Adaptively controlled 4.8 kbit/s speech coders with low distortion

Abstract

AbstractAs a low bit‐rate speech coding method to realize a satisfactory quality of reconstructed speech, this paper proposes a 4.8 kbit/s pitch‐controlled multipulse coding method, which adaptively controls the speech pitch prediction interval and represents the multipulse positions for the excitation signals with a small number of bits. As a method to control the pitch adaptively, an algorithm is derived and proposed where the pitch prediction interval is adjusted adaptively for each frame so that the pulse position is determined to minimize the perceptual error power representing subjectively important difference.In addition to the forementioned method, another method also is proposed to improve the speech quality, where the pitch‐controlled multipulse coding is proposed to control adaptively the coefficients of the synthesis filter. An algorithm is proposed for the adaptive control of the coefficients of the synthesis filter, to calculate anew the optimal filter coefficients for excitation pulses derived from the adaptive control of the pitch.A simulation is executed for the two methods, and the following results are obtained as an evaluation of SNRseg. It is 7.5 dB in the conventional MPC algorithm, 8.6 dB in the proposed pitch‐controlled MPC algorithm, and 9.8 dB in the pitch‐controlled MPC algorithm including the adaptive control of the synthesis filter coefficients. In other words, the improvement in the latter two is 1.1 and 2.3 dB, respectively. The performance is satisfactory also from the viewpoint of the cepstral distance. The improvement in the reconstructed speech quality also is verified for the proposed methods by the subjective evaluation.