# 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.

- **Patent:** US 2266401
- **Original title:** Signaling system
- **Owner:** International Standard Electric Corp
- **Granted:** 1941
- **Status:** Public domain (expired)
- **Times cited:** 181
- **Field:** telecommunications, semiconductors

## What it does

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 does NOT 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.

## Real-world examples

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

## 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.

## Frequently asked questions

### What does How Pulse Code Modulation Digitizes Analog Signals cover?

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

### Who owns patent US 2266401?

International Standard Electric Corp owns this patent, granted in 1941.

### When does this patent expire?

This patent has expired and is now in the public domain — anyone can use the invention freely.

### What is patent US 2266401 cited by?

This patent has been cited by 181 later patents that build on its ideas.

### What problem does this patent solve?

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.

### What does this patent NOT cover?

Does not cover the specific hardware circuitry used in modern microprocessors.

**Full plain-English explainer:** https://patentbrief.org/patent/us/2266401/pcm-pulse-code-modulation-reeves

**Original patent:** https://patents.google.com/patent/US2266401

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_Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._


## Related patents

Semantically similar inventions in the PatentBrief corpus:

- [How Digital Audio Compression Works](https://patentbrief.org/patent/us/5579430/digital-encoding-process) — A foundational method for compressing digital audio by transforming sound into spectral data and using variable-length codes to store it efficiently.
- [How Samuel Morse Patented the Electric Telegraph System](https://patentbrief.org/patent/us/1647/morse-telegraph) — Samuel Morse's 1840 patent for the electric telegraph, which enabled long-distance communication by sending electrical pulses over wires to represent letters.
- [How James Russell Invented the Digital Optical Disc](https://patentbrief.org/patent/us/3501586/optical-digital-recording-russell) — A 1966 invention that replaced physical needles on vinyl records with a laser beam reading digital data from a spinning disc.
- [How Cable Modems Fix Signal Distortions Before Sending Data](https://patentbrief.org/patent/us/6665308/apparatus-and-method-for-equalization-in-distributed-digital-data-transmission-systems) — A method for cable modems to pre-filter data so it arrives clearly at the central office, preventing signal errors caused by the messy physical wires between them.
- [How Early Cochlear Implants Used Digital Signals to Restore Hearing](https://patentbrief.org/patent/us/4063048/cochlear-implant-hearing) — A 1977 patent describing an electronic device that converts sound into digital pulses to stimulate the auditory nerve, bypassing a damaged inner ear.