In 1928, a Bell Labs engineer named Homer Dudley was staring at a problem that would define the century: a transatlantic telephone cable could carry only a few voice channels at a time, because each voice hogged thousands of hertz of bandwidth. Dudley asked a heretical question. What if the wire didn't have to carry the sound of the voice at all — only the instructions for rebuilding it on the far end?

His answer became US Patent 2,151,091, "Signal Transmission," filed October 30, 1935 and granted March 21, 1939. It is arguably the most consequential audio patent ever issued.

Dudley's insight was the source–filter model of human speech. When you talk, your vocal cords produce a buzzy pitched tone (or a hiss, for consonants like "sh"), and your mouth, tongue, and lips act as a slowly-changing filter that sculpts that tone into vowels and consonants. The source is fast and information-poor. The filter is slow and information-rich. Dudley's patent proposed sending only the filter shape — a handful of band-energy values updated a few dozen times per second — and reconstructing intelligible speech at the receiver using a locally-generated buzz and hiss. He called it the Voice Coder, shortened to Vocoder.

Bell Labs built a hand-played demo, the Voder (patent 2,121,142), and had a trained operator "play" speech on it at the 1939 New York World's Fair. Crowds heard a machine talk. It sounded uncanny — because it was: no human throat was moving.

Then the war came. The US Army Signal Corps took Dudley's patent and built SIGSALY, a 55-ton system deployed in 1943 that digitized, vocoded, and encrypted the Roosevelt–Churchill hotline using one-time-pad noise pressed onto phonograph records. It was the first secure digital voice link in history. Churchill's cabinet spoke through Homer Dudley's math.

The modern lineage is direct and enormous:

• Linear Predictive Coding (LPC), invented at Nagoya and Bell Labs in the 1960s–70s, is a mathematical refinement of Dudley's source-filter idea. Every GSM, CDMA, VoLTE, WhatsApp, Zoom, and Discord voice call is a vocoder call — usually a CELP variant descended straight from the 1935 patent.
• Auto-Tune (Andy Hildebrand, 1997) is a pitch-shifted vocoder. So is the "robot voice" of Kraftwerk, Daft Punk, Roger Troutman, and T-Pain. Wendy Carlos used a real analog vocoder on A Clockwork Orange in 1971.
• Text-to-speech systems from DECtalk (which gave Stephen Hawking his voice) through Siri, Alexa, and today's neural TTS all rest on the same analysis-synthesis paradigm — separating what is said from how it sounds.
• Modern neural audio codecs like Google's Lyra, Meta's Encodec, and the audio front-ends inside multimodal LLMs are, philosophically, deep-learned vocoders — trained to compress voice into a few kilobits per second by learning a latent source-filter representation Dudley would recognize.

What makes the patent so startling is its epistemology. In 1935, before Shannon's information theory, before the transistor, before the very idea of "digital audio" existed, Dudley proposed that meaning and signal were separable — that you could transmit a description of speech instead of the speech itself. Every compressed voice call and every synthetic voice you've ever heard is a footnote to that idea.