It is well known that the telephone channel introduces a convolved disturbing component in the observed signal which slowly varies with time. Several techniques have been proposed in the literature to reduce the telephone line effects, namely cepstral subtraction and highpass filtering of cepstral trajectories. In a previous work  we introduced a theoretical framework to clarify the link between the channel effects and the low frequencies in the cepstral trajectories and to interpret the encouraging results obtained by cepstral subtraction (an off-line approach) and highpass filtering of the cepstral trajectories. This paper proposes a new approach to reduce the telephone line effects. This approach implements an adaptive filter in a blind equalization scheme. On the basis of recognition results obtained on different speaker independent telephone databases, the blind equalization seems to outperform the classical on-line technique.
Bibliographic reference. Mokbel, C. / Jouvet, D. / Monné, J. (1995): "Blind equalization using adaptive filtering for improving speech recognition over telephone", In EUROSPEECH-1995, 1987-1990.