United States Patent [191 [45] Nov. 2, 1976

United States Patent  [191 [45] Nov. 2, 1976
United States Patent [191
3,989,897 ’
Nov. 2, 1976
Unwanted background noise in audio signals, and par
ticularly in musical material, is signi?cantly reduced
by a multiple band dynamic ?ltering system operated
[76] Inventor: Robert W. Carver, 3520 NE. 135th,
Seattle, Wash. 98115
[22] Filed:
Oct. 25, 1974
audio signal. A' plurality of voltage controlled band
[21] Appl. No.: 518,082
pass gates or ?lters are established in the audio signal
in response ‘to the fundamental and hannonic content
and degree of ‘correlation or nonrandomness of the
path for individually and selectively passing frequency
content of the audio signal falling within each fre
US. Cl. ................................ .. 179/1 P; 325/65;
328/163; 333/17 R
quency band. The fundamental and harmonic content
of the signal, typically a musical composition, is de
Int. CL’! ........................................ .. H04B 15/00
tected by a corresponding plurality of band-pass signal
Field of Search .................. .. 179/l P, l H, l D;
detectors, the outputs of which are connected to and
325/473, 65, 477, 480, 478; 328/163; 333/17
R; 84/DIG. 9
References Cited
detectors sense the presence of any fundamental or
harmonic signal content and cause the appropriate
............. .. l79/l P
179/1 P
Knowd et al.
179/1 P
Sacks ................................. .. 179/1 P
Primary Examiner—George G. Stellar
Attorney, Agent, or Firm—-Christensen, O'Connor,
Garrison & Havelka
for operatingv associated band-pass gates. Normally the
band-pass gates are closed blocking any background
noise present on the incoming signal. The band-pass
band-pass gates to open and pass the full frequency
content of the musical signal. The degree of correla
tion or nonrandomness of the audio signal is automati
cally monitored and the threshold detection of the
presence of signal content in any one of the given fre
quency bands is varied in accordance with the degree
of signal correlation.
8 Claims, 8 Drawing Figures
US. Patent
Nov. 2, 1976
Sheet 2 01-‘3
‘I1l| l
have merely provided a signal processing system for
selectively increasing the amplitude of the musical
material. However usually these systems require a
preencoding of 'the musical signal such as by special
encoding of frequency modulated signals, phonograph
In general the present invention relates to noise re
duction systems and more particularly to method and
apparatus for removing background noise from audio
recorded ‘signals and/or tape recorded signals. For ex
ample, one ‘system widely used today for tape recording
and reproducing systems provides for encoded record
signals, especially musical signals. _
ing of lower level musical passages at a higher ampli
tude, above the noise level or noise floor of the record
The presence of background noise, sometimes de
?ned as wholly random signal energy, accompanying
ing system. During playback the encoding/decoding
sirable but apparently unavoidable by-product of the
process converts the playback signal back down'to a
correct amplitude’ for the lower level passages. This
transmission, recording and/or reproduction of audio
processing does provide .a reduction of noise, however
signals. Many attempts have heretofore been made to
reduce the noise content by various frequency ?ltering
schemes. However, noise occurs at all frequencies
it can only be used in a “closed” system, i.e., where the
source signal has been properly encoded.
audio muscial signals has long been known as an unde
within the audible band and as such any attempted
?ltering of the noise usually results in some loss of the
musical frequencies.
Accordingly, it is an ‘object of the present invention
to provide a noise reduction system which preserves
the entire frequency response of the incoming musical
signal and does not introduce any objectionable psy
For example frequency ?lters designed to remove the
“scratch” noise during the reproduction from phono
graph records constitutes a simple noise ?lter. The high
cho-acoustic effects during the signal processing.
frequency “scratch” sounds from the record are ?l
tered out by cutting off the high frequency components
of the reproduced signal. lnherently, the “scratch”
Another object of the'present invention is to provide
a noise reduction system which operates in real time
and does not require any preencoding of the signal.
This processing is sometimes referred to as “open
ended” processing in that an audio signal from any
the reproduction;
A more sophisticated ?ltering system employs what is 30 source may be passed through the system with the
output thereof issuing a “clean” noise reduced signal in
known as dynamic ?ltering. Here the amount of rolloff
real time and without requiring redundant processing.
or cutoff of the high frequencies is adjusted bya con
Brie?y, these objects are achieved in accordance
trol voltage which is a function of the energy content of
with" the preferred embodiment of the invention by
the r‘eproducedor transmitted musical ‘signal.’ These
dynamic ?lters represent an improvement over the 35 multiple band dynamic frequency ?ltering or gating
that is controlled by the amplitude, harmonic content
simple “scratch ?lter;” however, known dynamic ?lter
and degree of correlation (as the term is de?ned here
ing techniques only remove the higher frequency noise
inbelow) of the audio/musical signal. A plurality of
and cause a certain amount of undesirable modulation
signal controlled band-pass ?lters or gates are adapted
of the audible higher frequencies.
Another attempt at noise reduction has been the 40 to receive and selectively pass frequencies of the music
which fall within the respective frequency bands. A
provision of multiple band dynamic‘?ltering. This, uses
?lter removes the high frequency content of the record
from the reproduced signal, diminishing the ?delity of
corresponding plurality of band-pass signal detectors
the dynamic ?ltering principle asdiscussed above to
are provided for sensing a threshold presence of signal
content within each of the foregoing frequency bands.
ated by the frequency content of the incoming signal. 45 When any signal frequency content is detected,v the
corresponding band-pass ?lter or gate is opened to
Thus during processing of the signal, the gates individu
gether with a series or multiplicity of controllable band
pass ?lters. The individual ?lters are dynamically oper
allow those frequencies to pass to the‘system’s output.
ally and collectively open and close allowing the musi
cal content of the signal to pass through while blocking
the noise content. Unfortunately, multiple band dy
namic ?ltering systems heretofore developed have ex
In real time with the incoming signal,‘ a circuit re
ceives and monitors the content of the incoming audio
50 signal and measures or estimates the degree of correla
tion ‘of the content of such signal. As used herein, the
hibited an objectionable audio “swish” sound coinci
term degree of correlation refers to the‘degree of perio
dent with each opening of one of the band-pass ?lters.
dicity, as contrasted with randomness, of the signal
The “swishing” sounds constitute a disturbing psycho
content.‘ Those signals which exhibit a relatively high
acoustic effect and have prevented this‘ type of ?ltering
system from gaining wide acceptance in the high ?del
ity equipment industry.
Also, the “swish” sounds heard during the opening
and closing of the band-pass ?lter gates demonstrates
degree of periodicity are signals which are predictable
and‘are thus considered to have a high degree of corre
lation. On the other hand signal content which is ran
dom, that is nonperiodic, is considered to have a rela
one of the inherent dif?culties in reducing or eliminat
tively low degree of correlation. Pure sinusoidal musi
ing noise from an otherwise high ?delity audio signal.
