Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority from and is related to the following prior application: Digital Quasi-RMS Detector, U.S. Provisional Application No. 60/284,460, filed Apr. 18, 2001. This application is also related to the following co-pending applications that are commonly owned by the assignee of the present application: Digital Hearing Aid System, U.S. patent application No. [application number not yet assigned], filed Apr. 12, 2002; and Inter-Channel Communication in a Multi-Channel Digital Hearing Instrument, U.S. patent application No. [application number not yet assigned], filed Apr. 18, 2002. These prior applications, including the entire written descriptions and drawing figures, are hereby incorporated into the present application by reference. 

   BACKGROUND 
   1. Field of the Invention 
   This invention generally relates to sound processing. More specifically, the invention provides a computationally efficient method of estimating the root-mean-square (“RMS”) content of a spectrally complex signal. 
   2. Description of the Related Art 
   The human ear gauges the loudness of a signal based upon its RMS content. Traditionally, the RMS content of a spectrally complex signal is calculated by computing the square root of the mean of the squared signal being processed. The square root calculation, however, is processor intensive, and thus places a heavy burden on miniaturized sound processing devices, such as digital hearing instruments. 
   SUMMARY 
   A digital quasi-RMS detector is provided that approximates the time-varying RMS energy of a signal. The digital quasi-RMS detector rectifies the signal and compares the rectified signal with an estimated present energy value of the audio signal. If the difference between the rectified signal and the estimated present energy value is not greater than zero, then the digital quasi-RMS detector multiplies the rectified signal by a first time constant to generate a first filtered signal and sums the first filtered signal with the estimated present energy value to determine the approximate RMS energy. If the difference between the rectified signal and the estimated present energy is greater than zero, however, then the digital quasi-RMS detector multiplies the rectified signal by a second time constant to generate a second filtered signal and sums the second filtered signal with the estimated present energy value to determine the approximate RMS energy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an exemplary digital hearing aid system according to the present invention; 
       FIG. 2  is a flowchart depicting an exemplary method for estimating the RMS energy of a spectrally complex signal; and 
       FIG. 3  is a graph illustrating the performance of an exemplary quasi-RMS detector compared to a typical average detector and a typical peak detector. 
   

   DETAILED DESCRIPTION 
   Turning now to the drawing figures,  FIG. 1  is a block diagram of an exemplary digital hearing aid system  12 . The digital hearing aid system  12  includes several external components  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 ,  28 , and, preferably, a single integrated circuit (IC)  12 A. The external components include a pair of microphones  24 ,  26 , a tele-coil  28 , a volume control potentiometer  24 , a memory-select toggle switch  16 , battery terminals  18 ,  22 , and a speaker  20 . 
   Sound is received by the pair of microphones  24 ,  26 , and converted into electrical signals that are coupled to the FMIC  12 C and RMIC  12 D inputs to the IC  12 A. FMIC refers to “front microphone,” and RMIC refers to “rear microphone.” The microphones  24 ,  26  are biased between a regulated voltage output from the RREG and FREG pins  12 B, and the ground nodes FGND  12 F and RGND  12 G. The regulated voltage output on FREG and RREG is generated internally to the IC  12 A by regulator  30 . 
   The tele-coil  28  is a device used in a hearing aid that magnetically couples to a telephone handset and produces an input current that is proportional to the telephone signal. This input current from the tele-coil  28  is coupled into the rear microphone A/D converter  32 B on the IC  12 A when the switch  76  is connected to the “T” input pin  12 E, indicating that the user of the hearing aid is talking on a telephone. The tele-coil  28  is used to prevent acoustic feedback into the system when talking on the telephone. 
   The volume control potentiometer  14  is coupled to the volume control input  12 N of the IC. This variable resistor is used to set the volume sensitivity of the digital hearing aid. 
   The memory-select toggle switch  16  is coupled between the positive voltage supply VB  18  and the memory-select input pin  12 L. This switch  16  is used to toggle the digital hearing aid system  12  between a series of setup configurations. For example, the device may have been previously programmed for a variety of environmental settings, such as quiet listening, listening to music, a noisy setting, etc. For each of these settings, the system parameters of the IC  12 A may have been optimally configured for the particular user. By repeatedly pressing the toggle switch  16 , the user may then toggle through the various configurations stored in the read-only memory  44  of the IC  12 A. 
   The battery terminals  12 K,  12 H of the IC  12 A are preferably coupled to a single 1.3 volt zinc-air battery. This battery provides the primary power source for the digital hearing aid system. 
