In reponse to this post on LWN.net: Simon - speech activated user interface for KDE (KDE.News):
KDE.News has a look at simon,
which is a speech-activated interface for KDE. It looks like an
interesting project, but, unfortunately, may suffer from some licensing
snags: "HTK, the toolkit responsible for the HMM [Hidden Markov
Model] evaluation is distributed under GPL-incompatible, restrictive
license that prevents redistribution. In order to install simon, one
must separately download HTK from their website which requires
registration. The source is available, [...]
bedhar (Simon developer) replies with:
Of course there are existing speech models.
You could even use speech models created by SPHINX-Train by using a
speech model converter to convert the model to HTK format (there is
such a converter available on sourceforge).
BUT: Speech models created by the HTK can be used _freely_ anyways. You
can create models using HTK and then basically use them for whatever
you want. This is also the reason why the voxforge initiative can build
their speech model using the HTK and still licence the model itself
under the GPL license.
The HTK plain text hmm format is well documented.
You can check out an example here: http://www.repository.voxforge1.org/downloads/Nightly_Bui...
(The file hmmdefs is the HMM model created by the HTK).
For the record: There is an open source initiative called ghmm which
tries to create a GPL licenced library for working with HMM models but
I contacted them and they said they were not ready for this kind of
usage and generally want to be more general-purpose than the HTK so I
am not sure if they will be soon/ever.
Also, the HTK is very high quality software and a good recognition rate is obviously the main goal for any speech recognition software - GPL or not.