The noise exists substantially continuously and uni
formly within the band-pass of the musical material.
cal sounds are highly ‘correlated while noise is com
pletely random and thus uncorrelated.
A circuit is provided for developing an electrical
control signal which is a function of the degree of cor
The noise may be of greater or lesser magnitude rela
relation of the incoming signal. This correlation func
tive to the strength of the audio information signal,
however it is always superimposed thereon and as a 65 tion signal is combined with another signal representing
the harmonic content of the incoming musical signal to
practical matter inseparable therefrom.
control the threshold level at which the presence of
Some systems have recognized the inseparability of
musical information‘ within each of the band-pass fre
the noise and information content of the signal and
quencies is detected. By automaticallyadjusting the
various threshold levels at which the band-pass ?lter
gates are opened it has been found that the incoming
signal which is passed to the output is usually if not
13. In this instance the detecting means are provided by
multiple band-pass detectors 22 having a’ plurality of
outputs 23 connected to. and for controlling the plural
always of such amplitude, frequency and degree of
correlation to subjectively “mask" the noise energy
ity of band-pass gates in each of channels 12 and 13.
Brie?y, detectors 22 will sense the presence of signal
which inherently accompanies the musical information
in each of the pass-bands.
spectively to the available band-pass frequencies pro
vided by the band-pass gates in each of channels 12 and
information within any one of the given frequency
bands and operate the band-pass gates of left and right
channels 12 and 13 to pass those frequencies. In the
absence of signal information in any one or more of the
These and further objects, features and advantages of
the apparatus and method according to the present
invention will become apparent-to those skilled in the
art from a consideration of the following detailed de
given pass-bands, the associated band-pass gates of
scription of an exemplary embodiment thereof.
FIG. 2 is a signal strength-versus frequency graph
channels 12 and :13 remain closed, blocking the pas
sage of background noise.
In order to insure the transmission of all signal infor
mation including the higher harmonics of signals with a
relatively high degree of correlation and certain noise
like, musical sounds that exhibit a relatively low degree
of correlation, means are provided for automatically
illustrating certain operating principles of the system
20 varying the threshold level at which detectors 22 sense
FIG. I is a generalized block diagram of the overall
noise reduction system of the preferred embodiment of
the invention.
‘ "
the presence of signal information. In this instance a
shown in FIG. 1.
. FIG. 3 is a more detailed block diagram of a‘ portion
of the system shown in FIG. I.
threshold positioning circuit 26, including correlation
estimation, is connected between summing ampli?er
FIG. 4 is a detailed ‘schematic of the threshold posi
tioning ampli?er circuit shown diagrammatically in
FIGS. land_3.
21 and multiple band-pass detectors 22. Circuit 26
automatically monitors the harmonic content and de
gree of correlation of the incoming audio signal and
adjusts the threshold responsiveness of detectors 22 in
. FIG. 5 is a detailed schematic of one of the non-linear
peak detectors shown diagrammatically in FIG. 3.
accordance therewith. In general this circuit has the
FIG. 6 is a detailed schematic diagram of the ?rst
effect of conditioning the band-pass gate of channels
12 and. 13 to pass lower amplitude high frequency har
several band-pass gates shown diagrammatically in
FIG. 3.
FIG. 7 is a graph illustrating how the ‘degree of corre
monics, even when they are beneath the background
moise level of the program, and to pass musical sounds
lation can be estimated from the spectral energy distri
of all frequencies that havea relatively low degree of
bution or content of the incoming signal.
FIG. 8 is another graph showing how the threshold
detection of, the various frequency constituents of the
correlation such as wire brushes, rushing water, etc.
35 which are similar to random noise but are of course -
part of the information content of the signal.
In order to more fully understand the basic operating
conditions of system 11, a discussion of the characteris
tics of noise and how it contaminates audio/musical
information will be helpful. Noise content or noise
energy in a signal is a completely random, noncoherent
event. The noise energy exists substantially continu
incoming signal is varied as a function of both the de
gree of correlation and as a function of frequency.
With reference to FIG. 1, the present invention is
embodied here in a noise reduction circuit or system_
ously and uniformly within the audible frequency band
for. use in a stereo preampli?er or other stereo repro
duction, transmission or recording system. Brie?y the
system includes a plurality of voltage controlled band
or more precisely within the band-pass of the system.
Music and in general‘ most information carrying signals
‘will have an energy spectrum which is neither random
pass gates or ?lter means provided in each of left and
nor continuous. That is, the music energy appears in
right channels 12 and 13. The unprocessed musical
signals are fed into the left and right channel inputs l6
and 17 and passed from there to the respective left and
discrete, predictable, energy bundles throughout the
right channels 12 and I3 which are operated to selec
at a particular frequency, we also know that even and
tively pass only certain frequency constituents of these
signals to ‘the left and right channel outputs l8 and 19.
odd harmonics of the particular fundamental frequency
audio band and is therefore noncontinuous. Addition
ally, if'some musical energy does appear, for example
will be present and moreover the location of such har
Although system 11 takes the form of, a stereo, two
monics in the pass-band will be known. Furthermore, it
' channel circuit, it will be apparent from the the follow
is known that musical energy from the same source will
not exist between these harmonics.
ing disclosure that the present invention may be em
bodied in a system having only a single musical channel
or any number of additional channels.
For the present system 11, the signals from inputs l6
and 17 are summed in a summing ampli?er 21 to de
velop a composite signal including the frequency con
stituents of both the left and right signal channels. This
composite signal is then processed to develop a series
of control signals for operating the left and right chan
nels l2 and 13. In general the control signals are devel
oped by a plurality of band-pass signal detecting means
adapted to detect the threshold presence of frequency
components of the incoming signal coresponding, re
In other words, with information carrying signals and
particularly music, it is possible to predict where the
frequency, content of the music is likely to occur, and if
a fundamental frequency is present, the location of one
or more harmonics thereof may also be established.
Also, and importantly, it is possible to predict where
the musical energy does not exist.
Because of the predictability of the frequency at
65 which the musical energy appears, it is possible to se
lectively pass only those frequency constituents which
make up the music content and block all_ other frequen
cies. Since noise appears in all of the available fre
brushes, hand clapping, rushing water, and uncorre
lated, completely random gaussean noise. Although it
quency bands, it will be appreciated that the closing or
blocking of those;frequency bands having an absence
of musical energy resultsina signi?cant reduction in
is desirable to remove as much of the random gaussean
noise as possible, the system should not introduce any
the noise content of the processed signal. For those
frequency bands which are opened to allow passage of 5 substantial attenuation of these and other musical or
information sounds having a relatively low degree of
the musical content, any noise in such band-pass is also
transmitted, however the music content, is usually of
response to the occurrence of musical frequencies or
correlation. For low level sounds of this type having‘an
amplitude in the vicinity of the, system’s noise ?oor, the
dif?culty of distinguishing between the pure gaussean
noise and the noise-like sounds is apparent. In general
this distinction is successfully accomplished in the em
content within each such pass-band. Thus in effect, a
series of frequency or band-pass windows are provided
bodiment of the invention described herein by measur
ing or estimating the degree of correlation of the in
by the band-pass gates of channels 12 and 13, each
coming signaland adjusting or varying the threshold
responsiveness of detectors 22.
suf?cient amplitude to mask the associatednoise.
