Such systems can be classified as to whether they require the user to "train" the system to recognise their own particular speech patterns or not, whether the system can recognise continuous speech or requires users to break up their speech into discrete words, and whether the vocabulary the system recognises is small (in the order of tens or at most hundreds of words), or large (thousands of words).
Commercial systems for speech recognition have been available off-the-shelf since the 1990s.
Systems requiring a short amount of training can (as of 2001) capture continuous speech with a large vocabulary at normal pace with an accuracy of about 98% (getting two words in one hundred wrong), and different systems that require no training can recognize a small number of words (for instance, the ten digits of the decimal system) as spoken by most English speakers. Such systems are popular for routing incoming phone calls to their destinations in large organisations.
However, it is interesting to note that despite the apparent success of the technology, few people use such speech recognition systems.
It appears that most computer users can create and edit documents more quickly with a conventional keyboard, despite the fact that most people are able to speak considerably faster than they can type.
Additionally, heavy use of the speech organs results in vocal loading.
Some of the key technical problems in speech recognition are that:
The "understanding" of the meaning of spoken words is regarded by some as a separate field, that of natural language understanding. However, there are many examples of sentences that sound the same, but can only be disambiguated by an appeal to context: one famous T-shirt worn by Apple Computer researchers stated:
A general solution of many of the above problems effectively requires human knowledge and experience, and would thus require advanced artificial intelligence technologies to be implemented on a computer. In any system it can suffice to use knowledge from linguistics to interpret the speech.