United States Patent (19)
`Paul
`
`54 ADAPTIVE NOISE CANCELLING
`RECEIVER
`75 Inventor: James E. Paul, Anaheim, Calif.
`73) Assignee: Rockwell International Corporation,
`El Segundo, Calif.
`21) Appl. No.: 884,093
`22 Filed:
`Mar. 6, 1978
`51 Int. C.’............................................... H04B 1/12
`52 U.S. Cl. .................................... 325/475; 325/477;
`325/480; 328/163
`58) Field of Search ................... 325/42, 62,323, 324,
`325/472-477,479,480; 179/1 P; 333/18, 70T,
`28 R, 28 T; 328/163, 167, 165; 364/724, 728
`References Cited
`U.S. PATENT DOCUMENTS
`7/1961 Wilcox ................................. 325/475
`2,991,358
`3/1964 Shirman ............................... 325/474
`3,126,449
`6/1968 Hellwarth et al. ................... 325/474
`3,387,222
`3,769,591 10/1973 Brown et al. ........................ 325/474
`3,803,357 4/1974 Sacks .................................... 179/1 P
`3,876,943
`4/1975 Watt et al. ........................... 325/479
`3,953,802 4/1976 Morris et al. ........................ 325/474
`4,052,559 10/1977
`Paul et al. ............................ 179/1 P
`OTHER PUBLICATIONS
`"Adaptive Noise Cancelling," Widrow et al., IEEE
`Proceedings, vol. 63, No. 12, Dec. 1975.
`Primary Examiner-Robert L. Griffin
`Assistant Examiner-Jin F. Ng
`
`56
`
`11
`45
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`4,177,430
`Dec. 4, 1979
`
`Attorney, Agent, or Firm-L. Lee Humphries; H.
`Frederick Hamann; Rolf M. Pitts
`(57)
`ABSTRACT
`A receiving system is described which has the capabil
`ity of substantially cancelling broadband noise, such as
`impulse noise, atmospheric noise, electrical line noise
`and receiver front end noise, from a selected radio fre
`quency passband. In a first embodiment, a desired radio
`frequency passband is selected and converted to a
`broadband intermediate frequency (IF) signal, which is
`applied in parallel to the inputs of first and second fre
`quency channels. When broadband noise is present in
`the broadband IF signal, the first frequency channel
`develops a desired audio signal in the presence of an
`undesired first broadband noise signal, while the second
`frequency channel develops an undesired second broad
`band noise signal which is correlated with the first
`broadband noise signal. An adaptive transversal filter is
`responsive to the second broadband noise signal and to
`an output signal for adaptively developing an estimate
`of the first broadband noise. The estimate of the first
`broadband noise signal is subtracted from the combina
`tion of the desired audio signal and the first broadband
`noise signal in order to cancel out the first and second
`broadband noise signals and substantially develop only
`the desired audio signal as the output signal. When no
`broadband noise is present in the broadband IF signal,
`the adaptive filter automatically shuts itself off and the
`first frequency channel only develops the desired audio
`signal. Thus, regardless of whether or not broadband
`noise is present, the output signal is substantially com
`prised of only the desired audio signal.
`16 Claims, 3 Drawing Figures
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`B.P.
`FTER
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`B.P.
`FILTER
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`ADAPTIVE
`FTER
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`LGE EXHIBIT NO. 1013
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`- 1 -
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`Amazon v. Jawbone
`U.S. Patent 8,467,543
`Amazon Ex. 1013
`
`

`

`U.S. Patent Dec. 4, 1979
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`Sheet 1 of 2
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`4,177,430
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`S'TSNNVHO
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`U.S. Patent Dec. 4, 1979
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`1.
`
`ADAPTIVE NOSE CANCELLING RECEIVER
`
`10
`
`15
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention relates to communication systems and
`particularly to a receiving system for substantially can
`celling undesired broadband noise in the same fre
`quency spectrum as a desired signal.
`2. Prior Art Statement
`Many different prior art devices, apparatuses and
`receiving systems have been proposed for separating,
`removing cancelling noise or interference from an input
`signal.
