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US 4,225,751 AGranted
Variable-angle, multiple channel amplitude modulation system
AssigneeHarris Corporation
InventorDavid L. Hershberger
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AssigneeHarris Corporation
InventorDavid L. Hershberger
Estimating the term…
A compatible AM stereo system employing a modified quadrature modulation scheme where the gain in the quadrature-phased channel, and thus the phase angle between the L and R modulated phase components of the composite modulated signal, is dynamically varied in accordance with changing modulation conditions. The average phase angle between the L and R modulated phase components is much greater than was previously possible in fixed gain systems, thus signal-to-noise ratio (SNR) in stereophonic receivers is improved. In the transmitter (FIGS. 3, 7A and 7B), a matrix circuit (40, 188) adds and subtracts L and R audio signals to produce sum (L+R) and difference (L-R) signals. An analog divider (54; 194, 200) adjusts the gain of the (L-R) signal in accordance with a gain control signal A.sub.Q. The (L+R) and gain adjuster (L-R) signals are transmitted on two quadrature-phased carriers by a quadrature AM (QAM) transmitter (50, 160). A distortion estimator (56, 190, FIGS. 5A and 5B) calculates the amount of distortion which the envelope of the composite modulated signal will produce in a conventional monophonic receiver and dynamically adjusts the gain control signal A.sub.Q to maximize gain without exceeding predetermined distortion constraints. A circuit (60, 210, FIGS. 9 and 10) generates a pilot signal and modulates it with the dynamically varying gain control. This pilot signal is added to, and thus transmitted with, the gain adjusted (L-R) signal. In one transmitter embodiment (FIG. 7A and 7B), a circuit (156, FIG. 8) is provided for compressing low amplitude audio signals so as to further improve SNR. A signal indicative of the amounts of compression is also modulated onto the pilot signal, and thus transmitted to the receiver. A filter network (154) provides a gap in the low frequency portion of the (L-R) signal into which the pilot signal may be inserted, and processes the (L+R) and (L-R) signals so that no loss in bass occurs as a result of this filtering. Stereophonic receivers (FIGS. 11-13) are disclosed which demodulate the pilot signal and control the gain of the recovered (L+R) and (L-R) signals in accordance therewith so as to track the varying phase angle and signal compression of the composite modulated signal, and thus optimally recover the L and R audio signals in their original form. The system may easily be converted into an improved independent sideband (ISB) system by the insertion of appropriate phase delays in the (L+R) and (L-R) signal paths.
A system for communicating at least first and second signals from a first location to a second location, comprising:
A modulator for modulating at least first and second signals onto a carrier signal to produce a modulated signal to be communicated to a plurality of demodulators, including first demodulators operative to separately recover said at least first and second signals from said modulated signal, and second demodulators operative to recover only a signal corresponding to the envelope of said modulated signal, said modulated signal producing distortion in said second demodulators to the extent that said envelope differs from the sum of said at least first and second signals, said modulator comprising:
Apparatus for modulating a carrier signal to produce a modulated signal including a carrier component and at least two modulated phase components, said modulated signal to be communicated to first and second types of demodulators, where a performance characteristic of said first type of demodulators improves as the phase angle between said phase components increases and a performance characteristic of said second type of demodulators deteriorates as said phase angle increases, said apparatus comprising:
Apparatus comprising:
A modulator for modulating one carrier signal in accordance with in-phase and quadrature-phase modulating signals, respectively, to provide a composite modulated signal to be communicated to first and second types of demodulators, wherein said first type of demodulator is operative to separately recover said in-phase and quadrature-phase modulating signals, and said second type of demodulator is operative to recover only a signal corresponding to the envelope of said composite modulated signal, it being desired that said signal recovered by said second type of demodulator be distorted by no more than preselected amount from one of said modulating signal, said modulator comprising:
A modulator for modulating one carrier signal in accordance with in-phase and quadrature-phase modulating signals, respectively, to provide a composite modulated signal to be communicated to first and second types of demodulators, wherein said first type of demodulator is operative to separately recover said in-phase and quadrature-phase modulating signals, and said second type of demodulator is operative to recover only a signal corresponding to the envelope of said composite modulated signal, it being desired that said signal recovered by said second type of demodulator be distorted by no more than a preselected amount from one of said modulating signals, said modulator comprising:
A modulator for modulating one carrier signal in accordance with in-phase and quadrature-phase modulating signals, respectively, to provide a composite modulated signal to be communicated to first and second types of demodulators, wherein said first type of demodulator is operative to separately recover said in-phase and quadrature-phase modulating signals, and said second type of demodulator is operative to recover only a signal corresponding to the envelope of said composite modulated signal, it being desired that said signal recovered by said second type of demodulator be distorted by no more than a preselected amount from one of said modulating signals, said modulator comprising:
A modulator for generating a composite modulated signal having a first signal predominantly carried in one sideband thereof and a second signal predominantly carried in the other sideband thereof, with the envelope of said composite modulated signal corresponding to the sum of said first and second signals with predetermined limits, comprising:
Apparatus for demodulating a composite modulated signal including at least first and second phase components having a varying phase angle therebetween, said composite modulated signal also including a pilot signal having a characteristic which varies as the phase angle between said phase components, comprising:
An AM stereo modulator comprising means for providing first and second signals, and means responsive to said first and second signals to provide two phase displaced AM carrier components with the amplitude of each of said AM components being modulated in accordance with a corresponding one of said first and second signals, and with the phase displacement between said AM components being varied in dependence upon characteristics of said first and second signals.
