Abstract:
Disclosed herein, among other things, are methods and apparatus for a level-dependent compression system for hearing assistance devices, such as hearing aids. The present subject matter includes a hearing assistance device having a buffer for receiving time domain input signals and a frequency analysis module to convert time domain input signals into a plurality of subband signals. A power detector is adapted to receive the subband signals and to provide a subband version of the input signals. A nonlinear gain stage applies gain to the plurality of subband versions of the input signals, and a frequency synthesis module processes subband signals from the nonlinear gain stage and to create a processed output signal. The device also includes a filter for filtering the signals, and a level-dependent compression module. The level-dependent compression module is adapted to provide bandwidth control to the plurality of subband signals produced by the frequency analysis stage.

Description:
PRIORITY APPLICATIONS 
       [0001]    This application is a continuation of and claims the benefit of priority to U.S. patent application Ser. No. 13/269,232, filed on 7 Oct. 2011, which application is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present subject matter relates generally to audio processing devices and hearing assistance devices, such as audio limiters, audio compressors, hearing aids, and in particular to a level-dependent compression system for audio processing and hearing assistance devices. 
       BACKGROUND 
       [0003]    In the past, single-channel as well as multi-channel audio amplification systems have been devised to compress the dynamic range of audio signals. However, both types of systems suffer from different, mutually exclusive limitations. Single-channel systems preserve spectral contrast but cannot provide adequate frequency-dependent compressive gain. In addition, such systems unnecessarily suppress or distort signal information in situations with low signal-to-noise ratios, where strong interfering components in remote frequency regions can control the gain. For the same reason, steady background sounds can acquire an objectionable modulation in the presence of fluctuating foreground sounds. Multi-channel systems, on the other hand, can provide frequency-dependent compression and can ensure audibility of weak signal components in the presence of wideband interferers if these systems are sufficiently fast-acting. However, by reducing spectral contrast across channels, they diminish spectral pattern information. 
         [0004]    One place where this is observed is in hearing correction devices, such as hearing aids. Persons with sensorineural hearing loss experience reduced sensitivity to faint, low-level sounds and loudness recruitment, i.e., an abnormally steep growth of perceived loudness with sound level. In addition, due to the partial loss of frequency-dependent compressive gain in the impaired auditory system, the level-dependent auditory frequency tuning is affected. Compared to the normally-functioning auditory system, the tuning is particularly degraded at low sound levels resulting in a more static tuning as a function of level. One goal of assistive technology is to compensate for these consequences of sensorineural hearing loss, in order to improve perceived sound quality and aided performance of hearing-impaired listeners on advanced auditory functions such as speech or music perception in complex auditory environments. However, conventional single-channel and multi-channel systems suffer from the aforementioned problems, which sometimes can even compound the difficulties experienced by hearing-impaired listeners. The reduction of spectral contrast by multi-channel systems, for example, will only exacerbate the challenges faced by the impaired auditory system with its degraded frequency resolution. 
         [0005]      FIG. 1  illustrates a basic prior art compression system. In the first stage, the incoming signal is buffered and spectrally analyzed, for example by using an FFT, warped FFT, or a time-domain filter-bank analysis (e.g., Kates, J. M., 2008, “Digital Hearing Aids,” Plural Publishing, San Diego, Calif.). Next, typically the signal power or signal envelope (for brevity, only signal power is referred to in the following) in each band is estimated by a power detector and smoothed by a power integrator which informs the subsequent gain calculation (throughout this application, “band” refers to static spectral bands). This gain is then applied to the individual band signals and the overall signal is re-synthesized by using an inverse FFT or a synthesis filter bank in conjunction with overlap add synthesis.  FIG. 2  shows an alternative prior art implementation where the compressive-gain calculation is “side-branched”, with the compressive-gain filter transformed into the time domain and applied via time-domain convolution. 
         [0006]    Static hybrid systems such as the one devised by White in U.S. Pat. No. 4,701,953 entitled “Signal Compression System” (1987), use broadly overlapping analysis filters for envelope/power detection and narrow synthesis filters, preserve spectral contrast and provide frequency-dependent gain functions, but still fail to provide adequate signal gain in situations with low signal-to-noise ratios. 