   The last external component is the speaker  20 . This element is coupled to the differential outputs at pins  12 J,  121  of the IC  12 A, and converts the processed digital input signals from the two microphones  24 ,  26  into an audible signal for the user of the digital hearing aid system  12 . 
   There are many circuit blocks within the IC  12 A. Primary sound processing within the system is carried out by a sound processor  38  and a directional processor and headroom expander  50 . A pair of A/D converters  32 A,  32 B are coupled between the front and rear microphones  24 ,  26 , and the directional processor and headroom expander  50 , and convert the analog input signals into the digital domain for digital processing. A single D/A converter  48  converts the processed digital signals back into the analog domain for output by the speaker  20 . Other system elements include a regulator  30 , a volume control A/D  40 , an interface/system controller  42 , an EEPROM memory  44 , a power-on reset circuit  46 , a oscillator/system clock  36 , a summer  71 , and an interpolator and peak clipping circuit  70 . 
   The sound processor  38  preferably includes a pre-filter  52 , a wide-band twin detector  54 , a band-split filter  56 , a plurality of narrow-band channel processing and twin detectors  58 A– 58 D, a summation block  60 , a post filter  62 , a notch filter  64 , a volume control circuit  66 , an automatic gain control output circuit  68 , a squelch circuit  72 , and a tone generator  74 . 
   Operationally, the digital hearing aid system  12  processes digital sound as follows. Analog audio signals picked up by the front and rear microphones  24 ,  26  are coupled to the front and rear A/D converters  32 A,  32 B, which are preferably Sigma-Delta modulators followed by decimation filters that convert the analog audio inputs from the two microphones into equivalent digital audio signals. Note that when a user of the digital hearing aid system is talking on the telephone, the rear A/D converter  32 B is coupled to the tele-coil input “T”  12 E via switch  76 . Both the front and rear A/D converters  32 A,  32 B are clocked with the output clock signal from the oscillator/system clock  36  (discussed in more detail below). This same output clock signal is also coupled to the sound processor  38  and the D/A converter  48 . 
   The front and rear digital sound signals from the two A/D converters  32 A,  32 B are coupled to the directional processor and headroom expander  50 . The rear A/D converter  32 B is coupled to the processor  50  through switch  75 . In a first position, the switch  75  couples the digital output of the rear A/D converter  32  B to the processor  50 , and in a second position, the switch  75  couples the digital output of the rear A/D converter  32 B to summation block  71  for the purpose of compensating for occlusion. 
   Occlusion is the amplification of the users own voice within the ear canal. The rear microphone can be moved inside the ear canal to receive this unwanted signal created by the occlusion effect. The occlusion effect is usually reduced by putting a mechanical vent in the hearing aid. This vent, however, can cause an oscillation problem as the speaker signal feeds back to the microphone(s) through the vent aperture. Another problem associated with traditional venting is a reduced low frequency response (leading to reduced sound quality). Yet another limitation occurs when the direct coupling of ambient sounds results in poor directional performance, particularly in the low frequencies. The system shown in  FIG. 1  solves these problems by canceling the unwanted signal received by the rear microphone  26  by feeding back the rear signal from the A/D converter  32 B to summation circuit  71 . The summation circuit  71  then subtracts the unwanted signal from the processed composite signal to thereby compensate for the occlusion effect. 
   The directional processor and headroom expander  50  includes a combination of filtering and delay elements that, when applied to the two digital input signals, form a single, directionally-sensitive response. This directionally-sensitive response is generated such that the gain of the directional processor  50  will be a maximum value for sounds coming from the front microphone  24  and will be a minimum value for sounds coming from the rear microphone  26 . 
   The headroom expander portion of the processor  50  significantly extends the dynamic range of the A/D conversion, which is very important for high fidelity audio signal processing. It does this by dynamically adjusting the operating points of the A/D converters  32 A/ 32 B. The headroom expander  50  adjusts the gain before and after the A/D conversion so that the total gain remains unchanged, but the intrinsic dynamic range of the A/D converter block  32 A/ 32 B is optimized to the level of the signal being processed. 
   The output from the directional processor and headroom expander  50  is coupled to the pre-filter  52  in the sound processor  38 , which is a general-purpose filter for pre-conditioning the sound signal prior to any further signal processing steps. This “pre-conditioning” can take many forms, and, in combination with corresponding “post-conditioning” in the post filter  62 , can be used to generate special effects that may be suited to only a particular class of users. For example, the pre-filter  52  could be configured to mimic the transfer function of the user&#39;s middle ear, effectively putting the sound signal into the “cochlear domain.”Signal processing algorithms to correct a hearing impairment based on, for example, inner hair cell loss and outer hair cell loss, could be applied by the sound processor  38 . Subsequently, the post-filter  62  could be configured with the inverse response of the pre-filter  52  in order to convert the sound signal back into the “acoustic domain” from the “cochlear domain.” Of course, other pre-conditioning/post-conditioning configurations and corresponding signal processing algorithms could be utilized. 
