Abstract:
A method and apparatus for active noise canceling. The method includes retrieving an input sample from at least one of a feedback or feedforward microphone digitized through the sigma-delta converter, retrieving the input sample and a related filter, wherein the filter is customized to the particular headset, outputting a filtered signal through a speaker without any interpolation and reducing order of CIC filters, and outputting a response sharply tapered down.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to a method and apparatus for a configurable active noise canceller. In one embodiment, the present invention relates to a configurable active noise canceller that may be used in a digital system 
     2. Description of the Related Art 
     Currently, due to latencies, analogue solutions are used in active noise cancelling devices, such as headsets. Even though such analog solutions tend to have a high bandwidth of noise cancellation, they offer limited tuning of the cancellation profile and music equalization. Furthermore, since music is equalized even when the active noise canceller is OFF, turning off the active noise canceller usually requires either a separate channel for music or turning off the music completely, which is an expensive solution. Hence, the music is usually turned off when the active noise canceller is not active. 
     Therefore, there is a need for a method and/or apparatus for an improved configurable active noise canceller. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention relate to a method and apparatus for active noise canceling. The method includes retrieving an input sample from at least one of a feedback or feedforward microphone digitized through the sigma-delta converter, retrieving the input sample and a related filter, wherein the filter is customized to the particular headset, outputting a filtered signal through a speaker without any interpolation and reducing order of CIC filters, and outputting a response sharply tapered down. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  embodiment depicting a block diagram of an active noise cancellation using a fixed controller at oversampled data rates; 
         FIG. 2  is a flow diagram depicting a method for active noise canceling; 
         FIG. 3  is an embodiment depicting a controller; 
         FIG. 4  is an embodiment depicting an alternate path for music equalization for non-active noise canceller; 
         FIG. 5  is an embodiment depicting a feedback active noise cancellation for a headset; 
         FIG. 6  is an alternate embodiment depicting a feedback active noise cancellation for a headset; 
         FIG. 7  is an embodiment of an analog implementation of an active noise cancellation controller; 
         FIG. 8  is an embodiment depicting an open loop response feedback of an analog active noise canceller; and 
         FIG. 9  is an embodiment depicting a wideband adaptive feedback digital active noise canceller with FXLMS. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is a a feedback active noise canceller using a fixed controller at oversampled data rates.  FIG. 1  is an embodiment depicting a block diagram of an active noise cancellation using a fixed controller at oversampled data rates. In this embodiment, the active noise canceller comprises analogue to digital converters, a digital signal processor, and digital to analogue converters. The analogue to digital converters convert the left and right internal, i.e feed-back, microphone signals into the digital domain and the left and right external i.e. feed-forward, microphone signals into the digital domain. The digital signal processor is configurable and programmable at sample rates much higher than the typical audio sample rate. The digital to analogue converters convert the noise and audio data into the analog domain and into the headphone speakers. 
       FIG. 2  is a flow diagram depicting a method for an active noise canceller using a fixed controller at oversampled data rates. The method starts at step  200  and proceeds to step  202 . At step  202 , the method  200  retrieves a digital input sample. The digitized input sample is from feedback or feedforward microphone and may be digitized through the sigma-delta converter. At step  204 , the method  200  retrieves and filter the input sample, the filtering may be customized to the particular headset. The filter may be computed automatically or manually tuned for a target response. At step  206 , the method  200  outputs the filtered signal without any interpolation and reduced order of CIC filters. At step  208 , the method  200  outputs a response sharply tapered down. The method  200  ends at step  210 . 
     For commercial headset active noise canceller solutions, a wideband implementation is necessary that may work with low-medium quality headset design. Oversampled data rates achieve both of these goals. The data may get sampled at 8-10 times the audio sample rate. These sample rates is much higher than the data rates used for audio applications. 
     As a result, active noise canceller may utilize hardware CIC filters for anti aliasing. A separate decimation component is avoided as the aliasing frequencies are close to 192 KHz. This is outside the range of hearing for humans. The decimation component also significantly contributes to the overall latency of the system. By not using a decimation filter the latency is minimized in the software processing. Also, oversampling allows for the use of hardware copy-paste filters for anti imaging, which avoids a separate interpolation component. Hence, the headphone and the microphone elements act as anti imaging/aliasing filters by filtering out higher frequencies, i.e. above 20 KHz. 
     In one embodiment, processing is performed at 384 KHz. At this sample rate we have an 8 sample delay in the ADC/DAC chain due to the CIC and the copy paste interpolation/decimation process. This corresponds to 20 us latency without using any filtering in the DSP. At these low delays, an analog-like controller design is implemented to perform noise cancellation.  FIG. 3  is an embodiment depicting a controller. In this embodiment, the noise cancellation of interest is assumed to be below 1000 Hz. When data is oversampled, the delays are negligible for controller operation, which reduces the significance of such delays. As a result, a digital low pass filter may be used for noise cancellation. Since the latency of the filters increase with group delay, the lower order digital filters perform better noise cancellation. This structure has the bandwidth of the analog implementation along with advantages of digital solutions, which include low complexity solution, fixed music equalization, alternate path for music equalization for non-active noise canceller cases are possible, and tunable active noise canceller response.  FIG. 4  is an embodiment depicting an alternate path for music equalization for non-active noise canceller. In  FIG. 4 , grey is the music path for active noise cancellation and black shows music path with active noise cancellation disabled 
     Oversampled data rates allow for low latency in the feedback path giving good wideband performance for noise cancellation. A digital control provides easily tunable cancellation and music response as compared to analog systems and allows for separate ANC-on and ANC-off music paths. This allows for separate equalization for the headphones when the ANC is disabled. In an analog setup, additional data path is required for this feature making it expensive in terms of power and number of components. 
     As a result, a single solution is possible across a large selection of headphones. This lowers the overall silicon costs and provides them with a tunable equalizer for the headphone response. This solution offers the bandwidth of cancellation comparable to an analog solution with the tenability of a digital ANC. 
       FIG. 5  is an embodiment depicting a feedback active noise cancellation for a headset.  FIG. 6  is an alternate embodiment depicting a feedback active noise cancellation for a headset. The objective of the controller is to generate anti-noise y(n) to drive the error e(n) to zero. The controller can be fixed or adaptive. The stability of the system is a function of the headphone acoustics (i.e. secondary path) and the controller response. 
     Under steady state conditions E(ω)=D(ω)−S(ω)W(ω)E(ω)→E(ω)=D(ω)/(1+S(ω) W(ω)). If S(ω) were flat and without phase shift E(ω) could be made small by applying a large gain W(ω) over the frequencies of interest. In a digital system, S(ω) includes the delays caused by ND conversion filtering and D/A conversion and the headphone acoustics. As the delays in the SP become significant the controller becomes in-efficient and the bandwidth of cancellation reduces. 
       FIG. 7  is an embodiment of an analog implementation of an active noise cancellation controller in accordance with the prior art. The noise path in the controller consists of a non-inverting amplifier (filter  1 ) followed by an inverting amplifier (filter  2 ).  FIG. 8  is an embodiment depicting an open loop response feedback of an analog active noise canceller. Filter  2  also pre-equalizes the music to compensate for the attenuation cased by ANC. 
     The digital feedback active noise canceller is implemented using a Filtered-X-Ims algorithm.  FIG. 9  is an embodiment depicting a wideband adaptive feedback digital active noise canceller with FXLMS. As described in  FIG. 9 , the FIR controller is adapted using the LMS algorithm to reduce the error e(n). The input to LMS is generated using e(n) and the secondary path estimate  SP . This signal is filtered by  SP  to align the error with the estimated desired signal. 
     Table 1 describes a comparison of analogue and digital active noise canceller solution. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Analog ANC 
                 Digital ANC 
               