System 11 operates to automatically and selectively
open a plurality of pass-bands in the audio spectrum in
window disposed side-by-side and slightly overlapping
its neighbor, and together encompassing at least a por
tion of the audio band. Each band-pass window can be
either opened or closed. In system 11, the various
band-passes are normally closed and opened only in the
event musical information appears having frequency
With reference to FIG. 3, these features are achieved
in thepresent embodiment by threshold positioning
circuit 26 including a correlation measurement or esti
mation means. This measures or estimates the degree
of periodicity of the incoming signal energy. Circuit 26
content which requires any one or more of the given
among other things causes a change in the threshold
bands. For example in response to a musical signal
level at which detectors 22 respond to the incoming
signal frequencies as a function of the degree of corre
having a lower frequency fundamental and one or more
higher frequency harmonics, the band-pass windows
provided by the gates of channels 12 and 13 associated
‘with such fundamental and harmonic frequencies will
be opened while all other frequency windows remain
As indicated above, a signal that is “noise-like” is
said‘have a relatively low degree of correlation. Exam
ples of such sounds are sibilant speech, wire brushes on
a drum head, hand clapping, waves crashing against a
beach, the midpoint of a human cough, and the sound
position changes from instant-to-instant the status of
the band~pass gates changes to accommodate the new 30 of rushing water. Some of these, such as the sound of
wire brushes are found in musical material and consti
frequency constituents.
tute an important part of the musical content.
This operation is illustrated in a general manner in
Gaussean noise also has a relatively low degree of
FIG. 2, wherein a series of band-pass windows 31, 32
correlation and, in fact, represents one extreme of the
33 and an upper frequency window 34 are provided. As
illustrated in the'?gure, the presence of a musical en 35 spectrum. Thus, completely random noise, or “hiss,” to
use. the nomenclature of the high ?delity equipment
ergy component 36 at the 2 KHZ frequency-will cause
?eld, is totally uncorrelated and may be assigned a
the opening‘of band-pass window 31 of channels 12
degree of correlation of zero (0) representing one end
and 13. Similarly the 8 KHz signal energy component
closed. As the frequency content of the musical com
of the spectrum.
37, which may be a harmonic of the 2 KI-Iz energy,
causes band-pass window 33 to open. In this example 40 On the other hand, a highly periodic signal such as a
sine wave or “sine-wave-like” signal is considered to
there is an absence of signal energy in the 4 KHz band
have a relatively high degree of correlation. Examples
pass and thus window 32 remains closed to block any
of such signals are the sound of a harp, a plucked gui
random noise in this frequency band from reaching the
tar, a piano, certain vocal consonant sounds, etc. Be
system’s output.‘
It is observed in FIG. 2 that a still further component 45 causeof this, a sine wave may be considered to be
completely correlated and thus will be assigned a maxi
38 of signal energy exists in the higher frequency band
mum degree of correlation of one (1). Since it is known
pass window 34. This higher frequency signal compo
from Fourier analysis that any periodic wave form is
nent may be a still further harmonic of energy compo
composed of a linear sum of one or more sine waves,
nents 36 and 37. It is also observed that energy compo
correlation having a degree of one (1) applies in gen
nent 38 has an amplitude which lies below the noise
eral to both a single sine wave and any linear sum of
level or noise ?oor 39 of the system. ‘
Usually the threshold levels of detectors 22 are ‘ad
justed so as to sense the presence of signal energy exj
ceeding the noise floor of the system as is the case‘for
energy components 36 and 37 of FIG. 2. In this manner
the pure random noise of the system does not trigger
sine waves. The numerical values assigned to totally
uncorrelated and totally correlated signals is arbitrary,
All sounds that occur in nature have a degree of
correlation somewhere between zero (0) and one (1).
Some sounds have values very close to zero, such as
“hissing” air between your teeth. Other sounds have a
the opening of the band-pass windows provided by the
gates of channels 12 and 13.
However as described more fully herein it is a feature
of the present embodiment of the invention that higher
frequency harmonics such as energy component 38
lying below the noise ?oor are recovered. The recovery
of the‘ higher frequency low level harmonics preserves
the overall frequency response of the system.
however, it does ‘facilitate an understanding of the prin
ciples' upon which the present invention is based.
degree of correlation whose value is very close to one
(1) such as the pure ringing of a struck glass goblet or
tuning fork. The degree of correlation of music, or in
general, any information-containing signal, varies con
tinuously from‘ moment to moment. An interesting
Another feature of this embodiment of the present 65 property of the degree of correlation is that its de?ni
tion depends upon the history of the signal. This means
invention is its ability to distinguish between highly
that the value degree of correlation depends on its
uncorrelated music or other information sounds having
immediately preceeding history. Accordingly, it is nec
a relatively low degree of correlation such as wire
essary for circuit 26 to continuously monitor and esti
voltage from any one or more of detectors 61-64 is
mate the correlation degree of of the incoming signal.
available for opening the associated signal gates 41-44
only when band-pass ?lters 51-54 apply a sufficient
threshold amount of signal energy to the inputs of the
In FIG. 3, channel 12 is shown to be composed of a
plurality of band-pass, voltage controlled ?lters or
gates 41, 42, 43 and 44 serially connected to receive
and selectively pass frequencies in the left hand audio
channel. In addition to the band-pass gates 41-44 oper
ated by detectors 22 in response to the threshold posi
tioning circuit 26, an additional low band, voltage con
respective detectors to cause an opening of signal gates
44-44. Normally, the presence of pure random or
gaussean noise at the inputs to band-pass ?lters 51-54
will be insuf?cient to reach the threshold level of re
sponsiveness of detectors 61-64 and gates 41-44.
trolled dynamic ?lter 46 may be provided for ?ltering
In this particular embodiment, each of the detectors
50, 61, 62, 63 and 64 has its output connected to one
out low frequency “hum” and “rumble”. Filter 46 is in
this instance controlled by a separate low frequency
controller 45 including low frequency ampli?er 47
of polarity inverting ampli?ers 66, 67, 68, 69 and 70 to
produce a conjugate pair of control voltages for operat
ing a particular type of diode control gate employed in
band-pass gates 41-46 of channels 12 and 13. Thus,
connected to the output of summing ampli?er 21, a
calibrating variable resistor 48 for adjusting the sensi
tivity of the low frequency ?ltering path, and a low
inverters 66 through 70 provide a plus polarity control
frequency ?lter 49 and averaging detector 50 the out
voltage at output terminals 71, 72, 73, 74 and 75 while
put of which is connected to and for controlling ?lter
the uninverted outputs from the detectors provide the
46. The presence of low frequency “hum” and “rum
conjugate negative control voltage at terminals 81, 82,
ble” is ampli?ed by low pass ampli?er 47 and detected 20 83, 84 and 85.
