Dual subframe quantization of spectral magnitudes

A method of decoding speech from a 90 millisecond frame of bits received across a satellite communication channel, the method comprising the steps of:

• dividing the frame of bits into two blocks of bits, wherein each block of bits represents two subframes of speech;
• applying error control decoding to each block of bits using redundant error control bits included within the block to produce error decoded bits which are at least in part protected from bit errors;
• using the error decoded bits to jointly reconstruct spectral magnitude parameters for both of the subframes within a block, wherein the joint reconstruction includes using a plurality of vector quantizer codebooks to reconstruct a set of combined residual parameters from which separate residual parameters for both of the subframes are computed, forming predicted spectral magnitude parameters from the reconstructed spectral magnitude parameters from a previous block, and adding the separate residual parameters to the predicted spectral magnitude parameters to form the reconstructed spectral magnitude parameters for each subframe within the block;
• and
• synthesizing a plurality of digital speech samples for each subframe using the reconstructed spectral magnitude parameters for the subframe.

An encoder for encoding speech into a 90 millisecond frame of bits for transmission across a satellite communication channel, the system including:

• a digitizer configured to convert a speech signal into a sequence of digital speech samples;
• a subframe generator configured to divide the digital speech samples into a sequence of subframes, each of the subframes comprising a plurality of the digital speech samples;
• a model parameter estimator configured to estimate a set of model parameters for each of the subframes, wherein the model parameters comprise a set of spectral magnitude parameters that represent spectral information for the subframe;
• a combiner configured to combine two consecutive subframes from the sequence of subframes into a block;
• a dual-frame spectral magnitude quantizer configured to jointly quantize parameters from both of the subframes within the block, wherein the joint quantization includes forming predicted spectral magnitude parameters from the quantized spectral magnitude parameters from a previous block, computing residual parameters as the difference between the spectral magnitude parameters and the predicted spectral magnitude parameters, combining the residual parameters from both of the subframes within the block, and using a plurality of vector quantizers to quantize the combined residual parameters into a set of encoded spectral bits;
• an error code encoder configured to add redundant error control bits to the encoded spectral bits from each block to protect at least some of the encoded spectral bits within the block from bit errors;
• and
• a combiner configured to combine the added redundant error control bits and encoded spectral bits from two consecutive blocks into a 90 millisecond frame of bits for transmission across a satellite communication channel.

A decoder for decoding speech from a 90 millisecond frame of bits received across a satellite communication channel, the decoder including:

• a divider configured to divide the frame of bits into two blocks of bits, wherein each block of bits represents two subframes of speech;
• an error control decoder configured to error decode each block of bits using redundant error control bits included within the block to produce error decoded bits which are at least in part protected from bit errors;
• a dual-frame spectral magnitude reconstructor configured to jointly reconstruct spectral magnitude parameters for both of the subframes within a block, wherein the joint reconstruction includes using a plurality of vector quantizer codebooks to reconstruct a set of combined residual parameters from which separate residual parameters for both of the subframes are computed, forming predicted spectral magnitude parameters from the reconstructed spectral magnitude parameters from a previous block, and adding the separate residual parameters to the predicted spectral magnitude parameters to form the reconstructed spectral magnitude parameters for each subframe within the block;
• and
• a synthesizer configured to synthesize a plurality of digital speech samples for each subframe using the reconstructed spectral magnitude parameters for the subframe.