Experiments of hands-free connected digit recognition using a microphone array

A scenario concerning hands-free connected digit recognition in a noisy office environment is investigated. An array of six omnidirectional microphones and a corresponding time delay compensation module are used to provide a beamformed signal as input to a hidden Markov model (HMM) based recognizer. Phone HMM adaptation is used to further reduce the mismatch between training and test conditions. Both real and simulated array signals, generated by means of the image method, were used to evaluate system performance. Real data were collected in a moderate noise environment with talkers at 1.5 m distance from the array. Results show that a digit accuracy over 95% can be achieved using the array and the HMM adaptation. This result has to be compared with 99.7% digit accuracy obtained by using a closetalk microphone instead of the array