STABILITY CONTROL METHOD FOR SECONDARY PITCH PREDICTION FILTER

摘要

PURPOSE:To maintain the effect of secondary pitch prediction sufficiently and to suppress oscillations caused by a secondary pitch prediction filter by detecting the coefficient of the secondary pitch prediction and performing switching to primary pitch prediction at the time of the value of the coefficient is in an unstable area. CONSTITUTION:The output of an adder 16 is delayed by M and M+1 through a delay device 17 and supplied to the secondary prediction filter 18 in an predicting and encoding circuit at the time of the secondary pitch prediction. the transfer function P2(z) of the filter 18 is expressed by P2(z)=beta1Z-M+beta2Z-M-1 after (z) conversion. In such a case, beta1 and beta2 are coefficients of pitch prediction and M is a pitch period. Then it is decided whether or not the coefficients beta1 and beta2 are in a stable area, i.e. whether or not ¦beta1¦+¦beta2¦1 and when it is decided that a coefficient deviates from the stable area, a value calculated from an autocorrelation function f(M) = is used as the coefficient to obtain the function of the primary pitch prediction filter. Consequently, the filter is made to function as the primary pitch prediction filter occasionally, the filter operates stable on the whole without losing the features of the secondary pitch prediction filter.