CN109547883B

CN109547883B - Circuit and method for determining an environmental impact on an electrical load device

Info

Publication number: CN109547883B
Application number: CN201811076794.4A
Authority: CN
Inventors: A·马丁·马林森
Original assignee: ESS Technology Inc
Current assignee: ESS Technology Inc
Priority date: 2017-09-14
Filing date: 2018-09-14
Publication date: 2021-01-22
Anticipated expiration: 2038-09-14
Also published as: CN109547883A

Abstract

Circuits and methods are provided for determining an environmental impact on an electrical load device. An improved system and method for reducing the ambient noise experienced by a user listening to a headset without the use of a microphone is disclosed. An "ambient noise signal" is obtained that is generated by a sound pressure wave of ambient noise acting on the earpiece transducer. In some embodiments, the ambient noise signal is inverted and fed back, and the inverted signal is added to the desired audio signal being sent to the headphones, thereby eliminating the ambient noise. In other embodiments, the processor receives the ambient noise signal and predicts the modifications required to cancel the ambient noise for the desired audio signal. The ambient noise signal may be obtained by comparing the actual signal across the earpiece transducer with the expected audio signal, or by detecting a change in the current across the transducer relative to the current generated to drive the transducer.

Description

Circuit and method for determining an environmental impact on an electrical load device

This application claims priority from provisional application No. 62/558,545 filed on 14.9.2017, provisional application No. 62/567,745 filed on 3.10.2017, and provisional application No. 62/568,299 filed on 4.10.2017, each of which is incorporated herein by reference in its entirety.

Technical Field

The present invention relates generally to electrical devices, and more particularly to determining and reducing environmental impact on the operation of electrical load devices.

Background

It is often desirable to determine environmental effects on the operation of electrical devices and, in some cases, to counteract such effects. For example, Active Noise Cancellation (ANC) is a desired function in headphones. (as used herein, "headset" includes any sound reproduction device worn over, on, or within a user's ear, including a headset, earpiece, or earbud). The effect of noise cancellation is to suppress ambient noise without altering the audio signal applied to the headset so that the user experiences a lower level of ambient noise, thereby improving the user's listening experience. Noise cancellation is particularly useful in situations where the ambient noise level is high, such as in aircraft, trains, and other similar environments.

There are three known types of ANC. In "feed forward" ANC, the microphone is placed away from the headset and receives ambient noise before the user receives it. In a "feedback" implementation of ANC, the microphone is placed near the headset, or even in the headset itself, and thus receives ambient noise in substantially the same way as the user receives it. Those skilled in the art will recognize limitations in both feed-forward and feedback ANC, as well as the use of "mixed" implementations of ANC, which include both feed-forward and feedback techniques in an effort to achieve better noise cancellation.

The present application relates to feedback ANC. In feedback ANC, a microphone near or within the headset receives ambient noise, thereby generating an ambient noise signal. The signal that is an inverted copy of the ambient noise signal is added to the desired audio signal such that the addition of the inverted copy in the desired audio program eliminates the perceived ambient noise to some extent. Thus, additional noise (i.e., an inverted copy) is added to the desired audio program to eliminate the ambient noise, and the user perceives the ambient noise to be at a lower level.

The amplitude and phase of the inverted noise cancellation signal are preferably selected to optimize this perceived ambient noise reduction. This is typically achieved by using some kind of adaptive feedback loop; in some embodiments, a finite impulse response Filter (FIR) is configured using a Least Mean Square (LMS) algorithm to optimally remove noise. Such techniques are well known in the art.

However, the need for a microphone to detect ambient noise results in limitations on the ability to successfully perform active noise cancellation. One is that the proximity of the microphone and headphone driver is critical to performance; the velocity of sound in air means that even small positional differences between the microphone and earpiece transducer cause delays that prevent the noise cancellation loop from canceling high frequency sounds.

Therefore, it would be useful to be able to perform active noise cancellation without the need for a microphone to detect ambient noise.

Disclosure of Invention

An improved system and method for determining a signal indicative of an environmental impact on an electrical load when the electrical load is operating based on an input signal is disclosed.

One embodiment discloses a circuit for determining a signal representative of an environmental impact on an electrical load when the electrical load is operating based on an input signal, comprising: a first differential amplifier having a first input receiving an input signal and a second input receiving an output of the first differential amplifier, the output of the first differential amplifier driving an electrical load such that the second input receives a signal representative of the input signal and including an environmental impact on the electrical load; a second differential amplifier having a first input receiving an input signal and a second input receiving an output of the second differential amplifier, the output of the second differential amplifier driving a load having an impedance equal to the impedance of the electrical load such that the second input receives a signal representative of the input signal; and a third differential amplifier having a first input receiving the output of the first differential amplifier and a second input receiving the output of the second differential amplifier, thereby generating a signal as the output of the third differential amplifier, the signal being the difference between the input signal and a signal applied to the electrical load by both the input signal and the environmental influence.

