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BACKGROUND OF THE INVENTION
In general the present invention relates to noise reduction systems and
more particularly to method and apparatus for removing background noise
from audio signals, especially musical signals.
The presence of background noise, sometimes defined as wholly random signal
energy, accompanying audio muscial signals has long been known as an
undesirable but apparently unavoidable by-product of the transmission,
recording and/or reproduction of audio signals. Many attempts have
heretofore been made to reduce the noise content by various frequency
filtering schemes. However, noise occurs at all frequencies within the
audible band and as such any attempted filtering of the noise usually
results in some loss of the musical frequencies.
For example frequency filters designed to remove the "scratch" noise during
the reproduction from phonograph records constitutes a simple noise
filter. The high frequency "scratch" sounds from the record are filtered
out by cutting off the high frequency components of the reproduced signal.
Inherently, the "scratch" filter removes the high frequency content of the
record from the reproduced signal, diminishing the fidelity of the
reproduction.
A more sophisticated filtering system employs what is known as dynamic
filtering. Here the amount of rolloff or cutoff of the high frequencies is
adjusted by a control voltage which is a function of the energy content of
the reproduced or transmitted musical signal. These dynamic filters
represent an improvement over the simple "scratch filter;" however, known
dynamic filtering techniques only remove the higher frequency noise and
cause a certain amount of undesirable modulation of the audible higher
frequencies.
Another attempt at noise reduction has been the provision of multiple band
dynamic filtering. This uses the dynamic filtering principle as discussed
above together with a series or multiplicity of controllable band-pass
filters. The individual filters are dynamically operated by the frequency
content of the incoming signal. Thus during processing of the signal, the
gates individually and collectively open and close allowing the musical
content of the signal to pass through while blocking the noise content.
Unfortunately, multiple band dynamic filtering systems heretofore
developed have exhibited an objectionable audio "swish" sound coincident
with each opening of one of the band-pass filters. The "swishing" sounds
constitute a disturbing psycho-acoustic effect and have prevented this
type of filtering system from gaining wide acceptance in the high fidelity
equipment industry.
Also, the "swish" sounds heard during the opening and closing of the
band-pass filter gates demonstrates one of the inherent difficulties in
reducing or eliminating noise from an otherwise high fidelity audio
signal. The noise exists substantially continuously and uniformly within
the band-pass of the musical material. The noise may be of greater or
lesser magnitude relative to the strength of the audio information signal,
however it is always superimposed thereon and as a practical matter
inseparable therefrom.
Some systems have recognized the inseparability of the noise and
information content of the signal and 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 recorded signals and/or tape recorded signals. For example, one
system widely used today for tape recording and reproducing systems
provides for encoded recording of lower level musical passages at a higher
amplitude, above the noise level or noise floor of the recording system.
During playback the encoding/decoding process converts the playback
signal back down to a correct amplitude for the lower level passages. This
processing does provide a reduction of noise, however it can only be used
in a "closed" system, i.e., where the source signal has been properly
encoded.
SUMMARY OF THE PREFERRED EMBODIMENT AND ITS OBJECTS
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
psycho-acoustic effects during the signal processing.
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 source may be passed through
the system with the output thereof issuing a "clean" noise reduced signal
in real time and without requiring redundant processing.
Briefly, these objects are achieved in accordance with the preferred
embodiment of the invention by multiple band dynamic frequency filtering
or gating that is controlled by the amplitude, harmonic content and degree
of correlation (as the term is defined hereinbelow) of the audio/musical
signal. A plurality of signal controlled band-pass filters or gates are
adapted to receive and selectively pass frequencies of the music which
fall within the respective frequency bands. A corresponding plurality of
band-pass signal detectors are provided for sensing a threshold presence
of signal content within each of the foregoing frequency bands. When any
signal frequency content is detected, the corresponding band-pass filter
or gate is opened to allow those frequencies to pass to the system's
output.
