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Claims  |
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We claim:
1. System for background noise suppression upon reproduction of program
audio signals in an audio circuit, particularly in a car radio, having
means (1) for providing said program audio signal;
means (2, 6; 3, 7) for transducing said program audio signals into acoustic
output;
a signal path between said signal providing means and signal transducing
means; and
means (10) for providing noise signals, representative of background noise
to be suppressed,
and wherein said signal path comprises
a multi-channel equalizer means (11) for separating the program audio
signals into a plurality of program audio signal frequency components, and
for passing said program audio signal frequency components in a plurality
of channels having a transfer amplitude which is controllable through the
equalizer means;
a frequency analysis stage (12) including
first bandpass means (100, 101, 102) receiving the audio signals for
separating the process audio signals into a plurality of frequency bands;
first envelope detector means (200, 201, 202) separately detecting the
envelopes of the respective program audio signal frequency bands;
second bandpass means (103, 104, 105) receiving the noise signals for
separating the noise signals into a plurality of frequency bands, said
second bandpass means having similar bandpass characteristics as said
first bandpass means;
second envelope detector means (203, 204, 205) separately detecting the
envelopes of the separated respective noise frequency bands;
divider means (210, 211, 212) separately comparing the detected envelopes
of the noise frequency bands with the detected envelopes of the program
audio frequency bands, and providing program audio signal/noise signal
comparison signals; and
a control stage (13) receiving said comparison signals and providing
control output signals for each of said frequency bands, said control
output signals being coupled (240, 241, 242; B) to the control inputs of
the equalizer and controlling the transfer amplitude, change of the
transfer amplitude and rate of the change of the transfer amplitude of the
respective frequency bands passing through the equalizer as a function of
said comparison signals.
2. The system of claim 1, wherein said divider means are analog divider
means dividing instantaneous values of the respective envelope detector
means connected thereto.
3. The system of claim 2, wherein said first and second envelope detector
means have rise and decay time constants for, respectively, rising and
decaying signals applied thereto from the respective bandpass means;
and wherein the time constants for a rising signal are shorter than the
time constants for a decaying signal.
4. The system of claim 2, wherein said first and second envelope detector
means have rise and decay time constants for, respectively, rising and
decaying signals applied thereto from the respective bandpass means;
and wherein the decay time constants of the envelope detectors are shorter
for higher frequency bands than for lower frequency bands.
5. The system of claim 1, wherein the control stage (13) comprises a
plurality of multi-stage comparator circuits (220, 221, 222), each
associated with and connected to receive the comparison signals in the
respective frequency bands;
weighting means (k.sub.i) connected to the respective stages of the
comparator circuits (220, 221, 222) and adding means (265) adding the
respectively weighted signals received from the weighting means; and
integrator means (230) integrating the added signals and providing
integrated output signals for coupling to said equalizer control input.
6. The system of claim 5, wherein the comparator circuits include
operational amplifiers (260-264), receiving at one input the output
signals of the respective divider means (210-212);
buffer amplifier means (270) interposed between the outputs of the
respective divider means and the operational amplifiers;
voltage divider means (280-285) being coupled to a second input of the
operational amplifiers, and providing voltage values of different
magnitude to said operational amplifiers;
resistor coupling means (290-294) of respectively different values coupling
the outputs of the operational amplifiers to said adder (265) and forming
said weighting means, said resistor coupling means determining the
integration speed and direction of said integrator means (230) for,
respectively, controlling the transfer amplitude, change of the trnasfer
amplitude, and rate of the change of the transfer amplitude of the program
signals in the respective channels of the equalizer.
7. The system of claim 6, further including channel interconnection
circuits (300-301) interconnecting the integrators (230, 231, 232) for
modifying the integration behavior of integrators in the channels
associated with higher frequency band upon substantial and rapid change of
the control signal applied to the control inputs of the equalizer of a
lower frequency band beyond predetermined values.
8. System for background noise suppresson upon reproduction of program
audio signals in an audio circuit, particularly in a car radio, having
means (1) for providing said program audio signal;
means (2, 6; 3, 7) for transducing said program audio signals into acoustic
output;
a signal path between said signal providing means and signal transducing
means; and
means (10) for providing noise signals, representative of background noise
to be suppressed,
and wherein said signal path comprises
a multi-channel equalizer means (11) for separating the program audio
signals into a plurality of frequency bands and for passing said frequency
bands in a plurality of transfer channels having a transfer amplitude
which is controllable through the equalizer means;
means for separating the program audio signals and the noise signals into a
plurality of respective audio program signal components and noise signal
components of respectively different frequency bands, and corresponding
essentially to the frequency bands of the multi-channel equalizer;
means for comparing, within like frequency bands, the respective noise
signal components and the program audio signal components and deriving a
plurality of control comparison signals; and
means for receiving the comparison control signals and providing control
output signals for each of the frequency bands, said control output
signals being coupled to control inputs of the equalizer means and
controlling the transfer amplitude, change of the transfer amplitude and
rate of the change of the transfer amplitude of the respective frequency
bands passing through the equalizer means as a function of said comparison
signals.