`The article of Bernard Widrow etal, entitled "Adapt
`ive Noise Cancelling: Principles and Applications',
`found on pages 1692-1716 of Proceedings of the IEEE,
`Vol. 63, No. 12, Dec. 1975, teaches the concept of
`adaptive noise cancelling. The adaptive noise cancelling
`20
`concept, as shown in FIG. 1 on page 1693, uses a two
`input device to which is respectively applied a primary
`input containing a signal corrupted by additive noise or
`interference, and a reference input containing noise
`correlated in some unknown way with the primary
`25
`noise. The reference input is adaptively filtered and
`subtracted from the primary input to obtain a resultant
`estimate of the signal. Basically, Widrow etal is a time
`waveform canceller.
`U.S. Pat. No. 4,052,559 discloses a one input, speech
`30
`filtering device which utilizes a transversal filter to
`provide an estimate of only the longer correlation time
`period "noise" that is contained in an input speech-bear
`ing signal and a subtractor to subtract the longer corre
`lation time-perod "noise" from the input speech-bearing
`35
`signal to obtain substantially only the shorter correla
`tion time-period speech component of the input speech
`bearing signal as the signal remnant. Unlike the device
`of Widrow etal, the speech filtering device of U.S. Pat.
`No. 4,052,559 is a linear predictor which predicts the
`correlated noise component that is contained in the
`input speech-bearing signal.
`U.S. Pat. No. 3, 126,449 basically discloses a nonlinear
`audio noise blanker, which detects noise bursts and
`interrupts the passage of audio during the noise bursts.
`45
`More specifically, this patent discloses a noise discrimi
`nator circuit having three parallel branches, each
`branch consisting of a different band pass filter in series
`with an integrator-detector/Schmitt trigger circuit
`combination. The outputs of the three parallel branches
`are selectively applied to various logic gates to develop
`a control signal which, by way of a one-shot multivibra
`tor and a holdover circuit, controls an audio gate for the
`purpose of accomplishing the noise discrimination func
`tion.
`55
`U.S. Pat. No. 3,803,357 discloses a noise filter device
`which operates as an automatic noise canceller based on
`audio band segmentation and selective gain recombina
`tion. The noise filter device comprises a plurality of
`gain controllable, contiguous, narrow band nonlinear
`60
`filters connected to a signal source, a noise tracker and
`a summing amplifier. The noise tracker is also con
`nected to the signal source and essentially controls the
`ability of each of the narrow band filters to pass a signal
`as a function of the noise level that the noise tracker
`detects in the signal. The summing amplifier combines
`the spectral outputs of the narrow band filters in the
`proper power phase relationship.
`
`4,177,430
`2.
`U.S. Pat. No. 3,953,802 discloses an adjacent channel
`rejector for use in communications systems. Basically,
`this adjacent channel rejector is a nonlinear noise
`blanker. In this patent the IF signal of the receiver is
`applied through an IF amplifier and demodulator to an
`inhibit gate. The IF signal is also applied to a high gain
`amplifier. The output of the high gain amplifier is ap
`plied through a narrow band filter/demodulator to a
`first input of an amplitude comparator, as well as
`through a wide band filter/demodulator to a second
`input of the amplitude comparator. When a received
`signal has a center frequency outside a predetermined
`channel bandwidth, the high gain amplifier is driven
`into saturation, subsequently causing the comparator to
`apply an inhibit signal to the inhibit gate to prevent the
`received signal from passing through the inhibit gate.
`U.S. Pat. No. 3,769,591 discloses a frequency selec
`tive pulse receiving system which performs a function
`similar to that performed by the system of U.S. Pat. No.
`3,953,802. This pulse receiving system comprises a main
`channel having a pass band to receive incoming signals,
`three auxiliary channels respectively having pass bands
`equal to, above, and below the desired pass band of the
`receiver, and a comparator circuit responsive to the
`output signals of the three auxiliary channels for con
`trolling the operation of a suppression device to either
`pass or suppress the output of the main channel as a
`function of the amplitude relationships of the output
`signals of the three auxiliary channels. In this manner
`the system automatically notches out interference.
`Each of the above-described systems disclosed in
`U.S. Pat. Nos. 3,126,449; 3,802,357; 3,953,802; and
`3,769,591 achieves noise cancellation by distorting,
`blanking, inhibiting or otherwise modifying the desired
`signal, as well as the noise, whenever noise is detected.
