1. Apparatus which receives spoken input training words and a subsequent spoken input command word and generates a correlation function that is indicative of the resemblance of the command word to each training word, comprising: a. feature extraction means for processing received input words and generating digital feature output signals on particular ones of a number of feature output lines, the particular ones being dependent upon the features present in an input word; b. means for storing, as a time dependent matrix, the presence or absence status of the feature signals which occur during each training word; c. means for storing, as a time dependent matrix, the presence or absence status of the feature signals which occur during the command word; and d. means for comparing, member by member, the command word matrix with each training word matrix and for generating a correlation figure which reflects each matrix comparison, the member comparisons taking account only of the presence of features in the matrices being compared.