oducts being introduced ca- pression technique. pable of connected word recognition. This can separate and recog- More advanced compression algorithms yielding 8:1 compression nise a series of words spoken without significant pauses. For ratios are available, but either the quality is poor or the hardware example, connected word recognition systems allow the caller to cost and complexity is too great for application in multi-channel speak a string of digits to the system. The speech recognition system voice processing system. then has to separate the individual digits in order to recognise them. Even if the SIR system has a recognition performance in the high 37.2.6 Text to speech conversion 90% region individual digits, the length of string that should be attempted is practically limited by the overall recognition accuracy This capability is used to convert computer based text (held in a that may be achieved. For example, if a recognition accuracy greater than 80% is desired and the single word performance is 95%, then database or disk files) into speech for transmission to the caller. a 4 digit string would have a 81% probability of being accurate. Normally the caller hears pre-recorded and replayed speech. How- ever, using this technique to speak the contents of a database may It is also quite difficult to identify which spoken digit is incorrect be impractical because of the huge repertoire involved. and then prompt for that digit's re-utterance by the caller. For these reasons connected digit recognition systems are usually pro- There are two main parts of the text to speech conversion process. grammed to prompt the caller to speak a large number a few (3 or The text is analysed and converted to phonetic equivalents. The 4) digits at a time. phonetic equivalents are then spoken to the caller by a formant voice synthesiser. This device mimics the resonant cavities of the human vocal tract by using variable filters (usually realised in DSPs) and 37.2.8 Word spotting simple impulse excitation of these filters. 