AU5545294A - Electronic simulator of non-linear and active cochlear signal processing - Google Patents

Description

ELECTRONIC SIMULATOR OF NON-LINEAR AND ACTIVE COCHLEAR SIGNAL PROCESSING

Cross-Reference To Related Applications

This application is a continuation-in-part of U.S. Serial No. 07/970,141, filed November 2, 1992, entitled "Electronic Simulator Of Non-Linear And Active Cochlear Spectrum Analysis".

Background and Summary of the Invention

Many researchers, including the inventor herein, have spent many years trying to understand the biophysi¬ cal mechanisms of the human ear to understand how the human ear works and also to help in developing a module, which may be physically constructed, which duplicates or simulates the operation of the ear. Many theories and models have been developed over the years which have progressed this development. The inventor herein has previously developed a biophysical model (See Goldstein, J.L. "Modeling Rapid Waveform Compression on the Basilar Membrane as Multiple-Bandpass-Nonlinearity Filtering, " Hearing Research 49, 39-60, attached as Appendix A.) which sought to explain the biophysical mechanisms of the ear in terms of a multiple band pass non-linear input/- output model for simulating the non-linear basilar mem¬ brane response. The approach in this prior model was to account for non-linear cochlear phenomena directly in terms of a mathematical model of I/O behavior rather than treating them as non-linear perturbations from a physi¬ cally based linear theory. This basic model, and later models, incorporate two non-linearly interacting filter systems that simulate the two distinct non-linear regimes observed in all non-linear phenomena at low-to-moderate and moderate-to-high stimulus sound levels. While this prior model and theory represented a dramatic departure from the prior art in itself, and helped to advance the work on understanding the ear as it provided new direc¬ tion, this model was not a complete answer in that it did not explain observed phenomena such as distortion prod¬ ucts and otoacoustic emissions, or otherwise provide any understanding of the operation of the organ of Corti. Furthermore, the model did not fully address the function of the basilar membrane as the model structure does not reflect the known biophysical properties of the cochlea. The invention disclosed in the parent improved on this earlier model bringing it further in congruence with the known biophysical mechanisms of the ear, including most particularly the organ of Corti and basilar mem¬ brane, and in the process achieved simulation of the heretofore unexplained distortion product and otoacoustic emissions. In an intermediate step of the invention disclosed in the parent, the inventor added bilateral signal processing to his prior model to more completely simulate the two signaling channels responsible for the "tips" and "tails" of Cochlear tuning curves. The bila¬ teral processing potentiates extension of the model to other phenomena, including combination tones (distortion products) and otoacoustic emissions. This intermediate step takes advantage of non-linear feedback while the full invention disclosed in the parent adds distributed amplification. The distributed amplification provides for non-linear addition of many signals from tip sources which are believed to function similarly to the organ of Corti. These organ of Corti filters, or tip sources, are connected at different locations along a filter-bank spectrum analyzer (a corollary to the outer hair cells and adjoining structures) and are non-linearly added through a propagating medium (a corollary to the basilar membrane) to provide distributed amplification. This model thus helps explain the non-linear input/output characteristic as observed by others in the basilar mem¬ brane mechanical response in the human ear. There are several additional inventive features disclosed in the parent, including a "zoom" capability, a sensitivity adjustment, and efferent neural control simu¬ lation. The invention disclosed in the parent includes a pair of matched all pole lattices with a plurality of tip couplers tapped into each lattice and interconnecting them at chosen "center frequencies". A scaling factor, or alpha, may be induced at any frequency to alter the response at that frequency and thereby match the model's output to any particular human ear output. Additionally, an efferent bias control, which is ordinarily set to zero, may also be used to scale the throughput of any one or more tip couplers to simulate the brain's ability in humans to "tune out" undesirable sounds or simulate "li¬ stening without hearing" as experienced in humans. Choo- sing the number of tip couplers (and hence the length of the matched lattices), and the "center frequencies" of each of the tip couplers permits the model builder to focus on any one or more range of frequencies for mea¬ surement with the model. Additionally, the model accom- modates the use of 12,000 tip modules which corresponds to the full complement of outer hair cells believed to be contained and operative in the organ of Corti, to thereby provide a full representation and simulation of the fre¬ quency range of the human ear. As this may be cumbersome or undesirable, a fewer number of tip couplers may be used and may be focused over a chosen portion of the frequency range of hearing to thereby minimize cost and complexity of the model while still simulating with great accuracy the desired response frequencies.

