Backward pitch prediction for low-delay speech coding

A backward-adaptive pitch prediction algorithm is described. It is used in conjunction with a backward-adaptive short-term predictor in a low-delay speech coding system operating at 16 kb/s. The backward-adaptive pitch prediction algorithm is a hybrid algorithm which combines backward block adaptive pitch prediction and backward recursive pitch prediction. The pitch predictor tap gains and the pitch period are periodically initialized by using a backward block adaptive algorithm. Between these initializations, however, both the tap gains and the pitch period are adapted using backward recursive algorithms. The tap gains are adapted using the well-known gradient algorithm, in a manner similar to the way the short-term predictor coefficients are adapted. The pitch period is adapted using a novel pitch tracking algorithm. By combining backward recursive adaptation with backward block adaptation, it was possible to increase the prediction gain of the pitch predictor and reduce the interval required between initialization of the pitch predictor parameters