Note: Descriptions are shown in the official language in which they were submitted.
12~79581
AUTOMATIC DYNAMIC EQUALIZING
The present invention relates in general to
dynamic equalizing and more particularly concerns novel
apparatus and techniques for achieving the advantages of
controlling frequency response of a sound amplifying
system as a function of level without introducing unde-
sired boominess in voice reproduction as disclosed in U.
- S. Patent No. 4,490,843, but independently of the volume
control. These advantages may thus be achieved with sys-
tems, such as after-market car stereo systems and powered
loudspeaker systems, such as the commercially available
BOSE ROOMMATE system, having a conventional volume con-
trol before the power amplification stages.
That patent explains how conventional loudness
contour circuits are based upon Fletcher-Munson curves
which relate the frequency response of a human to the
level of the sound being heard. It was discovered that
even though the Fletch-Munson equal-loudness curves
; predict that one should boost the frequencies between 200
and 500 Hz, very satisfactory musical performance was
obtained by limiting the frequencies to be boosted to
those below 150 Hz while avoiding the undesirable effects
of boominess in the reproduction of voices. There was
discovered and disclosed a family of frequency contours
which vary with volume control setting that improve the
perceived low level music reproduction and do not degrade
voice reproduction.
The present invention also represents an
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advance over the Aiwa dynamic super loudness tDSL)
circuit commercially available and described in service
manuals. This DSL circuit comprises an equalizer circuit
that boosts bass and treble, a detector circuit which
judges the strength of the sound level and a control cir-
cuit which suppresses the DSL characteristics when the
sound level is high. The control circuit is the same as
an automatic level control (ALC) circuit used for normal
recording, except that its attack time and recovery time
are extremely short. The DSL circuit output is dif-
ferentially combined with an input signal derived from
the volume control in the main amplifier circuit. When
the signal level is low, the DSL circuit f~eds a strong
signal having bass and treble greatly boosted. However,
when the signal level is high, the DSL circuit output is
suppressed.
It is an important object of this invention to
provide improved automatic loudness compensation.
It has been discovered from observing the
desired dynamic equalization frequency response curves in
FIG. 6 of the aforesaid patent that whenever the midband
gain drops by substantially 10 decibels, the gain in a
narrow band of the low bass frequency range should drop
by only 5 decibels, and this two-to-one ratio extends
over a wide range of gains. Apparatus according to the
invention comprises an input terminal for receiving sig-
nals from a unit with a conventional volume control,
compressing means having its input coupled to said input
terminal for compressing the dynamic range of signals on
the input terminal by substantially two, dynamic equali-
zation bandpass filter means coupled to the output of
said compressing means for providing a dynamically
equalized compressed signal, and combining means for
combining the signal on the input terminal with a dynam-
ically equalized compressed signal for providing an auto-
matically dynamically equalized signal for delivery to
the power amplifier and then the loudspeakers. According
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to another aspect of the invention, there may be input
filter means intercoupling the input terminal and the
compressing means for preventing the compressing means
from changing its gain for signals having spectral com-
ponents that should not alter compressor gain. Forexample, the input filter means may be a high pass filter
allowing the compressing means to vary its gain depending
only on mid and high frequency spectral component levels.
The compression ratio need not be fixed at 2:1, but miyht
be made some other ratio or vary as a function of level
to realize more exactly preferred desired equalization
curves. The attack and decay time of the compressing
means may be altered to mi,limize artifacts. The com-
pressing means may be in a channel common to both left
and right stereo channels to allow dynamic equalization
with a single dynamic equalization channel without
significantly altering stereo effects.
Numerous other features, objects and advantages
of the invention wil become apparent from the following
specification when read in connection with the accom-
panying drawing in which:
FIG. 1 is a block diagram illustrating the
logical arrangement of a system according to the inven-
; tion;
FIG. 2 is a graphical representation of a
family of equalization responses achieved with the system
of FIG. 1;
FIG. 3 is a block diagram illustrating thelogical arrangement of a stereo system according to the
invention in which both stereo channels share a common
automatic dynamic equalization system;
FIG. 4 is a block diagram illustrating the
logical arrangement of a variation of the system of FIG.
1 for controlling attack and decay time;
FIG. 5 is a combined block-schematic circuit
diagram of an exemplary working embodiment of the inven-
tion;
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FIG. 6 is a block diagram illustrating the
logical arrangement of a system having dynamic equaliza-
tion only in a bass channel; and
FIG. 7 is a combined block-schematic circuit
diagram of a monophonic automatic dynamic equalization
channel.
