| 摘要 |
In order to prevent errors and instability which may occur in a speech analysis and synthesis apparatus when the normalized predictive residual power falls to low levels, for example in high-pitched speech, the calculation of linear predictor coefficients from the autocorrelation coefficients of the speech sound is stopped when the normalized predictive residual power falls below a predetermined threshhold level. Either a variable stage synthesis filter is used having its number of stages determined by the number of linear predictor coefficients actually calculated, or a fixed number of stages can be used and a zero value filter stage coefficient supplied to those stages in excess of the number of coefficients calculated. Degradation of speech quality due to quantization and transmission errors can be alleviated by computing the normalized predictive residual power on the synthesis side from the transmitted predictor coefficients and using it to excite the input to the synthesis filter. In one embodiment especially suitable for high ambient noise conditions, both a sound source and a noise source are employed and two different conversion and window processing channels are provided; one for noise-affected speech and the other for pure noise. Autocorrelations in each channel are performed along with correllations between channels, and the autocorrelations and correllations are then appropriately combined to provide an autocorrelation coefficient of the speech sound. |
