# US 5,710,822 A · Frequency selective active adaptive control system > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,710,822 A - Document ID: US5710822A - Status: Granted - Kind: A - Application number: 08/553,186 - Filed: 1995-11-07 - Granted: 1998-01-20 - Earliest claimed: 1995-11-07 - Assignees: Digisonix, Inc.; Lord Corporation - Inventors: Shawn K. Steenhagen; Steve C. Southward; Kent F. Delfosse - Web page: https://patentgrep.com/patent/US5710822A ## CPC - G10K 11/17854: {the filter being an adaptive filter} - G10K 11/17817: {between the output signals and the error signals, i.e. secondary path} - G10K 11/17833: {by using a self-diagnostic function or a malfunction prevention function, e.g. detecting abnormal output levels} - G10K 11/1785: {Methods, e.g. algorithms; Devices (G10K11/1781, G10K11/1783 take precedence)} - G10K 11/17879: {using both a reference signal and an error signal} - G10K 11/17881: {the reference signal being an acoustic signal, e.g. recorded with a microphone} - G10K 2210/117: Nonlinear - G10K 2210/3028: Filtering, e.g. Kalman filters or special analogue or digital filters - G10K 2210/3039: Nonlinear, e.g. clipping, numerical truncation, thresholding or variable input and output gain - G10K 2210/3049: Random noise used, e.g. in model identification - G10K: SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR ## Abstract An active adaptive control system and method has frequency dependent filtering with a transfer characteristic which is a function of a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority. Band separation is provided for different tones. Power limit partitioning is provided for effectively distributing power between correction tones to maximize model performance. ## Claims ### Claim 1 An active adaptive control method comprising introducing a control signal from an output transducer to combine with a system input signal and yield a system output signal, sensing said system output signal with an error transducer providing an error signal, providing an adaptive filter model having a model input from a reference signal correlated to said system input signal, and a model output outputting a correction signal to said output transducer to introduce said control signal according to a weight update signal, adaptively leaking said weight update signal as a function of said correction signal relative to a given peak value according to a desired peak value signal such that said correction signal adaptively converges to a value limited by said peak value, filtering said correction signal by a filter having a transfer characteristic which is a function of a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority. ### Claim 2 (depends on 1) The method according to claim 1 wherein said transfer characteristic of said filter is an inverse function of said frequency dependent shaped power limitation characteristic. ### Claim 3 (depends on 2) The method according to claim 2 wherein said transfer characteristic of said filter is the inverse of said frequency dependent shaped power limitation characteristic. ### Claim 4 (depends on 2) The method according to claim 2 comprising comparing said correction signal against said desired peak value signal at a comparator to control adaptive leakage of said weight update signal, and supplying said correction signal to said comparator through said filter. ### Claim 5 (depends on 4) The method according to claim 4 comprising modeling said output transducer and the error path between said output transducer and said error transducer with a second adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, summing the output of said auxiliary noise source and said correction signal from said model output of said first model and supplying the resultant sum to said output transducer, to afford a post-summed correction signal after passage through said summer, and a pre-summed correction signal prior to passage through said summer, comparing said pre-summed correction signal against said desired peak value signal at said comparator to control adaptive leakage of said weight update signal, filtering said pre-summed correction signal through said filter prior to said comparing. ### Claim 6 (depends on 1) The method according to claim 1 wherein said transfer characteristic of said filter is a direct function of said frequency dependent shaped power limitation characteristic. ### Claim 7 (depends on 6) The method according to claim 6 wherein said transfer characteristic of said filter is said frequency dependent shaped power limitation characteristic. ### Claim 8 (depends on 6) The method according to claim 6 comprising supplying said correction signal through said filter to said output transducer, to afford a post-filtered correction signal after passage through said filter, and a pre-filtered correction signal prior to passage through said filter, comparing said pre-filtered correction signal against said desired peak value signal at a comparator to control adaptive leakage of said weight update signal. ### Claim 9 (depends on 8) The method according to claim 8 comprising modeling said output transducer and the error path between said output transducer and said error transducer with a second adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, summing the output of said auxiliary noise source and said pre-filtered correction signal from said model output of said first model and supplying the resultant sum to said filter, to afford a post-summed pre-filtered correction signal after passage through said summer and prior to passage through said filter, and a pre-summed pre-filtered correction signal prior to passage through said summer and prior to passage through said filter, comparing said pre-filtered pre-summed correction signal against said desired peak value signal at said comparator to control adaptive leakage of