Digital optical microphone and digital transducer

F.A. Ghelrnansarai

Indexing terms: Digital optical microphones, Digital transducers, Digital techniques, Optical techniques, Multireflection scanners

Abstract: The design of a new digital microphone using an optical technique is described. An inter- digital photoconductive detector is explained and a new optical scanner is designed for this micro- phone. The important parameters for developing a digital microphone are presented, followed by the required modifications to obtain a digital trans- ducer for the measurement of very small angles, displacements or pressure.

1 Introduction

High-quality audio systems use digital techniques to obtain the highest fidelity in reproduction. All existing microphones deliver analogue outputs proportional to the incident sound pressure. These analogue voltages are converted to digital signals by analogue to digital con- verters for use in digital mixing consoles, speech trans- mission, etc. Analogue microphones, however, have several disadvantages, the main ones being

(a) Occurrence of noise and distortion, which could be removed in a purely digital microphone

(b) it is relatively difficult to filter analogue signals compared with digital ones (A digital filter can be imple- mented as software, such as a subroutine on a digital computer and has inherent advantages of no drift, hand- ling low-frequency signals, no insertion loss, linear phase characteristics, . . .

(c) analogue microphones cannot be directly interfaced with digital systems.

The object is the design of a digital output microphone capable of directly converting a displacement of a dia- phragm into a digital signal by optical means. This device does not rely on an analogue to digital converter, thereby removing this intermediate stage and thus a potential source of noise and distortion.

The basic concept is the use of a laser beam (diode laser), that would be directed through an optical scanner and then incident on a photoconductive detector. The structure of the detector is designed in such a manner that it delivers a series of pulses which number depends on the scan length being a direct function of microphone membrane deflection (sound pressure). These pulses are converted to logic pulses and then counted by an up/down counter. The up/down input of the counter is controlled by a direction transducer. The outputs of the

0 IEE, 1995 Paper 1758G (E4 E10, E13), first received 11th May and in revised form 29th November 1994 The author is with the Department of Electrical Engineering and Electronics, Ferranti Building, UMIST, PO Box 88, Manchester M60 lQD, United Kingdom

I E E Proc.-Circuits Devices Syst., Vol. 142, No. 2, April 1995

counter display the amplitude of sound pressure in binary code.

2

A new design for a digital output microphone is devised, as shown in Fig. 1. This addresses two basic areas, that is the provision of an optical scanning system which can generate a relatively large scan angle and an optical detector, the output of which can be monitored to deter- mine the deflection of microphone membrane.

Design of a digital output microphone

[sound pressure J.

directional microphone diophragm transducer

Fig. 1 Block diagram of digital microphone

The digital microphone comprises a membrane which is displaced in response to sound generated variations in pressure, a reflective surface which is displaced in response to the displacement of the membrane, an optical scanner for directing the reflected beam to the detector (IDPC) and a detector for generating an output represen- tative of the membrane displacements.

When the IDPC is scanned by the laser beam, a series of pulses is obtained, the number of pulses depending upon the scan length of the IDPC. These pulses are con- verted to logic pulses and counted by an up/down counter. The up/down input of the counter is controlled by a direction sensor which detects the direction of the membrane displacements and activates an up or down input of counter.

A spherical capacitance transducer [l] is used as a direction sensor. A spherical conductor (steel ball) is used as a fixed electrode, and the moveable electrode is the microphone membrane. One of the main advantages of the spherical electrode is that the sample does not need to meet stringent, and often impractical requirements of being polished optically flat, since there is no parallel gap to be maintained. A stabilised DC potential is applied to the spherical electrode, and change in capacitance of the device results in a net charge flow to or from the elec- trode given by the relationship dq = Vdc. This charge flow is detected by a charge amplifier.

The author is grateful to Prof. L.E. Davis for his help and guidance during this research. Particular thanks are also owed to Dr E. Cohen.

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For example, for an 8 mm diameter electrode and a static air gap of lOpm, the transducer capacitance is 1.74 pF, and the sensitivity is 2.77 x lo-' Fm-', which for a 20VDC bias voltage and a charge amplifier producing 250mV pC-' leads to a device voltage sensitivity of 0.1385 mV nm-' [l-31.

Fig. 2 illustrates the direction sensor. Any vibration of a moveable electrode (membrane) results in a charge flow

counter

LWAQ '1

Fig. 2 Direction sensor

to or from the electrode and the output polarity of the charge amplifier would be positive or negative, respec- tively. This polarity variation is converted to logic pulses by a comparator. For positive and negative polarity, the output of the comparator will be almost 5 V (high) or 0 V (low), respectively. The output of the comparator is con- nected to the up/down input of the counter. Therefore, when the microphone membrane moves in or out, the up or down input of the counter is activated, respectively.

