The use of digital audio equipment is expanding from the home to the automobile. Conventional audio power amplifiers amplify signals from compact disks (CD), digital audio tapes (DAT), and digital audio signal processors (DASP) which have been converted to analog signals. We have developed a digital audio amplifier for automobile audio systems which directly drives the loudspeaker without D/A conversion. This is done by converting the pulse code modulation (PCM) source signal to a pulse width modulation (PWM) signal. With this amplifier, digital audio becomes truly digital, from input to output.
This amplifier features dedicated ICs for the PCM-to-PWM converter and 4X over-sampling and fourth-order Δ-∑ modulation techniques for the converter as well the output power amplifier driver. Although the PWM output resolution is only 8 bits, the bandwidth is wide from 20 Hz to 20 kHz and the total harmonic distortion (THD) is just 0.1% or less.