In real time with the incoming signal, a circuit receives and monitors the
content of the incoming audio signal and measures or estimates the degree
of correlation of the content of such signal. As used herein, the term
degree of correlation refers to the degree of periodicity, as contrasted
with randomness, of the signal content. Those signals which exhibit a
relatively high degree of periodicity are signals which are predictable
and are thus considered to have a high degree of correlation. On the other
hand signal content which is random, that is nonperiodic, is considered to
have a relatively low degree of correlation. Pure sinusoidal musical
sounds are highly correlated while noise is completely random and thus
uncorrelated.
A circuit is provided for developing an electrical control signal which is
a function of the degree of correlation of the incoming signal. This
correlation function signal is combined with another signal representing
the harmonic content of the incoming musical signal to control the
threshold level at which the presence of musical information within each
of the band-pass frequencies is detected. By automatically adjusting the
various threshold levels at which the band-pass filter gates are opened it
has been found that the incoming signal which is passed to the output is
usually if not always of such amplitude, frequency and degree of
correlation to subjectively "mask" the noise energy which inherently
accompanies the musical information in each of the pass-bands.
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
description of an exemplary embodiment thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a generalized block diagram of the overall noise reduction system
of the preferred embodiment of the invention.
FIG. 2 is a signal strength versus frequency graph illustrating certain
operating principles of the system shown in FIG. 1.
FIG. 3 is a more detailed block diagram of a portion of the system shown in
FIG. 1.
FIG. 4 is a detailed schematic of the threshold positioning amplifier
circuit shown diagrammatically in FIGS. 1 and 3.
FIG. 5 is a detailed schematic of one of the non-linear peak detectors
shown diagrammatically in FIG. 3.
FIG. 6 is a detailed schematic diagram of the first several band-pass gates
shown diagrammatically in FIG. 3.
FIG. 7 is a graph illustrating how the degree of correlation can be
estimated from the spectral energy distribution or content of the incoming
signal.
FIG. 8 is another graph showing how the threshold detection of the various
frequency constituents of the incoming signal is varied as a function of
both the degree of correlation and as a function of frequency.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1, the present invention is embodied here in a noise
reduction circuit or system for use in a stereo preamplifier or other
stereo reproduction, transmission or recording system. Briefly the system
includes a plurality of voltage controlled band-pass gates or filter means
provided in each of left and right channels 12 and 13. The unprocessed
musical signals are fed into the left and right channel inputs 16 and 17
and passed from there to the respective left and right channels 12 and 13
which are operated to selectively pass only certain frequency constituents
of these signals to the left and right channel outputs 18 and 19. Although
system 11 takes the form of a stereo, two-channel circuit, it will be
apparent from the the following disclosure that the present invention may
be embodied in a system having only a single musical channel or any number
of additional channels.
For the present system 11, the signals from inputs 16 and 17 are summed in
a summing amplifier 21 to develop a composite signal including the
frequency constituents 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 channels 12 and 13. In general the
control signals are developed by a plurality of band-pass signal detecting
means adapted to detect the threshold presence of frequency components of
the incoming signal coresponding, respectively to the available band-pass
frequencies provided by the band-pass gates in each of channels 12 and 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 plurality of band-pass gates in each of channels 12 and
13. Briefly, detectors 22 will sense the presence of signal 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 given
pass-bands, the associated band-pass gates of channels 12 and 13 remain
closed, blocking the passage of background noise.
In order to insure the transmission of all signal information 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 varying the
threshold level at which detectors 22 sense the presence of signal
information. In this instance a threshold positioning circuit 26,
including correlation estimation, is connected between summing amplifier
21 and multiple band-pass detectors 22. Circuit 26 automatically monitors
the harmonic content and degree of correlation of the incoming audio
signal and adjusts the threshold responsiveness of detectors 22 in
accordance therewith. In general this circuit has the effect of
conditioning the band-pass gate of channels 12 and 13 to pass lower
amplitude high frequency harmonics, even when they are beneath the
background moise level of the program, and to pass musical sounds of all
frequencies that have a relatively low degree of correlation such as wire
brushes, rushing water, etc. 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 characteristics 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 continuously and uniformly within the audible
frequency 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 nor continuous. That is, the music energy
appears in discrete, predictable, energy bundles throughout the audio band
and is therefore noncontinuous. Additionally, if some musical energy does
appear, for example at a particular frequency, we also know that even and
odd harmonics of the particular fundamental frequency will be present and
moreover the location of such harmonics in the pass-band will be known.
