Investigation of speech recognition over IP channels

In this paper we investigate the effects of IP channels on speech recognition systems and methods to recover the associated performance degradation. There are three major VoIP (voice over IP) distortion sources: speech encoding-decoding (codecs), packet loss and jitter (time-delay). To speech recognition systems distortions are mainly from packet loss and the speech codecs. Their effects on the recognizer´s performance are systematically investigated by using four different ITU-T recommended speech codecs. The results show that the speech codecs introduce bigger degradation than the packet losses (random and burst). To recover the codec degradations we have applied the MLLR adaptation and a data-mixed retraining method. These techniques reduce the degradation by about 50%.