In October 1938, a British engineer working at ITT's Paris laboratory quietly filed a patent that described, in complete detail, how to turn any analog signal — voice, music, video — into a stream of numbers, transmit those numbers, and reconstruct the original signal at the far end with essentially zero cumulative noise. He called it Pulse Code Modulation. His name was Alec Harley Reeves, and the patent was French Patent 852,183, followed by US Patent 2,272,070 (filed November 1939, granted February 1942) and British Patent 535,860.

The idea is the one that runs every phone call, streaming service, Blu-ray disc, and Zoom meeting today:

• Sample the analog waveform at regular intervals (Nyquist's 1928 theorem said how often).
• Quantize each sample to the nearest value from a finite set.
• Encode that value as a binary pulse train — literal 1s and 0s sent as on/off pulses on a wire or over the air.
• At the receiver, decode back to samples, then smooth the samples back into a waveform.

Reeves's patent drawings show exactly this: a sampler, a quantizer producing 5-bit codes, a serial pulse transmitter, and a decoder at the far end reading the pulses off a rotating commutator. He even understood the killer feature: because a receiver only has to distinguish pulse present from pulse absent, noise accumulated along the line can be regenerated away at every repeater. Analog signals degrade with distance. Digital ones don't.

The problem was that nothing on Earth could actually run it. A telephone-quality PCM signal needs about 64,000 bits per second. In 1938, the fastest practical electronics were mechanical commutators and slow vacuum tubes. Reeves built a proof-of-concept using gas-discharge tubes, but a real PCM telephone system would have required thousands of tubes running hot enough to melt themselves. The patent sat on the shelf.

Then the transistor arrived (1948), then the integrated circuit (1959), and suddenly Reeves's 1938 blueprint became buildable. Bell Labs' T1 carrier system (1962) — the first commercial digital telephone trunk line — is Reeves's patent implemented in silicon, 24 voice channels multiplexed as PCM at 1.544 Mbit/s. That same T1 line rate is still the atomic unit of telecom billing today.

From there PCM ate the world:

• Compact Disc (1982): 16-bit PCM at 44.1 kHz — Reeves's algorithm, just with more bits.
• Every WAV file, every AAC stream: AAC is PCM with a perceptual codec bolted on top; the underlying samples are Reeves.
• GSM, VoIP, Zoom, FaceTime: all sample-quantize-encode at the microphone.
• HDMI, DisplayPort, Ethernet: the "regenerate at every repeater" insight is what makes gigabit signals survive noisy copper.

Reeves lived until 1971 and watched his patent become the foundation of the digital age. He also spent his later career at Standard Telecommunication Laboratories in Harlow, where — in a delightful bit of continuity — his colleague Charles Kao would win the 2009 Nobel Prize for figuring out how to send Reeves's PCM pulses down glass fiber instead of copper wire.

Reeves's patent is the rare filing that was fully correct and completely unbuildable at the same time. The math was right in 1938. The transistors just hadn't been invented yet.