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.
Original patent title: “Signaling system”
A foundational 1938 patent describing how to convert continuous sound waves into a stream of digital numbers for transmission. Granted to International Standard Electric Corp in 1941 with 181 forward citations, and it is now in the public domain.
Key facts
Coverage
What does this patent actually cover?
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.
The gap
What does this patent 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
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.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
Digital telephony (VoIP)
Compact Disc (CD) audio
Pulse Code Modulation in telecommunications infrastructure
Why it matters
The bigger picture
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.
Filed
June 9, 1938
Granted
December 16, 1941
Market context
Who's building on this
Companies in this space
Every major telecommunications equipment manufacturer, including Nokia, Ericsson, and Cisco, builds on the principles established here. The core concept of digitizing signals is now a standard taught in every electrical engineering curriculum worldwide.
Market impact
This patent enabled the transition from analog to digital global infrastructure. It created the possibility for reliable long-distance communication and eventually paved the way for the digital revolution by allowing voice, data, and video to be treated as a unified stream of bits.
Claim 1 — Plain English
What this patent 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.
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.
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.
Patent Journey
From filing to expiry
PatentBrief Score
Impact Score
Moderate
Citation count
40/40
Highly cited
Claim breadth
0/20
Narrow claimsclaimsThe numbered statements at the end of a patent that legally define what the inventor owns.Read more →
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assigneeassigneeThe entity that owns the patent — usually the inventor's employer or a company.Read more →
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$42K – $134K
Midpoint $84K · expired or expiring · industry ×1.4
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
Concepts involved
Citations
Patent lineage
Cite this patent
Harley, R. A. (1941). How Pulse Code Modulation Digitizes Analog Signals (U.S. Patent No. 2,266,401). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/2266401/pcm-pulse-code-modulation-reeves
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
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Common Questions
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.
Same assignee
More from International Standard Electric Corp
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