Abstract:
This is a design for a microphone which directly outputs a one-bit digital audio signal. No electronic analogue-to-digital converter is required.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Ivention 
     This invention relates to microphones. 
     2. Description of the Prior Art 
     Known microphones convert an analogue sound waveform (i.e. physical variations in air pressure) into an analogue electrical audio signal. If a digital audio signal is required, the analogue signal has to be converted by a digital to analogue converter (DAC) into the digital audio signal. 
     This extra stage of analogue to digital conversion requires extra components and, more importantly, is not a lossless process. In other words, some of the information contained in the original analogue audio signal is lost by the conversion process, through conversion errors or noise. 
     It would be desirable to provide a microphone which generates a digital audio signal directly from the air pressure variations representing the actual sound. 
     SUMMARY OF THE INVENTION 
     This invention provides a microphone comprising: 
     a diaphragm movable in response to incident sound waves; 
     a position sensor for generating an electrical position signal indicative of the position of the diaphragm; 
     a thresholder for generating a one-bit digital signal indicating whether the position signal is above or below a threshold signal level; 
     a delay for delaying the digital signal; and 
     a diaphragm driver for moving the diaphragm in response to the digital signal and in an opposite sense to the motion of the diaphragm represented by the digital signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of a delta-sigma modulator; 
     FIG. 2 is a schematic diagram of a microphone according to a first embodiment of the invention; 
     FIG. 3 is a schematic diagram of a microphone according to a second embodiment of the invention; and 
     FIG. 4 is a schematic equivalent circuit to a part of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A known delta-sigma modulator is illustrated in FIG.  1 . An input analogue signal is supplied to a comparator  10  and from there to a feedback loop comprising a thresholder  20 , a delay  30  and a filter  40 . A one-bit signal representing the analogue signal is output by the delay  30 . 
     The microphone according to embodiments of the invention uses a similar principle to generate a one bit signal directly from physical sound vibrations. 
     In FIG. 2, a diaphragm  100  vibrates in response to incident sound waves. The motion of the diaphragm is sensed by an interferometer formed of a light source  110  directing a beam of light via a beam splitter  120  on to the diaphragm. A reference beam is also diverted from the beam splitter onto a photodiode  130 . 
     Light reflected from the diaphragm is diverted by the beam splitter onto the photodiode  130  where it is combined with the reference beam and converted to an electrical signal indicative of changes in the position of the diaphragm. The electrical signal is processed by a thresholder  140  and a delay  150  before being amplified by an amplifier  160 . 
     In other embodiments, two light beams in quadrature phase relationship could be used, to give an improved position sensing facility. 
     The diaphragm  100  is positioned between two charged plates  170 . The diaphragm is electrically conductive, and so an electrostatic force is applied to the diaphragm by the interaction of the signal output by the amplifier  160  (which charges the diaphragm) with the charged plates  170 . This part of the device operates in a similar manner to a known electrostatic loudspeaker. 
     So, by comparing FIGS. 1 and 2 it can be seen that the microphone acts in the same way as the DSM of FIG. 1, except that: 
     (a) the action of the filter  40  is provided by the mechanical response of the diaphragm  100 ; and 
     (b) the action of the comparator  10  is provided by the opposite responses of the diaphragm to incoming sound waves (an analogue signal) and the electrostatic forces applied by interaction with the charged plates  170 . 
     Accordingly, a one-bit signal representing the incoming sound signal is output from the delay  150 . 
     FIG. 3 schematically illustrates a microphone according to a second embodiment of the invention. 
     In FIG. 3, several of the parts  100 ,  140 ,  150 ,  160  and  170  are the same as those shown in FIG.  2 . However, rather than using an optical position sensor to detect the position of the diaphragm, a capacitative sensor is employed. 
     The capacitative sensing technique makes use of the capacitance between the diaphragm  100  and each of the plates  170 . A bridge arrangement is formed by connecting two further capacitors  200 ,  210 , of nominally identical capacitance, across the plates  170 . 
     A radio frequency (rf) source  220  is connected between the output of the driving amplifier  160  and the junction of the capacitors  200 ,  210 . The frequency of the rf source is selected to be well outside of the audio band—perhaps 5 MHz. A differential amplifier  230  is connected across the two plates  170 , with its output providing a position signal for input to the thresholder  140  as before. 
     An equivalent circuit is illustrated schematically in FIG. 4, where the capacitance between the diaphragm  100  and the plates  170  is illustrated as schematic capacitors  171 ,  172 . 
     As the diaphragm moves to one side, one of the capacitances  171 ,  172  increases and the other decreases. In this standard bridge arrangement, a voltage is developed across the inputs to the differential amplifier  230  indicative of the change in position of the diaphragm. This forms the position signal which is processed as described above with reference to FIG.  2 . 
     Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.