Furthermore, it is known that musical energy from the same source will not
exist between these harmonics.
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 which the musical energy
appears, it is possible to selectively pass only those frequency
constituents which make up the music content and block all other
frequencies. Since noise appears in all of the available frequency bands,
it will be appreciated that the closing or blocking of those frequency
bands having an absence of musical energy results in a significant
reduction in the noise content of the processed signal. For those
frequency bands which are opened to allow passage of the musical content,
any noise in such band-pass is also transmitted, however the music content
is usually of sufficient amplitude to mask the associated noise.
System 11 operates to automatically and selectively open a plurality of
pass-bands in the audio spectrum in response to the occurrence of musical
frequencies or content within each such pass-band. Thus in effect, a
series of frequency or band-pass windows are provided by the band-pass
gates of channels 12 and 13, each window disposed side-by-side and
slightly overlapping its neighbor, and together encompassing at least a
portion 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
content which requires any one or more of the given bands. For example in
response to a musical signal 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
closed. As the frequency content of the musical composition changes from
instant-to-instant the status of the band-pass gates changes to
accommodate the new frequency constituents.
This operation is illustrated in a general manner in FIG. 2, wherein a
series of band-pass windows 31, 32 33 and an upper frequency window 34 are
provided. As illustrated in the figure, the presence of a musical energy
component 36 at the 2 KHz frequency will cause the opening of band-pass
window 31 of channels 12 and 13. Similarly the 8 KHz signal energy
component 37, which may be a harmonic of the 2 KHz energy, causes
band-pass window 33 to open. In this example there is an absence of signal
energy in the 4 KHz band-pass and thus window 32 remains closed to block
any random noise in this frequency band from reaching the system's output.
It is observed in FIG. 2 that a still further component 38 of signal energy
exists in the higher frequency band-pass window 34. This higher frequency
signal component may be a still further harmonic of energy components 36
and 37. It is also observed that energy component 38 has an amplitude
which lies below the noise level or noise floor 39 of the system.
Usually the threshold levels of detectors 22 are adjusted so as to sense
the presence of signal energy exceeding 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 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 floor are recovered. The recovery of
the higher frequency low level harmonics preserves the overall frequency
response of the system.
Another feature of this embodiment of the present invention is its ability
to distinguish between highly uncorrelated music or other information
sounds having a relatively low degree of correlation such as wire brushes,
hand clapping, rushing water, and uncorrelated, completely random gaussean
noise. Although it is desirable to remove as much of the random gaussean
noise as possible, the system should not introduce any substantial
attenuation of these and other musical or information sounds having a
relatively low degree of correlation. For low level sounds of this type
having an amplitude in the vicinity of the system's noise floor, the
difficulty of distinguishing between the pure gaussean noise and the
noise-like sounds is apparent. In general this distinction is successfully
accomplished in the embodiment of the invention described herein by
measuring or estimating the degree of correlation of the incoming signal
and adjusting or varying the threshold responsiveness of detectors 22.
With reference to FIG. 3, these features are achieved in the present
embodiment by threshold positioning circuit 26 including a correlation
measurement or estimation means. This measures or estimates the degree of
periodicity of the incoming signal energy. Circuit 26 among other things
causes a change in the threshold level at which detectors 22 respond to
the incoming signal frequencies as a function of the degree of
correlation.
As indicated above, a signal that is "noise-like" is said have a relatively
low degree of correlation. Examples 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 of rushing water. Some
of these, such as the sound of wire brushes are found in musical material
and constitute an important part of the musical content.