3 Interdigital photoconductive detector (IDPC)

The structure of an IDPC is illustrated in Fig. 3. Metallic electrodes are deposited in an interdigital pattern on the

and electronic circuits

I

Referring to Fig. 3, the photodetector comprises a piece of semiconductor being divided into alternate con- ducting (InGaAs) and nonconducting (AI) sections spaced apart along its length. The conductivity of the semicon- ductor is greater when a conducting (InGaAs) section is illuminated than when a nonconducting (AI) section is illuminated [4-71.

The structure of the detector provides different amounts of laser beam reflection (and therefore transmission) in two consecutive sections. Consequently, there are two values of resistance R , and R, that corre- spond to the measured resistance at contacts when the conducting layer and aluminium finger are illuminated, respectively (R, < R,).

For instance, the measuring resistance of R, and R 2 for an n-type Ino,,,Gao~,,As with a free carrier concen- tration of 4.13 x loz1 m-,, d = 2 pm, w = 2.375 mm and 1 = 125pm (see Fig. 3) were 1.118 kn and 1.132kQ respectively. The measurement was carried out by using a Reichert microscope that employs a 12 V halogen lamp [2]. The structure of Fig. 3 may be processed by liftoff or etching.

The resistance variation of IDPC can be converted to logic pulses and then counted by an up/down counter, as shown in Fig. 4 [2].

First, the resistance variation is converted to an alter- nating voltage, and then to logic pulses by a comparator. The output of the comparator is connected to the clock (CK) input of an up/down counter.

When the sound pressure is zero, the laser beam is focused on the central metallic (AI) electrode. The nega- tive sound amplitude is displayed in 2s complement code. It is possible to modify the circuitry to generate the sign- magnitude code, which requires two separate detectors which are arranged adjacent to each other. One of the detectors is used to detect the positive sound pressure and the other to detect negative sound pressure. Fig. 5

Fig. 3 Structure of IDPC

surface of the conducting layer so that the alternate elec- trodes can be connected to a power supply. In this manner, the carriers generated between the conducting layers have the shortest distance to travel before they are collected. The conducting layer is an n-type indium gallium arsenide (Ino.53Gao.4,As) that is grown in a semi-insulating substrate of indium phosphide.

detector

Fig. 5 Arbitrary sound waveform

illustrates a waveform corresponding to an arbitrary sound. It will be seen that the waveform is rising in the time interval t o , and t , is falling in the time interval t,, t2 is rising in the time interval t 2 , t , is falling in the time interval t , to t , and then rises in the time interval t , and t,. When the amplitude of sound is zero ( to. t , and t,) the laser beam is on the central aluminium electrode.

direction sensor

I Fig. 4

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Circuit diagram ofdigital microphone

I E E Proc.-Circuits Devices Sysf., Vol. 142, No. 2, April 1995

When the sound amplitude is increasing between times to and t,, the laser beam will scan the detector. Scanning the detector in the time interval to and t, generates a series of pulses. These pulses are converted to logic pulses by the comparator. The resultant signals are applied to the clock input of upldown counter. Since the micro- phone membrane moves in, the output of the direction sensor is therefore high and this activates the up input of the counter. Thus, the counter counts up all the pulses occurring in the interval between to and t, and its output follows the increasing amplitude between to and t,.

In the interval t, to t,, the displacement direction of the membrane changes (moves out), and the laser beam retraces its path across the detector. Since the displace- ment direction of the membrane has been reversed, the output of the direction sensor will be low and therefore the down input of the counter will be activated. The acti- vated counter thus decrements.

In the interval between t, and t , , the up input of the counter is activated and the counter counts up. Between t3 and t4, the counter counts down. In the interval between times t4 and t,, the same counting process would be repeated and the outputs of counter display the 2s complement of the sound pressure.

Thus, this design provides a precise indication of the position of the laser beam on the photodetector. This indication can be used directly to generate a digital audio output which can then be further processed as appropriate.

Increasing the number of output bits increases the sensitivity and the dynamic range of microphone. Each additional bit adds 6 db of dynamic range. A 16-bit system would give a dynamic range of 96 db.

4

A novel type of optical scanner was devised to scan the IDPC. This scanner implements a pre-objective scanning that comprises a multireflection scanner, a beam expan- der and a scan lens. The optical scanner not only scans the IDPC without using any analogue driving voltage, but also magnifies the angular movement of the mem- brane.

