MXPA97000353A - Expansion of microphone to reduce noise defo - Google Patents

Abstract

An apparatus for reducing background noise in a telecommunications device is described, the telecommunications device includes a microphone for receiving corresponding electrical signals thereof, the acoustic signals include speech signals and noise signals. The apparatus of the present invention in one embodiment includes an amplifier having an input to receive electrical signals from the microphone and an output, the amplifier operates to receive the electrical signals at the first input and provides amplification to the electrical signals to produce electrical signals amplified The amplifier further operates to provide amplification to a first gain level and a second gain level, the first gain level exceeding the second gain level. The apparatus also includes a feedback device connected to the output of the amplifier and is also operably connected to control the operation of the amplifier. The feedback device operates to cause the amplifier to provide amplification of the first gain level when the feedback device receives amplified electrical signals at the output that have an amplitude lower than a predetermined threshold and causes the amplifier to provide amplification at the second gain level when the feedback device receives amplified electrical signals at the output that has an amplitude lower than the predetermined threshold

Description

MICROPHONE EXPANSION TO REDUCE BACKGROUND NOISE Field of the invention The present invention relates to the field of telephony, and in particular to the control of background noise reduction based on the detection of an audible speech signal presented to a microphone of the subscriber's receiving apparatus.

Background of the Invention An inherent problem in telephony is the presence of background noise. In general, the microphone of a telephone receives both vocal frequency signals and any background noise generated in the environment in which the microphone is located. Not only is background noise annoying to the calling party, but the called party is also disturbed by the effect that background noise has on the voice frequency signals received by the called party. In conference calls, each speaker is exposed to the combined background noise of all the speakers, and therefore the background noise is particularly disturbing. REF: 23684 The problem of background noise for the so-called subscriber also affects the feedback within the subscriber receiving apparatus known as "sidetone". The sidetone is a small portion of the caller's voice signal that is called backward to the telephone receiver to prevent the phone from sounding "dead" to the caller. The lateral tone also allows the speaker to adjust his voice to a desirable level. In this way, the conversations sound normal to the calling subscriber, even when one of the ears of the calling party is blocked to receive the vocal signals of the calling party through the air. However, when the calling party is not speaking, the sidetone operates to feed back the background noise of the calling party's environment which may interfere with the ability of the calling party to hear the vocal signals of the called party. Instinctively, the caller can place his or her hand over his or her other ear not placed over the telephone to block background noise. This technique does not work because the lateral tone allows background noise to be transmitted through the telephone receiver and into the ear of the caller who is listening. Several devices and methods have been developed to minimize or eliminate the problems associated with background noise. In the North American patent no. 3,691,311, for example, describes a telephone apparatus in which various levels of attenuation (or gain) are applied to the amplified signals received by the microphone of the subscriber's receiving apparatus. Specifically, the attenuation of the signals is low (high gain) when a speech signal is detected, and the attenuation of the signals is high (low gain) in the absence of a speech signal to effectively suppress background noise when the calling subscriber He is not speaking into the microphone. Another telephone for suppressing background noise when the calling subscriber is not speaking is described in the North American patent no. 5,235, 637. Another device, microcircuit Universal Vocal Signal Conditioner from AT &T Corp., Part No. LB1068AC / AW ("Conditioner"), intended to be used with commercial loudspeakers and to accommodate side tone and suppress noise background by varying the attenuation levels depending on the strength of the microphone signal. The Conditioner includes a variable gain amplifier that provides different levels of gain depending on whether the Conditioner detects the presence of vocal signals or background noise. In the AT & T Conditioner, a detection circuit determines whether an input signal is a voice frequency signal or simply background noise. The input signals are then provided to the variable gain amplifier. The detection circuit sets the gain level of the amplifier. Because the detection circuit operates with the input signal, the detection circuit must pre-amplify the input signal to levels suitable for detection. Up to this point, the Conditioner includes a preamplifier in the path of the detection circuit. The time it takes the Conditioner to switch from a high gain to a low gain, known as the attack time, is fourteen milliseconds. Although such relatively short attack times are acceptable in commercial loudspeakers and other applications, it is possible for the vocal frequency to be suppressed between syllables of voice frequency. It should be noted that the time it takes for a noise suppressor such as the AT &; T switching from a low gain to a high gain, known as the decay time, is also important. If the decay time is very long, the result is that the vocal frequency can be silenced by the undesirable suppression. Thus, it is desirable to provide a noise suppressor having attack and decay times that do not result in the suppression of the vocal frequency.

