Subband coding is a digital encoding method that bandpass filters the signal to be digitized into a set of subbands. Each subband signal is then independently encoded according to perceptual criteria for its frequency band. The digital signals are multiplexed for transmission and demultiplexed and decoded in the receiver. After decoding, the subband signals are summed to give a reproduction of the output signal. The technique permits a greater control over the spectral shape of the digital distortions in the digital encoding system. This paper presents a design for an all digital two-band subband coder to provide "commentary quality" speech and music over a single slot in a T1 carrier system. It provides a 7-kHz bandwidth at a transmission rate of 56 kb/s with no noticeable distortions or noise under normal "casual listening" environments. It provides a better quality and a more robust performance than a previously discussed analog coder. The design incorporates a number of new digital techniques, including quadrature mirror filtering and high quality analog-to-digital and digital-to-analog conversion circuits.
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