Variable-angle, multiple channel amplitude modulation system
First Claim
1. A modulator comprising:
- modulator means for providing a composite modulated signal having a subsonically varying quiescent carrier component, andadaptor means for separating said composite modulated signal into AF and RF components for application to AF and RF inputs of an AM transmitter, said adapter means including means responsive to said composite modulated signal to provide an envelope signal which varies as the envelope of said composite modulated signal, and means for subsonically adjusting the gain of said envelope signal so that its DC component remains substantially fixed, thereby reducing harmonic distortion resulting from AC coupling of said envelope signal into said AF input of said AM transmitter.
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Abstract
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 AQ. The (L+R) and gain adjusted (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 AQ 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 (FIGS. 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.
41 Citations
35 Claims
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1. A modulator comprising:
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modulator means for providing a composite modulated signal having a subsonically varying quiescent carrier component, and adaptor means for separating said composite modulated signal into AF and RF components for application to AF and RF inputs of an AM transmitter, said adapter means including means responsive to said composite modulated signal to provide an envelope signal which varies as the envelope of said composite modulated signal, and means for subsonically adjusting the gain of said envelope signal so that its DC component remains substantially fixed, thereby reducing harmonic distortion resulting from AC coupling of said envelope signal into said AF input of said AM transmitter. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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- 8. An AM stereo modulator comprising means for providing two phase-displaced carrier signals, means for amplitude modulating each of said carrier signals in accordance with a corresponding one of first and second modulating signals, means for linearly combining the resulting amplitude modulated carriers to provide a composite modulated signal, and means for varying the phase displacement between said two phase-displaced carrier signals in dependence upon characteristics of said first and second modulating signals, and further comprising means for varying the gain of said composite modulated signal so as to stabilize the gain of the component thereof which is in-phase with the quiescent carrier of said composite modulated signal.
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10. An AM stereo modulator comprising means for providing two phase-displaced carrier signals, means for amplitude modulating each of said carrier signals in accordance with a corresponding one of first and second modulating signals, means for linearly combining the resulting amplitude modulated carriers to provide a composite modulated signal, and means for varying the phase displacement between said two phase-displaced carrier signals in dependence upon characteristics of said first and second modulating signals, and further comprising adapter means for separating said composite modulated signal into AF and RF components for application to AF and RF inputs an AM transmitter, said adapter means including means responsive to said composite modulated signal to provide an envelope signal which varies as the envelope of said composite modulated signal, and means for adjusting the gain of said envelope signal so that the level of its D.C. component remains substantially fixed.
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11. An AM stereo modulator comprising means for providing two phase-displaced carrier signals, means for amplitude modulating each of said carrier signals in accordance with a corresponding one of first and second modulating signals, means for linearly combining the resulting amplitude modulated carriers to provide a composite modulated signal, and means for varying the phase displacement between said two phase-displaced carrier signals in dependence upon characteristics of said first and second modulating signals, wherein said means for providing said two phase-displaced carrier signals includes means for providing two phase-displaced carrier signals whose amplitudes vary in accordance with the secant of one-half the phase angle between said two phase-displaced carrier signals.
- 12. An AM stereo modulator comprising means for providing two phase-displaced carrier signals, means for amplitude modulating each of said carrier signals in accordance with a corresponding one of first and second modulating signals, means for linearly combining the resulting amplitude modulated carriers to provide a composite modulated signal, and means for varying the phase displacement between said two phase-displaced carrier signals in dependence upon characteristics of said first and second modulating signals, further including means for providing a pilot signal having a characteristic which varies in a known dependence upon said phase displacement, and means for combining said pilot signal with said composite modulated signal.
- 14. An AM stereo modulator comprising means for providing two phase-displaced carrier signals, means for amplitude modulating each of said carrier signals in accordance with a corresponding one of first and second modulating signals, means for linearly combining the resulting amplitude modulated carriers to provide a composite modulated signal, and means for varying the phase displacement between said two phase-displaced carrier signals in dependence upon characteristics of said first and second modulating signals, wherein said means for providing said two phase-displaced carrier signals comprises means for providing a first carrier signal, and phase shifter means responsive to said carrier signal and to a phase displacement control signal for providing two phase-displaced carriers equally phase displaced on either side of said first carrier signal by an amount which varies in accordance with said phase displacement control signal.
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18. 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 being varied between discrete incremental values in dependence upon characteristics of said first and second signals.
- 19. 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 between discrete incremental values 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 the relative gain varied in-phase and quadrature-phase modulating signals.
- 23. An AM stereo demodulator comprising means for receiving a composite modulated signal including at least two phase displaced AM carrier components, the amplitude of each of said components 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 composite modulated signal, said recovering means comprising means for providing first and second phase reference signals, each varying in phase so as to remain substantially in phase-quadrature with a corresponding one of said two phase displaced AM carrier components, and first and second demodulator means each responsive to said composite modulated signal for synchronously demodulating said composite modulated signal in accordance with a corresponding one of said phase reference signals whereby said first and second signals are thereby recovered from said composite modulated signal.
- 31. An AM stereo 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 recovered in-phase modulating signal in accordance with said variations in said pilot signal so that said recovered in-phase modulating signal varies in gain substantially the same as does said gain varying quadrature-phase component, whereby the ratio of the gains of said recovered in-phase and quadrature-phase modulating signals remains substantially constant.
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34. An AM stereo modulator comprising means for providing first and second signals, means responsive to said first and second signals to provide two phased displaced AM carrier components with the amplitude of each 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 accordance with a phase displacement control signal, means for providing sum and difference signals corresponding to the sum of and difference between said first and second signals, means for comparing said sum and difference signals with preset limits, and means for providing said phase displacement control signal in accordance with the results of said comparisons.
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35. An AM stereo demodulator comprising means for receiving a QAM signal whose in-phase component is modulated in accordance with the sum of first and second signals and whose quadrature-phase component is modulated in accordance with the difference between said first and second signals, said quadrature-phase component having a gain which varies with time, QAM demodulator means for demodulating said QAM signal to recover in-phase and quadrature-phase modulating signals therefrom, means for manually adjusting the relative gains of said recovered in-phase and quadrature-phase modulating signals, and means for adding said gain adjusted modulating signals to one another to recover a third signal and for subtracting said gain adjusted modulating signals from one another to recover a fourth signal.
Specification