Another embodiment discloses a circuit for determining a signal representative of an environmental impact on an electrical load and reducing the environmental impact on the electrical load when the electrical load is operating based on an input signal, comprising: a current output amplifier configured to output a current and an input signal to an electrical load; a voltage output amplifier configured to detect a current change in a resistor connected to the electrical load, the current change being caused by a voltage change at the electrical load due to an environmental impact on the electrical load; a sub-circuit configured to amplify a voltage across a resistor in a voltage output amplifier to generate a signal representative of an environmental effect and to send the signal representative to a processor; and a processor configured to adjust the input signal, thereby causing the current output amplifier to vary the current provided to the electrical load to reproduce the input signal without current flowing through the resistor.

Yet another embodiment discloses a method for determining a signal representative of an environmental impact on an electrical load when the electrical load is operating based on an input signal, comprising: providing an input signal as a first input to a first differential amplifier, a second input of the first differential amplifier receiving an output of the first differential amplifier, the output of the first differential amplifier driving an electrical load such that the second input receives a signal representative of the input signal and including an environmental impact on the electrical load; providing the input signal as a first input to a second differential amplifier, a second input of the second differential amplifier receiving an output of the second differential amplifier, the output of the second differential amplifier driving a load having an impedance equal to an impedance of the electrical load, such that the second input receives a signal representative of the input signal; and providing the output of the first differential amplifier as a first input and the output of the second differential amplifier as a second input to a third differential amplifier, thereby generating a signal as the output of the third differential amplifier, the signal being the difference between the input signal and a signal applied to the electrical load by both the input signal and the environmental influence.

Yet another embodiment discloses a method for determining a signal representative of an environmental impact on an electrical load and reducing the environmental impact on the electrical load when the electrical load is operating based on an input signal, comprising: outputting a current and an input signal from a current output amplifier to an electrical load; detecting, by a voltage output amplifier, a current change in a resistor connected to an electrical load, the current change caused by a voltage change at the electrical load due to an environmental impact on the electrical load; amplifying, by an amplifier circuit, a voltage across a resistor in a voltage output amplifier to generate a signal representative of an environmental effect and send the signal representative to a processor; and adjusting, by the processor, the input signal to cause the current output amplifier to vary the current provided to the electrical load to reproduce the input signal without current flowing through the resistor.

Drawings

Fig. 1A is an illustration of an exemplary prior art system with a microphone for detecting ambient noise, which may be used to perform feedback active noise cancellation.

Fig. 1B is a diagram of an exemplary system without a microphone for detecting ambient noise that may be used to perform feedback active noise cancellation, according to some embodiments.

Fig. 1C is an illustration of a headset that may be used in the system of fig. 1B, according to some embodiments.

Fig. 2 illustrates a feedback active noise cancellation circuit without a microphone for detecting ambient noise according to some embodiments.

Fig. 3 shows a feedback active noise cancellation circuit without a microphone for detecting ambient noise according to other embodiments.

Fig. 4 illustrates a method of feedback active noise cancellation according to some embodiments.

FIG. 5 illustrates another method of feedback active noise cancellation according to some embodiments.

Detailed Description

An improved system and method for determining a signal indicative of an environmental impact on an electrical load when the electrical load is operating based on an input signal is disclosed. There is provided a system and method showing how to use the system and method to reduce the ambient noise experienced by a user listening to a headset without using a microphone.

In various embodiments, the system and method make use of the fact that: an electrical load operating based on an input signal will typically respond in anti-phase and produce a signal in response to the effects of some environmental influence. This "inverted signal" can be used to detect a wide variety of conditions, and in many cases can also be used to improve these conditions.

For example, with respect to noise reduction, specifically, an electro-acoustic transducer for generating sound pressure waves (hereinafter referred to as "sound" or "audio") in response to an electrical audio signal (hereinafter referred to as "audio signal") will also operate in anti-phase and generate an audio signal in response to receiving sound (e.g., ambient noise around a user). A prior art microphone located in or near the headset is omitted and the "microphone effect" of the headset is used to provide a signal representative of the ambient noise to an adaptive feedback loop that optimizes noise suppression.

It is well known in the art that any transducer that produces sound in response to an audio signal, such as a transducer in a headphone or speaker, does this by: moves in response to an audio signal applied thereto and thus generates a sound corresponding to the audio signal. This process also works in reverse; when such a transducer is subjected to external sound, it in turn generates an electrical signal, although this signal is typically several orders of magnitude smaller than the signal used to drive the transducer.

This is the same principle as a microphone, which produces an electrical signal in response to sound. The anti-phase signal produced by a transducer that is typically used to produce sound in response to external sound may be considered an "echo signal" or an "ambient noise signal" to distinguish it from the audio signal that is typically applied to the transducer to cause it to produce sound.

The described system and method omits the microphone used in the prior art and instead utilizes this "microphone effect" of the headset to provide an ambient noise signal representative of ambient noise to the adaptive loop that optimizes noise suppression. As described below, since the ambient noise signal is much smaller than the audio signal, care must be taken to detect and amplify it to a signal large enough to be inverted and added to the audio signal.

