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Abstract

A digital audio communication system includes a digital modulation and demodulation scheme which efficiently uses the bandwidth and channel spacing of a cable television system. A 30 channel digital bit stream is demultiplexed into six groups of five channels. Each group of channels thereafter modulates a carrier by a quadrature partial response (QPR) process. The QPR signal, an amplitude modulated, double sideband, carrier suppressed (AM DSBSC) signal, is then transmitted over the cable system to a multiplicity of subscribers, each of which has a QPR demodulator. The demodulators are of the decision feedback type having a modified Costas loop carrier recovery circuit. The grouping of an odd number (five) of digital audio channels per QPR modulator minimizes error propagation due to the correlative QPR demodulation process. The decision feedback decoding is implemented in simple current nodes where a bilevel output from the decoded data is subtracted from a tertiary level output of a mixer. The resulting bilevel current is converted to a voltage, filtered, and limited to produce a logic output level. The modified Costas loop provides an error voltage based upon the difference of the received data states versus the nominal data states and the quadrant the data occupies. The error voltage is generated by differencing the outputs of two analog switches which have as their inputs analog signals representative of the amplitude level of the data and its inverse of one phase. The sign of the data bit of the opposite phase is used to control the switch and thus select which signal is used.

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