Abstract:
A system and method for actively changing the sound perceived by listeners in an audio environment. A single transducer is used as both a sensing microphone and as an output driver. In one embodiment, the invention is implemented as an active noise cancellation system. The sensed noise signals are phase shifted to provide a cancellation effect, combined with the desired audio program signals, and output to the transducer, thereby reducing the level of unwanted noise heard by they listener. In other embodiments, the system can be used to sense the frequency response of a listening room and make appropriate equalization adjustments to the output.

Description:
RELATED APPLICATION 
     This application claims priority from provisional application Ser. No. 60/825,734, filed Sep. 15, 2006, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to active noise cancellation systems for audio listening applications. 
     BACKGROUND 
     In audio listening applications, it is normally desirable to minimize the amount of background noise heard by the user. Methods for achieving such reduction fall into two main categories, passive and active. Passive noise reduction is accomplished by acoustically isolating the listener from the external noise source through the use of insulation or other sound blocking materials. However, the results are often unsatisfactory due to the difficulty of effectively blocking frequencies in the lower range of the audible spectrum. 
     Active noise reduction systems use the principle of phase reversal to cancel out unwanted signals. In these systems, a microphone is used to sense external background noise. This signal is then phase shifted to create a cancellation signal and added to the intended audio program signal sent to the speaker. The cancellation signal combines with the noise and effectively reduces or eliminates the level of unwanted noise perceived by the listener. 
     One shortcoming to active noise cancellation systems currently available is that a dedicated microphone must be incorporated to sense the noise heard by the user. For example, noise cancelling headphone sets will typically employ one microphone per ear piece and have their own power supply which energizes an electronic circuit to process the signal from the microphones and generate the cancellation signal. This additional circuitry increases the size and cost of such units and limits their marketability to consumers. Additional problems are presented due to the distance between the sensing microphone, the speaker, and the listener&#39;s ear, making cancellation of higher frequency noise signals problematic. 
     SUMMARY 
     The present invention solves the problems inherent in the prior art by capitalizing on the established principle that most speakers will act as microphones to a certain degree. Even though most speakers are designed for optimum output performance, external sound will interact with the speaker diaphragm to induce a corresponding electrical signal at the speaker terminals. This signal can then be isolated from the output signal through various processing techniques known in the art, inverted, and sent back to the speaker to create the noise cancelling effect. 
     By obtaining the noise signal from the output speaker itself, the need for a dedicated microphone to sense the external noise is eliminated. In one form, the noise cancelling circuitry can be incorporated into a source device, such as a personal music player. The user is then free to operate the device with a variety of standard headsets or speaker systems. The additional processing circuitry should add only a small cost to the driving device while still providing an acceptable level of noise reduction for the user. Another advantage of this approach is that there is no longer a physical distance between the output speaker and the microphone, thereby increasing the range of frequencies amenable to cancellation. 
     In another form, the present invention can be incorporated into a larger music source device, such as a home theater system. Again, the level of background noise penetrating the listening room from other parts of the house could be obtained from the output speakers and used to create a similar noise cancelling effect without the need for a dedicated measurement microphone. The invention could further be used in such systems to obtain the room frequency response data directly from the output speakers for use in corrective equalization techniques. 
     This summary is provided to introduce a selection of concepts in a simplified form that are described in further detail in the detailed description and drawings contained herein. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. Yet other forms, embodiments, objects, advantages, benefits, features, and aspects of the present invention will become apparent from the detailed description and drawings contained herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram depicting a digital implementation of the present invention. 
         FIG. 2  is a schematic diagram depicting a hybrid analog-digital implementation of the present invention. 
         FIG. 3  is a schematic diagram depicting a further implementation of the present invention incorporating an adaptive modeling filter and series resistor. 
         FIG. 4  is a schematic diagram depicting the present invention as incorporated into a personal music player. 
         FIG. 5  is a schematic diagram depicting the present invention as incorporated into a home theater system. 
         FIG. 6  is a flow diagram demonstrating one embodiment of the method claimed by the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting and understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. The present invention can be implemented with various mixtures of analog and digital circuitry. 
       FIG. 1  illustrates a hybrid analog-digital implementation and  FIG. 2  illustrates a digital implementation of the present invention. Note that these illustrations represent the implementation for a single channel and for a typical stereophonic audio system this circuitry is replicated for each channel. It is possible to combine information from both stereo channels to aid in canceling the external noise as the external noise will typically exist in both channels. 
