Vocabulary independent discriminative utterance verification for non-keyword rejection in subword based speech recognition

A method for use in a speech recognition system to verify whether input speech signals comprising digitized speech represents a probable keyword, said probable keyword being determined by a speech recognizer, said method comprising the steps of:

• processing said digitized speech into recognizer observation vectors;
• processing said recognizer observation vectors in a Hidden Markov Model (HMM) keyword recognizer, said HMM keyword recognizer having output signals representing said probable keyword, one or more subword segments of said probable keyword, and a likelihood score for each of said one or more subword segments;
• developing a plurality of subword verification likelihood ratio scores for said one or more subword segments of said probable keyword by generating two probability density functions (PDFs) for each subword, wherein a first of said PDFs represents a PDF of subword verification scores when said subword is correctly recognized and a second of said PDFs represents a PDF of subword verification scores when said subword is not correctly recognized;
• developing a keyword verification confidence score from said plurality of subword verification likelihood ratio scores for each of said one or more subword segments;
• verifying whether said probable keyword is present in said input speech signals by comparing said keyword verification confidence score to a threshold;
• and
• delivering as an output said probable keyword if said threshold test is met, and delivering as an output an indication that no keyword is detected if said threshold test is not met.