Riddle of the Sphinx: What Do I Need to Recognize Speech?
The most well-known open source speech recognizer is Carnegie Melon's Sphinx project, called CMUSphinx. However, Sphinx is a large and mature project, and as a primarily academic tool, it has many different experimental versions lying around. Just reading the project descriptions confuses me!
What do you need from the Sphinx suite? What is the best set of tools for your specific task? Here's a quick map to help!
1) A Trainer
C'mon get on the SphinxTrain! No matter what recognizer (also called a decoder) you choose from the list below, you must use SphinxTrain to set up your language data, called training your recognizer. SphinxTrain is written in C but also requires a few Perl scripts. I'm not sure why, but there is a directory for Python code in the source tree too. I'll look into that further later.
2) A Recognizer
This is where it gets really confusing. There are really only three choices, with a fourth special-case choice that seems unlikely.
2a) sphinx3 (C/Perl)
Sphinx3 is basically what you're going to want in most applications and experiments. It is the most mature, most used, and (I'm guessing) best supported version of the tool. It's written in C and has Perl script requirements. From my early readings in this area, it looks like when people say "CMUSphinx" or "Sphinx," they really mean "sphinx3."
2b) sphinx4 (Java)
Sphinx4 is sphinx3's little brother, growing up strong and fast, and written in Java. Sphinx4 wants to at least have feature parity with sphinx3, and is at least as fast in most aspects, a tad slower in others, and faster in still others. I'm not really sure why you would want to choose sphinx4 over sphinx3, unless you or your platform are more comfortable with Java instead of C. At this point sphinx4 is still seeing active development, whereas sphinx3 seems to have cooled just a bit. This is all anecdotal from the Subversion logs, though, so I may be wrong.
2c) PocketSphinx (embedded)
If you're wanting to use speech recognition on an embedded system like a handheld tool, mobile phone, automobile dashboard appliance, or other small form-factor computer, you'll want to use PocketSphinx, sphinx3's baby brother.
2d) sphinx2 (old-school)
Sphinx2 is more optimized for speed than accuracy and is not currently seeing much development. Everyone recommends you go to sphinx3 instead. Sphinx2 is still around though for a few experiments where blazingly fast real-time recognition is needed for tiny grammars.
Summary
So, you'll definitely need SphinxTrain, no matter what. After that, you'll most likely choose sphinx3 as the recognizer. I think this would put you in the largest majority of Sphinx users. However, sphinx4 is up and coming, and is a strong contender to overtake sphinx3 someday. If you're doing embedded work, use PocketSphinx, and in almost all cases you can ignore sphinx2.
Let me know which recognizer you use and for what projects!