The phone number of the North American patent no. 4,847,897 aims to eliminate the suppression of inadvertent vocal frequency. The North American patent no. No. 4,847,897 discloses a telephone having a noise detector, an expander and a variable gain amplifier, which provides varying levels of amplification depending on whether speech frequency is detected or simply background noise. The noise detector distinguishes the background noise of a speech frequency signal by examining the time-averaged characteristics of the electrical signal emanating from the microphone. The expander, in response to the detection of background noise, reduces the gain of the amplifier, and, in response to the detection of a speech frequency signal, increases the gain of the amplifier. More specifically, the telephone number of the North American patent no. 4,847,897 aims to avoid suppression of the vocal frequency by providing the circuits to compare a short-term average (approximately fifty milliseconds) of the microphone signal to a long-term average (approximately four seconds) of the microphone signal, assuming that the vocal frequency is usually not continuous for more than four seconds, and the interruptions of the vocal frequency usually do not exceed fifty milliseconds. However, like the AT & T Conditioner, noise detection is affected using the microphone signal, before the signal is passed through the variable gain amplifier. Also, the technique of averaging over time introduces additional costs to the telephone as well as prolonging the time to effect the average. Therefore, it is desirable to provide a noise suppressor that is inexpensive to manufacture and whose suppression method is efficient and easy to perform. It is also desired that a noise suppression apparatus be configurable to be capable of being used in a variety of environments. For example, the apparatus should be capable of suppressing the noise of commercial telephones, such as commercial loudspeakers, in which background noise can be particularly annoying to the called subscriber. The device should also be applicable to residential telephones, both traditional telephones and cordless telephones. In addition, it is desirable that the noise suppression apparatus be useful for cell phones.

Brief Description of the Invention The present invention provides a microphone expander that provides a gain adjustment that depends on whether speech signals are present. In particular, the signals are amplified by means of a high gain level or a low gain level depending on the level of a detected microphone signal. The microphone signal is detected after the gain adjustment has occurred, thereby reducing the required circuit components and also providing a hysteresis effect. A preferred embodiment of the present invention is an apparatus for reducing background noise in a telecommunications device, the telecommunications device includes a microphone that receives both vocal signals and background noise signals. The microphone is also operable to convert speech signals and background noise signals into electrical signals. The apparatus includes an amplifier having an input to receive the electrical signals and an output, the amplifier can be operated to receive electrical signals at the input and provide amplification to the electrical signals to produce amplified electrical signals. The amplifier can be further operated to provide amplification to a first gain level and a second gain level, wherein the first gain level exceeds the second gain level. The apparatus also includes a feedback device connected to the output of the amplifier and further operably connected to control the operation of the amplifier. The feedback device can be operated to cause the amplifier to provide amplification to the first gain level when the feedback device receives amplified electrical signals in the output having an amplitude greater than a predetermined threshold and causing the amplifier to provide amplification to the second level of gain when the feedback device receives amplified electrical signals at the output that have an amplitude less than a predetermined threshold. The characteristics discussed above, as well as the additional features and advantages of the present invention, will be more readily apparent by reference to the following detailed description and the accompanying drawings.

Brief Description of the Drawings Figure 1 illustrates a block diagram of an exemplary embodiment of a telephone handset of a telephone according to the present invention; Figure 2 illustrates a schematic block diagram of an exemplary embodiment of the microphone expander according to the present invention; Figure 3 illustrates a gain ratio of an operational amplifier used in the exemplary microphone expander circuit of Figure 2; and Figure 4 illustrates with further details a schematic diagram of the exemplary microphone expander of Figure 2.