said weight update signal. ### Claim 10 An active adaptive control method comprising introducing a control signal from an output transducer to combine with a system input signal and yield a system output signal, sensing said system output signal with an error transducer providing an error signal, providing an adaptive filter model having a model input from a reference signal correlated to said system input signal, and a model output outputting a correction signal to said output transducer to introduce said control signal according to a weight update signal, adaptively leaking said weight update signal as a function of said correction signal relative to a given peak value according to a desired peak value signal such that said correction signal adaptively converges to a value limited by said peak value, varying said desired peak value signal according to frequency. ### Claim 11 (depends on 10) The method according to claim 10 comprising comparing said correction signal against said desired peak value signal at a comparator to control adaptive leakage of said weight update signal, providing a frequency transfer function controlling said peak value of said desired peak value signal. ### Claim 12 (depends on 10) The method according to claim 10 comprising modeling said output transducer and the error path between said output transducer and said error transducer with a second adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, summing the output of said auxiliary noise source and said correction signal from said model output, to afford a post-summed correction signal after said summing, and a pre-summed correction signal prior to said summing, supplying said post-summed correction signal to said output transducer, comparing said pre-summed correction signal against said desired peak value signal at a comparator to control adaptive leakage of said weight update signal. ### Claim 13 An active adaptive control method for a system input signal having a plurality of tones, comprising separating said system input signal into at least first and second input tones, introducing a control signal from an output transducer to combine with said system input signal and yield a system output signal, sensing said system output signal with an error transducer providing an error signal, providing a first adaptive filter model having a model input from a first reference signal correlated to said first input tone, and a model output outputting a first correction signal to said output transducer to introduce said control signal according to a first weight update signal, adaptively leaking said first weight update signal as a function of said first correction signal relative to a first given peak value according to a first desired peak value signal such that said first correction signal adaptively converges to a value limited by said first peak value, providing a second adaptive filter model having a model input from a second reference signal correlated to said second input tone, and a model output outputting a second correction signal to said output transducer to introduce said control signal according to a second weight update signal, adaptively leaking said second weight update signal as a function of said second correction signal relative to a second given peak value according to a second desired peak value signal such that said second correction signal adaptively converges to a value limited by said second peak value. ### Claim 14 (depends on 13) The method according to claim 13 comprising filtering each of said first and second correction signals with a frequency dependent transfer characteristic. ### Claim 15 (depends on 14) The method according to claim 14 comprising determining a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority, and filtering each of said first and second correction signals with a filter having a transfer characteristic which is a function of said frequency dependent shaped power limitation characteristic. ### Claim 16 (depends on 15) The method according to claim 15 wherein each of said filters has a transfer characteristic which is an inverse function of said frequency dependent shaped power limitation characteristic. ### Claim 17 (depends on 15) The method according to claim 15 comprising providing a first said filter filtering said first correction signal, to afford a post-filtered first correction signal after passage through said first filter, and a pre-filtered first correction signal prior to passage through said first filter, providing a second filter filtering said second correction signal, to afford a post-filtered second correction signal after passage through said second filter, and a pre-filtered second correction signal prior to passage through said second filter, comparing said first post-filtered correction signal against said first desired peak value signal at a first comparator to control adaptive leakage of said first weight update signal, comparing said second post-filtered correction signal against said second desired peak value signal at a second comparator to control adaptive leakage of said second weight update signal. ### Claim 18 (depends on 17) The method according to claim 17 comprising summing said first and second pre-filtered correction signals and supplying the resultant sum to said output transducer. ### Claim 19 (depends on 17) The method according to claim 17 comprising modeling said output transducer and the error path between said output transducer and said error transducer with a third adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, and summing the output of said auxiliary noise source with said first and second pre-filtered correction signals and supplying the resultant sum to said output transducer. ### Claim 20 (depends on 13) The method according to claim 13 comprising filtering the input to said output transducer with a