`As a result, none of these systems could properly oper
`ate in the presence of intense and continuous broadband
`OSC.
`None of the above-described prior art published arti
`cle and U.S. patents teaches or suggests an adaptive
`noise cancelling receiver which includes the combina
`tion of a circuit for selecting a desired broadband fre
`quency signal containing a desired signal; first and sec
`ond frequency channels responsive to the broadband
`frequency signal for respectively generating first and
`second audio signals, with the first audio signal contain
`ing the desired signal in the presence of a first broad
`band noise and the second audio signal containing a
`second broadband noise that is correlated with the first
`broadband noise; an adaptive filter responsive to the
`second broadband noise and to an output signal for
`adaptively developing an estimate of the first broad
`band noise; and a combiner for combining the estimate
`of the first broadband noise with the first audio signal to
`substantially cancel out the first and second broadband
`noises and develop as the output signal a signal which
`substantially contains only the desired signal.
`SUMMARY OF THE INVENTION
`Briefly, an adaptive noise cancelling receiver is pro
`vided which has the capability of substantially cancel
`ling broadband noise, such as impulse noise, atmo
`spheric noise, electrical line noise and receiver frontend
`noise, from a selected radio frequency passband which
`contains a desired signal. In a first embodiment a prese
`lector selects a desired radio frequency passband which
`is converted to a broadband intermediate frequency
`(IF) signal. This broadband IF signal is applied to first
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`10
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`15
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`4,177,430
`3
`4.
`and second frequency channels, which are each com
`noise. The selected RF passband from the preselector
`prised of a filter/demodulator combination. The output
`13 is applied to a converter and intermediate frequency
`of the first frequency channel is a primary audio signal
`(IF) amplifier 15 to develop an amplified IF bandpass
`which is applied to a first input of a combiner. The
`signal according to techniques well understood in the
`output of the second frequency channel is a second
`art.
`audio signal. This second audio signal is applied to an
`The IF signal from the converter and IF amplifier 15
`adaptive filter to generate a filtered or reference signal
`is a broadband IF passband signal which is applied in
`which is applied to a second input of the combiner.
`parallel to primary and reference (frequency) channels
`Only the primary audio signal contains the desired sig
`19 and 21. It should be noted that the converter and IF
`nal. The output of the combiner, which is the difference
`amplifier 15 can also generate receiver front end noise.
`between the primary and reference audio signals, is
`By using a common front end, comprised of the prese
`supplied as an error signal to the adaptive filter. In its
`lector 13 and converter and IF amplifier 15, both of the
`operation, the adaptive filter cancels the common signal
`channels 19 and 21 are similarly affected by common
`components between the second audio signal and the
`receiver front end noise, as well as automatic gain con
`error signal by adaptively adjusting the amplitude,
`trol.
`phase and spectral transfer function of the second audio
`The primary channel 19 is comprised of a bandpass
`signal to match the corresponding components in the
`filter 23 to select a first band of frequencies within the
`error signal. As a result, the combiner cannot cancel the
`passband of the broadband IF signal and an audio de
`desired signal, but can only cancel broadband noise
`modulator 25 to develop a first or primary audio signal
`components that the common to both the primary and
`from the first band of frequencies. The reference chan
`reference audio signals. When no broadband noise is
`nel 2 is comprised of a bandpass filter 27 to select a
`present in the broadband IF signal, no noise compo
`second band of frequencies within the passband of the
`nents are common to both the primary and reference
`broadband IF signal and an audio demodulator 29 to
`audio signals and, as a result, the adaptive filter auto
`develop a second audio signal from the second band of
`matically shuts itself off. Thus, regardless of whether or
`frequencies. Channel selectivity is such that the selected
`25
`not broadband noise is present in the broadband IF
`second band of frequencies contains substantially no
`signal, the error signal is substantially comprised of only
`signal energy of either the desired signal or any other
`the desired signal. The desired signal is not filtered or
`received signal (other than possibly noise). The filters
`passed through any non-linear device, such as an inhibi
`23 and 27 can be sharp bandpass filters, such as mechan
`tor or gate, to achieve the noise cancellation.
`ical or crystal filters to select the first and second bands
`It is therefore an object of this invention to provide
`offrequencies. These first and second bands of frequen
`an improved noise cancelling receiver.