With reference now to the drawing and more
particularly FIG. 1 thereof, there is shown a block
diagram illustrating the logical arrangement of a system
according to the invention. The signal provided by an
audio signal source 11 having a conventional manual
volume control is transduced by loudspeaker 12 energized
by power amplifier 13 with perceived tonal balance cor-
responding substantially to that of the original sound
independently of volume control setting. The output of
audio signal source 11 is coupled directly to one input
of signal combiner 14 and to the input of 2:1 compressor
lS in a side channel. The output of compressor 15 is
coupled to a dynamic equali2ation bandpass filter 16,
typically centered at a low bass frequency corresponding
substantially to the nominal low frequency cutoff of the
system, usually the loudspeaker system nominal low fre-
quency cutoff, typically a frequency within the range of
55-70 Hz. The output of dynamic equalization bandpass
filter 16 is coupled to the other ;nput of signal com-
biner 14. The output of signal combiner 14 is coupled to
the input of power amplifier 13.
The system of FIG. 1 may be regarded as pro-
viding a main forward signal path which carries an
unmodified version of the audio signal provided by signal
source 11 through combiner 14 to power amplifier 13. In
addition there is a side path comprising compressor 15
and dynamic equalization bandpass filter 16 for pro-
cessing the input signal and delivering the processed
signal to the other input of combining means 14 for
adding back to the main forward path signal to create a
desired set of dynamic equalization curves, such as shown
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in FIG. 2. The gain of 2:1 compressor 15 decreases by
one decibel for every two decibels that the signal on its
input decreases. The specific form of dynamic equaliza-
tion bandpass filter 16 may be similar to that disclosed
in the aforesaid patent. However, the manual volume
controls are eliminated from the dynamic equalization
circuitry. Thus, the invention provides the dynamic
equalization function of the aforesaid patent that may be
used with a con~entional audio signal source having a
conventional manual volume control, such as a vehicle
tuner or portable cassette player, compact disk player or
receiver .
Referring to FIG. 3, there is shown a block
diagram illustrating the logical arrangement of a system
according to the invention in which left and right stereo
channels share the side channel comprising 2:1 compressor
15 and dynamic equalization bandpass filter 16. Left and
right input terminals llL and llR receive left and right
audio input signals, respectively, from a stereo audio
signal source and energize one input of left and right
output signal combining means 14L and 14R, respectively,
in the left and right forward through channels,
respectively. Input signal combining means 17 has left
and right inputs connected to left and right input
terminals llL and llR, respectively, to provide a com-
bined input signal in a common side channel that is
delivered to 2:1 compressor 15. The output of dynamic
equalization bandpass filter 16 is delivered to the other
inputs of left and right output signal combining means
14L and 14R, respectively, for delivery to left and right
output terminals 13L and 13~, respectively, connected to
respective left and right power amplifiers and loud-
speakers (not shown in FIG. 3), similar to power ampli-
fier 13 and loudspeaker 12 shown in FIG. 1. If desired a
filter, such as high pass filter 1~ shown as a broken-
lined box in FIG. 3, may be interposed between the output
of input signal combining means 17 and 2:1 compressor 15.
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This filter prevents compressor 15 from changing its gain
in response to spectral components that preferably should
not change gain. For example, a high pass filter 18,
typically having a cutoff frequency at 150 Hz, allows 2:1
compressor 15 to vary its gain only in response to mid
and high frequency spectral components, which control the
gain to provide an overall perceived loudness more
closely resembling the perceived loudness of the original
sound source heard live.
The compression ratio need not be restricted to
2:1, but might be some other ratio to realize more
exactly certain desired equalization curves. Further,
the comprission ratio might be made variable as a func-
tion of input level to fit even more closely desired
equalization curves. The attack and decay time of com-
pressor 15 may ~e altered to minimize artifacts.
Referring to FIG. 4, there is shown an alter-
nate embodiment of the invention having some of these
features. The system of FIG. 1 is modified by having a
controlled compressing system 20 in the side path. This
controlled compressing system includes control path
filter 18', that may correspond substantially to high
pass filter 18, and energizes level sensor 21. Level
sensor 21 includes delay circuitry for delaying the
change in output signal in accordance with a desired
attack time constant and decay time constant to provide a
delayed level signal to nonlinear network 22 having a
nonlinear transfer characteristic that provides a gain
control signal on line 23 to compressor 15'. A typical
suitable attack transient has a time constant of 0.1
second and a suitable decay transient typically has a
time constant of 0.1 second. Nonlinear network 22 may
have its transfer characteristic such as to more closely
approximate the fre~uency response as a function of maxi-
mum volume shown in FIG. 6 of the aforesaid patent.
Referring to FIG. 5, there is shown a schematiccircuit diagram of an actual operating embodiment of the
12~79~8~
invention corresponding substantially to the block dia-
gram of FIG. 3 with specific parameter values set forth.
Summing circuitry 17 comprising operation amplifier 31
sums the input signals to the left and right channels
received on input terminals llL and llR, respectively.
This sum signal is delivered to the 2:1 compressor 18
comprising half of an N~570 compressor chip. The com-
pressed -output is delivered to dynamic equalization
filter 16 comprising operational amplifier 32. Summers
14L and 14R sum the output of dynamic equalization filter
16 with the left and right input signals, respectively,
received on input terminals llL and llR, respectively.