frequency dependent transfer characteristic. ### Claim 21 (depends on 20) The method according to claim 20 comprising determining a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority, and filtering the input to said output transducer with a filter having a transfer characteristic which is a function of said frequency dependent shaped power limitation characteristic. ### Claim 22 (depends on 21) The method according to claim 21 wherein said transfer characteristic of said filter is a direct function of said frequency dependent shaped power limitation characteristic. ### Claim 23 (depends on 21) The method according to claim 21 comprising summing said first and second correction signals and supplying the resultant sum as a summed correction signal through said filter to said output transducer, to afford first and second pre-summed correction signals prior to said summing, and a post-summed correction signal after said summing and before passage through said filter. ### Claim 24 (depends on 23) The method according to claim 23 comprising comparing said first pre-summed correction signal against said first desired peak value signal at a first comparator to control adaptive leakage of said first weight update signal, and comparing said second pre-summed correction signal against said second desired peak value signal at a second comparator to control adaptive leakage of said second weight update signal. ### Claim 25 (depends on 13) The method according to claim 13 comprising summing said first and second correction signals and supplying the resultant sum as a summed correction signal to said output transducer, to afford a post-summed correction signal after said summing, and first and second pre-summed correction signals prior to said summing, comparing said first pre-summed correction signal against said first desired peak value signal at a first comparator to control adaptive leakage of said first weight update signal, comparing said second pre-summed correction signal against said second desired peak value signal at a second comparator to control adaptive leakage of said second weight update signal. ### Claim 26 (depends on 25) The method according to claim 25 comprising determining a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority, and filtering said post-summed correction signal supplied to said output transducer by a filter having a transfer characteristic which is a function of said frequency dependent shaped power limitation characteristic. ### Claim 27 (depends on 26) The method according to claim 26 comprising modeling said output transducer and the error path between said output transducer and said error transducer with a third adaptive filter model having a model input from an auxiliary noise source uncorrelated to said system input signal, and comprising summing the output of said auxiliary noise source with the outputs of said first and second models and filtering the output resultant sum by through said filter before passage to said output transducer. ### Claim 28 (depends on 13) The method according to claim 13 comprising varying each of said first and second desired peak value signals according to frequency. ### Claim 29 (depends on 28) The method according to claim 28 comprising comparing said first Correction signal against said first desired peak value signal at a first comparator to control adaptive leakage of said first weight update signal, comparing said second correction signal against said second desired peak value signal at a second comparator to control adaptive leakage of said second weight update signal, providing a first frequency transfer function varying said first desired peak value signal according to frequency, and providing a second frequency transfer function varying said second desired peak value signal according to frequency. ### Claim 30 (depends on 28) The method according to claim 28 comprising modeling said output transducer and the error path between said output transducer and said error transducer with a third adaptive filter model having a model input from an auxiliary noise source uncorrelated to said system input signal, summing the output of said auxiliary noise source and said first and second correction signals, to afford a post-summed correction signal supplied to said output transducer, a first pre-summed correction signal, and a second pre-summed correction signal, comparing said first pre-summed correction signal against said frequency dependent first desired peak value signal at a first comparator to control adaptive leakage of said first weight update signal, comparing said second pre-summed correction signal against said frequency dependent second desired peak value signal at a second comparator to control adaptive leakage of said second weight update signal. ### Claim 31 (depends on 13) The method according to claim 13 comprising separating said error signal into at least first and second error tones corresponding respectively to said first and second input tones, and combining said first reference signal with said first error tone to provide said first weight update signal, and combining said second reference signal with said second error tone to provide said second weight update signal. ### Claim 32 (depends on 31) The method according to claim 31 comprising providing said first error tone from a first error transducer, and providing said second error tone from a second error transducer. ### Claim 33 (depends on 13) The method according to claim 13 comprising variably balancing leakage of said first and second weight update signals to partition power distribution among said first and second correction signals to limit cumulative power to said output transducer. ### Claim 34 (depends on 33) The method according to claim 33 comprising determining an admissable region of values in a plot of said first correction signal versus