20. Apparatus as defined by claim 18 wherein said candidate feature array formation means is operative to form a candidate feature array for each of the following: a command word candidate consisting of each individual speech segment considered as a one segment word; a command word candidate consisting of each individual speech segment together with the previous speech segment, if any, considered as a two segment word; a command word candidate consisting of each individual speech segment together with the previous two speech segments, if any, considered as a three segment word; and a command word candidate consisting of each individual speech segment together with the previous three speech segments, if any, considered as a four segment word.
20. Apparatus as defined by claim 18 wherein said candidate feature array formation means is operative to form a candidate feature array for each of the following: a command word candidate consisting of each individual speech segment considered as a one segment word; a command word candidate consisting of each individual speech segment together with the previous speech segment, if any, considered as a two segment word; a command word candidate consisting of each individual speech segment together with the previous two speech segments, if any, considered as a three segment word; and a command word candidate consisting of each individual speech segment together with the previous three speech segments, if any, considered as a four segment word. 22. Apparatus as defined by claim 20 wherein said string matching means is operative to identify, at each segment of a string, the sequence of vocabulary words which provides the highest overall correlation up to that particular segment of the string when considering said particular segment and all previous segments of the string as being part of sequences of different word lengths.