Detailed Description of the Drawings Figure 1 shows a block diagram of an exemplary embodiment of a telephone set 10 according to the present invention. In this embodiment, the telephone set 10 comprises a wireless telephone set for use with a telephone base unit 11 and is functionally divided into a microphone portion 12 and a receiving portion 14. The telephone set 10 includes a conventional microphone 16 for receiving acoustic signals of the calling subscriber and any background noise located in the environment, and produce electrical signals from them. The electrical signals of the microphone 16 are amplified by an audio amplifier 17 and then provided to a microphone expander 18. The telephone set 10 is designed to operate in conjunction with the base unit 11, which communicates with the telephone set 10 via Radio frequency or RF signals. The base unit 11 is further connected to a telecommunication network, not shown. In general, the microphone expander 18 includes an amplifier 18a and a feedback device 18b. The amplifier 18a is a variable gain amplifier having an input to receive the electrical signals and an output connected to the compressor 20. The amplifier 18a according to the present invention operates to receive electrical signals and provide amplification to the received electrical signals to a First level of gain and a second level of gain, the first level of gain exceeds the second level of gain. The feedback device 18b is connected to the output of the amplifier and is operably connected in addition to control the operation of the amplifier 18a. Specifically, the feedback device 18b causes the amplifier 18a to provide amplification to the first gain level when the feedback device 18b receives amplified electrical signals at the output having an amplitude substantially greater than a predetermined threshold. Similarly, the feedback device 18b causes the amplifier 18a to provide amplification to the second gain level when the feedback device receives amplified electrical signals at the output having an amplitude substantially less than the predetermined threshold. In a preferred embodiment, discussed in detail below in connection with Figures 2 and 3, the feedback device 18b further operates to cause the amplifier to provide amplification to a third gain level when the feedback device 18b receives amplified electrical signals in the output having an amplitude approximately equivalent to the predetermined threshold. The amplified electrical signal generated by the microphone expander 18 is then compressed and shaped by a compressor 20 and a frequency shaper 22, respectively, by means and apparatus well known in the art. The compressed, shaped signal is then transmitted from the microphone portion 12 of the telephone set 10 to the base unit 11 by means of an RF transmitter 24. The base unit, as is well known in the art, typically includes an RF receiver, RF transmitter, frequency shaper, audio amplifier and hybrid circuit. The base unit 11 provides the signal transmitted to the telecommunication network, which in turn provides the signal to another phone, not shown. The base unit 11 also receives signals from other telephones through the telecommunication network. These signals are transmitted to the receiver portion 14 of the telephone set 10. The receiver portion 14 is comprised of components well known in the art. Specifically, the receiver portion 14 of the telephone apparatus 10 includes an antenna 26 and an RF receiver 28 to receive the RF signals generated by the base unit 11. The received signal is then shaped and expanded by a frequency shaper 30 and a expander 32, respectively. The shaped and expanded signal in the portion of the receiver 14 is then provided through an audio amplifier 34 to a headset 17 for displaying the audio signals to the calling subscriber. In addition, the calling subscriber is able to hear his own voice generated by the microphone 16 through the coupling of the lateral tone. The signals generated by the calling subscriber in the microphone portion 12 are provided to the base unit 11 through the RF transmitter 24 as described above. The circuits within the base unit 11 couple and transmit a portion of the signal transmitted back to the receiver portion 14, providing lateral tone. Referring now to Figure 2, there is illustrated a partial schematic diagram of an exemplary embodiment of the microphone expander 18 according to the present invention. The basic elements of the amplifier 18a include the resistors RA, RB and RF and an operational amplifier ("op-amp") 44. The feedback device 18b further includes a trip circuit 40, a low pass filter and a rectifier 42. The microphone expander 18 also includes an IN input and an OUT output. The op-amp 44 has first and second stages 45 and 46, respectively, and a common output stage 47. In general, the expander 18 is designed to provide background noise reduction during periods of silence, i.e., when the subscriber who is calling is not speaking into the microphone. The low pass filter comprises a resistor 51 and a capacitor 52 and is connected between the rectifier 42 and the trigger circuit 40. The trigger circuit 40 in the exemplary embodiment includes two bipolar junction transistors in differential pair configuration ("torque"). differential "). The trip circuit 40 is connected to a reference voltage, VREF at the input 54 and is operably connected to the op-amp 44 to controllably switch between the activation of the first input stage 45 and the second input stage. 46 in the form that will be described more fully later. In other words, the trigger circuit 40 is operably connected to control the gain applied by the microphone expander 18.

The first and second input stages 45 and 46 are each in an inversion configuration, sharing the same output stage 47. An RF feedback resistor connects the output OUT to an inversion input INI of the first input stage 45. INI inversion input of the first input stage 45 is also connected to a resistor RB, which in turn is connected to the IN input through another resistor RA connected in series. There is also a connection line 49 between the intersection of the resistors RA and RB and an inversion input IN2 of the second input stage 46. Thus connected, a first feedback path is defined in this RF mode, a second path of Feedback is defined in this modality by RF and RB. The gain of the IN input to the OUT output is determined by the ratio of the effective feedback resistance of the feedback path divided by the effective input resistance. Depending on which input stage 45 or 46 is selected by the trigger circuit 40, the gain is set according to the following relationships: For Low Gain: G = RF / (RA + RB) For High Gain: G = (RF + RB) / RA where V0Ut = VING.

In operation the microphone expander 18 receives electrical input signals VIN consisting of electrical vocal signals from the microphone circuit at the IN input and produces output signals V0Ut consisting of amplified electrical signals at the OUT output. To produce the amplified electrical signals, the Vout of the microphone expander amplifies the VrN by one of two high gain levels, or a low gain level. The selection of which gain level is applied is effected by the firing circuit 40, which selectively activates one of the two input stages 45 and 46, or some combination thereof. Op-amp 44 provides a low gain level when the first input stage 45 is inactive and the second input stage 46 is active, and provides a high gain level when the first input stage 45 is active and the second entry stage 46 is inactive. In general, the microphone expander 18 provides a high gain level when it detects output signals V0Ut at its output that exceed a predetermined threshold, which is indicative of vocal frequency or active conversation. In contrast, the expander 18 provides a low gain level when it detects output signals V0ut below the threshold, which is indicative of no vocal frequency, or in other words a pause in a conversation.