Building on the invention disclosed in the parent, the inventor has improved on his earlier cochlea simula¬ tion. Progress in the biophysical study of tonotopically organized mechano-motility responses from isolated outer hair cells (See Brundin, L. & Russell, I., Sound-Induced Movements and Frequency Tuning in Outer Hair Cells Iso- lated from the Guinea Pig Cochlea; Symposium Reprints:

Biophysics Of Hair Cell Sensory Systems, Duifhuis, H. et al., Eds., Groningen, June 28, 1993 - July 2, 1993, pp. 121-127) provides an empirical basis for implementing the distributed feedback configuration of the multiple-band- pass-nonlinear signal-processing model (DMFBPNL), claimed and described in the parent, with analog electronic tech¬ nology in closer agreement with the cellular biophysics of the human ear. Simulation of cochlear operation in the present invention replaces the source delay line in the DMFBPNL model with the differential delays within the passbands of the tonotopically organized organ of Corti filters.

Further, in this invention the distributed ampli¬ fication by the basilar membrane is implemented in closer conformity with biophysics. A non-linear additive direc¬ tional wave amplification means (modeling the basilar membrane and space of Nuel) responds to two different signals, each of which corresponds to a physical mecha¬ nism. One is a "global" signal to the organ of Corti filter model, which corresponds to the fast pressure wave within the inner ear cavity in response to stapes input. The second is a "local" input signal to the organ of Corti filter model which corresponds to the classical slow-wave response to the pressure gradient across the basilar membrane in response to stapes input. This model simulates the basilar membrane serving as a collector in the additive directional wave amplification of tuned responses by outer hair cells. The collector uses stan¬ dard engineering principles (well-known in the art) for non-uniform transmission lines, with the added condition that dissipation is required to attenuate the amplified waves as they travel beyond their position of maximum build-up.

Continuing the progressive replacement of func¬ tional subsystems by physical models, the organ of Corti filters are simulated using hair-cell and electro-moti- lity models found in the prior art (Davis, H. "A model for transducer action in the cochlea." Cold Spring Sym¬ posia on Quantitative Biology, vol. 30, 181-190 (1965); and Santos-Sacchi, J. "On the frequency limit and phase of outer hair cell motility: effects of the membrane filter." J. Neurosci, vol. 12, 1906-1916 (1992)). Fol¬ lowing these models, the present invention simulates the organ of Corti filters using an operational amplifier, an inverse transducer and a compressive transducer. The operational amplifier is used to implement the inverse transducer; however, the corresponding physical basis for this mechanism remains to be discovered. Further, the transducers are no longer memoryless, but include one or two integrations within the compressive transducer and an equal number of differentiations within the inverse transducer. (The electromotility function m(e) is de¬ fined by Santos-Sacchi, op. cit. )

While the principal advantages and features of the present invention have been described above, a more com- plete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow. Brief Description of the Drawings

Figure 1 is a schematic diagram representing the inventor's prior art model for simulating human ear re¬ sponse;

Figure 2 is a first embodiment of the invention disclosed in the parent which includes bilateral signal processing in a model for simulating human ear response; Figure 3 is a schematic depicting the inventor's interpretation of the biological function of the cochlea as disclosed in the parent;

Figure 4 depicts a schematic representation of an idealized example of the invention disclosed in the par- ent based upon in-phase addition of apically propagating "tip" responses;

Figure 5 is a schematic of the invention disclosed in the parent detailing the non-linear cochlear simula¬ tor; Figure 6 is a graph detailing the measured re¬ sponse of the invention disclosed in the parent;

