A class AD audio amplifier system (10) with reduced noise capability in
muting and unmuting events is disclosed. The amplifier system (10)
includes multiple audio channels (20), each of which can be constructed
to include a pulse-width-modulator (PWM) (24). The PWM modulator (24)
includes a pair of comparators (39A, 39B; 52+, 52-) that generate
complementary PWM output signals based upon the comparison between a
filtered difference signal and a reference waveform. When the system is
muted, a common mode voltage (CM_RAMP) is applied to the inputs of the
comparators (39A, 39B; 52+ 52-) to suppress the duty cycle at the
amplifier output, preferably to a zero duty cycle. In the transition from
a muted state to an unmuted state, the common mode voltage (CM_RAMP) is
ramped from the suppressing voltage to zero common mode voltage,
permitting the duty cycle of the complementary PWM signals to gradually
increase, thus reducing clicks and pops. The converse operation is
performed in the transition from unmuted to muted. Pulse-width-modulation
control logic (26) is also included to ensure that that the PWM "on" and
"off" pulses are of at least a minimum duration, and also to generate
compensating pulses on the complementary PWM line at low duty cycles.