The input signals V: N are provided to each of the input stages 45 and 46 at the inputs INI and IN2, respectively. A suitable deviation signal VC0M is applied to the non-inverting inputs of each of the input stages 45 and 46. The op-amp 44 provides at its output OUT the speech signal amplified by the gain level associated with any of the stages of entry 45 and 46 that is active. According to this mode, the high gain level is 6dB and the low gain level is OdB. The 6dB difference between high and low gain provides more uniform sound transitions than is possible with higher gain differences. In particular, if the gain difference is much greater, for example, 20dB, the transitions between the high and low gain may be more noticeable. In addition, the use of a large gain difference can make it more difficult to differentiate the active conversation from the noise at the OUT output of the op-amp. However, if the expander 18 is implemented in a loudspeaker, a gain difference of 20 dB may be more appropriate to achieve a high level of suppression. The amplified electrical signal Vout at the output OUT is provided as feedback through the rectifier 42, which in the present embodiment is a half-wave rectifier. The low pass filter then flattens the rectified signal to provide a voltage level substantially cd VRMS which is indicative of the amplitude level of the signal V0Ut provided at the output OUT. The voltage level cd VMS is applied to the input 53 of the circuit 40, which compares this signal with a reference voltage level VREF. In the exemplary mode, the half-wave rectifier provides an inverted rectified output, and thus, VMS is lower (more negative) for signals of greater amplitude V0ut-Co ^ result, if VRMS is substantially smaller than VREF, then V0ut it has an amplitude greater than the predetermined threshold and, as a result, the trigger circuit 40 provides a signal on the line 56 which makes the first input stage 45 activate. If, however, VRMS is substantially greater than VREF, then the trigger circuit 40 provides a signal to the input 56 of the op-amp 44 which causes the first input stage 45 to be inactivated and the second input stage 46 to be activated. . If the VMS level is somewhat close to the VREF level, then the trigger circuit 40 causes a combination of the first and second input stages 45 and 46 to be activated, which effectively provides a gain level between the levels of high and low gain. In particular, when both the first and the next input stages 45 and 46 are active, a portion of VIN receives low gain amplification and a portion of VIN receives high gain amplification, thereby providing a total gain level between the high and low gain level. The choice of the appropriate VREF and VCOM voltage levels will necessarily depend on the particular implementation, including the measured amplitude of the output signals for both active voice frequency and background noise input signals. Those skilled in the art can easily determine the appropriate levels for their implementation. It should be noted that the use of an inversion rectifier such as rectifier 42 will be by way of example only. Those skilled in the art can easily employ a non-inverting rectifier, in the case in which the trigger circuit 40 could be configured to activate the first input stage 45 when VRS is greater than VREF. Figure 3 shows the gain of op-amp 44 as a factor of the difference between VRMS and VREF. The curve between the point at which VRMS "VREF and VRMS" VREF allows a more flat or uniform transition between the low and high gain levels. The smooth transition causes a less abrupt gain switching than in other circumstances could cause a telephone conversation to be interrupted. The response shown in Figure 3 is caused mainly by the output response of the trigger circuit 40 of Figure 2. The trigger circuit 40 according to an exemplary embodiment described below in relation to Figure 4, comprises two transistors of bipolar junctions commonly connected in their respective emitters. The nature of the differential pair response of the bipolar junction transistor typically has the shape illustrated in FIG. 3. In an alternative embodiment, the firing circuit 40 may suitably have a step response that a uniform transition is not required. by the application. Another advantage of the present invention, not illustrated in Figure 3, is the hysteresis created by the measurement of the strength of the microphone signal at the amplifier output of the expander 18a. Consider a situation in which large amplitude signals indicative of active conversation are being provided to the IN input and the amplifier is provided with high gain. Small downward fluctuations or slight reductions in signal strength do not produce the activation of the second activation stage 46 (and reduce the gain) because the slightly reduced signal receives a high gain, which maintains V0Ut at a level greater so that VRMS is sufficiently lower than VREF. If a true low amplitude signal is applied to IN, then VRMS does not rise above VREF and expander 18 activates the second input stage 46. The hysteresis thus provides an improved operation by reducing the number of inadvertent or undesirable transitions. between the high and low gain levels. Also important for any suppression of background noise are the attack and decay times for the application of noise suppression. In the modality of Figures 2 and 3, both decay and attack times are 100 microseconds. The attack and decay times are determined by the time constant of the low pass filter created by the resistor 51 and the capacitor 52. The preferred attack and decay times were determined experimentally using human subjects and obtaining their subjective judgment on the application of several times. It should be noted that the microphone expander 18 of the present invention has attack and decay times that are significantly less than those of the prior art. For example, the AT & Conditioner; T, previously described here has an attack time of 14 milliseconds and a decay time of 140 milliseconds. Thus, the resolution of the present invention is much finer than that of the prior art, also resulting in many more occurrences of background noise suppression in a vocal frequency pattern as in the prior art. The rapid response results in fewer instances of undesirable vocal frequency suppression.