               
                   
               
             
             
               
                 Secondary Path 
                   
               
               
                 Low delay in secondary path 
                 At audio frequencies the delays in 
               
               
                   
                 secondary path and the controller is 
               
               
                   
                 significant due to A/D→anti- 
               
               
                   
                 alising→controller→D/A conversion 
               
               
                 Bandwidth 
               
               
                 High Bandwidth of cancellation. Due to 
                 The signal chain delays significantly 
               
               
                 low delays high cancellation is possible 
                 decrease the cancellation bandwidth 
               
               
                 by designing a controller with high gain 
               
               
                 in the frequency of interest 
               
               
                 Controller design. 
               
               
                 Due to low delays in the SP, the design 
                 The controller design needs to be 
               
               
                 of a controller is a low pass filter. 
                 adaptive/predictive to compensate for 
               
               
                   
                 delays in the secondary path. This is 
               
               
                   
                 computationally expensive 
               
               
                 Secondary Path Variability 
               
               
                 Since the secondary path (controller 
                 In the digital domain the changes in 
               
               
                 path) is very fast, small movements in 
                 secondary path due to headphone 
               
               
                 the headphone manifest as small changes 
                 movement become significant and the 
               
               
                 in the secondary path and the controller still 
                 controller has to readapt to the 
               
               
                 operates with good phase margins. 
                 changes [1]. This re-adaptation time is 
               
               
                   
                 very long leading to poor cancellation 
               
               
                   
                 performance. 
               
               
                 Narrow Band Performance 
               
               
                 The analog controllers need extensive 
                 Very good narrow band cancellation 
               
               
                 design for good narrowband 
                 can be achieved as the phase lag in 
               
               
                 cancellation. The controller design 
                 the secondary path can be 
               
               
                 needs to accommodate for the phase 
                 compensated by adaptive filters. The 
               
               
                 lag in the secondary path for exact 
                 variations in the secondary path are also 
               
               
                 noise cancellation for narrow band noise. 
                 easier to handle 
               
               
                 Music Playback 
               
               
                 Since the controller is fixed, 
                 Compensating for music is 
               
               
                 compensating for music playback is 
                 computationally due to the adaptive 
               
               
                 implemented as an equalizers to 
                 nature of the feedback path. The music 
               
               
                 equalize for low frequency attenuations 
                 is filtered using an inverse secondary 
               
               
                   
                 path and controller filter to negate the 
               
               
                   
                 attenuation caused by feedback. 
               
               
                   
               
             
          
         
       
     
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.