4.1 Multireflection scanner The basic concept is the use of the angular displacement of a diaphragm and its magnification by means of multi- reflection by the diaphragm. Fig. 6 shows the multi- reflection scanner. A metal reflection coating such as AI is applied on the surface of the membrane by evaporation. A fixed mirror is positioned in front of the diaphragm such that the laser beam is reflected a number of times between the two mirrors.

The drawback of this scanner is the nonlinearity. The angle of the reflected beam in each reflection is varied by k20 (0 is the angular deflection of membrane), -20 where the membrane moves in (amplitude of sound wave is positive) and +20 where the membrane moves out (amplitude of sound wave is negative). It can be shown that the total number of reflections (n,) for positive sound amplitude will be more than the number of reflections (n2) for negative sound amplitude (n, > n,) [2]. The non- linearity is proportional to the membrane deflection. The maximum nonlinearity occurs for the maximum displace- ment of the membrane [2].

The nonlinearity can be analysed mathematically [2] and it can be shown that it is proportional to the gap between the membrane and the stationary mirror, the

Optical scanner and a beam expander

IEE Proc.-Circuits Devices Syst., Vol. 142, No. 2, April 1995

first incident angle of the laser beam upon the membrane, the number of the reflections, and the angular displace- ment of the membrane [2]. The optimum values for the

d

- D - Fig. 6 Multireflection scanner

above parameters can be designed to minimise the non- linearity.

The nonlinearity of the scanner can be fully compen- sated by employing different lengths for the conducting and nonconducting (AI) sections on the detector [2].

4.2 Beam expander The beam expander comprises two circular mirrors, one to diverge the beam and the other to collimate it [2]. The focal points of diverging and collimating mirrors (F, and F, , respectively) should be coincident (see Fig. 7)

Dld = r2/r1

Fig. 7 Beam expander

where D is the expanded beam diameter, d is the beam diameter before expansion, r2 and r , are the radius of the converging (collimating) and diverging mirrors, respec- tively. C, and C, are the centres of curvature for diverg- ing and converging mirrors, respectively, and f2 is the focal length of collimating mirror.

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The beam expander is located after multireflections by the membrane and before the scan lens [2].

5

This Section briefly describes the important parameters in the design of a digital microphone. A ten-bit digital output microphone requires 1022 (2" - 2) conducting and 1023 (2" - 1) nonconducting sections (N is the number of output bits). n-type In,,,,Ga,,,,As with a free carrier concentration of 5 x ioi4 cm-,, 1 = 2 pm, w = 20 pm, d = 1.5 pm, may be employed as a photo- conductive detector (Fig. 3). The total length of the detec- tor will be 4.090 mm (assuming an equal length for conducting and nonconducting sections).

To calculate the required response time for the above detector, we consider the worst case, i.e. a sine wave sound with a high frequency (f= 15 KHz) and a large amplitude that is capable of scanning all the conducting and nonconducting sections on the detector. (It may be noted that this worst case assumption is unrealistic because high-frequency sound waves usually have a small amplitude). In this case the travelling time of the laser beam upon each individual section will be [2]

Design of a ten-bit digital microphone

1/4(15 KHzX1022.5) = 16.3 ns

Therefore, the response time of the detector must be less than 16.3 ns.

Assuming a carrier life time of 1 ns, the bandwidth and the gain-bandwidth product of the detector are equal to 159 MHz and 2.4 x 10" s-' [2, 4, 61. Consequently the gain of the detector will be 150.

The length of the positive part of the detector (for measuring the positive part of the sound wave) is half that of the total length, i.e. 2.045 mm. Suppose the focal length of the scan lens is 5 c m , then, the scan angle for scanning the positive part of detector is 2.34" (tan-' 2.045/50). By selecting a membrane radius of lOmm, the angular deflection of the membrane for a maximum membrane displacement of 30 pm (without causing overload) can be found as [2]

tan-' 30 x lO-,/lO = 0.17"

The number of reflections between the membrane and the stationary mirror is given by

2n(0.17") = 2.34",

The scan angle for seven reflections is changed to 2.38". The distance between the membrane and the station-

ary mirror is assumed to be 1 mm, and the first incident angle upon the membrane is taken at 30" (the minimum nonlinearity occurs around 30" [Z]).

To obtain seven reflections by the membrane, the computed length of the stationary mirror when the mem- brane moves in (approaches to stationary mirror) is 6.53 mm [2].

The required length of the mirror, when the membrane moves out is 7.33 mm [2]. If the length of the stationary mirror is assumed to be 6.53 mm, the number of reflec- tions for a negative sound wave (membrane moves out) will be six [2].

Thus the maximum scan angle for positive and nega- tive sound amplitude will be. 2.38" and 2.04", respectively.