`cies can be respectively varied to lie in adjacent, over
`lapping or separated portions of the frequency spectrum
`Another object of this invention is to provide a re
`ceiver capable of rejecting adjacent or nearby channel
`of the passband of the broadband IF signal. The pass
`interference (such as splatter) and broadband noise
`band of filter 23 can have a center frequency fo and a
`35
`(such as power line noise and atmospheric noise).
`bandwidth BWo, while the passband of filter 27 can
`A further object of this invention is to provide a
`have a center frequency f. and a bandwidth BW.
`communications receiver with an improved noise fac
`When broadband noise is present in the broadband IF
`signal being commonly applied to the inputs of the
`tor.
`primary and reference channels 19 and 21, the primary
`BRIEF DESCRIPTION OF THE DRAWINGS
`audio signal (s--n) from demodulator 25 contains the
`These and other objects, features, and advantages of
`desired signals plus a first broadband noise n, while the
`the invention, as well as the invention itself, will be
`second audio signal from demodulator 29 substantially
`come more apparent to those skilled in the art in the
`contains a second broadband noise m, which is corre
`light of the following detailed description taken in con
`lated with the noise n. The noise n is correlated with
`45
`sideration with the accompanying drawings wherein
`the noise n since broadband noise is highly correlated
`like reference numerals indicate like or corresponding
`across the adjacent or nearby frequency channels 19
`parts throughout the several views and wherein:
`and 21.
`FIG. 1 illustrates a simplified block diagram of the
`The second audio signal from demodulator 29 is ap
`plied to an adaptive filter 31, which can be an adaptive
`invention;
`transversal filter, to enable the filter 31 to generate a
`FIG. 2 illustrates a modification that can be made to
`FIG. 1 to produce a second embodiment of the inven
`filtered or reference audio signal. This reference audio
`signal if substantially an estimate fi of the broadband
`tion; and
`noise in contained in the primary audio signal. The refer
`FIG. 3 illustrates a modification that can be made to
`either FIG. 1 or FIG. 2 to produce a third embodiment
`ence audio signal or estimated noise f is subtracted in a
`55
`combiner 33 from the desired signal plus noises--n, or
`of the invention.
`primary audio signal, in order to substantially cancel
`DESCRIPTION OF THE PREFERRED
`out the first and second broadband noises and develop
`EMBOOMENTS
`an error signale which is substantially equal to s--n-h.
`Referring now to the drawings, FIG. 1 discloses a
`Since fi is an estimate of n, this error signal e is substan
`simplified block diagram of the invention.
`tially comprised of only the desired signals. This error
`One of a plurality of radio frequency (RF) passband
`signale, which is the output audio signal, is fed back to
`signals being received by antenna 11 is selected and
`the adaptive filter 31.
`amplified by a preselector or RF amplifier 13. Assume
`In its operation, the adaptive filter 31 cancels the
`that the selected passband signal at the output of the
`common signal components between the reference
`65
`preselector 13 contains both a desired signal and an
`audio signal (second broadband noise signal m) and the
`error signal e by adaptively adjusting the amplitude,
`undesired broadband noise, such as impulse noise, atmo
`spheric noise, electrical line noise or receiver front end
`phase and spectral transfer function of the reference
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`4,177,430
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`5
`audio signal (m) to match the corresponding compo
`In the discussion of this third embodiment, assume that
`nents in the error signale. The convergence criterion
`the broadband passband signal (IF from FIG. 1 or RF
`used in the adaptive filter 31 is the least mean square
`from FIG. 2) being commonly applied to the inputs of
`error. Thus, the error signal e is used to force the adapt
`the primary and reference channels 19 and 21 (FIG. 1)
`ive filter 31 to converge to a least mean square estimate
`contains both a desired signal and an undesired broad
`f of the first broadband noise signal n contained in the
`band noise, such as impulse noise, atmospheric noise,
`primary audio signal. The filter 31 seeks to minimize the
`electrical line noise or receiver front end noise.
`error signal e, but since its input, the reference audio
`It should be recalled that the second audio signal
`signal (m), contains no components of the desired signal
`from the demodulator 29 substantially contains a second
`s in the primary channel 19, the filter 31 can only esti
`broadband noise m which is correlated with the noise in
`mate the broadband noise n contained in the primary
`contained (with the desired signals) in the first or pri
`channel 19. As a result, the combiner 33 cannot cancel
`mary audio signal (s--n) from the demodulator 25
`the desired signal s, but can only cancel broadband
`(FIG. 1).