   The pre-conditioned digital sound signal is then coupled to the band-split filter  56 , which preferably includes a bank of filters with variable corner frequencies and pass-band gains. These filters are used to split the single input signal into four distinct frequency bands. The four output signals from the band-split filter  56  are preferably in-phase so that when they are summed together in summation block  60 , after channel processing, nulls or peaks in the composite signal (from the summation block) are minimized. 
   Channel processing of the four distinct frequency bands from the band-split filter  56  is accomplished by a plurality of channel processing/twin detector blocks  58 A– 58 D. Although four blocks are shown in  FIG. 1 , it should be clear that more than four (or less than four) frequency bands could be generated in the band-split filter  56 , and thus more or less than four channel processing/twin detector blocks  58  may be utilized with the system. 
   Each of the channel processing/twin detectors  58 A– 58 D provide an automatic gain control (“AGC”) function that provides compression and gain on the particular frequency band (channel) being processed. Compression of the channel signals permits quieter sounds to be amplified at a higher gain than louder sounds, for which the gain is compressed. In this manner, the user of the system can hear the full range of sounds since the circuits  58 A– 58 D compress the full range of normal hearing into the reduced dynamic range of the individual user as a function of the individual user&#39;s hearing loss within the particular frequency band of the channel. 
   The channel processing blocks  58 A– 58 D can be configured to employ a twin detector average detection scheme while compressing the input signals. This twin detection scheme includes both slow and fast attack/release tracking modules that allow for fast response to transients (in the fast tracking module), while preventing annoying pumping of the input signal (in the slow tracking module) that only a fast time constant would produce. The outputs of the fast and slow tracking modules are compared, and the compression parameters are then adjusted accordingly. The compression ratio, channel gain, lower and upper thresholds (return to linear point), and the fast and slow time constants (of the fast and slow tracking modules) can be independently programmed and saved in memory  44  for each of the plurality of channel processing blocks  58 A– 58 D. 
     FIG. 1  also shows a communication bus  59 , which may include one or more connections for coupling the plurality of channel processing blocks  58 A– 58 D. This inter-channel communication bus  59  can be used to communicate information between the plurality of channel processing blocks  58 A– 58 D such that each channel (frequency band) can take into account the “energy” level (or some other measure) from the other channel processing blocks. Preferably, each channel processing block  58 A– 58 D would take into account the “energy” level from the higher frequency channels. In addition, the “energy” level from the wide-band detector  54  may be used by each of the relatively narrow-band channel processing blocks  58 A– 58 D when processing their individual input signals. 
   After channel processing is complete, the four channel signals are summed by summation bock  60  to form a composite signal. This composite signal is then coupled to the post-filter  62 , which may apply a post-processing filter function as discussed above. Following post-processing, the composite signal is then applied to a notch-filter  64 , that attenuates a narrow band of frequencies that is adjustable in the frequency range where hearing aids tend to oscillate. This notch filter  64  is used to reduce feedback and prevent unwanted “whistling” of the device. Preferably, the notch filter  64  may include a dynamic transfer function that changes the depth of the notch based upon the magnitude of the input signal. 
   Following the notch filter  64 , the composite signal is coupled to a volume control circuit  66 . The volume control circuit  66  receives a digital value from the volume control A/D  40 , which indicates the desired volume level set by the user via potentiometer  14 , and uses this stored digital value to set the gain of an included amplifier circuit. 
   From the volume control circuit, the composite signal is coupled to the AGC-output block  68 . The AGC-output circuit  68  is a high compression ratio, low distortion limiter that is used to prevent pathological signals from causing large scale distorted output signals from the speaker  20  that could be painful and annoying to the user of the device. The composite signal is coupled from the AGC-output circuit  68  to a squelch circuit  72 , that performs an expansion on low-level signals below an adjustable threshold. The squelch circuit  72  uses an output signal from the wide-band detector  54  for this purpose. The expansion of the low-level signals attenuates noise from the microphones and other circuits when the input S/N ratio is small, thus producing a lower noise signal during quiet situations. Also shown coupled to the squelch circuit  72  is a tone generator block  74 , which is included for calibration and testing of the system. 