Figures 7a and 7b are schematic diagrams detailing lattice construction as utilized in the invention dis¬ closed in the parent; Figure 8 is a graph providing the relationship between tip coupler density and tip preamplifier gain as disclosed in the parent;

Figure 9 is a partial schematic of the model shown in Figure 5 and further detailing the interconnection between the tip line lattice and tail line lattice through the tip couplers disclosed in the parent;

Figure 10 is a schematic depicting the present invention which couples the bilateral signal processing model disclosed and claimed in the parent with an addi- tive directional wave amplifier incorporating analog electronic components for simulating human ear response; and

Figures 11a and lib are schematic diagrams of the present invention depicting physical simulations of the organ of Corti filters.

Detailed Description of the Preferred Embodiment

As shown in Figure 1, the inventor herein has previously developed a model for explaining and simulat¬ ing the cochlear response of the human ear. In essence, the model of Figure 1 is characterized by a unilateral non-linear signal processing of two signaling channels responsible for the "tips" and "tails" well demonstrated in the literature as being measured in cochlear frequency tuning curves. Using this prior art, cochlear spectrum analysis would be approximately simulated by a bank of independent non-linear filters, each tuned to a different audible frequency. Further details of the specific oper¬ ation and functional components of the model of Figure 1 are described in the inventor's prior article referenced above. However, it is important to note that signal processing occurs from left to right as shown in Figure 1 and there is no feedback loop nor counter signal flow demonstrated by the model. Nevertheless, the model is successful in simulating sound level dependent non-linear cochlear frequency analysis as measured in many psycho- physical and biophysical experiments.

The inventor's further work led to the invention disclosed in the parent which, in its first embodiment, incorporates bilateral signal processing by the alternate signal paths through the functions f and f^"1. This bila¬ teral signal processing potentiates extension of the model such that it can be used to explain other phenomena not previously explainable with the model of Figure 1, e.g. combination tones and otoacoustic emissions. As shown in Figure 2, points 2 and 3 may be thought of as taps in a propagating medium, further identified in later developments as shown herein as the basilar membrane. The non-linear feedback loop through which the bilateral signal processing occurs, i.e. through the alternate branches of f and f"¹, provides non-linear addition of a signal from the unilateral tip source at point 1 therein with a signal on the propagating "tail medium" at point th <_ree as shown therein. With this modification to the model of Figure 1, spectrum analysis by independent non¬ linear filters is retained and there is no interaction suggested between apically propagating responses. In other words, the low pass filter H3 admits low frequency signals through the middle ear and the tuned filter HI admits sounds at the center frequency ω_c and its response is non-linearly processed, as explained, with an output response filtered through band pass filter H2. Interac¬ tion between non-linear filters tuned to different fre¬ quencies is not suggested or explained in the model and schematic of Figure 2.

As shown in Figure 3, the non-linear amplification principle disclosed in the parent was extended to include a basilar membrane as a propagating medium which allows for the interaction between the sensed response of organ of Corti filters tuned to different frequencies. As shown therein, a plurality of tip filters HI are each tuned to a different center frequency CF_j which are then non-linearly coupled for bilateral processing to the basilar membrane. The measured responses are thus the result of a distributed non-linear amplifying effect. This bilateral signal processing is further exemplified by the double headed arrows connecting the cochlea (com¬ prising the organ of Corti and basilar membrane) with the middle ear and outer ear. This model closely parallels the actual physical construction of the cochlea and hence provides a model for construction of an electronic simu- lator for the cochlea. Of course, electronic circuitry simulating the middle ear and outer ear are well known in the art. See, for example, Chassaing R. and Horning D.W., (1990) Digital Signal Processing with the TMS 320C25; and Lin, Kun-Shan, Ed. (1987) Digital Signal Processing Applications with the TMS 320 Family, Vol. 1. The effect of bilateral processing is shown in Figure 6 to bring the response curve more into conformance with measured response for the ear.