Fig. 1A is an illustration of an exemplary prior art system with a microphone for detecting ambient noise, which may be used to perform feedback active noise cancellation. The amplifier 102 provides the amplified input audio signal to the headphones 104; however, the user also hears ambient noise. The microphone 106 receives ambient noise and provides a signal representative of the ambient noise to a circuit 108, such as an FIR filter typically configured with an LMS algorithm as described above, which circuit 108 produces a signal that is an inverted copy of the ambient noise signal. The inverted copy is then added to the input signal at adder 110 and the currently modified input signal 102 is provided to the earpiece 104. The intention is that an inverted copy of the ambient noise will cancel the ambient noise.

However, it is well known that feedback ANC has certain limitations. One limitation is that the proximity of the microphone and earpiece transducer produces sound that the user hears, and in particular the distance from the microphone to the transducer membrane is critical to performance. The velocity of sound in air means that even a 10 millimeter (mm) difference in position between the microphone and earpiece transducers prevents the ANC loop from canceling high frequency sound. Typically, such an arrangement of separate microphones and transducers results in an upper limit of noise suppression of about 1 kilohertz (kHz), i.e., frequencies of unwanted ambient noise above 1 kHz are difficult to suppress because even as little as 10 millimeters of difference between the microphone and the membrane results in sufficient delay to make the ANC loop unstable at that frequency.

Fig. 1B is an illustration of an example system that can be used to perform feedback active noise cancellation in accordance with some embodiments. As in the system of fig. 1A, in fig. 1B, the amplifier 112 also provides an amplified input audio signal to the headphones 114.

However, in fig. 1B, there is no microphone 106; instead, the transducer membrane in the earpiece 114 now receives ambient noise while it produces audio that the user hears. The detection circuit 116 measures the "echo signal" and, similar to the microphone 106 of fig. 1A, provides a signal representing the ambient noise to a circuit 118, typically also a FIR filter configured with an LMS algorithm, which circuit 118 also generates a signal that is an inverted copy of the ambient noise signal. As with the system of fig. 1A, the system of fig. 1B adds the inverted copy to the input signal at adder 120 and provides the currently modified input signal 112 to the earpiece 114.

Fig. 1C is an illustration of a headset that may be used in the system of fig. 1B, according to some embodiments. As described above, the amplifier 112 provides an amplified audio signal to the headphones 114, the detection circuit 116 measures an ambient noise signal representative of the ambient noise, and the circuit 118 produces an inverted copy of the ambient noise signal.

Fig. 1C also shows an electromagnetic coil 126 that receives the audio signal from the amplifier 112 and moves the transducer membrane 128 of the headset. However, the membrane 128 also experiences movement due to ambient noise, and this movement is converted by the coil 126 back into an electrical signal that is detected by the detection circuitry 116 and fed back to the circuitry 118 as described above.

This configuration improves on the prior art by eliminating any delay between ambient noise arriving at the microphone and at the membrane, since the microphone and the membrane are the same membrane. Thus, the effectiveness of noise cancellation is better, particularly at higher frequencies. In addition, the system of FIG. 1B allows any unmodified earpiece to be used with the ANC, thus avoiding the additional cost of a microphone.

The ambient noise signal may be detected by subtracting the expected audio signal that is to drive the transducer from the signal actually received by the transducer. Since the actual transducer signal contains an ambient noise signal, removing the output audio signal leaves only the ambient noise signal.

One method for such subtraction is to create a replica of the transducer load, subtract the current through the replica from the actual load current, and treat the remaining load current as an ambient noise signal. Fig. 2 illustrates a feedback active noise cancellation circuit 200 using this principle according to some embodiments.

The input audio signal is applied to the non-inverting inputs of two differential operational amplifiers ("operational amplifiers," referred to herein as "amplifiers") 202 and 204. The amplifier 202 has a feedback loop through a resistor 206 through which the output of the amplifier 202 is applied to a headset 208 and returned to the inverting input of the amplifier 202. It will be apparent to those skilled in the art that this will result in the amplifier 202 driving the resistor 206 so that both inputs of the amplifier 202 see the same signal, and the earpiece 208 will therefore see the same signal. Those skilled in the art will appreciate that the input audio signal will typically come from a digital-to-analog converter (DAC) that converts a digital audio signal to an analog signal, and the output will typically be passed to an analog-to-digital converter (ADC) that converts the analog signal to a digital signal; however, the described circuit may also be used with fully analog systems.

Amplifier 204 similarly has a feedback loop through resistor 210 to the inverting input of amplifier 204 and to ground through resistor 212, resistor 210 having the same value as resistor 206, and resistor 212 may be an adjustable resistor. Resistor 212 is selected or adjusted to have the same impedance as the earpiece 208, so the voltage on the inverting inputs of

amplifiers

202 and 204 will be the same in the absence of ambient noise. The current required to drive the headset flows through resistor 206 and an equal current flows through resistor 210 because the impedance of resistor 212 is equal to the impedance of headset 208.