9. The system of claim 8, further comprising an analysis stage (12), said
means (103-105) for separating the program audio signal and the noise
signal into the respective frequency bands and said means for comparing
forming part of said analysis stage bands; and
said analysis stage further including means (200-205) demodulating said
program audio signals, and said noise signals, in said frequency bands and
providing demodulated envelope curves to said means for comparing, for
comparing the instantaneous values of the demodulated program audio
signals and the noise signals within the respective frequency bands.
10. Method of suppressing background noise upon reproduction of program
audio signals in audio reproduction equipment, particularly car radios,
comprising the steps of
separating the program audio signals into a plurality of channels of
respectively different frequency bands;
deriving (10) background noise signals and separating said background noise
signals into a plurality of frequency bands, corresponding in band
frequency to said separated program audio signals;
comparing the instantaneous values, in the respective frequency bands, of
the noise signals with the program audio signals, and deriving a plurality
of comparison control signals in the respective frequency bands (V.sub.B,
V.sub.M, V.sub.H);
receiving the comparison control signals and providing control output
signals for each of the frequency bands;
coupling said control output signals to control inputs of a multi-channel
equalizer having a controllable transfer amplitude; and
controlling the transfer amplitude, change of the transfer amplitude and
rate of the change of the transfer amplitude of the respective frequency
bands passing through the equalizer as a function of said comparison
signals.
11. The method of claim 10, wherein said control step includes determining
signal rise and signal drop-off or decay of said comparison control
signals;
and controlling the rate of change of the signals passing through the
equalizer to result in decay times which are longer than said rise times
both for the program audio signal and the noise signal.
12. The method of claim 11, wherein said control step comprises controlling
the decay times to be shorter for higher frequency bands than for the
lower frequency bands. |
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Claims |
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Description |
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The present invention relates to suppression of reproduction of background
noises when acoustic signals are derived from electrical audio signals,
and more particularly to improving the audio reproduction of automobile
radios in which the listening area, particularly the passenger compartment
of the autombile, is subject to extraneous noise.
BACKGROUND
During reproduction of radio programs, tape-recorded programs, and the
like, by a car radio it frequently happens that the audio signal
reproduced by the loud speakers of the car radio is subject to extraneous
noise inteference of widely varying loudness or amplitude. Such extraneous
noise is superimposed above the audio program which is intended to be
heard, the audio program being entirely or partially masked by the
extraneous noise.
Extraneous noise depends on various factors, for example speed of the
vehicle, its construction, the quality of noise insulation, the inner
appointment of the passenger compartment, the number of passengers in the
passenger compartment, ventilation of the passenger compartment, the
position of windows, whether open or closed, air-conditioning apparatus
including its fan or ventilator, and the like. Any change in any one of
the above parameters usually leads the listener to readjust the volume
control of the car radio, for increased or decreased volume. If the driver
is the only operator of the car radio, this is frequently difficult
because the driver has to pay attention to road and traffic conditions,
and may require both hands for guidance and control of the vehicle;
various operating conditions of vehicles do not permit simultaneously
driving and operating a radio button. Additionally, the volume of the
reproduced signal varies frequently with respect to the volume of the
noise signal so rapidly that manual control is practically impossible.
The audio signal is reproduced with the fidelity characteristic of the
electronic reproduction apparatus. High-quality car radios provide high
fidelity outputs. Noise during passage of a piano concert, easily masks
the particular passage. The volume of reproduction should thus be raised
upon acceleration of the vehicle in order to permit the piano passage to
be readily perceptible. Even if the noise is at a level such that the
piano passage still can be heard, the music sounds distorted.
It has previously been proposed to provide automatic volume control for
vehicles in which a controller receives an input based on vehicle speed
and/or engine speed. It also been proposed to install a microphone or
similar apparatus in the engine compartment of the vehicle in order to
measure engine noise and to then cotrol the volume of reproduction as a
function of engine noise.
A microphone installed in the engine compartment of the vehicle does not
pick up all noises which occur, and referred to above. As a first
approximation, however, engine noise may be considered the primary
disturbance factor having the widest noise dynamics and the widest rate of
change of its amplitude during operation of a car radio.