     Referring to  FIG. 1 , digital audio source  100  is typical of those found in personal musical players or home theater systems and connects original audio program material to processing unit  201 . Processing unit  201  is a digital processor capable of performing various signal manipulating functions, including, but not limited to, equalization, level adjustment, filtering, and phase shifting. The output of processing unit  201  is directed to digital to analog converter (DAC)  202 , also typically found in most digital music players. The output of DAC  120  is connected to the input of amplifiers  203  and  204 . The output of amplifier  203  is connected to both speaker/headphone  500  (which can be any speaker or device containing one or more speakers, such as a pair of headphones or one or more speaker enclosures, to name just a few non-limiting examples) and one input of difference amplifier  205 . The output of amplifier  204  is connected to the remaining input of difference amplifier  205 . The output of difference amplifier  205  passes through low pass filter  206  before being converted to digital form by analog to digital converter (ADC)  207  and connected to processing unit  201 . The components connected between the digital audio source  100  and headphone  500  are collectively referred to as control unit  200 . 
     Amplifier  204  provides a reference analog source signal which can be subtracted from the signal present at the junction between the amplifier  203  and the headphone  500 . Because the headphone  500  functions as both a speaker to broadcast the analog source signal produced by amplifier  203  and as a microphone to produce a signal representing the combined broadcast analog source and noise existing at the headphone  500 , removing the analog source signal from the signal produced by the headphone  500  will leave a signal representing the external noise measured at the headphone  500 . Difference amplifier  205  produces an analog signal which is formed by subtracting the reference signal from amplifier  204  from the signal measured at headphone  500 . Therefore, the output of difference amplifier  205  contains only the noise signal from the headphone  500  acting as a microphone, i.e., the signal produced by external sound which has not been canceled. Optionally, a programmable termination or programmable gain may be applied to the output of amplifier  204  to match the termination of the specific attached headphone  500 . Low-pass filter  206  is typically set to suppress signals with frequencies above a few KHz. In practice, only signals up to a few KHz are able to be canceled because only sounds which have wavelengths on the order of or larger than the relevant length scales of a system are amenable to cancellation. The relevant length scales are determined by the distance between the microphone, speaker, and the ear. Since the microphone and speaker elements are physically coincident for this method (the microphone is the speaker), it is possible to achieve cancellation at higher frequencies than other methods which have physical separation between the microphone and speaker elements. 
     The processing element  201  generates a scaled and inverted version of the noise signal from ADC  207 , adds it to the signal from the digital audio source  100  and sends it to digital to analog converter (DAC)  202 . A technique for accomplishing this is further discussed hereinbelow with reference to  FIG. 3 . 
       FIG. 2  illustrates a more purely digital implementation. In this case, the differencing and low pass filtering are performed digitally by the processing element  201 . The implementation shown in  FIG. 1  may be less expensive since the implementation shown in  FIG. 2  may require an ADC of greater resolution and higher sampling rate. 
     To achieve good cancellation performance, it is desired to minimize the latency of the feedback loop (the time it takes for a signal to travel from the output of the processing element through the various elements and back to the output of the processing element). The latency should be a small fraction of the period of the sound being canceled, so that the cancellation signal is in phase with the external noise. That is, the cancellation circuit should react as quickly as possible to changes in the external noise, where the reaction delay requirement is set by the speed of change of the external noise. Higher audio frequencies require shorter delays. DAC  202  and ADC  207  will typically have dominant contributions to the latency. Many DACs and ADCs used for audio applications have latencies of several tens of microseconds or more and may not be suitable for use with the present invention. DACs and ADCs suitable for audio applications with latencies of a few microseconds or less are available. High latency DACs and ADCs may be applicable for use with the present invention if prediction techniques are used in the processing element  201 . That is, the processing element predicts the future external noise based on previous samples and generates a cancellation signal which will be in phase with the future external noise by the time the cancellation signal passes through the DAC to the headphone  500 . There are several prediction techniques known in the art. 
       FIG. 3  illustrates another embodiment of the present invention. Again, digital audio source  100  connects to a processing unit  201 . Within processing unit  201 , the input audio program material is connected to model filter  201 - 1  and one input of adder  201 - 6 . The output of model filter  201 - 1  is passed through low pass filter  201 - 2  and connected to one input of subtractor  201 - 4 . The output of subtractor  201 - 4  is passed through active noise cancellation unit  201 - 5  and directed to the remaining input of adder  201 - 6 . The output of adder  201 - 6  passes through DAC  202  and connects to the input of amplifier  203 . The output of amplifier  203  connects to a first terminal of series resister  208 . The second terminal of series resistor  208  is connected both to headphone  500  and the input of amplifier  209 . The output of amplifier  209  passes through ADC  207  and low pass filter  201 - 3  before being connected to the remaining input of subtractor  201 - 4  in a feedback loop. 
