Abstract

In a digital encoder, truncation is controlled by the fullness of the output rate-smoothing buffer. A signal compression system is shown for processing a stream of fixed length digital sample signals, such as audio signals, which system includes a linear digital compression filter for compression filtering the sample signal stream and generating a stream of compression-filtered signals. The compression-filtered stream is encoded by an encoder which implements a variable word length truncated code. The encoder output is supplied to a buffer for transmission to a communication channel. The encoder is controllably operable in a normal mode and in an out of range (OOR) mode. In the OOR operating mode an OOR code word together with the compression filtered signal is sent to the buffer. Buffer underflow is controlled by operation in the OOR mode when the buffer is empty or near empty. Buffer overflow is controlled by truncating sample signals supplied to the compression filter in response to a measure of buffer fullness when the fullness of the buffer exceeds a predetermined level. The level of truncation increases as buffer fullness increases. With a decrease in buffer fullness, a measure of the rate of change of buffer fullness may be used to accelerate the decrease in truncation for improved audio signal quality. Additionally, a measure of variance of the compression-filtered signal stream may be used in conjunction with control based upon a measure of buffer fullness in the control of truncation for improved audio signal quality.