US Patent Number 4701722 A is a perfect example of everything software patent opponents hate about software patents:
- It implements mathematical functions that are pretty well known.
- It covers a process of changing information not changing any physical object one could touch.
- It is owned by a company with a business model depending on licensing fees, not manufacturing things: What many people would call a “patent troll”
US 4701722 A is is one of the basic patents of Dolby B noise reduction awarded to Ray Dolby 
back in 1987. It’s not a software patent, it is a circuit patent. The usual arguments about why software should not be subject to patenting make little sense. If a circuit can be patented, then it should be possible to patent a program. The beginning of the study of computation as a mathematical subject was the discovery that a simple general purpose engine reading instructions from some sort of store can emulate any circuit. The early computers were “programmed” by physically moving wires around. The photo below shows ENIAC being programmed. A “program” then was obviously a circuit design. Technology advanced so that these circuits could be stored in memory and on disk drives. But that did not change the basic process – writing software is conceptually the same as wiring up an electrical circuit.
And around the same time Nyquist/Shannon showed that analog and digital signals could be equivalent to each other. Ray Dolby knew, and noted in this patent, that the analog signals transformed by his invention could be transformed into digital information and transformed back – as convenient. If there is an intrinsic flaw in the concept of software patents themselves, something fundamentally distinct in software that makes software inventions impossible, then the critics of software patents have failed to explain what that could be. See also Martin Goetz which I found via the anti-software patent arguments (1 and 2) of Tim B. Lee (who is not Tim Berners-Lee).