Method and apparatus for partial response demodulation
First Claim
1. A demodulator for a quadrature partial response (QPR) signal of the form Mi (t) * sin(ω
-
c t)+Mq (t) * cos(ω
c t), where Mi (t) and Mq (t) are encoded partial response information modulations of the inphase and quadrature phases of a carrier of frequency (ω
c t), said demodulator comprising;
means for decoding a data clock from the QPR signal and for generating a recovered data clock signal;
means for generating a demodulating carrier of frequency (ω
c t) with inphase and quadrature phases coherent with the QPR signal carrier;
first means for mixing the QPR signal with the inphase phase of said demodulating carrier, said first mixing means generating a current as a function of time including the component Mi (t);
means for determining whether Mi (t) is positive or negative at a sampling time determined by said recovered data clock signal;
means for generating a digital output depending upon the sign determination of Mi (t) where said digital output is one state if Mi (t) is positive and the other state if negative;
means for subtracting an amount of current equivalent to the previous state of said digital output generating means from the output of said first mixing means;
second means for mixing the QPR signal with the quadrature phase of said demodulating carrier, said second mixing means generating a current as a function of time including the component Mq (t);
means for determining whether Mq (t) is positive or negative at a sampling time determined by said recovered data clock signal;
means for generating a digital output depending upon the sign determination of Mq (t) where said digital output is one state if Mq (t) is positive and the other state if negative; and
means for subtracting an amount of current equivalent to the previous state of said digital output generating means from the output of said second mixing means.
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Accused Products
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.
49 Citations
9 Claims
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1. A demodulator for a quadrature partial response (QPR) signal of the form Mi (t) * sin(ω
-
c t)+Mq (t) * cos(ω
c t), where Mi (t) and Mq (t) are encoded partial response information modulations of the inphase and quadrature phases of a carrier of frequency (ω
c t), said demodulator comprising;means for decoding a data clock from the QPR signal and for generating a recovered data clock signal; means for generating a demodulating carrier of frequency (ω
c t) with inphase and quadrature phases coherent with the QPR signal carrier;first means for mixing the QPR signal with the inphase phase of said demodulating carrier, said first mixing means generating a current as a function of time including the component Mi (t); means for determining whether Mi (t) is positive or negative at a sampling time determined by said recovered data clock signal; means for generating a digital output depending upon the sign determination of Mi (t) where said digital output is one state if Mi (t) is positive and the other state if negative; means for subtracting an amount of current equivalent to the previous state of said digital output generating means from the output of said first mixing means; second means for mixing the QPR signal with the quadrature phase of said demodulating carrier, said second mixing means generating a current as a function of time including the component Mq (t); means for determining whether Mq (t) is positive or negative at a sampling time determined by said recovered data clock signal; means for generating a digital output depending upon the sign determination of Mq (t) where said digital output is one state if Mq (t) is positive and the other state if negative; and means for subtracting an amount of current equivalent to the previous state of said digital output generating means from the output of said second mixing means. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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c t)+Mq (t) * cos(ω
Specification