An AM stereo modulator comprising means for providing in-phase and quadrature-phase modulating signals, means for varying the relative gains of said in-phase and quadrature-phase modulating signals in accordance with at least one characteristic of said signals, and QAM modulating means for modulating the in-phase and quadrature-phase components of a QAM signal with said relative gain varied in-phase and quadrature-phase modulating signals, respectively.
An AM stereo modulator comprising audio matrix means for matrixing left and right audio signals to derive sum and difference signals therefrom, means for varying the gain of said difference signal in accordance with variations in at least a selected characteristic of said left and right audio signals, and means for applying said sum and gain varied difference signal to the in-phase and quadrature-phase modulating inputs, respectively, of a quadrature amplitude modulator.
An AM-ISB modulator comprising means for providing first and second signals, means responsive to said first and second signals to provide a modulated signal including upper and lower AM sidebands, each of said sidebands being amplitude modulated predominantly by a corresponding one of said first and second signals, but also by the other of said signals to an extent which varies in accordance with the amount by which the envelope of said modulated signal differs from the sum of said first and second signals.
A method of providing a stereo AM signal compatible for reception by a receiver employing an envelope detector to recover a monaural signal, comprising the steps of providing first and second audio frequency signals, generating a QAM signal from said first and second signals wherein the in-phase component of said QAM signal is amplitude modulated by the sum of said first and second signals and the quadrature-phase component of said QAM signal is amplitude modulated by the difference between said first and second signals, and varying the gain of said quadrature-phase component as necessary to prevent the envelope of said QAM signal from becoming distorted from a monaural form by more than a preselected amount.
An AM stereo demodulator comprising means for receiving at least two phase displaced AM carrier components, the amplitude of each said AM component being modulated in accordance with a corresponding one of first and second signals, with the phase displacement between said AM components varying in dependence upon characteristics of said first and second signals, and means for recovering said first and second signals from said AM carrier components in dependence upon said varying phase displacement.
A QAM demodulator comprising means for receiving a QAM signal having a quadrature-phase component whose gain varies in accordance with variations in a pilot signal also received by said means, QAM demodulator means for demodulating said QAM signal to recover in-phase and quadrature-phase modulating signals therefrom, and means for adjusting the gain of said quadrature-phase modulating signal in accordance with said variations in said pilot signal so as to derive a gain stable quadrature-phase modulating signal therefrom.
An AM stereo demodulator comprising means for receiving a QAM signal having an in-phase component whose amplitude varies in accordance with the sum of at least first and second signals and a quadrature-phase component whose amplitude varies in accordance with the difference between said at least first and second signals, the gain of said quadrature-phase component varying in accordance with a pilot signal also received by said receiver means, QAM demodulator means for demodulating said QAM signal to recover in-phase and gain varying quadrature-phase modulating signals therefrom, means for adjusting the gain of said recovered, gain-varying quadrature-phase modulating signal in accordance with said variations in said pilot signal so as to derive a gain stable quadrature-phase modulating signal therefrom, and matrix means for matrixing said in-phase and gain stable quadrature-phase modulating signals so as to derive said first and second signals therefrom.
An AM stereo demodulator comprising means for receiving a modulated signal including upper and lower AM sidebands, each of said sidebands being amplitude modulated predominantly by a corresponding one of first and second signals but also by the other to an extent which varies in accordance with characteristics of said first and second signals, and means for recovering said first and second signals from said modulated signal in dependence upon the varying extent to which each of said sidebands is modulated by the other of said signals.
Dependencies are inferred from the claim text.