Gaussean noise also has a relatively low degree of correlation and, in
fact, represents one extreme of the spectrum. Thus, completely random
noise, or "hiss," to use the nomenclature of the high fidelity equipment
field, is totally uncorrelated and may be assigned a degree of correlation
of zero (0) representing one end of the spectrum.
On the other hand, a highly periodic signal such as a sine wave or
"sine-wave-like" signal is considered to have a relatively high degree of
correlation. Examples of such signals are the sound of a harp, a plucked
guitar, a piano, certain vocal consonant sounds, etc. Because of this, a
sine wave may be considered to be completely correlated and thus will be
assigned a maximum degree of correlation of one (1). Since it is known
from Fourier analysis that any periodic wave form is composed of a linear
sum of one or more sine waves, correlation having a degree of one (1)
applies in general to both a single sine wave and any linear sum of sine
waves. The numerical values assigned to totally uncorrelated and totally
correlated signals is arbitrary, however, it does facilitate an
understanding of the principles upon which the present invention is based.
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 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
continuously from moment to moment. An interesting property of the degree
of correlation is that its definition depends upon the history of the
signal. This means that the value degree of correlation depends on its
immediately preceeding history. Accordingly, it is necessary for circuit
26 to continuously monitor and estimate the correlation degree of of the
incoming signal.
In FIG. 3, channel 12 is shown to be composed of a plurality of band-pass,
voltage controlled filters 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 operated by detectors 22
in response to the threshold positioning circuit 26, an additional low
band, voltage controlled dynamic filter 46 may be provided for filtering
out low frequency "hum" and "rumble". Filter 46 is in this instance
controlled by a separate low frequency controller 45 including low
frequency amplifier 47 connected to the output of summing amplifier 21, a
calibrating variable resistor 48 for adjusting the sensitivity of the low
frequency filtering path, and a low frequency filter 49 and averaging
detector 50 the output of which is connected to and for controlling filter
46. The presence of low frequency "hum" and "rumble" is amplified by low
pass amplifier 47 and detected by filter 49 and detector 50 to close
filter 46 whenever continuous low frequency energy above a desired "hum"
and "rumble" level is sensed. This low frequency hum and rumble filtering
is shown in combination with channel 12, detectors 22 and threshold
positioning circuit 26, however, it constitutes a separate, independent
circuit function which is preferably included in system 11 but may be
omitted without affecting the threshold positioning and correlation
estimating performed by circuit 26.
Multiple band-pass detectors 22 include a plurality of band-pass filters
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-pass
filter 51 of detectors 22 has a center frequency of 2 KHz for controlling
the operation of an associated 2 KHz gate 41 of channel 12. Similarly,
band-pass filters 52-54, which may be in the form of band-pass amplifiers,
are connected to and for controlling the associated signal gates 42-44.
The inputs of filters 51 through 54 are jointly connected to an output 56
from the threshold positioning circuit 26 so that the summed incoming
right and left hand channel signals are processed in circuit 26 and then
applied to the set of band-pass filters 51-54.
Filters 51-54 serve as a means for breaking down the incoming signal into
its frequency components, each component being located in one of the
established pass-bands. This frequency breakdown constitutes a type of
Fourier analysis where a given periodic wave form present at output 56 may
generate responses at one or more outputs of gates or filters 51-54
depending upon its Fourier makeup.
Detector means are provided for sensing the threshold presence of signal
energy in each of the pass-bands established by filters 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 filters 51-54 into time varying
direct current control 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 predetermined threshold of
signal energy in each band-pass. If the signal strength in any given
pass-band is less than the threshold responsiveness of its associated
detectors 61-64, the output voltage from the detector maintains the
corresponding signal gate 41-44 closed. A control voltage from any one or
more of detectors 61-64 is available for opening the associated signal
gates 41-44 only when band-pass filters 51-54 apply a sufficient threshold
amount of signal energy to the inputs of the 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 filters 51-54 will be
insufficient to reach the threshold level of responsiveness of detectors
61-64 and gates 41-44.
