You can freely build on How Early Television Systems Isolated Specific Colors for Special Effects

What this patent covers

This patent describes an analog circuit designed to identify a precise color within a video feed. It works by taking color difference signals and multiplying them by sine and cosine values of a control voltage, which effectively maps the color space to a coordinate system. By using a series of amplifiers, summers, and AND gates, the circuit creates an output pulse only when the incoming video signal matches the specific color selected by the operator. This pulse acts as a switch, telling the broadcast equipment to replace that specific color with another image, which is the fundamental mechanism behind chroma keying.

What is now free to use

All 1 claims of US 3678182 are in the public domain. Specifically:

The 1 dependent claim add narrowing limitations and are also free.

What is NOT covered

Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.

• Does not cover digital image processing or software-based color keying algorithms.

• Does not cover the actual display or projection of the background image.

• Does not cover methods for generating the original red, green, and blue color signals.

• Does not cover non-analog signal processing techniques.

Who is building on this today

While the specific analog circuitry is largely obsolete, the underlying logic of color space manipulation is the foundation for all modern video editing software. Companies like Adobe, Blackmagic Design, and Avid build upon these principles, though they now implement them through digital signal processing rather than discrete hardware components.

Products built on expired version of this technology

How to cite this patent in your documentation

Philips Broadcast Equipment Corp. US Patent 3678182. Chroma key circuit. Granted 1972, expired 1991. Now in the public domain.

Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.

PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.