Adaptive filter and filtering method for low bit rate coding

摘要

An improved filtering method for use in an enhancement filter in a mixed excitation linear prediction (MELP) speech coder or a postfilter in a codebook excitation linear prediction (CELP) speech coder is disclosed which includes two filters. The first filter (62) has a transfer function of ##EQU1## where P is the set of prediction coefficients, &agr; and . beta. are scaling factors, z is the inverse of the unit delay operation used in the transform representation of the transfer functions and sig- prob is signal probability estimator value and the second filter (65) has a transfer function of 1-&mgr;z.sup.-1 * sig-prob, where &mgr;= a scaling factor. The sig-prob is the signal probability value based on a comparison of power of the signals in a current frames to a long term estimate of noise power in signal probability estimator (63). The sig- prob value is 1 if the power of the signals is greater than the noise power plus 30 dB and the sig-prob is zero if the power is less than noise power plus 12 dB. Between these two conditions, sig-prob is (log gain-12 dB-noise gain)/18.