Figure 4 shows in greater detail a first embodiment of the microphone expander 18 that operates according to the present invention. Where appropriate, the reference numbers used in Figure 2 will also be used to identify the corresponding circuit elements in Figure 4. Unless otherwise specified, any transistors referred to hereinafter are bipolar junction transistors, each of which has a base, collector and emitter as is known in the art. The elements shown in Figure 4 can be suitably integrated on a single semiconductor substrate. In an alternative embodiment, however, the feedback resistances RA, RB and RF are not integrated on the substrate, but instead are externally connected to allow adjustment of the gain level by circuit designers. In addition to the circuit elements described below, the expander 18 also includes DC, FF (earth), COM, and BA voltage deviation points, signal points INI, IN2, RCT, RMS, and REF. The expander 18 as illustrated in Figure 4 also includes first and second current mirrors 53 and 54. In a semiconductor implementation of the expander, the deviation point voltages are all provided externally directly or indirectly through, for example. , of connections to spike or pin. In the following description, the voltage of any deviation point or signal point is represented as Vx where x is the reference used to identify the deviation point or signal point. For example, VCOM represents the voltage at the COM point. The trigger circuit 40 comprises the transistors Ql and Q2 commonly connected to their respective emitters. The trigger circuit 40 further includes another transistor Q4, the collector of which is connected to the emitters of the transistors Q1 and Q2. The emitter of Q4 is connected to ground. The base of transistor Ql is connected to point REF. The REF point is connected to COM through Rl, and to FF (earth) through a transistor Q3 and resistors R2A and R2B connected in series. The base of transistor Q3 is connected to the reference voltage point BA. Connected in this way, the voltage VBA forward transistor Q3 forward, and the resulting voltage drop through the emitter of transistor Q3 and resistors RIA and R2A set the current level through the collector-emitter junction of transistor Q3 . This current level defines the current through the resistor Rl, which therefore defines the voltage drop across Rl. Consequently VREF / which is equal to VCOM minus the drop on the resistance on the resistor Rl, can be set by altering the value of the resistance Rl. For example, by increasing the resistance of Rl, the voltage drop across Rl increases, and thus VREF decreases. In the present embodiment, the resistors Rl, R2A and R2B are each of 25 ohms. In addition, the voltage drop of the base to the emitter of transistor Q3 is approximately 0.75 volts. In addition, VCOM is approximately 1.8 volts and VBA is approximately 1.0 volt, all of which are provided externally. In this embodiment, then, the current through R2A and R2B, also through R1 is about 5 microamperes. As a result, VREF equals 1.8 - (5 X 10"6) (25k) or 1675 volts.The output of the trigger circuit 40 is the combination of the collectors of the transistors Ql and Q2.A part of the output, the collector of the transistor Ql is connected to the first current mirror 53. The other part of the output, the collector of the transistor Q2 is connected to the second current mirror 54. The first current mirror 53 is subsequently connected to the first input stage 45. , and the second current mirror 54 is further connected to the second input stage 46. In further detail, the first current mirror 53 consists of two PNP transistors Q5 and Q12, the emitters of which are connected to CC, and the bases which are connected to each other. The collector of transistor Q5 is connected to the output of the differential pair, and in particular, the collector of transistor Ql. The base and the collector of transistor Q5 are also connected to each other. The second current mirror 54 also consists of two PNP transistors Q6 and Q7, the emitters of which are also connected to the DC bypass point and the bases of which are connected to each other. The base and the collector of transistor Q6 are also connected to each other. In addition, the collector of transistor Q4 is connected to the output of trigger circuit 40, and in particular, to the collector of transistor Q2. The first input stage 54 consists of two transistors Q10 and Qll connected in the form of a differential pair, with a differential pair input, the base of the transistor Q10, forming the non-inverting input and the other input of the differential pair, the base of the transistor Qll, forming the investment entry. The non-inverting input is connected to COM and the investment input is connected to INI. The second input stage 46 includes two transistors Q8 and Q9 configured in a manner analogous to that of the first input stage 45, except that the inverting input of the second input stage is connected to IN2. The first and second input stages 45 and 46 each have the outputs connected to the op-amp of the output stage 47.

