| 摘要 |
<p>A compatible AM stereo system employing a modified quadrature modulation scheme where the gain in the quadraturephased 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 (Figures 3, 7A and 7B), a matrix circuit (40, 188) adds and substracts 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 adjuster (L-R) signals are transmitted on two quadraturephased carriers by a quadrature AM (QAM) transmitter (50, 160). A distortion estimator (56, 190, Figures 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, Figures 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 (Figures 7A and 7B), a circuit (156, Figure 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 (Figures 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.</p> |