As shown in Figure 4, an idealized example of the invention disclosed in the parent is presented wherein a plurality of band pass filters having a center frequency CFi provide a response to an input frequency F_s which is then non-linearly processed and summed sequentially from base to apex, through a series of low pass filters.

A more physically realizable representation and embodiment for the invention disclosed in the parent is shown in Figure 5. As depicted therein, a pair of match¬ ed lattices comprising a tip line lattice and a tail line lattice are interconnected by a plurality of tip modules (as shown in Figure 2) to provide non-linear bilateral signal processing therebetween at different frequency points. The tip line and tail line lattice are conven¬ tional all pole lattices as shown in Figure 7b. As shown in Figure 7a, a one pole lattice representing an ideal¬ ized section of a non-uniform acoustic tube has F_± and B as its forward and backward waves. A unit delay Z"¹ equals the transit time of the section. K_± is the reflec¬ tion coefficient that depends upon the ratio of cross- sectional areas of the idealized successive sections. For the all pole lattice as shown in Figure 7b, and as used as the tip line and tail line lattices of Figure 5, the forward delay is eliminated and the backward delay corresponds to twice the transit time. The scaling fac¬ tor for each section is normalized to unity. Except for the scale factor and delay, the form of the frequency response is unchanged, as demonstrated therein. As shown in Figure 5, the responses interact along the tail line lattice much as is believed to be the case in the basilar membrane of the human ear. Similarly, the non-linearly coupled tip line lattice and differentiator D(Z) provide a phase-matched filter-bank sound analysis that is believed to simulate the action of the outer hair cells and adjoining structures comprising the organ of Corti. Thus, the model, as shown in Figure 5, has some correspondence to the physical properties of the cochlea and hence provide insight into the actual physical mecha¬ nisms at work in the cochlea.

As shown in Figure 9, the correlation between the model of the invention disclosed in the parent and the cochlea itself leads to adjustments in the model which may be used to simulate responses measured in the human ear. For example, the filter responses of the tip line lattice must be normalized to the "center frequency" of each tip filter or tip module. In this event, losses in sensitivity of each of these tip filters or modules may be simulated by choosing a scaling factor alpha such that 0 <__ <** <__ 1. This scaling factor may be used to adjust the output at the "center frequency", corresponding to the response, as would be the case in the response of a dam¬ aged cochlea. Similarly, efferent neural control of the tip sensitivity can be simulated by providing a quiescent bias control at each of the tip modules, as shown. This efferent neural control is representative of the brain's ability to suppress the response of the ear to undesir¬ able sounds and to also simulate the results of inatten- tiveness, as when a person is listening but not hearing. Coupling of the backward propagation to the tip line from the tail line can be controlled by choosing beta such that 0 <_ β _< 1. Similarly, the tip preamplifier G may have its gain adjusted -fco correspond to the number of tip couplers used in implementing the simulator. This is shown in Figure 8 which allows that number to be as large as the 12,000 outer hair cells of the organ of Corti. In Figure 8, 600 represents the normal number of hair cells in a five percent section of the cochlea. The figure specifies the increase in gain required (G in Figure 5) to simulate normal sensitivity when the number of tip filters is reduced below 600 per five percent section. For example, 400 tip couplers can be uniformly spaced over the whole basilar membrane, whereupon G=19. Or, the 400 tip couplers can be "zoomed" onto one 5% region, whereupon G=l. This can result in reduced simulator com¬ plexity and cost in order to provide a model which simu¬ lates this response.