The rectifier 42 in the present embodiment has the following structure. The rectifier includes a differential pair of PNP QR2 and QR3 transistors, the emitters of which are connected to each other. The base of transistor QR2 is connected to OUT through a resistor RR1. The QR3 base is connected to COM through the RR4 resistor, and RCT through a RR3 resistor. The QR2 and QR3 collectors are each connected to an input of a current mirror consisting of the NPN QR4 and QR5 transistors. The collector of transistor QR2 is also connected to the base of an NPN QX transistor. The emitter of the transistor QX is connected to ground FF and the collector of the transistor QX is connected to the anode of the zener diode Zl. A capacitor CCR is also connected between the base and the collector of the transistor QX. The cathode of zener diode Zl is connected to RCT. The emitters of QR2 and QR3 are connected to each other and to an emitter of a transistor of deflection QR1. In a preferred embodiment of the present invention, resistance RR1 is a resistance of 8.6k ohm, resistance RR3 is a resistance of 45k ohm, and resistance RR4 is a resistance of lOk ohm. The CCR capacitor is a 2pf capacitor. In operation, the rectifier 42 receives the output signal V0Ut from the op-amp, and produces a rectified version thereof at its output, which is the point VRCt. The rectified output signal in this mode consists essentially of the negative V0Ut cycles. The low pass filter then flattens the negative cycles in voltage level plus or minus cd which represents an average, or more specifically, the average square root of the negative V0Ut cycles. This average square root voltage, Vms, is then provided to an input of the trigger circuit 40. Then as described above, the trigger circuit 40 activates either the first input stage 45, the second input stage 46 or some fraction of both depending on the difference between VRMS and VREF. With additional detail, consider a large output signal Vout, which corresponds to the active vocal frequency. The large output signal will include large negative turns in the voltage in the negative cycles. During these negative turns, QR2 tends to activate to a greater degree than QR3. As a result, the emitter-controller current through QR2 will tend to activate transistor QX. Because QX is activated, large amounts of current pass through the zener diode Z1, and the VRCT voltage is slowly pulled. Specifically, the current through zener diode Zl causes a large COM voltage drop across resistors RR3 and RR4, and as a result VRCT is relatively low compared to VCOM. The low voltage VRCT is then passed through the low pass filter, which produces a low Vms in relation to VCOM. Because VREF is slightly smaller than VCOM > as discussed above, the low VRHS at the base of Q2 will tend to interrupt transistor Q2 and activate transistor Q1. Under these conditions, the transistor Ql causes the current to flow through the transistor Q5 and thus through the transistor Q12. The current through the transistor Q12 provides a deviation current to the emitters of the transistors Q10 and Qll, which activates the differential input of the first input stage 45. The transistor Q2, however, allows little or no current to flow from the collector of Q6, which tends to turn off or deactivate both transistors Q6 and Q7. Because the transistor Q7 is turned off or deactivated, no current flows to the emitters of the transistors Q7 and Q8, which deactivates the input stage 46. As a result, the large signals in the V0ut, output having turns of Large negative voltage tend to activate the first input stage 45. When the first input stage 45 is activated, the VIN? input applied to the investment input INI provides the high gain level, as determined by the resistance RA, RB and RF of Figure 2. Consequently, the large signals typically associated with the active conversation, as opposed to the background noise, they cause the expander 18 to provide a high gain level of input to the output.

In the case of smaller signals at the OUT output, such as those that may be present when only the background noise is present, the QR2 transistor tends to be activated to a lesser degree. As a result, the QR3 transistor tends to activate to a greater degree, which, in turn, causes the current mirror defined by the QR4 and QR5 transistors to pull the voltage at the base of the low QX transistor. Under these conditions, the QX transistor will tend to turn off or deactivate, or at least, conduct current to a lesser degree. As a result, less current is supplied through the zener diode Zl and the voltage VRCT tends to rise towards VCOM. Relatively high VRCT voltage passes through the low pass filter and provides a VRMS voltage which approximates the VCOM level - Because VRMS is close to VC0M and VREF is slightly less than VCOM the VRMS voltage tends to activate the transistor Q2 and tends to make transistor Ql turn off or off. The transistor Q2 allows the current to flow from the current mirror 54, which in turn activates the second input stage 46. The transistor Ql, however, allows little or no current to flow from the current mirror 53, which in turn deactivates the first input stage 45. The second input stage 46, as discussed above in connection with Figure 2, provides a low gain of the IN2 input of the OUT output due to the configuration of the resistors RA, RB and RF of Figure 2. It may be preferable to replace the exemplary rectifier circuit 42 and the low pass filter configured with alternative means to convert the output signals to a dc voltage level. In particular, a full-wave rectifier having a current output added to the parallel resistor-capacitor circuit can be used in place of the exemplary rectifier 42 and the low-pass filter illustrated in Figure 4. An example of a suitable full-wave rectifier it can be found in many commercially available compander integrated circuits (compressor and expander), such as those manufactured by Motorola and Signetics. Although the embodiment of Figure 1 comprises a residential wireless telephone set, it should be appreciated by those skilled in the art that the microphone expander and background noise suppression method of the present invention can be used in any telephone set, either that its intended primary use is residential or commercial. Those skilled in the art can easily determine the appropriate levels of attenuation and the appropriate attack and decay times for the particular telephone and its intended environment. Once such appropriate levels are determined, the circuit can be easily modified as appropriate. It should also be appreciated that the microphone expander 18 need not reside in the telephone apparatus as illustrated in Figure 1. Although such placement may be preferable for cordless telephones so that the uncorrected microphone signal does not need to be transmitted to the base of the telephone, there is no requirement that the microphone expander reside in the telephone set. In addition, for phones that have most of their electronic devices located in the base, such as a traditional wired telephone, it is preferable to place the microphone expander 18 in the base of the telephone. Similarly, all electronic devices, including the microphone, can exist in the telephone set, such as in a cell phone. It should be further appreciated that the microphone expander of the present invention is inexpensive to manufacture and operates in an efficient manner. Because noise suppression is determined at the output of the expander, no amplifier is required between the audio amplifier connected to the microphone and the expander, as required in the prior art. Eliminating the preamplifier not only reduces the cost of the device, but also eliminates a redundant function, such as the preamp that also occurs within the expanders of the prior art. In addition, the response of the device is very efficient since it does not require averaging over time the signals received by the microphone. It should be appreciated that the above embodiments are merely illustrative and that those skilled in the art can easily review or develop other embodiments that embody the principles of the present invention and fall within the scope and spirit thereof. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (20)