by ?lter 49 and detector 50 to close ?lter 46 whenever
In this particular and preferred embodiment, the
‘continuous low frequency energy above a desired
threshold responsiveness of detectors 61 through 64 is
“hum” and “rumble” level is sensed. This low fre
quency hum and rumble ?ltering is shown in combina
varied through selective frequency ampli?cation of the
incoming signal by threshold positioning ampli?er 26.
tion with channel 12, detectors 22 and threshold posi 25 In other words, the processed signal available at output
tioning circuit 26, however, it constitutes a separate,
56 and applied to the inputs of band-pass ?lters 51
independent circuit function which is preferably in
through 54 has been selectively ampli?ed and/or atten
cluded in system 11 but may be omitted without affect
uated as a function of its spectral content and degree of
ing the threshold positioning and correlation estimating
performed by circuit 26.
Multiple band-pass detectors 22 include a plurality of
correlation so that detectors 61 through 64 will re
spond more readily, i.e., at a lower threshold, for those
frequencies which have been differentially ampli?ed by
band-pass ?lters 51, 52, 53 and 54, each having a pass
band substantially coextensive with the pass-bands of
signal gates 41-44. Thus, in this embodiment, band
circuit 26 and will respond less readily, i.e., at a higher
threshold, to those frequency components which have
received less ampli?cation or in some cases have been
pass'?lter 51 of detectors 22 has a center frequency of 35 subjected to relative attenuation. Thus, the incoming
2 KI-Iz for controlling the operation of an associated 2
audio information signal is processed by circuit 26 to
KHz gate 41 of channel 12. Similarly, band-pass ?lters
form a signal having an amplitude versus frequency
52-54, which may be in the form of band-pass ampli?
characteristic representing the parameters of harmonic
ers, are connected to and for controlling the associated
content and degree of correlation. This signal, available
signal gates 42-44. The inputs of ?lters 51 through 54
at the output of circuit 26, is merely a control signal at
are jointly connected to an output 56 from the thresh
this point and is not used in and of itself as any portion
old positioning circuit 26 so that the summed incoming
of the audio output signal. The actual signal channel
right and left hand channel signals are processed in
proceeds exclusively in this case between inputs 16 and
circuit 26 and then applied to the set of band-pass
17 and outputs 18 and 19, respectively, of channels 12
45 and 13.
?lters 51-54.
Filters 51-54 serve as a means for breaking down the
An input 86 of threshold positioning circuit 26 re
incoming signal into its frequency components, each
ceives the summed, ampli?ed input signals from sum
component being located in one of the established
ming ampli?er 21. In this instance, a linear, 6db ampli
pass-bands. This frequency breakdown constitutes a
?cation is provided by ampli?er 21 between input 16
type of Fourier analysis where a given periodic wave 50 and 17 and input 86 to circuit 26. This summed and
form present at output 56 may generate responses at
ampli?ed control signal derived from the input chan
one or more outputs of gates or ?lters 51-54 depending
nels is fed through a high-pass ?lter 87 which passes the
higher signal frequencies associated with band-pass
upon its Fourier makeup.
Detector means are provided for sensing the thresh
old presence of signal energy in each of the pass-bands
established by ?lters 51-54. In this embodiment, such
means are provided by a plurality of nonlinear peak
detectors 61, 62, 63 and 64. These detectors convert
the alternating current energy received from the band
pass ?lters 51-54 into time varying direct current con
trol voltages, the amplitudes of which are a function of
the amount of signal energy in each of the pass~bands.
More particularly, detectors 61-64 sense a predeter
gates 41 through 44 and rejects the frequency compo
55 nents below these levels. It has been found that most of
the random noise associated with reproduction, record
ing and transmission equipment for musical signals lies
at frequencies of l or 2 K112 and above. Accordingly,
the present embodiment of the invention provides for
noise reduction at the 2 KHZ level and higher. For this
purpose, high-pass ?lter 87 is designed in this case to
pass frequency components of 2 KHz or more.
The accentuated high frequencies are passed from
mined threshold of signal energy in each band-pass. If
the signal strength in any given pass-band is less than
?lter 87 to a voltage controlled ampli?er 88 having a
65 control input 89 responsive to the harmonic content
v the threshold responsiveness of its associated detectors
and degree of correlation of the incoming signal. More
particularly a control signal is applied .to input 89 of
61-64, the output voltage from the detector maintains
the corresponding signal gate 41-44 closed. A control
ampli?er 88 so as to cause the gain of the ampli?er to
responsiveness of detectors 22. For information sounds
which have a relatively high degree of correlation and
include lower frequency fundamentals and higher fre
continuously vary and thus continuously change the _
threshold sensitivity of detectors 22 as the spectral
distribution and degree of corrrelation of the incoming
musical signal vary with time.
quency harmonics, the response of circuit 26 is to dif
referring to several, general objectives sought from its
ferentially increase the gain of the higher frequency
components and slightly attenuate the lower frequency
operation. First, with reference to FIG. 2 it is desirable
, fundamentals and harmonics. This has a tilting effect
to set the threshold at which detectors 22 start to open
on the frequency response curve of the circuit and
causes the threshold level of the lower frequency com
ponents to be raised above the noise floor and the
The functioning of circuit 26 is best understood by
band-pass gates 41-44 at a level just slightly above the
noise level floor 39. By doing this, gates 41-44 are
opened only as the leading edge, sometimes referred to
threshold responsiveness of the higher frequency har
monics to be pushed below the noise ?oor. This
achieves the result desired under the second rule above
as the “attack” of the music increases to a level just
slightly above the noise level. Accordingly, when the
gates begin to open, the noise tends to be masked by
the increasing amplitude or strength of the musical
signal and little or no noise modulation is perceived by
‘whereby the higher frequency, lower amplitude har
monics are retrieved from beneath the noise, ?oor of
the system.
the listener. This is to be contrasted with a situation in
Circuit 26 also automatically and continuously moni
which the band-pass gate is opened immediately upon
tors or estimates the degree of correlation of the in
detecting any signal strength’ in any one of the band
pass frequencies. In such case there will be an objec
coming signal information. It this particular embodi
tionable “swish” sound as the noise itself is modulated
by the opening of the band-pass gate. This is the ?rst
general rule of operation.
ment, the degree of correlation is indirectly estimated
or sensed from the frequency makeup of the incoming
signal. It has been found that the degree of correlation
is roughly proportional to the instantaneous spectral
energy distribution of the audio/musical material. I
Secondly, this ?rst general rule must be modi?ed in
certain instances to avoid the loss of certain high fre 25 . This fact is illustrated in FIG. 7 which shows the
degree of correlation varying as a function of the spec
quency components of the incoming signal. One such
tral energy distribution of the input signals. For incom
instance is in the case of low amplitude harmonics of a
ing signals having the larger portion of their energy
lower frequency fundamental. This situation is illus
distributed in the relatively lower frequencies, for ex
trated in FIG. 2 in which a higher frequency harmonic
energy component 38 lies in amplitude below the noise 30 ample in this instance in the band-pass frequencies of 2
and 4 KI-Iz, the degree of correlation is usually closer to
level floor 39 while its associated lower frequency har
1. On the other hand where the energy is concentrated
monics and fundamental lie above the noise ?oor. It is
nearer the ‘higher end of the frequency bands, the de
important in this type of situation to recover the higher
gree of correlation drops toward zero. Circuit 26 moni
frequency harmonics from beneath the noise floor.