     As described hereinabove, it is difficult to perform noise cancellation on audio frequencies which have a corresponding period smaller than the time scale of the noise cancelling system. Therefore, low pass filtering is used to remove higher frequency signals and avoid instability in the system. However, low pass filtering components also introduce delay into the signal path, creating a tradeoff between the cutoff frequency of the low pass filters  201 - 2  and  201 - 3  and the performance of the system. DAC  202 , ADC  207  and active noise cancellation unit  201 - 5  also introduce significant delay into the signal path. In the embodiment shown in  FIG. 3 , model filter  201 - 1  is a filter which reproduces the delaying effects of the components in the signal path including any gain and delay which may be frequency dependent. Ideally, the output of low pass filter  201 - 2  is identical to that portion of the output of low pass filter  201 - 3  representing the output of adder  201 - 6 . The output signal of subtractor  201 - 4  therefore ideally consists of a digital signal representing the external noise signal to be cancelled. Active noise cancellation unit  201 - 5  uses an adaptive active noise control algorithm to create a cancelling signal which is then combined with the original source signal by adder  201 - 6 . Many such noise control algorithms are known in the art, such as the Filtered-X LMS algorithm. 
     The embodiment of  FIG. 3  also includes series resistor  208 , which prevents the signal from headphone  500  acting as a microphone from being suppressed when amplifier  203  is implemented as a voltage output device, such as an operational amplifier. The value of series resistor  208  should be on the order of headphone  500 , which is in the range of ten ohms to a few hundred ohms depending on the particular brand and model of headphones being used. Series resistor  208  can be implemented as a programmatically selectable resistance or can be of fixed value. 
     A typical headphone has the left and right channels sharing the ground connection which results in some small mixing of the left and right channels measured at the input of amplifier  209 . This is because the headphone cable  502  has a non-zero resistance, typically much less than the headphone speaker. The amount of mixing is determined by the ratio of the cable resistance to the resistance of the speaker. This mixing can be incorporated into model filter  201 - 1 . Model filter  201 - 1  should correspond to the characteristics of the value of series resistor  208  and the characteristics of headphone  500 , which will change when the headphone  500  is changed and is generally not known in advance. This means that model filter  201 - 1  must be at least partially constructed adaptively. Several techniques to accomplish this are known in the art. For examples, see Kuo, Sen, and Morgan, Dennis,  Active Noise Control systems: Algorithms and DSP Implementations . New York: Wiley, 1996. 
       FIG. 4  illustrates an embodiment of the present invention as incorporated into a personal music player  400 . Two channels, left and right, are implemented as indicated by the “L” and “R” suffixes of various components. Personal music player  400  contains a digital audio source  100  for each audio channel. Digital audio source  100  provides input to control unit  200  which processes both the source signal and sensed external noise signal. The output of both control units  200 L and  200 R are connected to jack  300 , into which a standard stereo headphone set  500  may be connected. Headphone connector  501  operatively couples the speakers  504 L and  504 R to music player  400  via cable  502 . Cable  502  splits near the speakers, with one audio channel sent over each of cables  503 L and  503 R. For each audio channel, control unit  200  utilizes the corresponding speaker  504  as both an output driver and a noise-sensing microphone. 
       FIG. 5  illustrates another embodiment of the present invention as incorporated in a home theater audio system. Again, a separate audio source  100  and control unit  200  are provided for each audio channel. In this example, four speakers  600  are shown, with suffixes LF, RF, LR, and RR indicating left-front, right-front, left-rear, and right-rear respectively. Each control unit  200  is connected to a connector  300 , which operatively couples speaker  600  to control unit  200  via speaker cable  601 , with each speaker  600  acting as both an output driver and noise sensing microphone. Furthermore, in this example, speakers  600  can be used not only to sense unwanted background noise from outside the listening room, but also to sense imperfections in the frequency response of the room itself. For example, a reference signal, such as white or pink noise, can be output to the speakers with the resulting room response again measured using the same speakers as microphones. The data from this operation can then be used to make equalization adjustments in the amplifier&#39;s output, as is known in the art 
       FIG. 6  is a flow diagram illustrating the method of removing unwanted noise described hereinabove. The process begins at start point  700  where the digital source audio input is received (stage  710 ). The signal is then processed (stage  720 ) and converted to analog form (stage  730 ). After proper amplification, the signal is then sent to the speaker (stage  740 ). At stage  750 , the system measures the actual signal present at the speaker terminals, which includes both the intended program signal and the noise signal. The original source signal is then subtracted from this measured signal to extract the noise signal component (stage  760 ). After low-pass filtering the noise signal and converting to digital form (stage  770 ), the noise signal is phase shifted substantially 180° (although other amounts of phase shift are contemplated by the present invention) to obtain a cancellation signal (stage  780 ) which is then added back to the original signal in a feedback loop and output to the speaker (stage  790 ), with the process ending at point  795 . 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain embodiments have been shown and described and that all equivalents, changes, and modifications that come within the spirit of the inventions as described herein and/or by the following claims are desired to be protected. 
     Hence, the proper scope of the present invention should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications as well as all relationships equivalent to those illustrated in the drawings and described in the specification.