In this particular embodiment, each of the detectors 50, 61, 62, 63 and 64
has its output connected to one of polarity inverting amplifiers 66, 67,
68, 69 and 70 to produce a conjugate pair of control voltages for
operating a particular type of diode control gate employed in band-pass
gates 41-46 of channels 12 and 13. Thus, inverters 66 through 70 provide a
plus polarity control voltage at output terminals 71, 72, 73, 74 and 75
while the uninverted outputs from the detectors provide the conjugate
negative control voltage at terminals 81, 82, 83, 84 and 85.
In this particular and preferred embodiment, the threshold responsiveness
of detectors 61 through 64 is varied through selective frequency
amplification of the incoming signal by threshold positioning amplifier
26. In other words, the processed signal available at output 56 and
applied to the inputs of band-pass filters 51 through 54 has been
selectively amplified and/or attenuated as a function of its spectral
content and degree of correlation so that detectors 61 through 64 will
respond more readily, i.e., at a lower threshold, for those frequencies
which have been differentially amplified by circuit 26 and will respond
less readily, i.e., at a higher threshold, to those frequency components
which have received less amplification or in some cases have been
subjected to relative attenuation. Thus, the incoming audio information
signal is processed by circuit 26 to form a signal having an amplitude
versus frequency characteristic representing the parameters of harmonic
content and degree of correlation. This signal, available at the output of
circuit 26, is merely a control signal at this point and is not used in
and of itself as any portion of the audio output signal. The actual signal
channel proceeds exclusively in this case between inputs 16 and 17 and
outputs 18 and 19, respectively, of channels 12 and 13.
An input 86 of threshold positioning circuit 26 receives the summed,
amplified input signals from summing amplifier 21. In this instance, a
linear, 6db amplification is provided by amplifier 21 between input 16 and
17 and input 86 to circuit 26. This summed and amplified control signal
derived from the input channels is fed through a high-pass filter 87 which
passes the higher signal frequencies associated with band-pass gates 41
through 44 and rejects the frequency components below these levels. It has
been found that most of the random noise associated with reproduction,
recording and transmission equipment for musical signals lies at
frequencies of 1 or 2 KHz 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 filter 87 is designed in this case to
pass frequency components of 2 KHz or more.
The accentuated high frequencies are passed from filter 87 to a voltage
controlled amplifier 88 having a control input 89 responsive to the
harmonic content and degree of correlation of the incoming signal. More
particularly a control signal is applied to input 89 of amplifier 88 so as
to cause the gain of the amplifier to 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.
The functioning of circuit 26 is best understood by referring to several,
general objectives sought from its operation. First, with reference to
FIG. 2 it is desirable to set the threshold at which detectors 22 start to
open 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 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
the listener. This is to be contrasted with a situation in which the
band-pass gate is opened immediately upon detecting any signal strength in
any one of the band-pass frequencies. In such case there will be an
objectionable "swish" sound as the noise itself is modulated by the
opening of the band-pass gate. This is the first general rule of
operation.
Secondly, this first general rule must be modified in certain instances to
avoid the loss of certain high frequency components of the incoming
signal. One such instance is in the case of low amplitude harmonics of a
lower frequency fundamental. This situation is illustrated in FIG. 2 in
which a higher frequency harmonic energy component 38 lies in amplitude
below the noise level floor 39 while its associated lower frequency
harmonics and fundamental lie above the noise floor. It is important in
this type of situation to recover the higher frequency harmonics from
beneath the noise floor. This desirable result is achieved in circuit 26
by sensing the existence of strong lower frequency fundamentals and
tilting the frequency response at output 56 to push the threshold
detection for these higher level harmonics below the noise floor. 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 amplitude lower frequency
harmonics and fundamentals which are strong enough to mask the noise that
is transmitted along with the weak harmonics.
