An Efficient Variable-Bit-Rate Low-Delay CELP (VBR-LD-CELP) Coder

Abstract

Two versions [1],[2] of Low-Delay Code-Excited Linear Predictive (LD-CELP) coders have been recently suggested as candidates for the CCITT 16 kbit/s speech coding standard. The goal of this standard is to cover a long list of possible applications like mobile radio, video-phone, Digital Circuit Multiplication Equipment (DCME), etc.. Many of these applications have different requirements so it has been difficult to define the performance requirements and objectives for a unique algorithm which will be suitable for a large variety of these applications. Thus it was suggested [3] and accepted as an objective of the standard that the adopted algorithm will have a nominal rate of 16 Kbit/s but can operate at bit rates higher and lower than the nominal rate. This may enable a more optimal implementation for each specific application and provide a better basis for acceptance of the standard by various user groups. For example, Variable-Bit-Rate (VBR) coding can be used to add an error correction/detection information for noisy channel applications like mobile radio. Another example is in DCME systems, where VBR coding can avoid speech clipping during overload traffic periods, and can improve speech quality during underload periods.