         [0007]    What is needed in the art is a way to provide level-dependent processing of sounds that optimizes both spectral contrast and gain. 
       SUMMARY 
       [0008]    Disclosed herein, among other things, are methods and apparatus for a level-dependent compression system for audio processing and hearing assistance devices, such as audio limiters, audio compressors, and hearing aids. The present subject matter includes a hearing assistance device having a buffer adapted for receiving time domain input signals and a frequency analysis module adapted to convert the time domain signals into a plurality of subband signals. A power detector is adapted to receive the subband signals and to provide a subband version of the input signals. The hearing assistance device includes a nonlinear gain stage adapted to apply gain to the plurality of subband versions of the input signals, and a frequency synthesis module adapted to process subband signals from the nonlinear gain stage and to create a processed output signal. The device also includes a filter adapted for filtering the input signals and the output signal, and a level-dependent compression module. According to an embodiment, the level-dependent compression module is adapted to provide bandwidth control to the plurality of subband signals produced by the frequency analysis stage. The level-dependent compression module is adapted to add a weighted power of a first subband signal to at least one other weighted subband signal in an adjacent subband, and to provide a final instantaneous-power estimate, in an embodiment. 
         [0009]    This Summary is an overview of some of the teachings of the present application and not intended to be an exclusive or exhaustive treatment of the present subject matter. Further details about the present subject matter are found in the detailed description and appended claims. The scope of the present invention is defined by the appended claims and their legal equivalents. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a basic compression system found in prior art devices. 
           [0011]      FIG. 2  is a side-branch compression system found in prior art devices. 
           [0012]      FIG. 3  is a level-dependent compression system using feedback bandwidth control according to one embodiment of the present subject matter. 
           [0013]      FIG. 4  is a level-dependent compression system using feed-forward bandwidth control according to one embodiment of the present subject matter. 
           [0014]      FIG. 5  is a power summation system for channel n of a static-filterbank level-dependent compression system using summation of a plurality of neighboring static bands, according to one embodiment of the present subject matter. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The following detailed description of the present subject matter refers to subject matter in the accompanying drawings which show, by way of illustration, specific aspects and embodiments in which the present subject matter may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present subject matter. References to “an”, “one”, or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment. The following detailed description is demonstrative and not to be taken in a limiting sense. The scope of the present subject matter is defined by the appended claims, along with the full scope of legal equivalents to which such claims are entitled. 
         [0016]    The present subject matter includes method and apparatus for a level-dependent compression system for audio processing and hearing assistance devices, such as audio limiters, audio compressors, and hearing aids. The following examples will be provided for a hearing aid, which is only one type of hearing assistance device. It is understood however, that the disclosure is not limited to hearing aids and that the teachings provided herein can be applied to a variety of audio processing and hearing assistance devices. 
         [0017]    The present invention relates to a signal compression system and method, particularly suitable for compression of audio signals such as speech and music. In various embodiments, the present subject matter provides the use of level-dependent analysis channels to control the compressive-gain signal as a function of frequency. In various embodiments, the present level-dependent analysis channels are channels with level-dependent bandwidths. In various embodiments, powers from bands of a static bandwidth are weighted and summed according to signal level to operate on an effectively broader frequency range than a single analysis band. In various applications, the level-dependent bandwidths are a function of signal level to provide compression as a function of frequency and signal level. 
         [0018]    The present subject matter applies to compression systems using both uniformly and non-uniformly scaled analysis filterbanks. In addition, the present subject matter applies to compression systems using both unbranched and side-branched architectures. 
         [0019]    In various embodiments, this system provides an improved solution for the trade-off dilemma between preserved spectral contrast and applying frequency-specific gain compared to prior systems. The present subject matter is useful in a variety of applications involving compression of signals generally. 
