Paper on theory of a Digital microphone.
"A basic concept of direct converting digital microphone
An electroacoustic system which directly converts analog acoustic signals to digital electric signals is described. The system consists of a subtractor, a sampling and holding circuit, a sigma?delta modulator as a comparator, an accumulator, and a local direct digital-to-analog converting transducer similar to a typical electronic analog-to-digital converter. The subtractor is an electrostatic device which has a diaphragm, driving electrodes, and a detecting electrode. The surface area of the driving electrodes corresponds to the significant bits in the digital signal, as an electroacoustic digital-to-analog converter. The detecting electrode produces an electrical signal proportional to the displacement of the diaphragm driven by subtracting the received acoustic signal from the electrostatically driven force. This is regarded as a subtractor. The detected signal is amplified and sampled-held and modulated by the sigma?delta procedure and generates a signal of +/-1 bit, which is added to the accumulator memory by a high clock frequency. The output of the accumulator is the digital signal output and is also fed to the driving electrodes. A 4-bit conceptual system was developed to affirm this concept. © 1999 Acoustical Society of America."
"A basic concept of direct converting digital microphone
An electroacoustic system which directly converts analog acoustic signals to digital electric signals is described. The system consists of a subtractor, a sampling and holding circuit, a sigma?delta modulator as a comparator, an accumulator, and a local direct digital-to-analog converting transducer similar to a typical electronic analog-to-digital converter. The subtractor is an electrostatic device which has a diaphragm, driving electrodes, and a detecting electrode. The surface area of the driving electrodes corresponds to the significant bits in the digital signal, as an electroacoustic digital-to-analog converter. The detecting electrode produces an electrical signal proportional to the displacement of the diaphragm driven by subtracting the received acoustic signal from the electrostatically driven force. This is regarded as a subtractor. The detected signal is amplified and sampled-held and modulated by the sigma?delta procedure and generates a signal of +/-1 bit, which is added to the accumulator memory by a high clock frequency. The output of the accumulator is the digital signal output and is also fed to the driving electrodes. A 4-bit conceptual system was developed to affirm this concept. © 1999 Acoustical Society of America."