THE INVENTION
It is an object to so improve a car audio system that the reproduction of a
program signal will be less subject to noise disturbance than heretofore.
Briefly, a multi-channel equalizer is inserted in the path of the program
signal. The equalizer, as well known, has a plurality of channels,
subdivided into frequency bands, which channels are individually
adjustable. The program signal is separated into a plurality of frequency
bands or components, typically three frequency bands or components and
corresponding, for example, to the frequency bands of a three-channel
equalizer. A noise signal is derived, for example from a microphone
installed in the engine department of the vehicle which, likewise, is
subdivided by bandpass filters into frequency bands having the same band
width as the band width of the subdivided program signal. The noise signal
bands and the program signal bands are then divided into each other, in
other words compared, and individual separate control signals,
corresponding to the respective frequency bands, are then derived which,
in a control stage, provide control output signals for the control inputs
of the equalizer to control the transfer amplitude of the respective
channels of the equalizer with respect to level of transfer amplitude,
change of transfer amplitude, and rate of change of transfer amplitude,
all as a function of the comparison signals.
The system and method has the advantage that the program signal is
selectively controlled with respect to its frequencies so that only those
frequency ranges which are masked by noise are increased in amplitude; if
specific frequency ranges are not affected by noise, their amplitude can
be reduced. Control of the reproduction amplitude, thus, is
frequency-selective.
In accordance with a preferred feature of the invention, the electrical
noise signal is derived from a microphone located in the engine
compartment of the veihcle. Other sources of noise signals may be used
without departing from the scope of the present invention:
Drawing, illustrating embodiments of the invention
FIG. 1 is a basic block circuit diagram of the system in accordance with
the present invention;
FIG. 2 is a detailed block diagram of an analysis stage and a control stage
for control of an equalizer in the signal path of a car radio;
FIG. 3 is a detailed schematic block diagram of a portion of the control
stage of FIG. 2; and
FIG. 4 is a more detailed diagram of the portion shown in FIG. 3.
DETAILED DESCRIPTION
A stereo signal source 1 which, for example, may be a car radio, a tape
deck, or the like, is coupled through two channels R, L for right and left
stereo channels, respectively, to a two-channel audio amplifier 2, 3 to
feed respective loudspeakers 6, 7. The stereo signal source 1 provides
stereo program audio signals for reproduction by the loudspeakers 6, 7.
Noise compensation circuits 4, 5 are interposed in the signal paths of the
respective channels R, L. Measuring feedback lines 8, 9 connect from the
audio amplifiers 2, 3 to the respective noise compensation circuits 4, 5.
A noise pick-up or microphone 10 is provided, picking up electrical noise
signals which are applied to the noise compensation circuits 4, 5.
In accordance with the present invention, the levels of the noise signals
available at terminal 10' and the program audio signals from the signal
source 1 are compared in respective frequency bands and, in accordance
with a preferred embodiment of the invention, in three separate individual
frequency bands. Three associated control circuits then receive the
differences in level between program audio signals and noise signals. The
control system is so arranged that differences in level between the
program audio signals and the noise signals are maintained in a temporal
average at predetermined levels which can be adjusted and controlled with
respect to the specific channels associated with each frequency band. This
permits associating the audio programming signal with the frequency
spectrum and the level of the disturbance or noise signals, although the
dynamics of the programming signals may also be somewhat reduced.
The respective circuits 4, 5 are shown in detail in FIG. 2. Each one of the
noise compensation circuits 4, 5 includes a three-band equalizer 11 in the
signal path between the program audio signal input and the program audio
signal output. A controlled amplifier, shown only schematically, is
serially connected with the equalizer 11. The noise compensation circuit
further comprises a signal analysis stage 12 and a control stage 13. The
signal analysis stage 12 has three octave bandpass circuits 100, 101, 102,
receiving the output signal from the threeband equalizer through
respective calibrating amplifiers or circuits 110, 111, 112. Similar
bandpass filters 103, 104, 105 are connected to terminal 10', for example
through a suitable amplifier (not shown). The bandpass filters 103-105
likewise pass in three octave bands. Envelope detectors 200, 201, 202 and
203, 204, 205 are, respectively, connected to the outputs of the
respective bandpass circuits 100-102 and 103-105 for the noise signals.
The output signals from the envelope detectors are connected in the
respective frequency band channels to analog dividers 210, 211, 212 for
comparison of the amplitudes of the signals from the respective envelope
detectors.
The control stage 13 receives as its input, for each frequency band, a
threshold circuit 220, 221, 222, in which the output signal from the
analog dividers 210, 211, 212 is compared with predetermined thresholds.