Thirdly, for signals having a relatively low degree of correlation, for
example signals having a correlation coefficient in the range of 0.1 to
0.3, it is desirable to push the threshold of response below the noise
floor. This is true because musical sounds having a low degree of
correlation, such as a wire brush, are very noise-like and the faithful
reproduction of such sounds inherently requires the transmission of the
pure random gaussean noise therewith. However, since the musical sounds
again such as a wire brush are noise-like, these sounds substantially mask
the underlying gaussean noise or pure "hiss" and there is no
psycho-acoustic impression of increased noise content. For pure "hiss" in
the absence of having a low degree of correlation musical or other
information sounds, the threshold remains just slightly above the noise
level in accordance with the first rule discussed above.
Now to achieve these operating results, circuit 26 includes various
frequency responsive networks discussed herein, which provide the
following frequency response at output 56. For pure random noise or "hiss"
the frequency response of circuit 26 is substantially flat with a gain set
so as to be just below the threshold of responsiveness of detectors 22.
For information sounds which have a relatively high degree of correlation
and include lower frequency fundamentals and higher frequency harmonics,
the response of circuit 26 is to differentially increase the gain of the
higher frequency components and slightly attenuate the lower frequency
fundamentals and harmonics. This has a tilting effect on the frequency
response curve of the circuit and causes the threshold level of the lower
frequency components to be raised above the noise floor and the threshold
responsiveness of the higher frequency harmonics to be pushed below the
noise floor. This achieves the result desired under the second rule above
whereby the higher frequency, lower amplitude harmonics are retrieved from
beneath the noise floor of the system.
Circuit 26 also automatically and continuously monitors or estimates the
degree of correlation of the incoming signal information. It this
particular embodiment, 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.
This fact is illustrated in FIG. 7 which shows the degree of correlation
varying as a function of the spectral energy distribution of the input
signals. For incoming signals having the larger portion of their energy
distributed in the relatively lower frequencies, for example in this
instance in the band-pass frequencies of 2 and 4 KHz, the degree of
correlation is usually closer to 1. On the other hand where the energy is
concentrated nearer the higher end of the frequency bands, the degree of
correlation drops toward zero. Circuit 26 monitors this spectral energy
distribution and produces a control signal applied at input 89 of
amplifier 88 for adjusting the amplifier's gain as a function of the
degree 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 correlation
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.
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 flat throughout the frequency
spectrum and with the gain positioned just slightly under the threshold of
detectors 22 as represented by solid line 92. The background noise content
of the incoming signal is evenly amplified by circuit 26 but with
insufficient gain to cause detectors 22 to open any of the band-pass gates
41-44.
Now assume that information content appears on the incoming signal and is
sufficiently correlated to exhibit a degree of correlation of 0.8. This
indicates a relatively highly correlated information content with the
signal energy being concentrated in the relatively lower fundamental
frequencies but with the possibility of lower level higher frequency
harmonics existing beneath the noise floor. Accordingly, as illustrated by
dotted line 93 in FIG. 8, circuit 26 tilts the frequency response between
its input 86 and output 56 so as to slightly attenuate the lower frequency
components and accentuate the higher frequencies. This requires the lower
frequency fundamentals and harmonics to achieve an amplitude above the
threshold indicated at 92 in order to cause detectors 22 to open the gates
41-44, i.e., presenting a higher threshold to the relatively lower
frequencies, and increasing the gain of the higher frequency harmonics to
provide a lower threshold of sensitivity thereto.
Now assume that the information content of the incoming signal becomes more
random and exhibits a lower degree of correlation of 0.2. In this case, as
exemplified by dotted line 94 in FIG. 8, the frequency response of circuit
94 remains relatively flat throughout the audio spectrum and the gain is
increased so as to present a lower threshold of responsiveness of
detectors 22. This corresponds to the situation in which information
sounds of a relatively low degree of correlation, such as wire brushes
rubbing on a drum head, rushing water, hand clapping, require a reduction
of the detector's threshold to a level below the noise floor in order to
faithfully reproduce the signal information. It is observed again however
that there is no loss of the overall psycho-acoustic perception of noise
reduction in this latter instance because the noise-like information,
sounds substantially, if not totally, like the underlying pure noise or
"hiss."
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