Overview
Alan Dower Blumlein (1903–1942) filed a patent in 1931 covering binaural sound so comprehensively that it still describes how stereo recording works: coincident-pair microphone technique, the shuffling circuit, and the 45/45 method of cutting two channels into one groove. He also worked on the EMI television system and on H2S radar, and died in a crash while testing it. He held 128 patents by 38.
What You Need
- Lived: 1903–1942, Britain
- Anchor year: 1931 — the binaural sound patent
- Strand: overlooked — killed young, then classified
Steps
The problem as it stood
Recorded sound was monophonic and gave no sense of where anything was. It was not obvious that spatial information could be captured and reproduced at all, let alone through a single groove.
What he actually did
Blumlein's 1931 patent set out a coincident pair of directional microphones at an angle, capturing direction as intensity difference; a shuffling circuit to convert between sum-and-difference and left-and-right; and the 45/45 groove, cutting two channels into the two walls of a single groove at right angles.
How it worked
Placing both microphones at effectively the same point means direction is encoded purely as level difference, with no timing difference — so the result stays coherent when summed to mono, which mattered enormously for broadcast. The 45/45 groove is equally elegant: the two walls of one groove carry two channels, and a mono stylus reads their sum. Both are still in use.
What it made possible
Stereo, as actually implemented from the late 1950s. The 45/45 system became the stereo LP standard; coincident-pair remains a standard classical recording technique. He also contributed substantially to the EMI 405-line television system.
What happened to him
He died on 7 June 1942 when the Halifax bomber he was flying in, testing H2S airborne radar, crashed. His death was not announced because the work was secret, and the secrecy delayed recognition of his contribution for many years.
Where the credit landed
Blumlein's 1931 patent set out the coincident-pair technique, the shuffling circuit and the 45/45 stereo groove, and stereo was not commercially implemented until the late 1950s — by which time he had been dead for fifteen years. His work was assigned to EMI, his death was concealed for wartime reasons, and his radar work stayed classified. Assignment plus secrecy plus dying at 38 is close to a worst case for a reputation.
Pro Tips
- Coincident-pair encodes direction as level difference, so it stays coherent in mono.
- 45/45 puts two channels in one groove — still the stereo LP standard.
- He filed the patent 27 years before stereo records were sold.
Knowledge Base
What You'll Learn
The most complete single patent in the history of recorded sound, filed before anyone wanted it.
Why mono compatibility drove the design
In the 1930s and for decades after, most listeners were mono. Any stereo system that sounded wrong when summed was commercially dead. Coincident-pair produces no timing differences between channels, so summing causes no phase cancellation; the 45/45 groove sums to a lateral cut a mono cartridge reads correctly. Blumlein designed for a transition he did not live to see, and the constraint produced the more elegant engineering.
Where This Fits
This guide covers one specific part of the history of media technology. The wider picture — how each link in the chain from capture through transmission to display was actually built, who built it, and why the credit so often landed somewhere else — is in A History of Broadcast Technology: The Chain From Capture to Screen, which frames the discipline as a whole and links out to the detailed guides underneath it, including this one. If you are starting from scratch rather than solving a specific problem, read that first and come back here.
FAQ
Q: Who invented stereo sound?
A: Alan Blumlein set out the essentials in a 1931 patent — coincident-pair recording, the shuffling circuit and the 45/45 groove. Stereo was not commercially implemented until the late 1950s, fifteen years after his death.
Q: What is the 45/45 system?
A: Cutting two audio channels into the two walls of a single record groove, each at 45 degrees to the surface. It became the stereo LP standard and remains so.