The length of the positive part of the detector is 2.045 mm. Therefore, the required focal length of the scan lens should be 49.2 mm (2.045/tan (2.38)). This focal length with a negative scan angle of 2.04" produces a

n = 6.88 or 7 reflections

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scan length equal to 1.752mm for the negative part of the detector.

Since the number of conducting and nonconducting sections on the negative part of the detector is 1022.5 (0.5 indicates that one half of the central AI section is on the negative part of the detector because when the amplitude of sound wave is zero, the laser beam is focused on the middle of the central AI electrode), to compensate the nonlinearity, the length of the mentioned sections should be modified to

1.752 mm/1022.5 = 1.7 pm

Consequently, the length of the conducting and noncon- ducting sections on the positive and negative parts of the detector are 2 pm and 1.7 pm, respectively.

As stated earlier, in the worst case the travelling time of the laser beam upon each individual section is 16.3 ns which means the period of the generated pulses at the output of the detector is 32.6 ns (the travelling time of the laser beam upon a pair of conducting and nonconducting sections). Consequently, the rise (fall) time of the direction sensor should be equal to or less than 32.6 ns [2].

A direction sensor with a 4 mm diameter electrode, a static air gap of 4 pm, a 20 V DC bias voltage and a charge amplifier producing 250 mV pC- ' provides a sensitivity of 0.09 mV nm-' and a bandwidth of 5 MHz [l, 2) that can be employed in a ten-bit digital micro- phone.

6 Digital transducer

The digital transducer uses the same principles as those defined earlier. It can be seen that the concept defined in the earlier application has wider applications than the digital microphone. For example, such a system can be used as a digital angular transducer, a digital displace- ment transducer or a digital pressure transducer.

A digital angular transducer may present a higher resolution in measuring small angles in comparison to the conventional angular transducer that uses the prin- ciple of interferometry, [2, 51. Referring to Fig. 8, a

R I

Fig. 8

collimator

Digital angular transducer

diagram of the digital angular transducer is shown. A mirror is pivoted at one end at a fixed point. A laser beam is reflected a number of times between the pivotal mirror and the fixed mirror, and emerges to pass through a beam expander and a scan lens to the detector. The same principle of detection is used, as described in the earlier application. This arrangement can be used to measure small angular displacements of the movable mirror or an object fixed to it. The angular displacement 0 is magnified by the multireflection of the beam. Hence, the scanning length of the detector is a direct function of 0 and the value of 6 can be delivered as a digital binary

IEE Proc.-Circuits Devices Syst., Vol. 142, No. 2, April 1995

output. Any nonlinearity can be compensated by using different lengths of conducting and nonconducting sections on the detector [2].

7 Conclusions

This paper studies the design of a novel type of micro- phone and, generally, a transducer capable of delivering digital output without employing an analogue voltage or analogue to a digital converter, thereby removing this intermediate stage and thus a potential source of noise and distortion.

An IDPC is used as an optical detector that provides a high-gain bandwidth product. Also a novel type of optical scanner is based on multireflections used for optical scanning.

The digital microphone does not require complicated signal processing and can be developed to 16-bit output.

The digital microphone may be developed as a digital transducer. The digital transducer uses the same prin- ciples. For example, such a system can be used as digital

IEE Proc.-Circuits Devices Syst., Vol. 142, No. 2, April 1995

angular transducers, a digital displacement transducer or a digital pressure transducer.

8 References

1 AINDOW, A.M., COOPER, J.A., DEWHURST, R.J., and PALMER, S.B.: ‘A spherical capacitance transducer for ultrasonic displacement measurement in NDE, J . Phys. E. Sci. Instrum, 1987, 20, pp. 204-209

2 GHELMANSARAI, F.A.: ‘A digital microphone using an optical technique’. PhD thesis, UMIST (University of Manchester Institute of Science and Technology), 1993

3 SMYTHE, W.R.: ‘Static and dynamic electricity’ (Maraw-Hill, 1968.3rd edn.)

4 FORREST, S.R.: ‘The sensitivity of photoconductor receivers for long wavelenath ovtical communications’, J . Lightwave Technol., 1983, LT-3, (2 i pp. j47-360

1992 5 GHELMANSARAI, F.A.: ‘Digital transducer’. UK Patent 9209141,

6 SLAYMAN, C.W., and FIGUEROU, L.: ‘Frequency and pulse response of a novel high speed interdigital surface photoconductor (IDPC)’, IEEE Electron Darice Lett., 1981, 2, (9, pp. 112-114

7 YEH, C.: ‘Handbook of fiber optics theory and applications’ (Academic Press, 1990)

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