1. An apparatus for reducing background noise in a telecommunications device, the telecommunications device includes a microphone to receive acoustic signals, acoustic signals include vocal signals and background noise signals, the microphone can be further operated to convert the acoustic signals in corresponding electrical signals, the apparatus is characterized in that it comprises: a) an amplifier having an input to receive the electrical signals and an output, the amplifier operable to receive the corresponding electrical signals from the microphone at the input and provide the amplification of the signals corresponding electrical signals to produce amplified electrical signals at the output, the amplifier operates to provide amplification at a first gain level and a second gain level, the first gain level exceeding the second gain level; and b) A feedback device connected to the output of the amplifier and further operably connected to control the operation of the amplifier, the feedback device operates to cause the amplifier to provide amplification to the first gain level when the feedback device receives electrical signals amplified at the output having an amplitude greater than a predetermined threshold and causes the amplifier to provide amplification at the second gain level when the feedback device receives amplified electrical signals at the output having an amplitude less than the predetermined threshold.

2. The apparatus according to claim 1, characterized in that the amplifier further includes a first input stage connected to the input and a second input stage connected to the input and wherein the amplifier provides the first gain level when the first stage of input is activated and the second input stage is deactivated and the amplifier provides the second gain level when the second input stage is activated and the first input stage is deactivated.

3. The apparatus according to claim 2, characterized in that the amplifier further comprises an operational amplifier and wherein the output is further connected to the first input stage through a feedback path and the output is further connected to the second stage of input through a second feedback path, and where the first feedback path affects the first gain level and the second feedback path affects the second gain level.

4. The apparatus according to claim 1, characterized in that the feedback device includes means for converting the amplified electrical signals to a dc voltage level and wherein the feedback device operates to cause the amplifier to provide amplification to a selected level of the amplifier. first gain level and the second gain level if the amplified electrical signals are below the predetermined threshold using the dc voltage level and a cd reference voltage level.

5. The apparatus according to claim 4, characterized in that the means for converting the amplified electrical signal to a voltage level of cd includes a rectifier and a low pass filter.

6. The apparatus according to claim 2, characterized in that the feedback device includes means for converting the amplified electrical signals to a dc voltage level, and wherein the feedback device operates to cause activation of a selection of the first stage. input and the second input stage using the cd voltage level and a cd reference voltage level.

7. The apparatus according to claim 6, characterized in that the feedback device further includes a trigger circuit operably connected to the first input stage, the second input stage, and the means for converting the amplified electrical signals to a level cd voltage, and wherein the trigger circuit alternately and controllably activates and deactivates the first input stage and the second input stage depending on whether the cd voltage level exceeds the reference voltage level of cd.

8. The apparatus according to claim 7, characterized in that the tripping circuit comprises first and second bipolar junction transistors ("BJT"), each BJT has a base, a collector and an emitter, and wherein the first emitter of the BJT is connected to the second transmitter of the BJT, the first base of the BJT is connected to a reference voltage source of cd, and the second base of BJT is operably connected to the means for converting the amplified electrical signals to a voltage level of CD.

9. The apparatus according to claim 1, characterized in that the first gain level exceeds the second gain level by approximately 6dB.

10. The apparatus according to claim 5, characterized in that the low pass filter comprises a resistor and a capacitor.

11. The apparatus according to claim 10, characterized in that the low pass filter has a time constant of approximately 100 microseconds.

12. The apparatus according to claim 3, characterized in that the operational amplifier and the feedback device are integrated on a single semiconductor device.

13. The apparatus according to claim 1, characterized in that the amplifier is integrated on a single semiconductor device.

