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A Quadrature Amplitude Modulation (QAM) method and apparatus including a QAM transmit modulator with at least one unbalanced mixer, which creates an asymmetric two-dimensional (2-D) QAM symbol constellation. The asymmetrical symbol constellation provides baseband symbol clock signal leakage sufficie

A Quadrature Amplitude Modulation (QAM) method and apparatus including a QAM transmit modulator with at least one unbalanced mixer, which creates an asymmetric two-dimensional (2-D) QAM symbol constellation. The asymmetrical symbol constellation provides baseband symbol clock signal leakage sufficient to facilitate quick and simple baseband symbol clock recovery and signal channel compensation at the QAM receiver without significantly degrading the system bit-error rate (BER). While slightly degrading static BER, overall system performance is improved when considering baseband symbol clock recovery and received signal compensation for an imperfect signal channel. This allows QAM to be deployed in systems where QAM is otherwise prohibitively expensive and improves overall system performance for any existing QAM system application without additional bandwidth, cost or complexity.

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1. In a communication system including a data transmitter coupled through a signal channel to a data receiver, a method for transferring data through the signal channel comprising the steps of: encoding the data to produce a first baseband modulating signal I(t) and a second baseband modulating sign

1. In a communication system including a data transmitter coupled through a signal channel to a data receiver, a method for transferring data through the signal channel comprising the steps of: encoding the data to produce a first baseband modulating signal I(t) and a second baseband modulating signal Q(t) whose amplitudes together represent a time series of complex symbols (I, Q) each selected from a two-dimensional (2-D) constellation of symbols distributed on the phase plane about the origin such that at least one of the baseband modulating signals has a substantially non-zero mean amplitude;multiplying the first baseband modulating signal I(t) by a first baseband symbol clock signal to produce a first modulation product signal and multiplying the second baseband modulating signal Q(t) by a second baseband symbol clock signal to produce a second modulation product signal, where the phases of the first and second baseband symbol clock signals are generally fixed in quadrature;summing the first and second modulation product signals to produce a transmitter output signal;coupling the transmitter output signal through the signal channel to the data receiver; anddemodulating the first and second modulation product signals at the data receiver to recover the series of complex symbols (I, Q). 2. The method of claim 1 wherein the 2-D symbol constellation is square and disposed asymmetrically about one of the two phase plane axes. 3. The method of claim 1 wherein the signal channel is selected from a group consisting essentially of an electrical conductor, an optical fiber and a free-space electromagnetic wave propagation path. 4. The method of claim 1 further comprising the steps of: detecting the first baseband symbol clock signal from the transmitter output signal at the data receiver; andproducing a recovered baseband symbol clock signal that is synchronized with the first baseband symbol clock signal at the data receiver. 5. A communication system comprising: a data transmitter, including an input for accepting data, a Quadrature Amplitude Modulation (QAM) encoder coupled to the data input for producing, responsive to the data, a first baseband modulating signal I(t) and a second baseband modulating signal Q(t) whose amplitudes together represent a time series of complex symbols (I, Q) each selected from a two-dimensional (2-D) constellation of symbols distributed on the phase plane about the origin such that at least one of the baseband modulating signals has a substantially non-zero mean amplitude, a QAM modulator coupled to the QAM encoder for multiplying the first baseband modulating signal I(t) by a first baseband symbol clock signal to produce a first modulation product signal and multiplying the second baseband modulating signal Q(t) by a second baseband symbol clock signal to produce a second modulation product signal, where the phases of the first and second baseband symbol clock signals are generally fixed in quadrature, and for summing the first and second modulation product signals to produce a transmitter output signal, and an output for coupling the transmitter output signal to a signal channel; anda data receiver, including a signal input coupled to the signal channel for accepting the transmitter output signal, and a QAM demodulator coupled to the signal input for recovering the series of complex symbols (I, Q) from the first and second modulation product signals. 6. The system of claim 5 wherein the 2-D symbol constellation is square and disposed asymmetrically about one of the two phase plane axes. 7. The system of claim 5 wherein the signal channel is selected from a group consisting essentially of an electrical conductor, an optical fiber and a free-space electromagnetic wave propagation path. 8. The system of claim 5 further comprising: in the data receiver, a baseband symbol clock detector coupled to the signal input for detecting the first baseband symbol clock signal from the transmitter output signal; anda baseband symbol clock recovery oscillator coupled to the baseband symbol clock detector for producing a recovered baseband symbol clock signal synchronized with the first baseband symbol clock signal. 