This desirable result is achieved in circuit 26 by sensing 35 tors this spectral energy distribution and produces a
control signal applied at input 89 of ampli?er 88 for
the existence of strong lower frequency fundamentals
and tilting the frequency response at output 56 to push
the threshold detection for these higher level harmon
ics below the noise ?oor. It has‘been found that there is
very little psycho-acoustic perception of noise in this
case, even though the threshold is positioned below the
noise floor, so long as there are adjacent, higher ampli
tude lower frequency harmonics and fundamentals v
which are strong enough to mask the ‘noise that is trans
mitted along with the weak harmonics.
adjusting the ampli?er’s gain as a function of the de
gree of correlation. This is best illustrated in FIG. 8
which shows the frequency response or weighted, gain
of the output of threshold positioning circuit 26 for
incoming information signals having a degree of corre
lation of 0.2 and 0.8 and also shows the response of
circuit 26 in the absence of any information content in
the incoming signal.
Thirdly, for signals having a relatively low degree of
correlation, for example signals having a correlation
Accordingly, for pure “hiss" or noise, absent any
information content, the frequency response and gain
of circuit 26 is indicated by dotted line 91 and shows
the response to‘ be substantially ?at throughout the
coef?cient in the range of 0.1 to 0.3, it is desirable to
frequency spectrum and with the gain positioned just
push the threshold of response below the noise floor.
This is true because musical sounds having a low de 50 slightly under the threshold of detectors 22 as repre
gree of correlation, such as a wire brush, are very noise
sented by solid line 92. The background noise content
like and the faithful reproduction of such sounds inher
ently requires the transmission of the pure random
gaussean noise therewith. However, since the musical
of the incoming signal is evenly ampli?ed by circuit 26
sounds again such as a wire brush are noise-like, these 55
but with insuf?cient gain to cause detectors 22 to open
any of the band-pass gates 41-44.
Now assume that information content appears on the
sounds substantially mask the underlying gaussean
incoming signal and is suf?ciently correlated to exhibit
noise or pure “hiss” and there is no psycho-acoustic
a degree of correlation of 0.8. Thisi‘indicates a rela
impression of increased noise content. For pure “hiss”
in the absence of having a low degree of correlation
tively highly correlated information content with the
signal energy being concentrated in the relatively lower
fundamentalfrequencies but with the possibility of
lower level higher frequency harmonics existing be
musical or other information sounds, the threshold 60
remains just slightly above the noise level in accor
dance with the ?rst rule discussed above. ,
Now to achieve these operating‘ results, circuit 26
includes various frequency responsive networks dis
cussed herein, which provide the following frequency
response at output 56. For pure random noise or “hiss”
neath the noise ?oor. Accordingly, as illustrated by
dotted line 93 in FIG. 8, circuit 26 tilts the frequency
response between its input 86 andtoutput 56 so as to
slightly attenuate the lower frequency components and
accentuate the higher frequencies. This requires the
the frequency response of circuit 26 is substantially ?at
lower frequency fundamentals and harmonics to
with a gain set so as‘to be just below the threshold of
achieve an amplitude above the threshold indicated at
92 in order to cause detectors 22 to open the gates
low frequency ?lter and peak detector network 163
41-44, i.e., presenting a higher threshold to the rela
wherein both of these networks are responsive to the
tively lower frequencies, and increasing the gain of the
feedback signal from junction 159 through a transistor
higher frequency harmonics to provide a lower thresh
164 connected with resistor 166 in an emitter-follower
old of sensitivity thereto.
The relatively frequency'?lter and detector network
162 includes ?lter components formed by serially con
nected capacitor 167 and resistor 168 and parallel-con
nected capacitor 169 and resistor 171. A peak detec
Now assume that the information content of the in
coming signal becomes more random and exhibits a
lower degree of correlation of 0.2. In this case, as ex
empli?ed by dotted line 94 in FIG. 8, the frequency
response of circuit 94 remains relatively ?at through
tion diode 172 is connected between the RC network
out the audio spectrum and the gain is increased so as
formed by capacitor 167 and resistor 168 and the RC
network formed by capacitor 169 and resistor 171 to
detect the energy content in the relatively higher fre
quencies of the signal passing through circuit 26 as
to present a lower threshold of responsiveness of detec
tors 22. This corresponds to the situation in which
information sounds of a relatively low degree of corre
lation, such as wire brushes rubbing on a drum head,
modi?ed by ?lter 87, ampli?er 88‘ and ampli?er 154.
Similarly, relatively low frequency ?lter and detector
network 163 includes a blocking capacitor 173, peak
detection diodes 176 and 177, capacitors 178 and 179,
rushing water, hand clapping, require a reduction of
the detector’s threshold to alevel below the noise floor
in order to faithfully reproduce the signal information.
It is observed again however that there is no loss of the
a resistor 181 and a diode 182 which is connected
across the ?lter network composed of resistor 181 and
capacitors 178 and 179 .to form a nonlinear ?lter re
sponsive to the peak detection of diodes 176 and 177.
overall psycho-acoustic perception of noise reduction
in this latter instance because the noise-like informa
tion, sounds substantially, if not totally, like the under
lying pure noise or “hiss.”
Although a number of circuit con?gurations may be
used to provide the foregoing operating characteristics
for circuit 26, a preferred and relatively inexpensive
While network 162 detects the relatively high fre
frequency constituents of the incoming signal. The
form of this circuit is illustrated in FIG. 4 and includes
high-pass ?lter 87 which may be provided by a capaci
tor 151 and resistor 152 for differentially weighting the
frequency response of the circuit in favor of the higher
frequencies. Voltage controlled ampli?er 88 is in this
instance a log ampli?er in which the output voltage
therefrom tracks the logarithm of the input voltage.
quencyenergy content of the signal, network 163 de
tects the energy content carried by the relatively lower
networks 162 and 163 produce control signals that vary
in opposite polarity senses, wherein the control signals
are summed at a summing junction formed at the con
30 nection of resistor 183 to the base of transistor 184.