Amplifier 214 is not a normal operational amplifier, but an instrumentation amplifier ("instrumentation amplifier") that amplifies the difference between the two input signal voltages while rejecting any signal that is common to the two inputs, and therefore can typically measure small signals in noisy environments. An example of such an instrumentation amplifier is the AD524 amplifier from analog devices corporation of america.

As is well known in the art, while a normal amplifier has two inputs, non-inverting and inverting, an instrumentation amplifier has four inputs. Two of these inputs are similar to those on a normal amplifier and are the

inputs

216 and 218 of the amplifier 214 in fig. 2. The other two

inputs

220 and 222 of amplifier 214 provide differential feedback paths that may be configured with various gains to balance the differential signals from

inputs

216 and 218. The output of the amplifier 214 is fed back to its non-inverting feedback input 220, while the inverting feedback input 222 is grounded. In this configuration the output of the amplifier 214 will be the product of the difference between the two

inputs

216 and 218 of the amplifier 214 and the gain of the amplifier 214.

Since amplifier 214 receives the outputs of

amplifiers

202 and 204 at its

inputs

216 and 218, the output of amplifier 214 will be zero when these outputs are the same in the absence of ambient noise. However, when ambient noise is present, the pressure from the ambient noise sound waves on the earpiece transducer causes the current through resistor 206 to change, while the current through the matched impedance of resistor 212 does not change. The outputs of

amplifiers

202 and 204 are the same when only an audio signal is present, and are different when ambient noise is present. The difference is an ambient noise signal that can then be amplified by amplifier 214 and sent to conventional circuitry, such as circuit 118 of fig. 1B, for inversion, which can also be an FIR filter using the LMS algorithm.

Using an instrumentation amplifier as the amplifier 214 rather than a conventional amplifier is desirable because, as described above, the ambient noise signal is very small, much smaller than the audio signal used to drive the earpiece transducer. For example, a typical earplug having an impedance of 6 to 600 ohms may be driven by a signal of up to 500 millivolts (mV), but when used as a microphone only produces an inverted audio signal of 5 to 50 microvolts (μ V). Therefore, the ratio of the driving audio signal to be detected to the inverted audio signal can be as high as 100,000 to 1(500mV to 5 μ V).

It can be seen that this ratio is too large for ambient noise signals to be present or detected while the transducer is temporarily displaced when audio is played in the headphone. This has not proved to be the case, since experiments have shown that the ambient noise signal generated by ambient sound arriving at the transducer always appears as a small signal superimposed on the audio signal used to drive the transducer and can be separated from the audio signal. The large gain of the instrumentation amplifier is expected to increase the voltage of the inverted audio signal to the approximate output of amplifier 214 for further processing according to the prior art as described above.

However, even currently available instrumentation amplifiers do not provide optimal gain in view of the large difference between the audio signals typically used to drive the transducers and the ambient noise signals that may be provided by these transducers. Thus, another method of determining the ambient noise signal may be more easily implemented.

Another way to determine the ambient noise signal is to try to measure the actual ambient noise signal and estimate what the audio signal should be to drive the ambient noise signal to zero. Since this can be achieved without amplifying and inverting the ambient noise signal, a high gain of the instrumentation amplifier is not required.

Fig. 3 illustrates a feedback active noise cancellation circuit 300 according to some embodiments using this principle. The circuit 300 includes three

sub-circuits

302, 304, and 306. Subcircuit 302 provides current to drive earphone 308; the sub-circuit 304 receives an ambient noise signal; and sub-circuit 306 amplifies the ambient noise signal and converts it back into a digital signal to feed to processor 310.

Subcircuit 302 is a current output amplifier that drives an earphone 308. In addition to replicating the output stage, it is used primarily as a conventional amplifier. The audio signal is input to processor 310 and processed as will be further explained below, and the output of processor 310 is input to DAC 312. The DAC 312 converts the digital audio signal into an analog audio signal suitable for driving the transducer, and the analog audio signal is input to an amplifier 314, which amplifier 314 in turn drives two

output stages

316 and 318.

The output stage 316 drives a variable load resistor 320, as discussed further below, while the output stage 318 drives the earpiece 308. When operating as illustrated, the output stage 318 provides all of the current required to properly drive the transducer in the earpiece 308.

In the illustrated embodiment, output stages 316 and 318 operate in the same manner, except that output stage 318 provides 20 times the current of output stage 316. In some cases, the output stage 318 may include 20 instances of output stage circuitry. As is apparent from fig. 3, the output stage 316 and the output stage 318 must also receive the same output voltage from the amplifier 314.

Those skilled in the art will appreciate that it is desirable to keep DAC 312 within its "sweet spot," i.e., the linear portion of its range. Many semiconductor devices, and thus standard DACS, typically have a supply voltage of 3.3 volts. As understood by those skilled in the art, an operating voltage of 1 volt is therefore generally considered to be within the linear portion of the DAC range. Under the control of the processor 310, the variable resistor 320 operates as a control mechanism to achieve this.