       Approaches Using Tunable Bands 
       [0020]      FIG. 3  is a level-dependent compression system using feedback bandwidth control according to one embodiment of the present subject matter. In contrast to the prior approaches, the present level-dependent compression system provides tuning of the compression analysis channels that depends on the level of the incoming sound. In the system illustrated in  FIG. 3 , this is realized by changing the bandwidths of the initial frequency-analysis channels recursively, according to the power in each channel. In various embodiments, a feedback system is employed to perform bandwidth adjustment. For example, in various embodiments, the power in a given channel at a given time determines the bandwidth of that given channel at a later time. In one approach the bandwidth is updated for the next time frame (the immediately following time frame), corresponding to the embodiment in  FIG. 3  with identical clocks tA and tB. In this embodiment, the bandwidth update lags the signal by one frame. In various embodiments, the bandwidth update is performed by a feedback loop cycling multiple times during a given frame (at a higher clock speed) to reduce or avoid the lag. In various embodiments, the feedback loop is down-sampled to allow the bandwidth to update every M frames (M is an integer greater than 1). This corresponds to the embodiment in  FIG. 3  with clock tB running slower than clock tA. The bandwidth change can be implemented by changing filter parameters. In one embodiment, the bandwidth change is performed by changing parameters of finite impulse response (FIR) filters. In another embodiment, the bandwidth change is performed by changing parameters of infinite impulse response (IIR) filters. 
         [0021]    The bandwidth-power function should be continuous, but does not need to be monotonous. Possible choices include, but are not limited to, sigmoid curves, piecewise linear, exponential or power-law functions. In various embodiments with feedback a maximum change in bandwidth with power, i.e., the maximum absolute slope of the bandwidth-power function, is limited such that, for a white-noise input, the change in bandwidth corresponding to a 1-dB change in power results in an additional change of within-channel power of less than 1 dB. This ensures that the feedback loop is stable and converging in time. It is understood that other bandwidth-power functions may be used without departing from the scope of the present subject matter. 
         [0022]      FIG. 3  shows system  300  that includes a signal buffer  312  to receive input signals. In the embodiment of a hearing aid application, the input signal is acoustic information that is received by a transducer such as a microphone or radio receiver. In the embodiment of an audio processing application, the input signal is acoustic information that is received by a transducer, either in real-time or pre-recorded. The signal side-branches to a frequency analysis block  302  which generates sub-channel signals for power detector  304 . The sub-channel signals are received by power detector  304  which provides power estimates as a function of frequency (or sub-channel) as input to bandwidth control  316 . Based on the sub-channel power, the bandwidth control  316  calculates and updates the bandwidth-control parameters of the frequency analysis block  302 . The sub-channel signals from power detector  304  are sent to power integrator  305  which smoothes the power signals in time to minimize distortion (e.g., the power integrator could be a one-pole low-pass filter). The smoothed signals from power integrator  305  are sent to non-linear gain  306 , which calculates the gain according to prescriptive gain information for the wearer. The resulting sub-channel gains are converted to the time domain by frequency synthesis  310 . The resulting output of frequency synthesis  310  is sent to filtering  314  which applies the time-domain filter to the signal from buffer  312 . The output of filtering  314  is a processed sound using at least one embodiment of the present subject matter for level-dependent compression. Other configurations are possible and may vary without departing from the scope of the present subject matter. 
         [0023]      FIG. 4  is a level-dependent compression system using feed-forward bandwidth control according to one embodiment of the present subject matter. This level-dependent compression system provides tuning of the compression analysis channels that depends on the level of the incoming sound. In the system illustrated in  FIG. 4 , this is realized by changing the bandwidths of the frequency-analysis channels non-recursively, according to the power within bands of a static filterbank. In various embodiments, a feed-forward system is employed to perform bandwidth adjustment. For example, in various embodiments, the power in a given static band at a given time determines the bandwidth of the corresponding channel at the same time (this is the case in  FIG. 4  with identical clocks tA and tB). In various embodiments, the feed-forward bandwidth control is down-sampled to allow the bandwidth to update every M frames (M is an integer greater than 1). This corresponds to the embodiment in  FIG. 4  with clock tB running slower than clock tA. The bandwidth change can be implemented by changing filter parameters. In one embodiment, the bandwidth change is performed by changing parameters of finite impulse response (FIR) filters. In another embodiment, the bandwidth change is performed by changing parameters of infinite impulse response (IIR) filters. 
         [0024]    The bandwidth-power function should be continuous, but does not need to be monotonous. Possible choices include, but are not limited to, sigmoid curves, piecewise linear, exponential or power-law functions. It is understood that other bandwidth-power functions may be used without departing from the scope of the present subject matter. 