More particularly, the output of the relatively low
frequency nonlinear detection network 163 is pres
ented at a junction 186 and summed with the output of
This nonlinear ampli?cation performed by log ampli
the relatively higher frequency detection network 162
?er 88 serves to consolidate or reduce the overall dy 35 available at output .junction 187 by means of resistors
namic amplitude range of the incoming signals to a
183 and 188 with the signals thus summed being fed to
level suitable for serving as a control signal at the‘out
and for controlling input 89 of log ampli?er 88 through
put 56 of circuit 26. Although the amplifier does intro
duce a log relationship between the output and input,
the emitter-follower formed by transistor 184 and resis
the ‘ampli?er gain is also varied as a function of control
input 89 which is a linear control function. In other
words, ‘for a constant input voltage to ampli?er 88 at
input 153, the output of the ampli?er will vary by an.
amount directly proportional to the change in the con
When the lower- frequency energy predominates in
the incoming signal received by circuit 26, network 163
detects this condition and produces a positive signal
change at junction 186 that increases the voltage
thereat in a- positive sense for summing with the signal
tor 189.
trol voltage applied at control input 89.
From the output of voltage controlled log ampli?er
88, the control signal is fed to a spectral weighting
from network 162. On the other hand, in the event the
-signal received by circuit 26 is loaded with high fre
quency energy, the network 162 detects this condition
ampli?er circuit 154 for further accentuating the high
and produces a negative signal change at the junction
frequency response relative to the lower frequency
and resistor 157 and an ampli?er 158. Accordingly, the
between diode 172 and the resistor and capacitor 171
and 169 respectively, that decreases the voltage thereat
in a negative sense and where this negative sense signal
is extended to, junction 187 for summing with the out
signal as processed by ?lter 87, ampli?er 88 and spec
put from relatively low frequency nonlinear detector
response. For this purpose, circuit 154 includes a seri
ally connected RC network including capacitor 156
tral weighting ampli?er circuit 154 results in a signal at
junction 159 varying as the logarithm of the signal
applied at input 86 and weighted in favor of the higher
frequency component.
creases in a positive. sense, this causes a control voltage
on'input 89 which decreases the gain of log ampli?er
88. On the other hand, a negative voltage change ap
This signal available at junction 159 is fed back to
and for controlling the gain of ampli?er 88 through a
nonlinear ?lter and peak detector circuit 161 operating
plied to the base of transistor 184 produces an increase
to vary the gain of circuit 26 as a function of the rela
tive degree of correlation of the incoming signal and to
tilt the frequency response of the circuit during the
presence of highly correlated, relatively low frequency
fundamental and harmonic signals for the purposes
described more fully above. In particular, circuit 161 as
shown in FIG. 4 includes a relatively high frequency
?lter and peak detector network 162 and a relatively
163. When the net voltage applied to the base of tran
sistor 184 through summing resistors 183 and 188 in
in gain of ampli?er 88. When the high and low fre
quency energy content is fairly uniformly distributed,
then there is no'net positive or negative signal variation
applied to transistor 184 and the gain of ampli?er 88
remains at a uniform'medium level.
As a practical matter, the relatively low frequency
responsive detector and ?lter network 163 is also re
sponsive to higher frequency energy content along with
the lower frequency energy. For this purpose a clamp
In general this is achieved by network 102 which in
ing diode 191 and resistor 192 are connected across the
output of junction 186 from network 163 so as to limit
cludes a slower time constant resistive-capacitive net
work 113 and a pair of back~to~back transfer diodes
116 and 117 connecting networks 106 and 113. Usu
the maximum positive excursion of the output voltage
at junction 186 so that in response to predominant high
frequency signalenergy content and network 162 pro
duces a negative voltage change which prevails at the
summing junction of resistors 183 and, 188 for increas
ing the gain of ampli?er 88.
ally intermodulation distortion is caused by relatively
low frequency amplitude variations in an otherwise
continuous signal. in order to eliminate these from the
response of the detectors, transfer network 102 is rela
tively unresponsive to slowly varying amplitudes in
For signal information of relatively low degree corre
lation received by circuit 26, which condition is sensed
by a predominant abundance of high frequency energy
content as detected by network 162, the overall gain of
which the amplitude deviation is relatively small and
quite responsive to rapidly varying amplitudes where
the amplitude change is signi?cant. For this purpose
network 113 includes a resistor 118 and a capacitor
119 selected to have a relatively large time constant
dance with the above noted operating principles. Simi 5 with respect to network 106. For example a time con
stant equal to or greater than 1 second has been found
larly, when the signal energy is predominantly of a low
adequate. Transfer diodes 116 and 117 create an am
frequency, network 163 senses this condition and re
plitude window equal to the sum of the forward voltage
duces the gain of ampli?er 88 for tilting the frequency
drop for each diode in which an output 121 of the
response of circuit 26 accentuating the higher fre
circuit 26 increases to cause a commensurate reduction
in the threshold detection level as is desired in accor
quency, lower level harmonics and attenuating the
circuit does not track the low frequency amplitude
larger amplitude, lower frequency fundamentals and
variations of the incoming signal. For signi?cant ampli
tude variations at input 111, the forward voltage drop
harmonics. This has the net affect of decreasing or
reducing the threshold of the higher frequency har
of the diodes 1 16 and 117 is substantially exceeded and
output 121 directly tracks the peak variations of circuit
106 through the transfer diodes. As indicated above,
this nonlinear detection is necessary to prevent the
control voltage developed at the output of detectors 22
monics to retrieve them from beneath the noise floor of
the system while insuring that the lower frequency
fundamentals and harmonics are of sufficient ampli
tude to mask any higher frequency “hiss” or pure gaus
sean noise passed by the system.
With reference to FIG. 5, each of the nonlinear peak
detectors 61-64 may be provided by the combination
of a half-wave voltage doubling peak detector 101 and
a nonlinear transfer network 102. Half-wave voltage
from modulating the envelope of the audio signals pass
ing through channels 12 and 13 which would be a form
of intermodulation program distortion. The control
voltage available at output 121 is used to develop the
conjugate pair of control voltages at the outputs of
each‘of detectors 61-64 by connecting the various
polarity inverters 67-70 to form outputs 72-75 and
feeding the uninverted control voltage directly to out~
puts 82-85 to form the other control voltage polarity of
doubling peak detector 101, in this instance, includes
diodes 103 and 104 and a parallel capacitive-resistive
network 106 connected between the anode and cath 35
ode of the diodes as shown. An input network including
the conjugate pair.