In the illustrated embodiment, the variable resistor 320 has an impedance that is 20 times the impedance of the earpiece 308. As will be seen, this corresponds to the current output of the output stage 318 being 20 times the current output of the output stage 316.

Assume that the earpiece 308 has an impedance of 60 ohms. At a desired voltage of 1 volt, the current through the earphone 308 provided through the output stage 318 will be slightly higher than 16 milliamps. Because the output stage 316 receives 1/20 of the current received by the output stage 318, 800 milliamps will flow through the output stage 316.

The variable resistor 320 has an impedance that is 20 times the impedance of the earpiece 308, i.e., 1200 ohms. Because this impedance of the variable resistor 320 is significantly lower than the impedance of the feedback resistor, 800 milliamps from the output stage 316 will flow primarily through the variable resistor 320. The 800 milliamps across the 1200 ohm variable resistor 320 causes the same expected voltage from DAC 312 of 1 volt.

Next, assume that the earphone 308 is replaced by a new earphone having an impedance of 6 ohms. The current through the output stage 318 at 1 volt, which also passes through the output stage 318, will now be 160 milliamps. The current through the output stage 316, which again is 1/20 of the current through the output stage 318, will be 8 milliamps.

However, when 8 milliamps are passed through a 1200 ohm impedance variable resistor 320, a voltage from DAC 312 of 10 volts would now be expected, well above the operating voltage of the 3.3 volt system. Processor 310 may recognize that this is not possible and even pushing DAC 312 to its upper voltage of 3.3 volts will cause DAC 312 to go out of its linear operating range and may change variable resistor 320 to 120 ohms so that a current across variable resistor 320 of 8 milliamps again anticipates a voltage from DAC 312 of 1 volt.

In this way, the current mode DAC 312 remains within its linear operating range for all of the different expected loads of the headset 308.

Subcircuit 304 is a voltage output amplifier that also drives a headset 308. The DAC 322 receives the audio signal and in turn drives an amplifier 324, the amplifier 324 being of a conventional differential to single-ended configuration. Resistor 326 receives the output of amplifier 324 so that any load current required from amplifier 324 must flow through resistor 326. The feedback point of amplifier 324 is such that the output voltage at the node connecting resistor 326 and earpiece 308 must be equal to the signal from voltage-mode DAC 322.

In this embodiment, resistor 326 has an impedance of 300 ohms, which is significantly higher than the impedance of earpiece 308. However, it is expected that no current will flow through resistor 326 into earpiece 308 because, as explained below, the action of circuit 300 causes all of the load current required by the earpiece to flow out of subcircuit 302. The only current flowing through resistor 326 is the current from the ambient noise signal, and this allows the impedance of resistor 326 to be substantially higher than the impedance of earpiece 308.

The differential amplifier arrangement around amplifier 324 is used to ensure that the voltage at the headset is exactly the voltage determined by DAC 322. Subcircuit 306 measures the current that amplifier 324 is providing to achieve the output voltage; this allows processor 310 to adjust subcircuit 302 to provide current to achieve the output voltage, rather than flowing from amplifier 324. In operation, subcircuit 306, processor 310, and subcircuit 302 cooperate to suppress all current flowing out of amplifier 324 through resistor 326. Thus, the nominal value of the voltage across resistor 326 is zero.

It is possible to suppress the voltage across resistor 326 because processor 310 is provided with an audio input signal — the same audio input signal applied to DAC 322. The processor 310 adjusts the current provided from the sub-circuit 302 through an LMS or similar algorithm until the average of the voltage across the resistor 326 is zero. Thus, after the LMS algorithm has converged, the audio signal flowing through the processor and into the sub-circuit 302 provides an estimate of all the current required by the headphone load.

No current from the differentially configured amplifier 324 is required to drive the audio content to the headphones 308; rather, all of the current to the drive earphone 308 is provided by the predicted current from the sub-circuit 302. The amplifier 324 only provides the difference between the predicted current and the actual current required by the headset. If the predicted current is completely correct, no current flows through resistor 326.

However, the ambient noise picked up by the headset 308 from the environment cannot be predicted by the processor 310 and is therefore not present in the current output of the sub-circuit 302. Therefore, any ambient noise current must flow through resistor 326.

Amplifier 328 and its associated resistors are distortion reducing circuits, as further described in commonly owned U.S. patent No. 9,595,931, assigned to the assignee of the present application. This portion of the sub-circuit 304 may optionally be omitted, but without this portion of the sub-circuit 304, the sub-circuit 304 will not function as well as desired.

Subcircuit 306 receives the voltage from resistor 326 through resistor 332 and the voltage of the drive current from output stage 318 through resistor 330. Amplifier 334 amplifies any difference between the two voltages and passes the amplified result to ADC 336. It will be apparent that if there is no ambient noise, there will be no current flowing through resistor 326, so the two voltages will be the same and the output of both amplifier 334 and ADC 336 will be zero.