         [0025]      FIG. 4  shows system  400  that includes a signal buffer  420  to receive input signals. In the embodiment of a hearing aid application, the input signal is acoustic information that is received by a transducer such as a microphone or radio receiver. In the embodiment of an audio processing application, the input signal is acoustic information that is received by a transducer, either in real-time or pre-recorded. The signal side-branches to a frequency analysis block  402  which generates subband signals for power detector  404 . The subband signals are received by power detector  404  which provides power estimates as a function of frequency (or subband) as input to bandwidth control  406 . Based on the subband power, the bandwidth control  406  calculates and updates the bandwidth-control parameters of the frequency analysis block  408 . Frequency analysis block  408  generates sub-channel signals for power detector  410  which provides power estimates as a function of frequency (or sub-channel) as input to power integrator  412 . Power integrator  412  smoothes the power signals in time to minimize distortion. The smoothed signals from power integrator  412  are sent to non-linear gain  414 , which calculates the gain according to prescriptive gain information for the wearer. The resulting sub-channel gains are converted to the time domain by frequency synthesis  416 . The resulting output of frequency synthesis  416  is sent to filtering  418  which applies the time-domain filter to the signal from buffer  420 . The output of filtering  418  is a processed sound using at least one embodiment of the present subject matter for level-dependent compression. Other configurations are possible and may vary without departing from the scope of the present subject matter. 
       Approaches Using Weighted Static Bands 
       [0026]    Alternatively, the frequency-analysis stage  202  can remain static as in  FIG. 2 , but instead using level-dependent filtering realized by a modified power detector  500 , as illustrated in  FIG. 5 . For a given compression channel with number n, power estimates P n  from the frequency-analysis band number n and its adjacent bands n−1, n−2, n+1, n+2, etc. are weighted and summed. This yields the instantaneous power {tilde over (P)} n  in channel n. In this way, compression channel n operates on a wider frequency range than the single analysis band n. The weights w n,k  for channel n are chosen as a function of the target bandwidth b n  for this channel, according to the weight-bandwidth function: w n,k =W n,k (b n ), with w n,k ∈[0,1].The weights can be symmetrically or asymmetrically distributed across the lower and upper neighboring bands. For example, if non-zero weights were chosen only for band n and its higher-frequency neighbors (n+1, n+2, etc), channel n would be widened only to the high-frequency side. 
         [0027]    Since the level-dependence of the bandwidths is realized through power summation, it is most convenient to measure the channel bandwidths in terms of equivalent rectangular bandwidths. If the bands in  202  have equal maximum passband transmission, the ERB of compression-channel n will be the weighted sum of the ERBs of the individual bands contributing to that channel, with weights w n,k . The target bandwidth b n  for channel n is given by the bandwidth-power function B n , which should be continuous, but does not need to be monotonous. It is understood that other bandwidth-power functions may be used without departing from the scope of the present subject matter. There are two possible choices for the input received by the bandwidth-power function. The bandwidth can be chosen to depend on the channel power: b n =B n ({tilde over (P)} n ), or, alternatively, to depend on the band power: b n =B n (P n ). The former results in feedback bandwidth control while the latter results in a feed-forward bandwidth control. 
         [0028]    In  FIG. 5 , the power estimates from a plurality of bands, including band n, from subband power detectors  512 ,  514 ,  516  are weighted ( . . .  522 ,  524 ,  526 , . . .) and summed with a summing node  528 . The resulting instantaneous power {tilde over (P)} n  is sent to the power integrator  530 . 
         [0029]    Another embodiment of the present subject matter includes a compression system which employs two parallel filterbank paths, one filterbank with narrow and one with broad channels, and then either weights and sums their corresponding power estimates with level-dependent weights or calculates two non-linear gain signals based on the power estimates from the two filterbanks and then weights and sums these gain signals with level-dependent weights. At low sound levels, for example, the gain is predominantly determined by the filterbank with narrow channels, while the gain at high sound levels is determined by the filterbank with broad channels. 
         [0030]    Further Considerations 
         [0031]    Compression speeds and bandwidth-power functions of the compression channels are chosen according to the objectives of the compression system. For example, the compression speed should mirror the rate of the information-carrying power fluctuations in the signal to be compressed, which can differ for speech and music. The present subject matter is not limited to the use of a particular compression speed or bandwidth-power function. However, various embodiments of the present subject matter include one or more of fast-acting compression (resolving phonemic level variations of speech) and/or channels widening with increasing level when the system is employed to compensate for hearing impairment. In various embodiments, time constants on the order of tens of milliseconds are employed to perform the fast-acting compression. 