Each threshold corresponds to a different speed or rate of change of the
voltage at the ouputs 240, 241, 242 of integrators 230, 231, 232 connected
to the outputs of the threshold circuits 220, 221, 222. The value of the
voltages can be indicated and shown on a display 250, 251, 252 for each
one of the frequency band channels. The output signals at terminals 240,
241, 242 form a control signal for the control elements of the three-band
equalizer and are connected thereto by a suitable cable B.
The equalizer, which is controlled by d-c control signals, permits separate
control of three frequency bands with separating frequencies at 300 Hz and
2000 Hz. When all the control inputs to the equalizer have zero voltage
signals thereon, the overall damping, independent of frequency, is 20 dB.
Starting with this overall damping, and band-selective, the damping of 20
dB can be decreased so that, when all the control inputs to the equalizer
have maximum voltage thereon, a frequency-independent damping of 0 dB
between the input and output of the equalizer will be in the path of the
program audio signal. A typical equalizer is linear, with a sensitivity of
2 dB per volt, so that a control signal varying between 0 and 10 volts
provides for frequency-dependent transfer of the program signals through
the equalizer in accordance with the respective control voltages supplied
to the control inputs thereof.
The analysis stage likewise has three channels, with separating frequencies
at 300 Hz and 2000 Hz. These separating frequencies, determined by the
bandpass filters 100-102 and 103-105 are the same both for the program
signal as well as for the noise signal.
OPERATION
The program audio signal is transmitted to the analysis stage over three
calibration circuits 110, 111, 112, to be then connected to the respective
bandpass circuits 100, 101, 102. The calibration circuits 110, 111, 112
are controllable in 2 dB steps for damping between 0 to 18 dBd and are
used to set the average level differences. The bandpass circuits 100-102
are octave bandpass circuits with center frequencies of 100 Hz for the
bass channel, 700 Hz for a midrange channel, and 4400 Hz for the high, or
treble channel. The bandpass filters, thus, are octave bandpasses of the
fourth degree. The subsequent envelope detectors 200-202 and 203-205
provide for full-wave rectification; the signals are weighted with signal
rise and signal decay time constants. The signal rise and signal decay
time constants are optimized for the respective demodulators with respect
to frequency and the characteristics of the signal. A suitable
optimization are time constants in accordance with Table 1.
Reference to Table 1, forming part of this specification, shows that the
rise time of the envelope detectors 200-202 and 203-205 for both signals
is short with respect to the decay time; and that the decay periods in the
higher frequency bands are shorter than in the lower frequency bands.
The noise signal applied to terminal 10' is directly connected, possibly
after suitable amplification, to the octave bandpass circuits 103-105. The
time constants of the envelope detectors connected to the bandpass
circuits 103-105 differ from the time constants of the envelope detectors
for the program audio signal, as clearly seen in Table 1. The envelopes
derived from the program audio signal and from the noise signal are then
divided between each other in analog dividers 210-212 in order to evaluate
the level differences between the signals connected to the dividers. In
the dividers, the instantaneous signal amplitude of the program audio
signal is compared with the instantaneous signal amplitude of the envelope
of the noise signal. The respective amplitude relationships then form
control signals V.sub.B for the bass channel, V.sub.M for the mid-range
channel and V.sub.H for the high, or treble channel. Connecting lines
between the dividers of the analysis stage 12 and subsequent circuits in
the control stage 13 in FIG. 2 have been omitted for clarity of the
drawing, with the terminals merely having been equally labelled.
The control stage 13 has three threshold circuits 220, 221, 222,
respectively for the bass, mid-range and high, or treble channels. The
control stage further includes integrators 230, receiving the outputs from
the threshold circuits, and similar integrators 231, 232
channelinterconnect circuits 300, 301, and output displays 250, 251, 252.
Intermediate displays formed by light-emitting diodes LD.sub.B, LD.sub.M
and LD.sub.H are provided. The output control signals S.sub.B, S.sub.M and
S.sub.H are available at respective terminals 240, 241, 242 which are
connected by a multi-line cable B to the control inputs of the three-band
equalizer.
Any one of the threshold circuits 220, 221, 222 (FIG. 2) of the control
stage are shown in detail in FIG. 3. The output voltage from the
respective divider 210, 211, 212, and shown merely as voltage V in FIG. 3,
is compared in a five-stage comparator 260-264 with five threshold levels,
which correspond to level differences between the program audio signal and
the disturbance signal of, respectively, 15 dB, 10 dB, 3 dB, -3 dB and -10
dB. The five individual comparators, typically operational amplifiers,
control respective light-emitting diodes (LEDs) LD0-LD4 for optical
supervision of the operation of the circuit. The output signals of the
five comparators are weighted with factor k.sub.i and added in the adder
265, with the corresponding sign, to be then integrated in integrator 230.