If there is ambient noise, then there will be a current passing, and the voltage across resistor 326 and the output of amplifier 334 will be non-zero. In this case, the output of amplifier 334, and therefore the output of ADC 336, represents ambient noise.

The output of ADC 336 allows processor 310 to suppress portions of the digital output from ADC 336 due to audio content. The processor does this by means of the known LMS algorithm or similar. The processor 310 finds the correlation between the audio content and the digital output of the ADC 336 and minimizes the audio content present in the output of the ADC 336 by driving the predicted current of the sub-circuit 302 and thus removing the current from the resistor 326.

Thus, after the processor has converged, the output of the ADC 336 represents only the ambient audio signal, i.e., not part of the microphone action of the headset due to the audio content. Only the microphone signal is present as an ambient noise signal at the output of ADC 336.

In this embodiment, the processor 310 is a Digital Signal Processor (DSP). When the processor 310 receives the ambient noise signal from the ADC 326, the processor 310 uses the ambient noise signal to cause the sub-circuit 302 to output a current from the amplifier 318 that best meets the current required to reproduce the audio signal in the headphone 308, without the current through the resistor 326, and therefore without a signal related to the audio content from the ADC 326.

The feedback ANC described herein may be described as a method. Fig. 4 illustrates a method 400 of such feedback ANC, in accordance with some embodiments. In step 402, an audio signal is applied to the input of each of the two differential amplifiers, as described above with respect to the circuit 200 of fig. 2; each amplifier has a feedback loop with its output to its other input. The output of the first amplifier drives the electroacoustic transducer, so the feedback to the first amplifier input includes any changes in voltage due to the effect of ambient noise on the electroacoustic transducer. The output of the second amplifier drives a load having the same impedance as the electroacoustic transducer and is therefore not affected by this ambient noise.

At step 404, the outputs of the two amplifiers are compared and amplified using a third differential amplifier (preferably an instrumentation amplifier as above). As mentioned above, the outputs of the first and second amplifiers each comprise an audio signal, but the output of the first amplifier also comprises a voltage change of the electroacoustic transducer, which voltage change is indicative of the influence of ambient noise on the electroacoustic transducer. Therefore, the difference between the outputs of the first and second amplifiers is a signal representing ambient noise.

At step 406, the signal representing the ambient noise is then inverted, as in the prior art, and the inverted signal is added to the audio signal. The inverted signal cancels out or at least significantly reduces the effect of ambient noise on the electro-acoustic transducer and thus how much noise is heard by the user.

Fig. 5 illustrates another method 500 of feedback ANC according to some embodiments. At step 502, the current and audio signals are output from a current output amplifier, such as the subcircuit 302 of fig. 3, to an electroacoustic transducer.

At step 504, a voltage output amplifier, such as the sub-circuit 304 in fig. 3, detects a change in current flowing in a resistor connected to the electroacoustic transducer caused by a change in voltage at the electroacoustic transducer due to the effect of ambient noise on the electroacoustic transducer.

At step 506, an amplifier circuit, such as sub-circuit 306 in fig. 3, amplifies the voltage across the resistor in the voltage amplifier to generate and send a signal representative of the ambient noise to the processor.

Finally, at step 508, the processor adjusts the gain of the audio signal to cause the current output amplifier to vary the current provided to the electroacoustic transducer to reproduce the audio signal without current flowing through the resistor.

Those skilled in the art will understand how the components described above with respect to fig. 2 and 3 are configured to perform the methods of fig. 4 and 5, respectively.

Those skilled in the art will appreciate that although the present application describes examples of noise reduction in an electroacoustic transducer, the described circuits and methods may be used to detect changes in voltage of any electrical load (e.g., any type of electrical component, transducer, motor, etc.) due to any external or environmental factor (e.g., pressure, temperature, humidity, application of physical forces, aging of components, etc.). Furthermore, the described circuit and method have the advantage that even in case the audio signal is 100000 times the above-mentioned ambient noise signal, very small changes in the voltage on the electroacoustic transducer due to ambient noise can be detected. The prior art circuits and methods do not achieve this level of accuracy.

The disclosed systems and methods have been described above with reference to several embodiments. Other embodiments will be apparent to those skilled in the art in view of this disclosure. Certain aspects of the described methods and apparatus may be readily implemented using configurations or steps other than those described in the above-described embodiments, or in combinations of elements other than or in addition to those described above. It will also be apparent that in some cases the order of the processes described herein may be changed without changing the overall results of the performance of all of the described processes, and that different types of air scrubbing systems may be used.

For example, those skilled in the art will appreciate that as with the prior art feedback ANC discussed herein, the feedback ANC of the present application may be used with feed-forward ANC to achieve hybrid ANC. It should also be understood that there are a number of algorithms that will produce a Least Mean Square (LMS) of the signal. Some such algorithms will converge faster than others, while others will be less sensitive to residual errors. Those skilled in the art will be able to select an appropriate LMS algorithm for a particular application.