9. In a communication system for transferring data through a signal channel to a data receiver, a data transmitter comprising: an input for accepting data;a Quadrature Amplitude Modulation (QAM) encoder coupled to the data input for producing, responsive to the data, a first baseband modulating signal I(t) and a second baseband modulating signal Q(t) whose amplitudes together represent a time series of complex symbols (I, Q) each selected from a two-dimensional (2-D) constellation of symbols distributed on the phase plane about the origin such that at least one of the baseband modulating signals has a substantially non-zero mean amplitude;a QAM modulator coupled to the QAM encoder for multiplying the first baseband modulating signal I(t) by a first baseband symbol clock signal to produce a first modulation product signal and multiplying the second baseband modulating signal Q(t) by a second baseband symbol clock signal to produce a second modulation product signal, where the phases of the first and second baseband symbol clock signals are generally fixed in quadrature, and for summing the first and second modulation product signals to produce a transmitter output signal; andan output for coupling the transmitter output signal through the signal channel to the data receiver. 10. The data transmitter of claim 9 wherein the 2-D symbol constellation is square and disposed asymmetrically about one of the two phase plane axes. 11. The data transmitter of claim 9 wherein the signal channel is selected from a group consisting essentially of an electrical conductor, an optical fiber and a free-space electromagnetic wave propagation path. 12. In a remote inspection system including a video transmitter coupled through a signal channel to a video receiver, a method for transferring a video signal through the signal channel comprising the steps of: encoding the video data to produce a first baseband modulating signal I(t) and a second baseband modulating signal Q(t) whose amplitudes together represent a time series of complex symbols (I, Q) each selected from a two-dimensional (2-D) constellation of symbols distributed on the phase plane about the origin such that at least one of the baseband modulating signals has a substantially non-zero mean amplitude; multiplying the first baseband modulating signal I(t) by a first baseband symbol clock signal to produce a first modulation product signal and multiplying the second baseband modulating signal Q(t) by a second baseband symbol clock signal to produce a second modulation product signal, where the phases of the first and second baseband symbol clock signals are generally fixed in quadrature; summing the first and second modulation product signals to produce a transmitter output signal; coupling the transmitter output signal through the signal channel to the data receiver; anddemodulating the first and second modulation product signals at the data receiver to recover the series of complex symbols (I, Q). 13. The method of claim 12 wherein the 2-D symbol constellation is square and disposed asymmetrically about one of the two phase plane axes. 14. The method of claim 12 wherein the signal channel is selected from a group consisting essentially of an electrical conductor, an optical fiber and a free-space electromagnetic wave propagation path. 15. The method of claim 12 further comprising the steps of: detecting the first baseband symbol clock signal from the transmitter output signal at the video receiver; andproducing a recovered baseband symbol clock signal that is synchronized with the first baseband symbol clock signal at the video receiver. 16. In a remote inspection system including a video transmitter coupled through a signal channel to a video receiver, a data modulator in the video transmitter for transferring a video signal through the signal channel, the data modulator comprising: an input for accepting data;a Quadrature Amplitude Modulation (QAM) encoder coupled to the data input for producing, responsive to the data, a first baseband modulating signal I(t) and a second baseband modulating signal Q(t) whose amplitudes together represent a time series of complex symbols (I, Q) each selected from a two-dimensional (2-D) constellation of symbols distributed on the phase plane about the origin such that at least one of the baseband modulating signals has a substantially non-zero mean amplitude; anda QAM modulator coupled to the QAM encoder for multiplying the first baseband modulating signal I(t) by a first baseband symbol clock signal to produce a first modulation product signal and multiplying the second baseband modulating signal Q(t) by a second baseband symbol clock signal to produce a second modulation product signal, where the phases of the first and second baseband symbol clock signals are generally fixed in quadrature, and for summing the first and second modulation product signals to produce a transmitter output signal. 