Digital quadrature amplitude and vestigial sideband modulation decoding method and apparatus
First Claim
1. A synchronous detector for recovering a demodulated output digital data signal from a transmitted input digital data signal regardless of whether the transmitted input digital data stream is modulated according to a vestigial sideband or a quadrature amplitude modulation scheme, the synchronous detector comprisinga tuner tuned to a center carrier frequency of a modulated data spectrum, the tuner comprising real and imaginary arms anda decoder for receiving digital data sequences from the real and imaginary arms, the decoder selectively operating according to the duration of a transmitted data symbol.
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Abstract
A synchronous detector for demodulating and decoding a digital data signal modulated either according to a vestigial sideband or a quadrature amplitude modulation scheme is based upon the principle of recognizing that a vestigial sideband signal may be regarded and treated as an offset-keyed QAM signal. The detector comprises a tuner for tuning to a center frequency symmetrically displaced within the transmitted digital data signal frequency spectrum and a decoder circuit selectively operating to reconstruct the transmitted digital data stream according to the duration of a transmitted data symbol. A related method of demodulating and decoding a modulated digital data stream comprises analogous steps of tuning to a center carrier frequency and selectively switching between in phase and quadrature arms of the demodulator such that the switch resides in one position or the other for a duration equal to half the duration of a transmitted data symbol. A related modulator for modulating a digital data signal either according to a vestigial sideband or quadrature amplitude modulation scheme comprises a switch for selectively switching an input digital data signal between real and imaginary arms and a modulator for modulating at center frequency. In one modulator embodiment, a noise cancellation circuit may be coupled between the real and imaginary arms.
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Citations
18 Claims
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1. A synchronous detector for recovering a demodulated output digital data signal from a transmitted input digital data signal regardless of whether the transmitted input digital data stream is modulated according to a vestigial sideband or a quadrature amplitude modulation scheme, the synchronous detector comprising
a tuner tuned to a center carrier frequency of a modulated data spectrum, the tuner comprising real and imaginary arms and a decoder for receiving digital data sequences from the real and imaginary arms, the decoder selectively operating according to the duration of a transmitted data symbol.
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7. A modulator for modulating a digital data stream according to either a VSB or QAM modulation scheme, the modulator comprising
a control circuit for alternately switching an input digital data stream to be modulated between a real arm and an imaginary arm and a modulator for modulating the digital data stream such that the carrier frequency is approximately symmetrically centered about the modulated data amplitude versus frequency spectrum.
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13. A method of demodulating and decoding a digital data stream modulated according to either a vestigial sideband or a quadrature amplitude modulation scheme, the method comprising the steps of
tuning in each of an in phase and a quadrature arm of a demodulator to a center frequency of the modulated data spectrum and reconstructing the digital data stream according to the symbol rate of the transmitted digital data stream.
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17. A method of modulating a digital data signal according to either a vestigial sideband or quadrature amplitude modulation scheme comprising the steps of
switching to real and imaginary arms of a modulator according to the duration of a transmitted data symbol and modulating the digital data stream about a carrier frequency symmetrically centered in the modulated digital data signal spectrum.
Specification