In addition, although the present application discusses methods and apparatus for performing ANC with an earpiece transducer, it will be apparent to those skilled in the art that any means of converting electrical signals to sound pressure waves may be operated in reverse, i.e., converting sound pressure waves to electrical signals, and thus the described methods and apparatus may be applied to any type of speaker, whether stand alone or on a wall. It is also expected that the relative position and movement of objects in the sound field of such loudspeakers can be determined by sufficient digital signal processing, thereby providing many possibilities for applications in the field of "smart homes" being developed.

Furthermore, although the present application discusses an audio signal, it will be apparent to those skilled in the art that any sound pressure modulation that is different from the intended content of the audio signal may be determined; for example, various types of bio-signals may be recovered using the methods and apparatus described herein.

Those skilled in the art will also appreciate that the described circuits and methods function by developing a model of the load (herein, the headset) when the load is only responsive to the drive signal. A correlation between the load and the drive signal is found and used to predict the load current. Errors in the prediction represent useful information. Although the example herein is a microphone action of an electroacoustic transducer, as mentioned above, this is by no means the only possibility: for example, the motor may be an electrical load driven by the circuit, and the load current is predicted to provide a constant force output current. Any variation in the mechanical load will be apparent in the deviation of the prediction from the actual load current. For example, a mechanical finger driven by a motor will exhibit a significant deviation from the predicted current upon finger touch. While the prior art discloses some techniques for detecting such changes, the ratio of the change to the input signal is limited to about 100 to 1, and as noted above, the circuits and methods disclosed herein may detect changes where the ratio to the input signal is 100000 to 1.

In addition, the circuit 300 herein includes a processor 310, the processor 310 having frequency dependency — an essential feature of the LMS algorithm used in the FIR filter in the prior art; it will be apparent to those skilled in the art that the state variables of the convergence algorithm represent the frequency dependence of the load current. The variability of the load, including its frequency dependence, can also be used to determine degradation over time, changes in ambient temperature, and the like.

It should also be appreciated that the described methods and apparatus may be implemented in numerous ways, including as a process, an apparatus, or a system. The methods described herein may be implemented by program instructions for instructing a processor to perform the methods, and the instructions are recorded on a computer-readable storage medium, such as a hard disk drive, a floppy disk, an optical disk such as a Compact Disk (CD) or a Digital Versatile Disk (DVD), a flash memory, and the like. Some of the methods may be incorporated into hardwired logic, if desired. It should be noted that the order of the steps of the methods described herein may be varied and still be within the scope of the present disclosure.

It is understood that the examples given are for illustrative purposes only and may be extended to other implementations and implementations with different conventions and techniques. While a number of embodiments are described, there is no intent to limit the disclosure to the embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents as apparent to those skilled in the art.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. Various features and aspects of the above-described invention may be used alone or in combination. Moreover, the present invention may be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will be appreciated that the terms "comprising," "including," and "having," as used herein, are specifically intended to be construed as open-ended terms.

Claims

1. A circuit for determining a signal representative of an environmental impact on an electrical load when the electrical load is operating based on an input signal, comprising:

a first differential amplifier having a first input receiving the input signal and a second input receiving an output of the first differential amplifier, the output of the first differential amplifier driving the electrical load such that the second input receives a signal representative of the input signal and including an environmental impact on the electrical load;

a second differential amplifier having a first input receiving the input signal and a second input receiving an output of the second differential amplifier, the output of the second differential amplifier driving a load having an impedance equal to the impedance of the electrical load such that the second input receives a signal representative of the input signal; and

a third differential amplifier having a first input receiving an output of the first differential amplifier and a second input receiving an output of the second differential amplifier, thereby generating as an output of the third differential amplifier a signal that is a difference between the input signal and a signal applied to the electrical load by both the input signal and the environmental influence.

2. The circuit of claim 1, wherein the third differential amplifier is an instrumentation amplifier.

3. The circuit of claim 2, wherein the instrumentation amplifier has a third input receiving an output of the instrumentation amplifier and a fourth input connected to ground.

4. The circuit of claim 1, wherein the input signal is an audio signal, the electrical load is an electroacoustic transducer, and the environmental impact is environmental noise acting on the electroacoustic transducer.

5. A method for determining a signal representative of an environmental impact on an electrical load when the electrical load is operating based on an input signal, comprising:

providing the input signal as a first input to a first differential amplifier, a second input of the first differential amplifier receiving an output of the first differential amplifier, the output of the first differential amplifier driving the electrical load such that the second input receives a signal representative of the input signal and including an environmental impact on the electrical load;

providing the input signal as a first input to a second differential amplifier, a second input of the second differential amplifier receiving an output of the second differential amplifier, the output of the second differential amplifier driving a load having an impedance equal to an impedance of the electrical load, such that the second input receives a signal representative of the input signal; and

providing the output of the first differential amplifier as a first input and the output of the second differential amplifier as a second input to a third differential amplifier, thereby generating a signal as the output of the third differential amplifier that is the difference between the input signal and a signal applied to the electrical load by both the input signal and the environmental influence.