`noise components (n and f) that are common to both the
`To reiterate, the invention makes use of the fact that
`primary and reference audio signals. By this means, the
`the desired signal components in the primary audio
`desired signals is not affected by the broadband noise
`signal is not correlated with the second broadband noise
`cancellation, and any output noise in the error signale,
`m (in the second audio signal), but that the first broad
`after the filter 31 converges, can never be greater than
`band noise component n in the primary audio signal is
`the noise n.
`correlated with the second broadband noise m. Note
`When no broadband noise is present in the broadband
`20
`that the broadband noise signals m and n are correlated
`IF signal, no noise components are common to the
`with each other by virtue of the assumption that they
`primary and reference audio signals and, as a result, the
`emanate from the same source, but are linearly modified
`adaptive filter 31 automatically shuts itself off, while the
`prior to being demodulated in the channels 19 and 21.
`primary (frequency) channel 19 only develops the de
`In the embodiment of FIG. 3, the first or primary
`sired audio signals. Thus, regardless of whether or not
`25
`audio signal (s--n) is sampled by a sampler 34 to pro
`broadband noise is present, the error signal e (output
`duce a discrete or sampled signal si-ni, comprised of a
`audio signal) is substantially comprised of only the de
`desired signal sample si and the undesired first broad
`sired audio signal.
`band noise sample ni. At the same time, the second
`Unlike the operations discussed in some of the above
`audio signal, which is substantially comprised of the
`identified prior art patents, the desired signals is not
`30
`second broadband noise m is sampled by a sampler 37 to
`filtered or passed through any non-linear device, such
`produce a discrete noise sample ni. Each of the sam
`as an inhibiter or gate, to achieve the noise cancellation.
`plers 34 and 37 can be, for example, an analog-to-digital
`The embodiment of FIG. can be modified in accor
`converter or a sample and hold implementation such as
`dance with the changes indicated in FIG. 2 to produce
`a charge coupled diode mechanization.
`a second embodiment of the invention. More specifi
`The adaptive transversal filter 3i is utilized to adjust
`cally the output of the preselector 13 can be directly
`the phase and amplitude of the noise sample macross all
`applied as a selected broadband RF passband signal to
`frequencies in the passband of the reference channel 21
`the primary and reference channels 19 and 21.
`to develop the reference audio signal or estimate f of
`This RF embodiment, as indicated in FIG. 2, could be
`the first broadband noise sample ni. The reference audio
`utilized where a fixed, preselected RF passband signal is
`signal or estimated noise sample ini is subtracted from
`to be received at all times in such applications as, for
`the primary audio sample si-ni in the combiner 33 to
`example, microwave and data link communications. In
`cancel the noise sample ni. As a result, the output of the
`these applications no IF frequency bandpass signal
`combiner 33 is a discrete audio error signal et which is
`would have to be detected, since passbands within the
`substantially comprised of the desired signal samplesi.
`fixed, preselected RF passband signal could be directly
`45
`The discrete audio error signal ei is converted to an
`detected within the channels 19 and 21. Thus, the band
`output analog audio signal e (or s) by a desampler 35.
`pass filters 23 and 27 would be implemented to respec
`The desampler 35 can be, for exampie, a digital-to
`tively select first and second bands of frequencies
`analog converter or a zero-order hold implementation.
`within the passband of the preselected broadband RF
`The discrete audio error signal ei is also gain-scaled by
`signal. These first and second bands of frequencies can
`a desired adaptation constant u in a multiplier 36 before
`be respectively varied to be in adjacent, overlapping or
`being fed back to adaptively adjust the filter coefficients
`separated portions of the frequency spectrum of the
`of the adaptive transversal filter 31. Thus, in response to
`passband of the broadband RF signal. Channel selectiv
`the discrete second broadband noise sample mi and to
`ity in the RF embodiment is such that the selected sec
`the gain-scaled discrete error signal sample , ei, the
`ond band of frequencies contains substantially no signal
`55
`filter 31 develops the reference audio signal or discrete
`energy of either the desired signal or any other received
`noise sample estimate ini which is used to cancel the
`signal (other than possibly noise).