Integrator 230, upon response of any one of the comparators, changes its
output voltage with a speed or at a rate which corresponds to the
respective weighting factor k.sub.i , introduced in the weighting circuits
260', 261', 262', 263', 264'.
Suitable values for factors k.sub.i, in dB per second, which results in the
respective speed of change of damping in the controlled equalizer, is
shown in Table 2, for an optimized circuit.
As can be clearly seen, the speed of change from +10 dB and -10 dB should
be selected to be substantially higher than if other signal conditions
prevail.
The equalizer display 250, fed by the integrator output 230, is a
10-division LED scale and shows the actual damping of the equalizer in ten
steps of 2 dB each between -20 dB to 0 dB damping.
Some disturbance noises have a substantial low frequency component. This
may result in low damping of the bass channel or, in other words,
controlling the equalizer to have minimum damping in the base channel,
whereas the mid-range and high, or treble channels retain their overall 20
dB damping. This results in an undesired, "booming" type of reproduction.
Tone compensation interconnect circuits 300, 301 (FIG. 2) sense
substantial decrease in damping of the equalizer of the respective bass
and mid-range channels and then control the respective mid-range and high,
or treble channels for lesser damping as well. Fixed relationships of
decrease in damping or rise in output amplitude can be set between the
respective B and M channels, for example of from 10 to 3 dB; and between
the M and H channels, for example from 10 to 7.25 dB. This means, for
example, that if the base channel is raised by 20 dB, the mid-range
channel, due to the interconnection, will be opened by 6 dB; and the high,
or treble channel will be opened by about 4.5 dB.
FIG. 4 shows a detail of the circuit of FIGS. 2 and 3. The comparator
circuits 260-264 are shown as operational amplifiers, receiving the input
voltage V through a buffer amplifier 270. A voltage divider chain 280-285
is connected between a reference voltage and ground or chassis and
determines five threshold levels. Resistors 290-294 in the outputs of the
comparators determine the rate of change or speed of change associated
with each threshold level which will appear at the output 240 of
integrator 230 and, thus, control the equalizer adjustment. The first
operational amplifier 260 has a capacitor 290' connected in parallel to
the resistor 290 which, in case of response of operational amplifier 260,
results in rapid and abrupt drop of the equalizer position by 7 dB.
Response of the comparator or operational amplifier 260 occurs when the
program audio signal is 15 dB above the disturbance signal. This rapid
drop of the equalizer adjustment may be compared to an emergency volume
control.
The diode-transistor chain connected in parallel to the integrating
capacitor 230, and forming the integrator, limits the output voltage to a
maximum of 10 volts.
The LED display 250 is driven by a driver circuit JC1, LM3914, which is a
standard component.
The direct input to the operational amplifier 230', forming part of the
integrator 230, is interrupted at terminal X1. The output signal from the
integrator 230, available at terminal 240, is connected to an operational
amplifier forming part of the tone compensation interconnect circuit 300.
The output terminal of the operational amplifier of the tone compensation
or channel interconnection circuit is shown as X2 and is connected to a
terminal X1 of the input to the integrator 231 (FIG. 2) of the mid-range
channel. The output of integrator 231 of the mid-range channel, terminal
241, is then connected to a similar circuit 301 which, in turn, has its
output connected to the input to the integrator 232 (FIG. 2). Terminal
241' is the respective reference signal with which the output signal from
the respective channel is compared to determine if the next higher channel
should be controlled for lesser damping if attenuation of the lower
channel in the equalizer is excessively reduced.
FIG. 4, additionally, shows circuit components and voltage levels suitable
for an actual construction of the circuit and using standard and
commercially available components.
Various changes and modifications may be made within the scope of the
inventive concept.
TABLE 1
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Mid-range
High, or treble
Bass Channel Channel Channel
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Program Audio Signal:
.sup.t rise
1 ms 1 ms 1 ms
.sup.t decay
1500 ms 500 ms 500 ms
Noise Signal:
.sup.t rise
1 ms 1 ms 50 ms
.sup.t decay
500 ms 300 ms 200 ms
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TABLE 2
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Mid-Range High, or Treble
Stage: Bass Channel Channel Channel
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+15 dB 7,0 dB 7,0 dB 7,0 dB
+10 dB 16,0 dB/s 48 dB/s 86 dB/s
+3 dB 2,0 dB/s 6 dB/s 14 dB/s
-3 dB 2,3 dB/s 5 dB/s 9 dB/s
-10 dB 8,5 dB/s 33 dB/s 54 dB/s
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