6. The method of claim 5, wherein the third differential amplifier is an instrumentation amplifier.

7. The method of claim 6, further comprising: providing an output of the instrumentation amplifier as a third input to the instrumentation amplifier, and connecting a fourth input of the instrumentation amplifier to ground.

8. The method of claim 5, wherein the input signal is an audio signal, the electrical load is an electroacoustic transducer, and the environmental impact is environmental noise acting on the electroacoustic transducer.

9. A circuit for determining a signal representative of an environmental impact on an electrical load and reducing the environmental impact on the electrical load when the electrical load is operating based on an input signal, comprising:

a current output amplifier configured to output a current and the input signal to the electrical load;

a voltage output amplifier configured to detect a current change in a resistor connected to the electrical load, the current change being caused by a voltage change at the electrical load due to an environmental impact on the electrical load;

a subcircuit configured to amplify a voltage across a resistor in the voltage output amplifier to generate a signal representative of the environmental impact and to send the signal representative to a processor; and

a processor configured to adjust the input signal to cause the current output amplifier to vary the current provided to the electrical load to reproduce the input signal without current flowing through the resistor.

10. The circuit of claim 9, wherein the current output amplifier further comprises:

a digital-to-analog converter converting a digital input signal into an analog input signal;

a differential amplifier receiving and amplifying the analog input signal; and

a first output stage receiving the amplified input signal and providing a current and the amplified input signal to the electrical load.

11. The circuit of claim 10, wherein the current output amplifier further comprises:

a second output stage providing a current to a variable resistor, the current to the variable resistor being a fraction of the current provided to the electrical load by the first output stage, and an initial impedance of the variable resistor being equal to a multiple of an impedance of the electrical load, the multiple being an inverse of the fraction.

12. The circuit of claim 11, wherein the processor is further configured to: adjusting the impedance of the variable resistor when the voltage across the variable resistor changes due to a change in the impedance of the electrical load.

13. The circuit of claim 9, wherein the voltage output amplifier further comprises:

a differential amplifier receiving and amplifying the analog input signal; and wherein the one or more of the one,

the resistor connects the output of the differential amplifier to the electrical load, allowing any change in current in the electrical load due to the environmental effect to produce a voltage across the resistor.

14. The circuit of claim 9, wherein the sub-circuit further comprises:

a differential amplifier receiving and amplifying a voltage corresponding to any change in current across the resistor; and

an analog-to-digital converter to convert the amplified voltage to a digital signal and to provide the amplified voltage as an input to the processor.

15. The circuit of claim 9, wherein the input signal is an audio signal, the electrical load is an electroacoustic transducer, and the environmental impact is environmental noise acting on the electroacoustic transducer.

16. A method for determining a signal representative of an environmental impact on an electrical load and reducing the environmental impact on the electrical load when the electrical load is operating based on an input signal, comprising:

outputting a current and the input signal from a current output amplifier to the electrical load;

detecting, by a voltage output amplifier, a current change in a resistor connected to the electrical load, the current change caused by a voltage change at the electrical load due to an environmental impact on the electrical load;

amplifying, by an amplifier circuit, a voltage across a resistor in the voltage output amplifier to generate a signal representative of the environmental impact and send the signal representative to a processor; and

adjusting, by the processor, an input signal to cause the current output amplifier to vary a current provided to the electrical load to reproduce the input signal without current flowing through the resistor.

17. The method of claim 16, wherein outputting the current and the input signal from a current output amplifier to the electrical load further comprises:

converting the digital input signal into an analog input signal by a digital-to-analog converter;

amplifying the analog input signal by a differential amplifier; and

providing a current and an amplified input signal to the electrical load through a first output stage that receives the amplified input signal.

18. The method of claim 17, wherein the electrical load is an electroacoustic transducer and the input signal is output from a current output amplifier to the electrical load, and wherein the outputting further comprises:

providing current to a variable resistor from a second output stage, the current to the variable resistor being a fraction of the current provided by the first output stage to the electrical load, an initial impedance of the variable resistor being equal to a multiple of the impedance of the electrical load, the multiple being the inverse of the fraction.

19. The method of claim 18, further comprising: adjusting, by the processor, an impedance of the variable resistor when a voltage across the variable resistor changes due to a change in the impedance of the electrical load.

20. The method of claim 16, wherein detecting a current change in a resistor by a voltage output amplifier further comprises:

amplifying the analog input signal by a differential amplifier; and

connecting the output of the differential amplifier to the electrical load through a resistor, thereby allowing any change in current in the electrical load due to the environmental effect to produce a voltage across the resistor.

21. The method of claim 16, wherein amplifying, by an amplifier circuit, the voltage across the resistor further comprises:

amplifying, by a differential amplifier, a voltage corresponding to any change in current across the resistor; and

the amplified voltage is converted to a digital signal by an analog-to-digital converter and provided as an input to the processor.

22. The method of claim 16, wherein the input signal is an audio signal, the electrical load is an electroacoustic transducer, and the environmental impact is environmental noise acting on the electroacoustic transducer.