Information
-
Patent Grant
-
6427014
-
Patent Number
6,427,014
-
Date Filed
Friday, October 23, 199825 years ago
-
Date Issued
Tuesday, July 30, 200221 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Isen; Forester W.
- McChesney; Elizabeth
Agents
- Frommer Lawrence & Haug LLP
- Frommer; William S.
- Gordon Kessler
-
CPC
-
US Classifications
Field of Search
-
International Classifications
- H04R300
- H04R1904
- H04B1000
- H04B1002
-
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.
Claims
- 1. A microphone comprising:(i) a diaphragm movable in response to incident sound waves; (ii) a position sensor for generating an electrical position signal indicative of said position of the diaphragm; (iii) a thresholder for generating a one-bit digital signal indicating whether said position signal is above or below a threshold signal level; (iv) a delay for delaying said digital signal; and (v) a diaphragm driver for moving said diaphragm in response to said digital signal and in an opposite sense to motion of said diaphragm represented by said digital signal.
- 2. A microphone according to claim 1, in which said position sensor is an optical position sensor.
- 3. A microphone according to claim 1, in which said position sensor is a capacitative position sensor.
- 4. A microphone according to claim 3, in which said diaphragm driver comprises one or more electrically charged plates adjacent to said diaphragm, and a driver circuit for supplying an electrical signal to said diaphragm in dependence on said delayed digital signal.
- 5. A microphone according to claim 4, in which said position sensor comprises means for detecting a change in a capacitance between said one or more plates and said diaphragm.
- 6. A microphone according to claim 5, comprising two charged plates disposed on opposite sides of said diaphragm, a capacitance between each charged plate and said diaphragm forming a respective arm in a bridge measuring circuit.
Priority Claims (1)
Number |
Date |
Country |
Kind |
9722548 |
Oct 1997 |
GB |
|
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Number |
Name |
Date |
Kind |
4395593 |
Flanagan |
Jul 1983 |
A |
5051799 |
Paul et al. |
Sep 1991 |
A |
5548658 |
Ring et al. |
Aug 1996 |
A |
5621806 |
Page et al. |
Apr 1997 |
A |