To complete this filter arrangement the fricative (hissing) part of the speech output is Word spotting systems continually examine callers' speech in an formed from a filtered and amplitude controlled noise generator. attempt to recognise an embedded word. This approach has the There are usually five formant filters which model the human advantage that it may be able to find one of the words in an voice tract and as few as three can be varied and still achieve unstructured reply by the caller to a prompt. For example, the reasonable speech quality. system might prompt the caller to say "yes" or "no". If the caller The process of translating from text into a phonetic representation replies "Well, yes, then". a normal speech recogniser would not find is well developed. Some text-to-speech converters are capable for the embedded "yes", whereas a word spotting system would. pronouncing over 100,000 words correctly, and can use the context of the words to derive the correct pronunciation. 37.2.9 Speaker verification (voice printing) Subsequent prosodic processing of these phonetic repre- sentations, introduces the intonations, inflexions and the rise and This is a system where the voice print of a caller is taken in an fall of the volume of the speech to make the output more acceptable. enrolment session and subsequently used to authenticate the caller's identity. The caller is prompted to speak a word or phrase which is However, to date the output quality achieved by this process still then compared with the samples taken during the enrolment session. has a pronounced robot like quality, and as a result this capability is Speaker verification system performance is classified in two not widely substituted for replaying pre-recorded speech. ways. The percentage of false acceptances and the per centage of false rejections. The system is usually biased to reject rather than 37.2.7 Speech recognition accept people. Systems with a performance of a few per cent false rejections, and a fraction of a per cent false acceptance