   The output of the squelch circuit  72  is coupled to one input of summation block  71 . The other input to the summation bock  71  is from the output of the rear A/D converter  32 B, when the switch  75  is in the second position. These two signals are summed in summation block  71 , and passed along to the interpolator and peak clipping circuit  70 . This circuit  70  also operates on pathological signals, but it operates almost instantaneously to large peak signals and is high distortion limiting. The interpolator shifts the signal up in frequency as part of the D/A process and then the signal is clipped so that the distortion products do not alias back into the baseband frequency range. 
   The output of the interpolator and peak clipping circuit  70  is coupled from the sound processor  38  to the D/A H-Bridge  48 . This circuit  48  converts the digital representation of the input sound signals to a pulse density modulated representation with complimentary outputs. These outputs are coupled off-chip through outputs  12 J,  12 I to the speaker  20 , which low-pass filters the outputs and produces an acoustic analog of the output signals. The D/A H-Bridge  48  includes an interpolator, a digital Delta-Sigma modulator, and an H-Bridge output stage. The D/A H-Bridge  48  is also coupled to and receives the clock signal from the oscillator/system clock  36  (described below). 
   The interface/system controller  42  is coupled between a serial data interface pin  12 M on the IC  12 , and the sound processor  38 . This interface is used to communicate with an external controller for the purpose of setting the parameters of the system. These parameters can be stored on-chip in the EEPROM  44 . If a “black-out” or “brown-out” condition occurs, then the power-on reset circuit  46  can be used to signal the interface/system controller  42  to configure the system into a known state. Such a condition can occur, for example, if the battery fails. 
   In audio processing applications, such as the digital hearing aid instrument shown in  FIG. 1 , it is desirable that the system be able to estimate (or calculate) the root-mean-square (RMS) value of the input audio signal. This is important because the human ear gauges the loudness of an audio signal according to its RMS power. This RMS calculation may be performed in the wideband twin detector  54 , the narrowband twin detectors  58 A– 58 D, and the AGC-O block  68 . Calculating the RMS value of a spectrally complex signal is problematic, however, because of the processing power required to compute the square root component of the RMS calculation. The present invention overcomes this problem by estimating the RMS calculation using a quasi-RMS detector. The quasi-RMS detector may be implemented in blocks  54 ,  58 A– 58 D and  68 , and comprises a hybrid of an average detector with a peak detector to thereby avoid making the square root calculation. 
   An average detector can be implemented using rectification of an AC signal followed by low-pass filtering. The rectifier converts the AC signal into a DC component and an AC component. Low-pass filtering smoothes the rectified waveform by attenuating the AC component and leaving the DC component intact. The magnitude of the DC component is proportional to the amplitude of the original signal, and is therefore useful for metering applications. The low-pass filter can be thought of as a “leaky integrator.” In addition to smoothing the AC component, the average detector also responds gradually to changes in the magnitude of the original signal, meaning that the DC component actually waxes and wanes with a time constant inversely proportional to the frequency of the filter&#39;s pole. If the original signal is periodic and of constant amplitude, the output of the low-pass filter is directly proportional to the arithmetic mean of one complete cycle of the rectified waveform. 
   Simple mathematical analysis, however, shows that an average detector underestimates the true RMS content of a periodic signal having a peak-to-RMS ratio greater than 1, and that this error increases as the peak-to-RMS ratio increases. Therefore, in audio metering applications, an average detector alone will not provide an accurate estimate of the true RMS content of a speech-like signal. In applications where signal-processing decision thresholds are meant to correspond to the ranges of human hearing, these thresholds will be in error. 
   A peak detector can be implemented by augmenting the average detector described above as follows. If the amplitude of the rectified signal exceeds that at the output of the low-pass filter, the rectified signal is passed directly to the peak detector&#39;s output. If the amplitude of the rectified signal is less than that at the output of the low-pass filter, then the low-pass filter output is passed directly to the peak detector&#39;s output. The effect of this modification is to force the detector output to respond instantaneously to sudden increases in amplitude of the incoming signal, and to decay with the inherent time constant of the low-pass filter whenever the amplitude of the incoming signal suddenly drops. 
   In terms of speech-like audio signals, a peak detector will over-estimate the RMS content, and once again, in applications where signal-processing decision thresholds are meant to correspond to the ranges of human hearing, these thresholds will be in error. Thus, neither an average detector nor a peak detector, alone, can accurately estimate the true RMS content of a signal. 