`undesired broadband noise ni from the primary audio
`In the RF embodiment, indicated in FIG. 2, any
`signal.
`broadband noise would be present in the broadband RF
`passband signal being commonly applied to the inputs
`Within the filter 31, the discrete noise signal mi from
`60
`the sampler 37 is applied through a sequence of Z-1
`of the primary and reference channels 19 and 2. Other
`(one sample time delay) biocks 391,392,..., 39 N (which
`wise, the operation of the primary and reference chan
`together form a delay line) to respectively develop
`nels 19 and 21, adaptive filter 31 and combiner 33 are
`various sample time delayed signals ini-1, m2,..., mi-N)
`identical to those described in relation to the embodi
`at their equally time-spaced output taps 411, 412, ...,
`ment of FG. I.
`65
`4 in, respectively. The number i represents the ith time
`FIG. 3 illustrates a modification that can be made to
`instant or sample, and the number N represents the
`either of the embodiments of FIGS. 1 and 2 to produce
`a specific, discrete third embodiment of the invention.
`number of the last tap in the filter 31.
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`(4)
`
`A
`
`N
`X aii mij
`j = 1
`where mi are past samples of the input second audio
`signal mi and i=1,2,..., N.
`The error signal et developed from subtracting the
`estimated noise sample fit from the primary audio signal
`si--ni is
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`4,177,430
`8:
`7
`In order for the filter 31 to be stable, the following
`Respectively coupled to the taps 411,412,..., 41 Nare
`... , 43N for
`adaptation constant | limits should be satisfied
`coefficient computer circuits 431, 432, .
`updating the weights or coefficients colio)2,i ... conias
`0<h<2/(NP)
`(3)
`a function of the gain-scaled error signal, u, ei, from the
`multiplier 37. Since all of the coefficient computer cir
`where N= the number of taps in the filter 31 and P is
`cuits 431, 432, . . . , 43N are similar in structure and
`the filter input power Emi).
`operation, only the circuit 43, will be discussed.
`The following equations mathematically define the
`The coefficient computer circuit 431 is comprised of a
`overall operation of the embodiment of FIG. 3.
`multiplier 451, a summer 471 and a one sample time
`10
`delay block 491. The sample time delayed signal mi-1
`from tap 411 is applied to the multiplier 451. The prod
`uct signal u, ei, which is a function of the error ei, is
`multiplied by the delayed signal mi-1 in the multiplier
`451 to develop weight update signal associated with the
`15
`tap 411. This weight update signal, which at the present
`instant of time (i or now) is equal tou, ei mi-1, is summed
`in the summer 471 with the present value (at time instant
`i) of the presently updated coefficient or weight a) 1,
`from the output of the one sample time delay block 491.
`It should be noted that the output of the summer 471 is
`the weight update signal coli-1 at the present time i for
`the oil filter coefficient or weight that will occur at the
`next instant of time (i+1). The algorithm for determin
`ing the value of the updated weight or filter coefficient
`25
`o1 at the output of the summer 471 for the next instant
`of time is given by the equation:
`
`20
`
`35
`
`c),i-1 = ()1,i- Leini-1
`(1)
`30
`The output of the summer 471 is applied to the input of
`the delay block 491 with the output of the delay block
`491 being the filter coefficient coli.
`In this manner the coefficient computer circuits 431,
`432,..., 43Ndevelop the updated filter coefficients coli,
`o2,i . . . , coni. The sample time delayed signals mi-1,
`mi-2, . . . , mi-N are respectively multiplied by these
`updated filter coefficients coli, a)2,i ..., contin multipli
`ers 511, 512, ... , 51N to develop adaptively weighted
`signals coli mi-1, a2,i mi-2, . . . (oNi mi-N, respectively.
`These weighted signals from the multipliers 511, 512, ..
`., 51N are summed together in a summation circuit 53 to
`develop a least mean square (LMS) estimate ni of the
`primary noise sample ni.