resistor 107 and a capacitor 108 receives the signal
With reference to FIG. 6, voltage controlled band
energy from an associated one of band-pass ?lters
pass filters or gates 41-44 are in this instance provided
51-54 and applies the representative voltage therefrom
across network 106. Diodes 103 and 104 allow capaci 40 by a cascaded series of voltage controlled notch ?lters
covering the relatively higher frequency audio bands
tor 109 of network 106 to charge up to a level propor
from 2 Kl-lz to 20 KHz, each ?lter slightly overlapping
its nearest neighbor. Band-pass gate 41 is illustrated in
detail in FIG. 6 with the succeeding band-pass gates
tional to the peak magnitude of the signal applied at
input terminal 111 and thereafter assume a high imped
ance condition which causes capacitor 109 to dis
charge, if at all, through its associated parallel resistor
112 of network 106. Thus, capacitor 109 is rapidly
charged to a peak-value function by diodes 103 and
being of substantially identical construction, with
merely different component values to achieve the dif
ferent band-pass frequencies. At the trailing end of this
cascaded series of band-pass ?lters or gates 41-44, a
104 and thereafter is discharged in accordance with the
conventional low frequency dynamic ?lter 46 is con
time constant formed by the values of capacitor 109
50 nected as shown in FIG. 3 to complete the serial ?lter
and resistor 112 of network 106.
ing, and a buffer ampli?er 126 may be employed to
The time constant of network 106 is selected to be
pass the audio signal from the last ?lter of the chain to
relatively fast, e.g., less than 1 millisecond in order to
permit rapid, faithful tracking of the band-pass signal
audio output 18.
With particular reference to band-pass gate 41 as
dynamic response for each of detectors 61 through 64 55 shown in FIG. 6, each voltage controlled ?lter or gate
is‘provided at its input with an impedance matching
which will follow rapidly the “attack” and “decay” of
emitter/follower, here provided by transistor 127 and
musical signals without undesirable overshoot. How
resistor 128. The band-pass itself is provided by a rela
ever, it has been found that such a rapid response of the
information applied to the detector. This results in a
detector sometimes causes intermodulationv distortion
between the information signal passing through each of
.the correlators and the control signal generated by
circuits 26 and detectors 22.
Accordingly, in this preferred embodiment of the
present invention, each of detectors 61-64 is provided
tively sharply tuned, three pole feed-back notch ?lter
network 129 which may be selectively bypassed by'a
voltage controlled‘diode network 131 which is con
nected to the output of an associated nonlinear peak
detector, in this instance detector 61 of detectors 22.
In the absence of a control voltage applied to net
with a nonlinear transfer network 102 which has been 65 work 131, the audio signal proceeds through each
notch ?lter such as ?lter 129 and those frequency com
found to permit both the rapid tracking of peak detec
ponents within the band established by the ?lter are
tor 101 and yet eliminate most, if not all, intermodula
tion distortion otherwise originating from this circuit.
essentially shorted out to ground and thus removed
from the signal spectrum. Alternatively gates 4144
the reproduction system will be passed to the output of
channels 12 and 13.
In the foregoing manner it will be appreciated that
this embodiment of the present invention provides a
and their associated notch ?lters, such as notch ?lter
129 may be thought of as a sequence of side-by-side
frequency windows covering the audio spectrum from
2 KHZ through 20 KHZ and above, wherein each of 5 method of suppressing background noise and particu
these windows may be selectively opened and closed
larly purely random noise which has contaminated an
depending upon the frequency makeup of the incoming
audio/musical signal. This is accomplished even though
signal. The lower frequency active or dynamic ?lter 46
in some cases the signal may include information con
is independently responsive to the low frequency con
tent having both relatively high and low degrees of
troller 45 as discussed above.
correlation i.e., noise-like, information content to
The frequency band over which each of the notch
?lters is effective can be established and changed by
gether with the wholly uncorrelated, noise content such
selecting the resistive and capacitive components such
as resistors 132, 133 and 134 and capacitors 135, 136,
As a ?rst step in this process, the threshold presence
as “hiss”.
of the incoming signal is detected in each of a plurality
of frequency bands where the selection and number of
bands may be at the option of the designer. Simulta
neously with the threshold presence detection, the
incoming signal is monitored for its degree of correla
and 137 in a well known manner. Diode switching
network 131 includes back-to-front parallel diodes 141 .
and 142 and blocking capacitors 143, 144 and 146 and
is connected in shunt around notch ?lter 129 as illus
trated. Normally network 131 assumes a high imped
ance condition placing notch ?lter 129 in the circuit
and effecting removal of the particular frequency com
ponents associated with that ?lter. This corresponds to
a condition in which the ?lter gate is closed.
In order to open the gate, network 131 is driven by
balanced control inputs 147 and 148 to a lower imped
ance condition shunting notch ?lter network 129.
tion. The more highly correlated signals are those ex
20 hibiting the greatest periodicity such as a sine wave or
a waveform composed of the sum of many sine waves
according to Fourier’s theorem.
The threshold at which signal energy is detected in
each of the established bands is thereupon varied as a
function of the degree of correlation of the audio/musi
cal signal. For signals having a high degree of correla
tion the threshold is generally increased, except in the
More particularly this is achieved by connecting inputs
147 and 148 to the associated conjugate pair of control
case of higher frequency lower level harmonics for
outputs from detectors 22, namely in this instance to
which the threshold is decreased, while for relatively
output terminals 72 and 82 from nonlinear peak detec 30 low degree correlated, noise-like sounds the threshold
is decreased across the entire spectrum.
tor 61. The presence of a threshold control voltage at
the output of the associated detector causes the diodes
of network of 131 to become forward biased and as
sume the necessary low impedance condition for by
passing the notch ?lter. Although a single voltage con
The complex detection of signal energy presence is
thereupon used to selectively pass those frequency
components of the processed audio signal falling into
35 the frequency bands in which such energy was detected
trol diode may be used in network 131, here the front
to-back parallel diodes 141 and 142 controlled by the
over the variable threshold.
The resulting psycho-acoustic effect of this signal
processing is to present a substantially noise-free audi
o/musical signal which preserves the full frequency
conjugate pair of output voltages from the detectors 22
causes a cancellation of the residual positive and nega
tive control voltages applied to circuit 131. This can
spectrum of the original program. In those cases where
the threshold detection of the noise contaminated in
coming signal has been lowered so as to simultaneously
cellation afforded by the two-diode balanced switching
network avoids the injection of any extraneous control
signals into the information signal path.
allow the passage of pure “hiss” or gaussean noise to
The aforementioned threshold responsiveness of de
tectors 61-64 in conjunction with gates 41-44 is estab
lished in this embodiment by the overall gain of the
various circuits between output 56 of circuit 26 and the
incipient bypassing of the various notch ?lters forming
gates 41—44. The system may be set up and properly
the system’s output, the information content of the‘
signal either has a relatively low degree of correlation
adjusted by varying the correlation threshold adjust
.and is thus noise-like, such as in the case of wire brush
sounds, rushing water sounds, etc., or musical material
predominated by high energy lower frequency funda
mentals and the associated higher frequency harmon
ment provided by variable resistor 90 of threshold posi
tioning ampli?er circuit 26 as shown in FIGS. 3 and 4.
