A trade secret that can be reverse engineered by a tinkerer with an oscilloscope in an afternoon.
This is more or less the type of situation patents were invented for: a simple invention anyone can copy once it's invented, but difficult (I assume) to invent in the first place.
It's why I don't like most software patents, but why I think codecs, especially the modern ones, should be patentable. They're complicated engineering challenges requiring you to make numerous tradeoffs and I feel that just because the result is an algorithm doesn't mean that it shouldn't be patentable.
Other software patents I'm more dubious of, but I feel comfortable saying H.264 should be patentable.
Totally agree. However in many cases, the main technology is invented at universities with public funding. Also, codecs are a means of communication, and I don't think it should be patentable because it can cause problems once everybody settles on a single standard. See the MPEG nightmare, where professional cameras have a license attached to any movie shot with them.