`The estimated sample fit of the primary noise nisam
`45
`ple is subtracted from the primary audio signal sample
`si-ni, in the combiner 33 to develop the error signal
`sample or signal remnant ei, which is minimized in a
`LMS sense. This error signal ei is gain-scaled by the
`desired adaptation constant u in the multiplier 36 to
`produce the product signal u, ei, which is fed back to the
`coefficient computer circuits 431, 432, . . . , 43N to adap
`tively adjust the weights or coefficients coli a)2 . . . ,
`oN of the adaptive transversal filter 31.
`When the noises m and n are not correlated with each
`55
`other, the adaptive transversal filter 31 "shuts off" by
`converging or adjusting all of its weights or coefficients
`(a) 1,i, a)2,i ..., coni) to zero. Never will the converged
`adaptive filter 31 add noise power to the primary audio
`signal. The desired signal si is never modified or dis
`torted since it only passes through the combiner 33.
`The adaptation constant u of the filter 31 is related to
`the input power Ps to the filter 31 and the adaptation
`time constant T (in samples) of the filter 31 by the ex
`pression
`65
`
`50
`
`T= 1/(2LP)
`
`(2)
`
`ei=si+ni-ii
`(5)
`The jth filter (31) coefficient at time sample 1+i is
`computed from the ith coefficient at time sample i as
`follows
`
`(6)
`aii-- 1 = a ii-hueinii
`The invention thus provides an adaptive noise cancel
`ling receiver which has the capability of substantially
`concelling broadband noise from a selected radio fre
`quency passband which contains a desired signal com
`ponent. The selected radio frequency passband can be
`downconverted to a broadband IF signal before being
`commonly applied to primary and reference frequency
`channels. In some applications the selected radio fre
`quency passband can be directly applied as a broadband
`RF signal to the primary and reference frequency chan
`nels. The primary and reference frequency channels are
`responsive to the input broadband frequency signal for
`respectively generating primary and second audio sig
`nals, with the primary audio signal containing the de
`sired signal in the presence of a first broadband noise
`and the second audio signal containing a second broad
`band noise that is correlated with the first broadband
`noise but not with the desired signal. The first and sec
`ond broadband noises are correlated with each other
`since they emanate from the same source. An adaptive
`filter adjusts the phase and amplitude of the second
`broadband noise across all frequencies in the passband
`of the input broadband frequency signal to the reference
`channel to generate a reference audio signal which is an
`estimate of the first broadband noise. A combiner com
`bines the primary and reference audio signals to sub
`stantially cancel out the first and second broadband
`noises and develop an output audio signal which sub
`stantially contains only the desired signal. When the
`first and second "broadband' noises are not correlated
`with each other, the adaptive filter turns off by con
`verging its weights or coefficients to zero.
`While the salient features have been illustrated and
`described in several embodiments of the invention, it
`should be readily apparent to those skilled in the art that
`modifications can be made within the spirit and scope of
`the invention as set forth in the appended claims.
`I claim:
`1. An adaptive noise cancelling receiver comprising:
`
`- 7 -
`
`

`

`O
`
`15
`
`30
`
`20
`
`4,177,430
`10
`9
`means responsive to received radio frequency signals
`said second frequency channel includes a second
`for selectively developing a desired broadband
`bandpass filter for passing a second band of fre
`frequency signal containing a desired signal;
`quencies in a second part of the frequency spec
`a first frequency channel responsive to the broadband
`trum of the broadband intermediate frequency
`frequency signal for generating a first audio signal
`signal, and a second demodulator for demodulating
`in a first band of frequencies containing the desired
`the second band of frequencies to develop the sec
`signal in the presence of a first broadband noise;
`ond audio signal.
`a second frequency channel responsive to the broad
`7. The receiver of claim 6 wherein:
`band frequency signal for generating a second
`said adaptive filter is an adaptive transversal filter
`having adjustable coefficients, said adaptive trans
`audio signal in a second band of frequencies con
`taining a second broadband noise which is corre
`versal filter changing each of its coefficients as a
`function of the output audio signal in order to adap
`lated with the first broadband noise;
`tively develop a least mean square estimate of the
`an adaptive filter responsive to the second broadband
`noise and to an output audio signal for adaptively
`first broadband noise.
`8. An adaptive noise cancelling receiver comprising:
`developing an estimate of the first broadband noise;
`and select means for selecting a desired radio fre
`and
`quency passband containing a desired radio fre
`means for combining