A circuit for converting from an input serial pulse code modulated (PCM) digital signal to an output pulse width modulated (PWM) digital signal for driving a switching audio amplifier requiring a pulse width modulated input signal, the circuit comprising a sample rate converter receiving the input serial PCM digital signal at a first sampling frequency and converting the input serial PCM digital signal to a second serial PCM digital signal at a second frequency if the first sampling frequency is lower than the second frequency, a digital filter stage for up-sampling the second serial PCM digital signal to a third frequency and converting the second serial PCM digital signal to a parallel digital signal, a volume control stage receiving the parallel digital signal and generating a volume adjusted parallel digital signal in accordance with a digital volume command control signal, a digital cross-point estimator stage for calculating a cross-point between the volume adjusted parallel digital signal and a digital ramp signal and generating a parallel digital signal representing a desired pulse width modulation of the switching audio amplifier, a quantizing stage for quantizing the parallel digital signal representing the desired pulse width modulation into a quantized parallel digital signal representing the pulse width modulation to be applied to the switching audio amplifier; and a PWM generation stage for converting the quantized parallel digital signal into a PWM signal for driving the switching audio amplifier.