Building on the invention disclosed in the parent, the inventor has improved on his earlier cochlea simula- tion. The present invention utilizes empirical data from the study of tonotopically organized mechano-motility responses from isolated outer hair cells (See Brundin, L. & Russell, I., Sound-Induced Movements and Frequency Tuning in Outer Hair Cells Isolated from the Guinea Pig Cochlea; Symposium Reprints: Biophysics Of Hair Cell Sensory Systems, Duifhuis, H. et al., Eds., Groningen, June 28, 1993 - July 2, 1993, pp. 121-127) to implement principals of cochlear system operation disclosed in the parent with analog electronic technology in closer agree- ment with cellular biophysics of the human ear. As de¬ picted in Figure 10, the source delay line of the inven¬ tion disclosed in the parent is replaced with the dif¬ ferential delays within the passbands of the bilateral signal processors (which model tonotopically organized organ of Corti filters). Further, the present invention utilizes a non-linear additive directional wave amplifi¬ cation means, which responds to two different signals, each of which corresponds to a physical mechanism. One is a "global" signal to the organ of Corti filter model which corresponds to the tonotopically-tuned phasic mechano-motility responses by the outer hair cells to the fast pressure wave within the inner ear in response to stapes input. The second is a "local" input signal to the organ of Corti filter which corresponds to a clas¬ sically defined slow-wave response to the pressure gradi- ent across the basilar membrane in response to stapes input. Tuned phasic length-modulation responses of outer hair cells inject phasic signals into the space of Nuel by modulating the separation between the reticular lamina and the basilar membrane. Being bound by the basilar membrane, the space of Nuel supports traveling waves similar to the classical slow-wave response to the pres¬ sure gradient across the membrane.

The amplification principles in the present inven¬ tion require dissipation means to attenuate the amplified waves as they travel beyond their position of maximum build-up. Dissipation is introduced in the model with the shunt capacitors C_t and can be added in parallel with the series inductors 1 . The invention simulates the basilar membrane serving as a collector in the additive directional wave amplification of tuned responses by outer hair cells. The collector uses standard engineer¬ ing principles (well-known in the art) for non-uniform transmission lines.

As shown in Figures 11a and lib, the present invention further simulates the biophysical implementa¬ tion of organ of Corti filters using the hair-cell and electro-motility models found in the prior art (Davis, H. "A model for transducer action in the cochlea." Cold Spring Symposia on Quantitative Biology, vol. 30, 181-190 (1965); and Santos-Sacchi, J. "On the frequency limit and phase of outer hair cell motility: effects of the mem¬ brane filter." J. Neurosci, vol. 12, 1906-1916 (1992)). As shown in Figure lib, the left side thereof corresponds to the compressive transducer f and the right side there- of corresponds to the inverse transducer f^-1 as shown in the schematic in Figure 11a. In these models, an operat- ional amplifier is used to implement the inverse trans¬ ducer. However, the corresponding physical basis for this mechanism is unknown. Further, the transducers are no longer memoryless, but include one or two integrations within the compressive transducer and an equal number of differentiations within the inverse transducer. (The electromotility function m(e) is defined by Santos- Sacchi, op. cit. )

The invention was demonstrated using VLSI simula- tion technology. The preferred embodiment is the rec¬ ommended implementation. VLSI simulation required a powerful general purpose computer, while the inventor considers DSP technology more practical.

It is understood that other technology may be used to implement the invention. Also, one of ordinary skill in the art, knowing the desired non-linear response as included in the tip couplers, could readily design and implement a custom DSP chip for interconnecting the two all pole lattices. There are various changes and modifications which may be made to the invention as would be apparent to those skilled in the art. However, these changes or modifications are included in the teaching of the disclo¬ sure, and it is intended that the invention be limited only by the scope of the claims appended hereto.

Claims (26)

What Is Claimed Is:

1. In a sound analyzer having means for receiving a complex sound input comprised of a plurality of fre¬ quencies and means for producing an output representative of the frequency response of a human ear, the improvement comprising an analog non-linear additive directional wave amplification means connected between said receiving means and said output producing means.

2. The sound analyzer of Claim 1 wherein said amplification means including means for producing a plu¬ rality of distributed outputs and further comprising means connected between said distributed outputs for dis¬ sipating said distributed outputs.