17. The data modulator of claim 16 wherein the 2-D symbol constellation is square and disposed asymmetrically about one of the two phase plane axes. 18. A remote inspection system comprising: a video transmitter, including, a video camera for producing video data, and a Quadrature Amplitude Modulation (QAM) coupled to the video camera, including a symbol encoder coupled to the video camera for producing, responsive to the video data, a first baseband modulating signal I(t) and a second baseband modulating signal Q(t) whose amplitudes together represent a time series of complex symbols (I, Q) each selected from a two-dimensional (2-D) constellation of symbols distributed on the phase plane about the origin such that at least one of the baseband modulating signals has a substantially non-zero mean amplitude, a QAM modulator coupled to the QAM encoder for multiplying the first baseband modulating signal I(t) by a first baseband symbol clock signal to produce a first modulation product signal and multiplying the second baseband modulating signal Q(t) by a second baseband symbol clock signal to produce a second modulation product signal, where the first and second baseband symbol clock signals are generally fixed in quadrature, a summer coupled to the QAM modulator for summing the first and second modulation product signals to produce a transmitter output signal, and a signal output for coupling the transmitter output signal to a signal channel; anda video receiver, including a video signal input coupled to the signal channel for producing a receiver input signal responsive to the transmitter output signal, a QAM demodulator coupled to the video signal input for recovering the video data from the receiver input signal, and a video display coupled to the QAM demodulator for producing images responsive to the video data. 19. The system of claim 18 wherein the 2-D symbol constellation is square and disposed asymmetrically about one of the two phase plane axes. 20. The system of claim 18 wherein the signal channel is selected from a group consisting essentially of an electrical conductor, an optical fiber and a free-space electromagnetic wave propagation path. 21. The system of claim 18 further comprising: in the video receiver, a baseband symbol clock detector coupled to the video signal input for detecting the first baseband symbol clock signal from the receiver input signal; anda baseband symbol clock recovery oscillator coupled to the baseband symbol clock detector for producing a recovered baseband symbol clock signal synchronized with the first baseband symbol clock signal. 22. A pipe inspection system comprising: a video transmitter, including a video camera adapted to produce video data, and a Quadrature Amplitude Modulation (QAM) modulator coupled to the video camera, including a symbol encoder for producing, responsive to the video data, a first baseband modulating signal IT(t) and a second baseband modulating signal QT(t) whose amplitudes together represent a time series of complex transmitter symbols (IT, QT) each selected from a two-dimensional (2-D) constellation of symbols distributed on the phase plane about the origin such that at least one of the baseband modulating signals has a substantially non-zero mean amplitude, a baseband symbol clock oscillator for producing first and second baseband symbol clock signals generally fixed in quadrature, a dual multiplier coupled to the symbol encoder and baseband symbol clock oscillator for multiplying the first baseband modulating signal IT(t) by the first baseband symbol clock signal to produce a first modulation product signal and for multiplying the second baseband modulating signal QT(t) by the second baseband symbol clock signal to produce a second modulation product signal, a summer coupled to the dual multiplier for summing the first and second modulation product signals to produce a transmitter output signal, and a filter coupled to the summer for producing a filtered transmitter output signal; a mechanical cable assembly coupled to the video transmitter for urging the video transmitter through a pipe under inspection and including an electrical conductor coupled to the QAM modulator for accepting the filtered transmitter output signal; anda video receiver, including a signal conditioner coupled to the electrical conductor for producing a baseband receiver input signal representing the filtered transmitter output signal, a QAM demodulator coupled to the signal conditioner, including a baseband symbol clock detector for detecting the first baseband symbol clock signal from the baseband receiver input signal, a baseband symbol clock recovery oscillator coupled to the baseband symbol clock detector for producing a first recovered baseband symbol clock signal generally synchronized with the first baseband symbol clock signal and for producing a second recovered baseband symbol clock signal generally fixed in quadrature with the first recovered baseband symbol clock signal, a dual multiplier coupled to the baseband symbol clock recovery oscillator for multiplying the baseband receiver input signal by the first and second recovered baseband symbol clock signals to produce first and second demodulation product signals, respectively, a dual filter coupled to the dual multiplier for producing, responsive to the first and second demodulation product signals respectively, first and second baseband demodulated signals, IR(t) and QR(t), whose amplitudes together represent a time series of complex receiver symbols (IR, QR), and a decoder coupled to the QAM demodulator for recovering the video data from the first and second demodulated signals, IR(t) and QR(t), and a video display coupled to the QAM demodulator for producing images responsive to the video data. 23. The system of claim 22 wherein the 2-D symbol constellation is square and disposed asymmetrically about one of the two phase plane axes.