Electrical audio signal processing systems and devices – Circuitry combined with specific type microphone or loudspeaker – With electrostatic microphone
Reexamination Certificate
1997-11-12
2001-07-24
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Circuitry combined with specific type microphone or loudspeaker
With electrostatic microphone
C381S111000, C381S122000, C381S123000
Reexamination Certificate
active
06266424
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to battery circuits for electret microphones having uniform performance.
Electret microphones are widely used today in voice communications for their small size, light weight and high output. These microphones require a DC supply in order to operate because of an internal JFET circuit common to all electrets. Also available are the conventional dynamic microphones which are larger, heavier, with much lower output levels but require no external DC power.
In “integrated” or built-in applications such as corded, cordless and cellular telephones, electret mic bias is provided internally without difficulty. Stand-alone desk microphones or headsets are frequently used with telephones, PC sound cards or DVD (Digital Voice over Data) modems. There may be no provision to obtain DC bias from the input of the equipment that the microphone is connected to. If DC bias is an option, it will require reference to a technical manual and opening of the cabinet in order to move a switch or jumper to enable the bias path. Many consumers are reluctant to go inside the equipment and prefer a microphone solution that is ready to use.
A schematic diagram of a simple “battery box” circuit
100
for electret microphone operation is shown in FIG.
1
. The circuit includes an electret mic
102
having an electret element
104
and a JFET transistor
106
. A resistor
108
provides a DC path for the JFET drain current and isolates the audio output from the very low impedance of battery
110
. The JFET drain terminal has a very high output impedance, so the resistor
108
brings the output down to a known, lower level typically 1-5 k&OHgr;. The range of currents for the chosen electret element
104
will determine the required battery voltage, such that after the voltage drop through the resistor, enough voltage remains at the drain for the JFET to operate in the active region. Typical operating current for a Gentex Model 3065 electret microphone is 100-300 &mgr;A with R at 2 k&OHgr; from a 3V battery.
A capacitor
112
separates the DC path from the input of audio amplifier circuitry
114
and must be large enough for the input impedance of the amplifier and the lowest audio frequency of interest. For example, 1 &mgr;F will high pass 100 Hz into 1500 &OHgr; at −3 dB.
As long as the battery voltage in
FIG. 1
is sufficient to keep the FET in the active region, normal operation will continue. The user, however, has no indication or knowledge of battery condition and eventually reduced voltage will cause FET saturation, lower output level and audio distortion. Before the condition is severe enough to be recognized by the user, speech intelligibility to human listeners and speech recognition by software programs will be adversely affected. Adaptive software which “learns better as it goes” will begin storing distorted data at low battery voltages and when the batteries are replaced, the user may sound “foreign” relative to the stored information. If the circuit of
FIG. 1
were connected to equipment configured for dynamic microphones only, the much higher output level from the electret mic could cause overload and distortion.
SUMMARY OF THE INVENTION
Accordingly, in accordance with one embodiment of the invention there is provided a battery circuit for coupling a battery source to an electret microphone. The circuit includes impedance circuitry for providing appropriate bias impedance for a desired microphone sensitivity, and disconnect circuitry for automatically disconnecting the battery source from the electret microphone in response to detecting a low voltage state in order to prevent distortion and low output level of the microphone.
In an alternative embodiment of the invention there is provided a battery circuit for coupling a battery source to an electret microphone which includes first impedance circuitry for providing appropriate bias impedance for a desired microphone sensitivity, disconnect circuitry for automatically disconnecting the battery source from the electret microphone in response to detecting a low voltage state, and second impedance circuitry for driving corresponding inputs with minimal loss.
In another alternative embodiment of the invention there is provided a battery circuit for coupling a battery source to an electret microphone which includes first impedance circuitry for providing appropriate bias impedance for a desired microphone sensitivity, disconnect circuitry for automatically disconnecting the battery source from the electret microphone in response to detecting a low voltage state, second impedance circuitry for driving corresponding inputs with minimal loss, and a compensation circuit which operates to extend battery source life and prevent lower output at lower voltage.
REFERENCES:
patent: 5170151 (1992-12-01), Hochstein
patent: 5185789 (1993-02-01), Hanon et al.
patent: 5239579 (1993-08-01), Schuh
patent: 5577129 (1996-11-01), Ehara
patent: 5579397 (1996-11-01), Ikeda et al.
Papadopoulos Costas
Truesdell David J.
Mei Xu
Samuels, Gauthier & Stevens, LLP.
VXI Corporation
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