said second correction signal, and coordinating control of leakage of said first and second weight update signals to maintain said first and second correction signals in said admissable region. ### Claim 35 (depends on 34) The method according to claim 34 comprising determining the boundary of said admissable region along a boundary line according to the sum of said first and second correction signals being equal to a predetermined maximum value. ### Claim 36 (depends on 34) The method according to claim 34 comprising determining the boundary of said admissable region along a boundary line and determining the optimum point on said boundary line for balancing said first and second desired peak value signals from a starting point off of said boundary line comprising projecting from said starting point to said boundary line along a projection line intersecting and perpendicular to said boundary line, the intersection of said projection line and said boundary line being said optimum point. ### Claim 37 (depends on 34) The method according to claim 34 comprising determining the boundary of said admissable region along a boundary line and maintaining said first and second correction signals on said boundary line. ### Claim 38 (depends on 34) The method according to claim 34 comprising determining the boundary of said admissable region along a boundary line and determining the optimum point on said boundary line for balancing said first and second desired peak value signals from a starting point off of said boundary line comprising projecting from said starting point to said boundary line along a projection line extending from the origin of said plot through said starting point and intersecting said boundary line, the intersection of said projection line and said boundary line being said optimum point. ### Claim 39 (depends on 34) The method according to claim 34 comprising determining the boundary of said admissable region along a boundary line and determining the optimum point on said boundary line for balancing said first and second desired peak value signals from a starting point off of said boundary line comprising determining an error surface around said starting point and projecting from said starting point to said boundary line along a projection line intersecting said boundary line and tangent to said error surface, the intersection of said projection line and said boundary line being said optimum point. ### Claim 40 An active adaptive control system comprising an output transducer introducing a control signal to combine with a system input signal and yield a system output signal, an error transducer sensing said system output signal and providing an error signal, an adaptive filter model having a model input from a reference signal correlated to said system input signal, and a model output outputting a correction signal to said output transducer to introduce said control signal according to a weight update signal, adaptive leak means adaptively leaking said weight update signal as a function of said correction signal relative to a given peak value according to a desired peak value signal such that said correction signal adaptively converges to a value limited by said peak value, a filter filtering said correction signal by a transfer characteristic which is a function of a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority. ### Claim 41 (depends on 40) The system according to claim 40 wherein said filter has a transfer characteristic which is an inverse function of said frequency dependent shaped power limitation characteristic. ### Claim 42 (depends on 41) The system according to claim 41 wherein said filter has a transfer characteristic which is the inverse of said frequency dependent shaped power limitation characteristic. ### Claim 43 (depends on 41) The system according to claim 41 comprising a comparator comparing said correction signal against said desired peak value signal to control adaptive leakage of said weight update signal, and wherein said correction signal from said output of said model is supplied through said filter to said comparator. ### Claim 44 (depends on 43) The system according to claim 43 comprising a second adaptive filter model modeling said output transducer and the error path between said output transducer and said error transducer, said second adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, a summer summing the output of said auxiliary noise source and said correction signal from said model output of said first mentioned model and supplying the resultant sum to said output transducer, to afford a post-summed correction signal after passage through said summer, and a pre-summed correction signal prior to passage through said summer, a comparator comparing said pre-summed correction signal against said desired peak value signal to control adaptive leakage of said weight update signal, wherein said pre-summed correction signal is supplied through said filter to said comparator. ### Claim 45 (depends on 40) The system according to claim 40 wherein said filter has a transfer characteristic which is a direct function of said frequency dependent shaped power limitation characteristic. ### Claim 46 (depends on 45) The system according to claim 45 wherein said filter has a transfer characteristic which is said frequency dependent shaped power limitation characteristic. ### Claim 47 (depends on 46) The system according to claim 46 wherein said correction signal is supplied from said model output through said filter to said output transducer. ### Claim 48 (depends on 45) The system according to claim 45 wherein said filter filters said correction signal supplied to said output transducer, to afford a post-filtered correction signal after passage through said filter, and a pre-filtered correction signal prior to passage through said filter, a comparator