How Pulse Code Modulation Digitizes Analog Signals

A foundational 1938 patent describing how to convert continuous sound waves into a stream of digital numbers for transmission.

What it covers

This patent describes the process of Pulse Code Modulation (PCM). It works by sampling an analog signal—like a human voice—at regular intervals and assigning a numerical value to the amplitude of the signal at each point. These numbers are then transmitted as a series of pulses, which can be reconstructed back into the original sound wave at the receiving end. This method allows for clearer communication because the digital pulses are easier to filter from noise than a continuous analog signal.

What it doesn't cover

• —Does not cover the specific hardware circuitry used in modern microprocessors.
• —Does not cover analog-to-digital conversion methods that do not use pulse modulation.
• —Does not cover data compression algorithms like MP3 or AAC.

The clever bit

Reeves realized that if you sample a signal frequently enough and represent it as discrete pulses, you can ignore the noise that accumulates between stations, as long as you can still distinguish between the pulses.

Why it matters

This is the bedrock of modern digital communication. Without Alec Harley Reeves' invention of PCM, the internet, cellular networks, and digital audio would not exist in their current form. It shifted the world from analog transmission, which degrades over distance, to digital transmission, which can be perfectly replicated.

Real-world examples

• 1.Digital telephony (VoIP)
• 2.Compact Disc (CD) audio
• 3.Pulse Code Modulation in telecommunications infrastructure