14. A method for controlling the amplification of electrical signals in a telecommunication device having a microphone, the method is characterized in that it comprises: a) providing the electrical signals of the microphone to an amplifier; b) employing the amplifier to amplify the electrical signals showing one of at least two gain levels, at least two gain levels include a high gain level and a low gain level; c) providing the amplified electrical signals to a feedback device; d) employing the feedback device to determine a relationship between an amplitude of the amplified electrical signals and a predetermined threshold; and e) causing the amplifier to amplify the subsequent signals using the high gain level if the amplitude of the amplified electrical signals exceeds the predetermined threshold.

15. The method according to claim 14, characterized in that it further comprises the step of f) causing the amplifier to amplify the subsequent signals using the low gain level if the predetermined threshold exceeds the amplitude of the amplified electrical signals.

16. The method according to claim 15, characterized in that it further comprises the step of g) of causing the amplifier to amplify the subsequent signals using a combination of the low gain level and the high gain level if the amplitude of the amplified electrical signals is approximately equivalent to the predetermined threshold.

17. The method according to claim 14, characterized in that the step of determining the ratio between the amplified electrical signals and a predetermined threshold includes: converting the amplified electrical signals to a dc voltage level indicative of the amplitude of the amplified electrical signals; compare the dc voltage level with a cd reference voltage level indicative of the predetermined threshold.

18. An apparatus for reducing background noise in a telecommunications device, the telecommunications device includes a microphone to receive acoustic signals, acoustic signals include vocal signals and background noise signals, the microphone further operates to convert the acoustic signals into signals corresponding electrical devices, the apparatus is characterized in that it comprises: a) an operational amplifier having a first input stage operably connected to receive the electrical signals, a second input stage operably connected to receive the electrical signals, and an output, the operational amplifier operates to provide amplification to the electrical signals to produce amplified electrical signals at the output, the operational amplifier further operates to provide amplification at a first gain level when the first input stage is activated and the second input stage is deactivated, and provide amplification to a second gain level when the second input stage is activated and the first input stage is activated; b) a rectifier connected to the output of the operational amplifier, the rectifier operates to receive the amplified electrical signals and generate rectified signals thereof; c) a low pass filter operably connected to the rectifier to receive the rectified signals and operable to generate a dc voltage level thereof, the voltage level of cd is representative of a magnitude of the amplified electrical signals; and d) a trip circuit connected to the low pass filter to receive the dc voltage level and further connected to control the activation and deactivation of each of the first input stage and the second input stage, and wherein the circuit of trigger activates and controllably and alternately deactivates the first input stage and the second input stage depending on whether the cd voltage level exceeds a reference voltage level of cd.

19. The apparatus according to claim 18, characterized in that the tripping circuit comprises first and second bipolar junction transistors ("BJT"), each BJT has a base, a collector and an emitter, and wherein the first emitter of the BJT is connected to the second transmitter of the BJT, the first base of the BJT is connected to a reference voltage source of cd, and the second base of the BJT is operably connected to the low pass filter.

20. A telephone device of telecommunications device operable to reduce background noise, the telecommunications device is characterized in that it comprises: a) a microphone to receive acoustic signals, the acoustic signals include voice signals and background noise signals, the microphone operates to convert the acoustic signals into corresponding electrical signals; b) an amplifier that has an input to receive the electrical signals and an output, the amplifier operates to receive the corresponding electrical signals from the microphone at the input and provides amplification of the corresponding electrical signals to produce amplified electrical signals at the output, the amplifier operates to provide amplification at a first gain level 'and a second gain level, the first gain level exceeding the second gain level; c) a feedback device connected to the output of the amplifier and further operably connected to control the operation of the amplifier, the feedback device operates to cause the amplifier to provide amplification to the first gain level when the feedback device receives the signals electric amplifiers in the output that have an amplitude greater than a predetermined threshold and causes the amplifier to provide amplification to the second gain level when the feedback device receives amplified electrical signals in the output having an amplitude less than the predetermined threshold; d) means for providing the amplified electrical signals to a telecommunication network; e) means for receiving signals from the telecommunication network; and f) means for displaying the received signals in the form of an audio signal. SUMMARY OF THE INVENTION An apparatus for reducing background noise in a telecommunications device is described, the telecommunications device includes a microphone for receiving acoustic signals and generating the corresponding electrical signals thereof, the acoustic signals include vocal signals and background noise signals. The apparatus of the present invention in one embodiment includes an amplifier having an input to receive the electrical signals from the microphone and an output, the amplifier operates to receive the electrical signals at the first input and provides amplification to the electrical signals to produce electrical signals amplified The amplifier further operates to provide amplification to a first gain level and a second gain level, the first gain level exceeding the second gain level. The apparatus also includes a feedback device connected to the output of the amplifier and is also operably connected to control the operation of the amplifier. The feedback device operates to cause the amplifier to provide amplification to the first gain level when the feedback device receives amplified electrical signals at the output that have an amplitude lower than a predetermined threshold and causes the amplifier to provide amplification at the second gain level when the feedback device receives amplified electrical signals at the output having an amplitude less than the predetermined threshold.