1. Field of the Invention
The present invention relates to communications; more specifically, voice encoding.
2. Description of the Related Art
A voice encoder (vocoder) is used to encode voice signals so as to minimize the amount of bandwidth that is used for transmitting over communication channels. It is important to minimize the amount of bandwidth used per communication channel so as to maximize the number of channels available within a given range of spectrum. Present vocoders using an adaptive codebook structure, such as those in the class of code excited linear predictive (CELP) vocoders, encounter an instability problem when receiving periodic, sinusoidal or tonal inputs coupled with a communications channel error. Since all communication channels experience errors, this type of problem manifests itself relatively frequently for the above inputs. For example, when a DTMF (Dual Tone Multifrelquency) signal is received with errors, many vocoders become unstable and produce an output that exhibits amplitude oscillation.