   A quasi-RMS detection algorithm falls somewhere between average detection and peak detection. A quasi-RMS detector can be implemented by augmenting the peak detector described above as follows. If the amplitude of the rectified signal exceeds the detector&#39;s present output level, then the low-pass filter&#39;s time constant is reduced. If the amplitude of the rectified signal is less than the detector&#39;s present output level, then the low-pass filter&#39;s time constant is increased. As the detector responds to a fluctuating input signal, its low-pass filter time constant adaptively switches between two preset values. The effect of this modification is to force the detector output to respond quickly, but not instantaneously, to sudden increases in amplitude of the incoming signal using the smaller of the two filter time constants. This is defined as the detector&#39;s attack time constant. Sudden decreases in signal amplitude invoke the slower filter time constant, which now governs the detector&#39;s response time. This is defined as the detector&#39;s release time. 
     FIG. 2  is a flowchart  100  depicting an exemplary method for estimating the RMS energy of a spectrally complex signal using the quasi-RMS detector described above. This is a software implementation of the quasi-RMS detector. It should be understood, however, that these software method steps could also be implemented in a hardware circuit. 
   The method begins at step  102 . At step  104 , the next sample in the spectrally complex signal to be measured is obtained. The sample is then rectified (by taking the absolute value of the sample) at step  106 . The rectified sample is then subtracted from a running, present estimate of the signal energy to obtain a difference value “DIFF” at step  108 . At step  110 , the method determines whether the DIFF function is greater than zero. If it is not, then the DIFF value is multiplied by a release time constant K 1  at step  112 . If the DIFF function is greater than zero, then the DIFF value is multiplied by an attack time constant K 2  at step  116 . The result of the multiplication from step  112  or  116  is summed with the previous, running estimate of the signal energy at step  118  to generate an estimated RMS value. Steps  112 ,  116  and  118  thus result in a low-pass filter with an adaptively switching time constant. After filtering, control passes back to step  104  to obtain the next sample of the complex signal. 
   The release time constant K 1  is equal to 1/(sampling frequency*programmable time constant). The sampling frequency could be, for example, 32 KHz, and the programmable time constant could be 4 mSec, giving a K 1  value of 1/128. In the digital domain, multiplying by this 1/128 K value can be easily implemented in a binary shift register by shifting right by 7 register positions, which is a very efficient operation. 
   The attack time constant K 2  is equal to ¼ times the release time constant K 1 , which implements an RMS energy level detector. If K 2  were equal to 1, then the low-pass filter would be operating as a peak detector, and if K 2  were equal to K 1 , then the filter would be operating as an average detector. In the digital domain, the multiplication function in step  116  is then easily implemented by shifting right by 5 bit positions in a shift register operation. Thus, using the quasi-RMS algorithm described above, an estimate of the RMS content of a complex signal can be obtained without any complex square root, division, or even multiplication functions. 
   Heuristically, it is found that a quasi-RMS detector as described above gives good agreement with the response of a true RMS detector to speech-like signals when its attack time constant is approximately one quarter of its release time constant. Fine tuning of the residual error is afforded by adjusting one of the two time constants. Overall response time can then be adjusted without destroying the quasi-RMS behavior by scaling both time constants up or down while preserving their mutual ratio. This accuracy is obtained with substantially no more complexity than a peak detector or average detector, involving rectification and linear filtering. The algorithmic complexity is much lower than a true RMS detector, which requires a hardware-intensive means for exponentiation in addition to linear filtering. 
     FIG. 3  is a graph  199  illustrating the performance of an exemplary quasi-RMS detector  201  compared to a typical average detector  202  and a typical peak detector  203 . The x-axis of the graph  199  represents a range of crest factors (peak to RMS ratios). For example, a square wave may have a 0 dB crest factor, a speech signal may have a 12 dB crest factor, and a highly peaky signal may have an 18 dB crest factor. The y-axis of the graph  199  represents the error (in dB) exhibited by the exemplary RMS detectors  201 ,  202 ,  203  over the range of crest factors. Also illustrated is a performance curve for an ideal RMS meter  200  which exhibits a 0 dB error over the range of crest factors. 
   The range of crest factors over which an RMS detector  201 ,  202 ,  203  can estimate an RMS value for a complex waveform that is close to the true RMS value (0 dB error) is a good indication of performance. The exemplary quasi-RMS detector  201  may estimate RMS values that remain within 1 dB of true RMS over the entire range of low crest factor signals. The average detector  202 , however, may vary from true RMS by 12 dB, and the peak detector  203  may vary by 15 dB. It should be understood, however, that the error values shown in  FIG. 3  are provided for illustrative purposes only, and are not intended as a limitation on the performance of the quasi-RMS detector. 
   This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.

Technology Category: 3