a system known as SIGSALY—a top-secret project developed by the United States to ensure that conversations between Allied leaders could not be intercepted by enemy forces.
The birth of secure digital voice communication
SIGSALY was the world’s first digital voice encryption system. Developed by Bell Telephone Laboratories and operational from 1943, it enabled secure, real-time voice conversations between high-ranking Allied commanders and political leaders. This was long before the age of digital computers or the internet, making it a remarkable technological milestone.
The system worked by converting voice signals into digital code using a method called pulse-code modulation (PCM)—a concept that still underpins modern digital audio. These digital signals were then encrypted using random noise stored on synchronised phonograph records, creating a secure one-time pad encryption system that remains unbreakable even by today’s standards.
Brisbane: a critical node in the SIGSALY network
Only twelve SIGSALY terminals were built and deployed globally during the war, and Brisbane was one of those elite locations. The others being: Washington, San Francisco, London, Algiers, Paris, Manila, Guam, Hawaii, Ottawa, New Delhi, Pentagon. The decision to install a SIGSALY terminal here reflects the city’s status as the headquarters of the South West Pacific Area (SWPA) under General Douglas MacArthur. Ensuring secure communication between MacArthur and President Roosevelt in Washington, D.C. was of the highest priority.
The Brisbane terminal was likely located within a high-security U.S. military communications facility in the city centre, possibly in or near the AMP Building on Queen Street, which hosted several key Allied communications units during the war.
How it worked
The SIGSALY system in Brisbane transmitted encrypted voice signals via shortwave radio to a receiving terminal in San Francisco. From there, the signal was relayed via landline to Washington. The entire process allowed senior commanders to speak in near real-time, securely, across thousands of kilometres—a feat unheard of at the time.
Each SIGSALY system was enormous—filling a room with over 40 tonnes of equipment, including vacuum tube amplifiers, turntables, modulators, and racks of wiring. The systems were operated by highly trained personnel who ensured exact synchronisation between the encryption records at both ends of the call.
Never broken, never compromised
Despite Axis efforts to intercept and decode Allied communications, SIGSALY transmissions were never successfully broken. The system’s one-time pad method ensured complete secrecy, making SIGSALY a cornerstone of Allied wartime strategy.
Its existence remained classified for decades and was only declassified in the 1970s, revealing the extent to which Allied command had invested in secure communication technologies.
A legacy of innovation
SIGSALY’s legacy is immense. It laid the foundations for modern digital voice technologies, including secure military communications, encrypted satellite links, and even everyday applications like VoIP. It also marked the beginning of digital encryption, a field that continues to shape our world today.
Brisbane’s inclusion in the SIGSALY network highlights the city’s critical importance to Allied operations in the Pacific. It also complements the story of Camp Columbia in Wacol, which served as the headquarters for Dutch and American forces during the war and as a key base for military planning, coordination, and communications.
The name
SIGSALY was a codename, not an acronym—so it doesn’t officially “stand for” anything in the way that acronyms like RADAR or NATO do.
However, the name has an interesting background:
- It was originally called the “X System” or “Project X” during its top-secret development at Bell Labs.
- Later, it became known as “SIGSALY”, which was a random code word assigned by the U.S. Army Signal Corps.
- The “SIG” part loosely refers to signals, as in “Signal Corps”, but the name itself was deliberately meaningless to obscure the project’s purpose.
Some of the engineers jokingly said “SIGSALY” stood for “SIGnal Army SYstem”, but this was never official—just internal humour. The U.S. military often used such arbitrary codenames to maintain secrecy.
See: also Wikipedia
Sources:
David Kahn – The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet (1996 edition)
James Bamford – The Puzzle Palace (1982)