comparing said pre-filtered correction signal against said desired peak value signal to control adaptive leakage of said weight update signal. ### Claim 49 (depends on 48) The system according to claim 48 comprising a second adaptive filter model modeling said output transducer and the error path between said output transducer and said error transducer, said second adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, a summer summing the output of said auxiliary noise source with said pre-filtered correction signal and supplying the resultant sum to said filter. ### Claim 50 An active adaptive control system comprising an output transducer introducing a control signal to combine with a system input signal and yield a system output signal, an error transducer sensing said system output signal and providing an error signal, an adaptive filter model having a model input from a reference signal correlated to said system input signal, and a model output outputting a correction signal to said output transducer to introduce said control signal according to a weight update signal, adaptive leak means adaptively leaking said weight update signal as a function of said correction signal relative to a given peak value according to a desired peak value signal such that said correction signal adaptively converges to a value limited by said peak value, frequency transfer means varying said desired peak value signal according to frequency. ### Claim 51 (depends on 50) The system according to claim 50 comprising a comparator comparing said correction signal against said desired peak value signal to control adaptive leakage of said weight update signal, said frequency transfer means controlling said peak value of said desired peak value signal. ### Claim 52 (depends on 50) The system according to claim 50 comprising a second adaptive filter model modeling said output transducer and the error path between said output transducer and said error transducer, said second adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, a summer summing the output of said auxiliary noise source and said correction signal from said model output of said first mentioned model, to afford a post-summed correction signal after passage through said summer, and a pre-summed correction signal prior to passage through said summer, said post-summed correction signal being supplied to said output transducer, a comparator comparing said pre-summed correction signal against said desired peak value signal to control adaptive leakage of said weight update signal. ### Claim 53 An active adaptive control system for a system input signal having a plurality of tones, comprising separating means separating said system input signal into at least first and second input tones, an output transducer introducing a control signal to combine with said system input signal and yield a system output signal, an error transducer sensing said system output signal and providing an error signal, a first adaptive filter model having a model input from a first reference signal correlated to said first input tone, and a model output outputting a first correction signal to said output transducer to introduce said control signal according to a first weight update signal, first adaptive leak means adaptively leaking said first update signal as a function of said first correction signal relative to a first given peak value according to a first desired peak value signal such that said first correction signal adaptively converges to a value limited by said first peak value, a second adaptive filter model having a model input from a second reference signal correlated to said second input tone, and a model output outputting a second correction signal to said output transducer to introduce said control signal according to a second weight update signal, second adaptive leak means adaptively leaking said second weight update signal as a function of said second correction signal relative to a second given peak value according to a second desired peak value signal such that said second correction signal adaptively converges to a value limited by said second peak value. ### Claim 54 (depends on 53) The system according to claim 53 comprising filter means filtering said first and second correction signals with a frequency dependent transfer characteristic. ### Claim 55 (depends on 54) The system according to claim 54 wherein said filter means has a transfer characteristic which is a function of a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority. ### Claim 56 (depends on 55) The system according to claim 55 wherein said filter means comprises first and second filters each having a transfer characteristic which is an inverse function of said frequency dependent shaped power limitation characteristic. ### Claim 57 (depends on 53) The system according to claim 53 comprising a first filter filtering said first correction signal with a frequency dependent transfer characteristic, to afford a post-filtered first correction signal after passage through said first filter, and a pre-filtered first correction signal prior to passage through said first filter, a second filter filtering said second correction signal with a frequency dependent transfer characteristic, to afford a post-filtered second correction signal after passage through said second filter, and a pre-filtered second correction signal prior to passage through said second filter, a first comparator comparing said first post-filtered correction signal against said first desired peak value signal to control adaptive leakage of said first weight update signal, a second comparator comparing said second post-filtered correction signal against said desired peak value signal to control adaptive leakage of said second weight update signal. ### Claim 58 (depends on 57) The system according to claim 57 comprising a