15. A computer-readable storage device having instructions stored which, when executed by a computing device, cause the computing device to perform operations comprising: generating a feature coefficient based on a speech signal from a user; comparing the feature coefficient to a user-specific codebook associated with the user, to yield a similarity value, wherein the user-specific codebook utilizes utterances from both the user and a group of non-users; and when the similarity value meets a threshold: adding the speech signal to a database of reference speech signals associated with the user-specific codebook; and adding the feature coefficient to the user-specific codebook.
15. A computer-readable storage device having instructions stored which, when executed by a computing device, cause the computing device to perform operations comprising: generating a feature coefficient based on a speech signal from a user; comparing the feature coefficient to a user-specific codebook associated with the user, to yield a similarity value, wherein the user-specific codebook utilizes utterances from both the user and a group of non-users; and when the similarity value meets a threshold: adding the speech signal to a database of reference speech signals associated with the user-specific codebook; and adding the feature coefficient to the user-specific codebook. 18. The computer-readable storage device of claim 15 , wherein the feature coefficient is one of a cepstrum coefficient and a linear predictive coding coefficient.