Demodulators – Amplitude modulation demodulator – Having specific distortion – noise or other interference...
Patent
1978-12-18
1980-09-30
Olms, Douglas W.
Demodulators
Amplitude modulation demodulator
Having specific distortion, noise or other interference...
329167, 329178, 332 23A, 332 40, H04H 500
Patent
active
042257514
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 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.
REFERENCES:
patent: 2903518 (1959-09-01), Kahn
patent: 3067293 (1962-12-01), Moore
patent: 3102167 (1963-08-01), Barton
patent: 3218393 (1965-11-01), Kahn
patent: 3233044 (1966-02-01), Hopper
patent: 3377559 (1968-04-01), Stewart
Harris Corporation
Olms Douglas W.
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