summer summing said first and second pre-filtered correction signals and supplying the resultant sum to said output transducer. ### Claim 59 (depends on 58) The system according to claim 58 comprising a third adaptive filter model modeling said output transducer and the error path between said output transducer and said error transducer, said third adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, wherein said summer sums the output of said auxiliary noise source with said first and second pre-filtered correction signals and supplies the resultant sum to said output transducer. ### Claim 60 (depends on 53) The system according to claim 53 comprising a filter filtering the input to said output transducer with a frequency dependent transfer characteristic. ### Claim 61 (depends on 60) The system according to claim 60 wherein said filter has a transfer characteristic which is a function of a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority. ### Claim 62 (depends on 61) The system according to claim 61 wherein said filter has a transfer characteristic which is a direct function of said frequency dependent shaped power limitation characteristic. ### Claim 63 (depends on 62) The system according to claim 62 wherein said filter has a transfer characteristic which is said frequency dependent shaped power limitation characteristic. ### Claim 64 (depends on 60) The system according to claim 60 comprising a summer summing said first and second correction signals and supplying the resultant sum as a summed correction signal to said output transducer, and wherein said filter filters said summed correction signal. ### Claim 65 (depends on 53) The system according to claim 53 comprising a summer summing said first and second correction signals and supplying the resultant sum as a summed correction signal to said output transducer, to afford a post-summed correction signal after passage through said summer, and first and second pre-summed correction signals prior to passage through said summer, a first comparator comparing said first pre-summed correction signal against said first desired peak value signal to control adaptive leakage of said first weight update signal, a second comparator comparing said second pre-summed correction signal against said second desired peak value signal to control adaptive leakage of said second weight update signal. ### Claim 66 (depends on 65) The system according to claim 65 comprising a filter filtering said post-summed correction signal supplied to said output transducer by a transfer characteristic which is a function of a frequency dependent shaped power limitation characteristic maximizing usage of available output transducer authority. ### Claim 67 (depends on 66) The system according to claim 66 comprising a third adaptive filter model modeling said output transducer and the error path between said output transducer and said error transducer, said third adaptive filter model having a model input from an auxiliary noise source uncorrelated with said system input signal, a summer summing the output of said auxiliary noise source with the outputs of said first and second models, and wherein said filter filters the output resultant sum from said summer by a transfer characteristic which is a direct function of said frequency dependent shaped power limitation characteristic before passage to said output transducer. ### Claim 68 (depends on 53) The system according to claim 53 comprising frequency transfer means varying said first and second desired peak value signals according to frequency. ### Claim 69 (depends on 68) The system according to claim 68 comprising a first comparator comparing said first correction signal against said first desired peak value signal to control adaptive leakage of said first weight update signal, a second comparator comparing said second correction signal against said second desired peak value signal to control adaptive leakage of said second weight update signal, a first frequency transfer function varying said first desired peak value signal according to frequency, and a second frequency transfer function varying said second desired peak value signal according to frequency. ### Claim 70 (depends on 68) The system according to claim 68 comprising a third adaptive filter model modeling said output transducer and the error path between said output transducer and said error transducer, said third adaptive filter model having a model input from an auxiliary noise source uncorrelated to said system input signal, a summer summing the output of said auxiliary noise source and said first and second correction signals, to afford a post-summed correction signal supplied to said output transducer, a first pre-summed correction signal, and a second pre-summed correction signal, a first comparator comparing said first pre-summed correction signal against said frequency dependent first desired peak value signal to control adaptive leakage of said first weight update signal, a second comparator comparing said second pre-summed correction signal against said frequency dependent second desired peak value signal to control adaptive leakage of said second weight update signal. ### Claim 71 (depends on 53) The system according to claim 53 comprising separating means separating said error signal into at least first and second error tones corresponding respectively to said first and second input tones, a first combiner combining said first reference signal with said first error tone to provide said first weight update signal, and a second combiner combining said second reference signal with said second error tone to provide said second weight update signal. ### Claim 72 (depends on 71) The system according to claim 71 comprising a first error transducer providing said first error tone, and a second error transducer providing said second error tone.