         [0032]    If the level-dependent compression channels are widened sufficiently with increasing level, the proposed level-dependent system will preserve spectral contrast for high-level portions of sound such as vowels and vowel-consonant transitions in speech which are coded in terms of spectral-pattern cues. Furthermore, this system will prevent distortion of short-term spectral changes in high-level sounds such as frequency glides or formant transitions in speech and music. Since the compression channels will be narrow at low input levels, the system can provide adequate gain to low-level signals such as consonants in speech surrounded by spectral interferers. Furthermore, narrow channels at low levels will prevent objectionable modulation of steady background sounds by foreground sounds. If the system is sufficiently fast-acting, it can restore audibility of weak sounds rapidly following intense sounds such as weak consonants following intense vowels. It can also restore audibility in complex situations where multiple talkers are speaking at different levels. Hence, this system increases the potential for listening in both spectral and temporal dips, and taking into account the preservation of spectral contrast at high levels, it combines the advantages of both single-channel and multi-channel compression without suffering from their respective disadvantages. 
         [0033]    It should be noted that an asymmetric widening of the compression channels towards the high-frequency side with increasing level can compensate specifically for increased upward spread of masking which is often observed in hearing-impaired listeners. High-frequency sound components falling into a given compression channel will reduce the gain applied to sound components at lower frequencies and thus reduce upward spread of masking. 
         [0034]    In addition, the proposed system can normalize loudness perception in hearing-impaired listeners to a larger extent than prior systems. Normal-hearing listeners show a differential growth of loudness for narrowband and wideband sounds, due to the level-dependent bandwidth of auditory filters. For wideband stimuli at low levels, remote frequency components are compressed independently, since they fall into narrow, independent auditory filters. At higher levels, filters are broader and remote frequency components will be compressed jointly, even for wideband stimuli. As a consequence, differences in loudness between narrowband and wideband sounds decrease with increasing level. Since hearing-impaired listeners show broadened and more static auditory filters than normal-hearing listeners, they do not show the same differential growth of loudness. However, compression using channels which widen with increasing level can restore differential loudness growth for aided hearing-impaired listeners. The normalization of loudness perception may improve perceived sound quality as well as performance on involved auditory tasks such as speech perception in complex environments. 
         [0035]    The combination of level-dependent channels and fast-acting compression also bears advantages in audio limiting and output compression limiting: If the instantaneous power in a given compression channel is high, the channel will be widened and thus, power summation across frequency is accounted for by this channel. This allows for a higher limiting threshold level (the level at which compression limiting is activated) and for a smaller clipping margin (the difference between the maximum allowed band output level and broadband saturation level), resulting in improved perceived sound quality. 
         [0036]    The present subject matter is demonstrated for hearing aids. It is understood however, that the disclosure is not limited to hearing aids and that the teachings provided herein can be applied to a variety of audio processing and hearing assistance devices, including but not limited to, behind-the-ear (BTE), in-the-ear (ITE), in-the-canal (ITC), receiver-in-canal (RIC), or completely-in-the-canal (CIC) type hearing aids. It is understood that behind-the-ear type hearing aids may include devices that reside substantially behind the ear or over the ear. Such devices may include hearing aids with receivers associated with the electronics portion of the behind-the-ear device, or hearing aids of the type having receivers in the ear canal of the user, including but not limited to receiver-in-canal (RIC) or receiver-in-the-ear (RITE) designs. The present subject matter can also be used in hearing assistance devices generally, such as cochlear implant type hearing devices and such as deep insertion devices having a transducer, such as a receiver or microphone, whether custom fitted, standard, open fitted or occlusive fitted. It is understood that other hearing assistance devices not expressly stated herein may be used in conjunction with the present subject matter. 
         [0037]    This application is intended to cover adaptations or variations of the present subject matter. It is to be understood that the above description is intended to be illustrative, and not restrictive. The scope of the present subject matter should be determined with reference to the appended claims, along with the full scope of legal equivalents to which such claims are entitled.