Speech coding parameter smoothing method

#The present invention is generally related to speech coding systems and more specifically to a method for improving the perceptual quality of such systems.

#Speech coding systems operate by generating an encoded representation of a speech signal for communication over a channel or network to one or more system receivers (i.e., decoders). Each system receiver reconstructs the speech signal by decoding the received signal. The quantity of information communicated by the system over a given time period defines the system bandwidth and affects the quality of the reconstructed speech. The objective of most speech coding systems is to provide the best trade-off between reconstructed speech quality and system bandwidth, given various conditions such as the signal quality of the input speech (i.e., the original speech signal which is to be coded), the quality of the communications channel itself, bandwidth limitations, and cost.

#The speech signal is commonly represented by a set of parameters which are quantized for transmission. These parameters may be either scalar or vector parameters. In many typical system encoders, a lookup is performed in a preconstructed table (commonly referred to as a codebook) in order to identify the table entry which best matches the parameter to be coded. Then, the index (i.e., the entry number) of the best matching codebook entry is transmitted to the receiver(s) for decoding. In a conventional receiver, an identical codebook to the one contained in the transmitter (i.e., the encoder) is used to reconstruct the parameter values from the transmitted indices, by retrieving the entries identified by each transmitted index. Upon retrieval of the parameter values, they are often interpolated and the resulting upsampled parameter sequence is provided as input to the speech synthesis portion of the speech decoder.

#FIGS. 1A-1C show illustrative line spectral frequency (LSF) trajectories for the word "dune." FIG. 1A shows original, unquantized trajectories; FIG. 1B shows quantized trajectories; and FIG. 1C shows trajectories which have been smoothed in accordance with an illustrative embodiment of the present invention.

#FIG. 2 shows an illustrative embodiment of a speech coder (including both the transmitter and the receiver portions) which may advantageously employ the principals of the present invention.

#FIG. 3 shows an illustrative implementation of the predictor parameter decoder of the receiver of FIG. 2 providing constrained smoothing in accordance with an illustrative embodiment of the present invention.

#Introduction

#Specifically, the illustrative embodiment of the present invention described herein comprises a method of decoding codebook indices obtained by the receiver of a speech coding system. In a conventional speech decoder, the codebook index refers to a particular parameter value entry of the codebook, and this value is used by the decoder as the resultant parameter value. (In the context of the present invention, parameter values may comprise scalar values, vector values or both.) In contrast, in accordance with the illustrative embodiment of the present invention, the resultant decoded value for a particular received index may also depend on indices received before and/or after the particular index being decoded.

#During quantization of parameters by an encoder, the value selected from the codebook is the one nearest to the unquantized value, according to some predetermined objective measure. Based on this predetermined measure, therefore, a region of values in which the unquantized parameter value must have been located can be defined around each quantized value. As is known to those skilled in the art, this region is called the Voronoi region, and the quantized value is referred to as the "centroid" of the region. (Note that if the unquantized parameter were to have fallen outside this region, then a different quantized value would necessarily have been selected.) Thus, just as each transmitted index can be associated with a particular quantized value or centroid, each transmitted index can alternatively be associated with a particular Voronoi region as a whole. Since the original parameter values necessarily fall within the Voronoi regions associated with the transmitted indices, it is advantageous to constrain the decoded values to fall within these same Voronoi regions. Thus, a sequence of decoded parameter values should generally be considered to fall within a sequence of Voronoi regions.