In general the correlation threshold adjustment pro
vided' by variable resistance 90 is adjusted to a center
position and a relatively clean high quality modern
phonograph record is played through the system. Pref
55 sean noise or “hiss.”
While only one particular embodiment of the present
invention has been disclosed herein, it will be readily
erably the record should have a musical selection
which contains an abundance of high frequencies. By
carefully listening to the playback of the record, and
simultaneously adjusting correlation threshold resistor
90, a setting may be reached in which the high fre
quency treble is faithfully reproduced and yet the un
ics. In either of these instances in which the threshold is
pushed below the noise floor of the system, the infor
mation content has either a sufficient amplitude or is
suf?ciently noise-like even though at a lower ampli
tude, to substantially if not entirely mask the pure gaus
apparent to persons skilled in the art that numerous
changes and modi?cations may be made thereto with
out departing from the spirit of the invention. For ex
ample, in the disclosed system and method, the plural~
ity of voltage controlled pass-band ?lters forming chan
nels 12 and 13 have been provided by a limited number
of band-pass gates, here having a one octave relation
the incoming signals and unacceptable loss of high 65 ship. In the alternative any number of gates may be
employed in lieu of the one octave gates 41 through 44
frequency response will occur. On the other hand, a
and covering a broader or narrower frequency spec
setting too far in the other direction will result in too
trum within the audio band. The greater number of
low a threshold and all of the random noise present in
derlying random noise disappears. To one side of the
proper setting, too high a threshold will be presented to
gates will result in a further reduction in the back
said frequency components of said control signal
groundnoise at the system’s output.
have exceeded, a predetermined‘threshold.
Similarly, the system’s performance may‘, be en
hanced by providing separate control circuits, namely ‘
v3. Thefrioise reduction system of claim 2, wherein
said threshold, positioning circuit means comprises:
separate threshold positioning ampli?er circuits and ‘5
high pass frequency‘ ?lter means connected to said
input of said threshold positioning circuit means;
detection circuits such as circuit 26 and detectors 22
for each of the two or more audio channels of the sys
tem. In
this embodiment
by ampli?erthe
21 two
to generate
stereo ‘channels
a single set
control signals which are jointly applied as shown in 10
‘ an output and a control input;
spectral weighting ampli?er circuit means connected
between the output of said voltage controlled am
FIG. 1 to channels 12 and 13. In an alternative system,
even the control channels would be maintained sepa
pli?er and the output of said threshold positioning
rate from one another, by merely duplicating the con
trol channel including circuit 26 and detectors 22 for
each of the right and left hand audio channels.
Accordingly, the foregoing disclosure and descrip
tion of the preferred embodiment of the invention is for
illustrative purposes only and does not in any way limit
the invention which is de?ned only by the following
I claim:
1. A method of suppressing background noise in an
audio frequency signal comprising the steps of:
producing an electrical control signal derived from
said audio signal in which the frequency compo
' voltage controlled ampli?er means having an input
connected to said high pass ?lter means and having
circuit means and including an additional high pass
frequency ?lter means; and
non-linear frequency ?lter and-peak detector means
connected between said output of said positioning
circuit means and said control input of said voltage
controlled ampli?er means for detecting amplitude
versus frequency characteristics of said control
signal and for varying the gain of said voltage con
trolled ampli?er means in response to said charac
4. The noise reduction system of claim 3, wherein
said threshold positioning circuit means further com
a manually controlled threshold adjustment means
nents of said control signal are continuously varied
connected to said output of said threshold position
ing circuit means for adjustably setting a nominal
frequency components of said audio signal as a
gain level for said control signal at said output
function of the instantaneous amplitude verses
relative to the audio signal received at said input.
frequency content of said audio signal;
5. The noise reduction system of claim 2, wherein
detecting a threshold presence of said frequency
said plurality of band-pass threshold signal detecting
components of said control signal in each of a plu
means each comprise:
rality of frequency bands; and
a band-pass ?lter connected to said output of said
selectively passing frequency components of said
threshold positioning circuit means; and
audio signal falling within each of said plurality of 35
a non~linear peak detector means connected to said
frequency bands only when a threshold presence of
relative to the amplitudes of the corresponding
said frequency components of said control signal
have been detected within the corresponding fre
quency band.
2. A noise reduction system for an audio signal com 40
a half-wave voltage doubling peak detector and a
non-linear transfer circuit connected thereto, said
a plurality of electrically-controlled ?lter means,
each having a de?ned frequency band, for receiv
ing said audio signal and for selectively passing
frequency components thereof that fall within each
of said bands;
a threshold positioning circuit means having an input
for receiving said audio signal and for modifying it
to produce a control signal at an output, said con
trol signal including frequency components of said
peak detector including a resistive-capacitive stor
age network having a predetermined response time
and said non-linear transfer circuit including a
resistive-capacitive storage network having a re
sponse time less than that of said predetermined
response time such that said peak detector means
tracks variations in the frequency components of
said control signal without causing intermodulation
of the frequency components of said audio signal
passed by said plurality of electrically controlled
audio signal in which the amplitudes of such com
ponents are continuously varied, relative to the
amplitudes of the corresponding frequency compo
band-pass ?lter for detecting peak voltage excur
sions of frequency components of said control sig
nal that exceed said predetermined threshold.
6. The noise reduction system of claim 5, wherein
said peak detector means comprises:
?lter means.
nents of said audio signal, such continuous varia
tion being a function of the amplitude versus fre 55 7. The noise reduction system of claim 2, wherein
said plurality of ?lter means comprises:
quency content of said audio signal; and
a cascaded series of voltage controlled notch ?lters in
plurality of band-pass threshold signal detecting
which the input of a ?rst of said notch ?lters re
means, one being associated with and connected to
ceives said audio signal and an output of a last of
each of said ?lter means, each said detecting means
having a pass-band corresponding to said fre 60
quency band of the associated said ?lter means,
each said signal detecting means connected to said
threshold positioning circuit means for receiving
said control signal and being responsive to fre
quency components falling within its pass-band,
said ?lter means being operated by its associated
said detecting means to pass said frequency com
ponents when the latter detects that the level of
said notch ?lters issues the audio signal with noise
8. The noise reduction system as de?ned in claim 2,
wherein said audio signal is composed of frequency
components originating from related channels,
and said system further comprises:
an additional plurality of electrically controlled ?lter
means for receiving the portion of said audio signal
from one of said channels and said ?rst named
plurality of electrically controlled ?lter means re
ceiving the portion of said audio signal from an
other of said channels, said additional plurality of
?lter means being connected in parallel with said
?rst named plurality of ?lter means to correspond
both said portions of said audio signal and having
an output issuing the summation of said portions,
said input of said threshold positioning circuit
ing ones of said plurality of signal detecting means;
summing ampli?er means having inputs for receiving
means connected to said output of said summing
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