3. The sound analyzer of Claim 2 wherein said dissipating means comprises a shunt capacitor and a se¬ ries inductor.

4. In a sound analyzer having means for receiving a complex sound input comprised of a plurality of fre¬ quencies and means for producing an output representative of the frequency response of a human ear and an amplifi- cation means connected between said receiving means and said output producing means, the improvement comprising means for generating an input to said amplification means for mixing with said complex sound input.

5. The sound analyzer of Claim 4 wherein said generated input corresponds to a slow pressure-gradient wave of the human ear.

6. The sound analyzer of Claim 4 wherein said input generating means includes a feedback connection between said dissipating means and said amplification means.

7. In a sound analyzer having means for receiving a complex sound input comprised of a plurality of fre¬ quencies and means for producing an output representative of the frequency response of a human ear, the improvement comprising a plurality of transducers with memory capa- bility, said plurality of transducers being connected in parallel between said receiving means and said output producing means.

8. In a sound analyzer having means for receiving a complex sound input comprised of a plurality of fre¬ quencies and means for producing an output representative of the frequency response of a human ear, the improvement comprising said output means having means for producing acoustical distortion products as demonstrated to emanate from the human ear.

9. The sound analyzer of Claim 8 wherein said output means has means for producing spontaneous emis¬ sions as demonstrated to emanate from the human ear.

10. The sound analyzer of Claim 9 wherein said output means has means for producing an output which simulates the non-linear interaction between said sponta¬ neous emissions and external sounds as demonstrated to occur in the human ear.

11. In a sound analyzer having means for receiving a complex sound input comprised of a plurality of fre¬ quencies and means for producing an output representative of the frequency response of a human ear, the improvement comprising means for varying the sensitivity of said analyzer with respect to said frequency of said complex sound input.

12. The sound analyzer of Claim 11 wherein said output comprises a plurality of discrete outputs, each of said outputs being representative of said frequency re¬ sponse at a pre-selected frequency, and said sensitivity varying means comprises varying the number of discrete outputs over a particular frequency range.

13. The sound analyzer of Claim 12 wherein said sensitivity varying means further comprises means associ¬ ated with each of said discrete outputs for varying the strength of said output.

14. The sound analyzer of Claim 13 further compris¬ ing means for selectively attenuating the output at all frequencies.

15. In a sound analyzer having means for receiving a complex sound input comprised of a plurality of fre¬ quencies and means for producing an output representative of the frequency response of a human ear, the improvement comprising a non-linear distributed amplifier means con¬ nected between said receiving means and said output pro¬ ducing means.

16. The sound analyzer of Claim 15 wherein said amplifier means comprises a pair of matched all pole lattices.

17. The sound analyzer of Claim 16 wherein said amplifier means further comprises a plurality of non¬ linear couplers interconnected between said pair of lat¬ tices.

18. The sound analyzer of Claim 17 wherein said output means comprises a plurality of taps into one of said lattices, each of said taps providing an output at a particular frequency.

19. The sound analyzer of Claim 18 further compris¬ ing means for selecting the number of taps and means for selecting the frequency of the output at each of said taps to thereby adjust the sensitivity and frequency range of said sound analyzer.

20. The sound analyzer of Claim 19 wherein one of said lattices has means for propagating a signal in two directions therethrough.

21. The sound analyzer of Claim 20 wherein said one lattice having means for propagating a signal in two directions is closer to said input than said other lat¬ tice.

22. The sound analyzer of Claim 21 wherein said lattice having a plurality of taps utilizes propagation of signals in only one direction.

23. The sound analyzer of Claim 22 further compris¬ ing a variable-gain preamplifier interconnected between said lattices.

24. The sound analyzer of Claim 23 further compris¬ ing an outer ear circuit connected to an inner ear cir¬ cuit, with the output of said inner ear circuit being connected to each of said lattices.

25. In a sound analyzer having means for receiving a complex sound input comprised of a plurality of fre¬ quencies and means for producing an output representative of the frequency response of a human ear, the improvement comprising means for processing signals in both direc¬ tions between input and output.

26. The sound analyzer of Claim 25 wherein said signal processing means includes means for non-linearly amplifying said signals.