A spectral enhancement procedure for the wideband/Narrowband tandem

This paper describes a procedure that reduces the spectral distortion in LPC encoded speech preprocessed by a CVSD coder. In this type of tandem configuration (wide-band/narrowband), the CVSD process introduces extraneous wideband noise and a general broadening of the formant bandwidths. When coupled with the formant distortion introduced by the LPC process, the tandem speech appears buzzy, muffled, and of lower quality than either system considered alone. By low-pass filtering the CVSD speech, on a formant adaptive basis, and narrowing the bandwidths of the primary formants, F1 and F2, the input signal to the LPC synthesizer more closely resembles the original unprocessed signal. This spectral enhancement procedure produces a higher quality CVSD/LPC signal than previously realized.