Operationai amplifiers 31-34 may be any general purpose
operational amplifier, such as 1/4 of a TL074 chip.
The present invention is functionally and
structurally different from the Aiwa DSL system. The
latter system uses essentially a conventional ALC circuit
used in tape recorders, except for the shorter attack and
decay transients. Such a circuit keeps a nearly constant
output for a wide range of inputs until the input drops
below a given level. Below that level, input and output
levels track. In contrast, the present invention uses
compressor means having a gain inversely proportional to
the level of the signal on the input terminal, preferably
arranged so that for every 2 dB increase in input level,
there is a 1 dB increase in output level over a wide
range of input levels. The results of this difference is
that in the systems having the DSL circuitry, below a
certain level at which substantially no boost is
imparted, the bass boost imparted is substantially the
same regardless of input signal level whereas in the pre-
sent invention, there is progressively more bass boost as
the input signal level decreases over a wide range of
input signal levels.
I Referring to FIG. 7, there is shown a block
diagram illustrating the logical arrangement of a system
having dynamic equalization in only a bass channel. The
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automatic dynamic equalization circuitry 41 comprises
input summer 17 that combines the left and right stereo
signals received on left input terminal llL and right in-
put terminal llR to provide a combined signal that is
delivered to variable gain amplifier 42, the input of
detector 43 and one input of output summer 44. The out-
put of detector 43 is applied to the gain control input
of variable gain amplifier 42 to control the gain of
amplifier 42 in accordance with the amplitude of the com--
bined input signal. 60 Hz bandpass filter 45 couples theoutput of amplifier 42 to the other input of output
summer 44. Low pass filter 46 couples the dynamically
equal:;zed signal from output summer 44 to bass power
amplifier 47 that drives woofer 48.
Left and right high pass filters 51L and 51R
couple the left and right signals on left and right input
terminals llL and llR, respectively, to left power ampli-
fier 52L and right power amplifier 52R, respectively,
which energize left tweeter 53L and right tweeter 53R,
respectively. This arrangement provides automatic dynam-
ic equalization in the bass channel where needed in the
essentially nondirectional bass channel while maintaining
the separation of the left and right signals at higher
frequencies where a listener perceives directionality, and
dynamic equalization is ordinarily not needed*to preserve
the stereo effect perceived by the listener.
Referring to FIG. 6, there is shown a schematic
circuit diagram of the automatic dynamic equalization
circuitry 41. Resistors 51 and 52 comprise summer 17 and
provide a combined monophonic source signal to amplifier
53. Amplifier 53 and transconductance amplifier 56 with
components resistor 54, capacitor 55, resistor 57 resis-
tor 58, resistor 59 and resistor 60 comprise linear
variable gain stage 42. The gain of variable gain stage
42 is inversely proportional to the D.C. current injected
into pin 5 of amplifier 56.
The combined monophonic source signal is also
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g
applied to level detector amplifier 61 through blocking
capacitor 62'. Amplifier 61 and associated components
including diode 62, resistor 63, capacitor 64, resistor
65, transistor 66, diode 67, 68 and 69, capacitor 70,
resistor 71, transistors 72 and 73 and resistor 74 com-
prise a closed loop precision negative peak detector cir-
cuit with specific attack, hold and release characteris-
tics used to control the current to terminal 5 of
transconductance amplifier 56 and thus the gain of the
linear variable gain stage 42.
A negative rising signal amplitude at the non-
inverting input (+) of amplifier 61 causes amplifier 61
to conduct through series diodes 67, 68 and 69,
Darlington configured transistors 72 and 73 and resistor
74. Negative loop feedback forces the voltage across
resistor 74 to equal that at the noninverting input (-)of
amplifier 61. This action occurs quickly, with a typical
attack time of one millisecond.
After the signal amplitude at the noninverting
input of amplifier 61 drops, holding capacitor 70 main-
tains peak voltage across resistor 74 for about four
seconds until transistor 66 conducts and discharages
capacitor 70 through resistor 63.
Transistor 66 begins to conduct when the
voltage across negatively charged capacitor 64, dis-
charging through resistor 65, becomes positive enough to
forward-bias the bass-emitter junction of transistor 66.
Diodes 68 and 69 provide a 1.2 voltage reverse bias
across the bass-emitter junction of transistor 66 during
initial peak charging of capacitors 64 and 70.
Transistor 73 converts the peak detected
: voltage across resistor 74 to current for controlling the
gain of transconductance amplifier 56. Diode 85 and
resistors 84 and 86~88 provide a minimum negative D.C.
bias of 50 millivolts at the noninverting input of
amplifier 61, which limits the minimum amount of
injection current to amplifier 56.
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Amplifier stage 99 comprises 60 Hz bandpass
filter 45 and is characterized by appropriate high fre-
quency psychoacoustic roll off characteristic. Resistors
100, 101 and 102 and amplifier 103 comprise output summer
44.
Overall operation is such that for peak signal
inputs of a volt or more, the frequency response at the
output of output summer 44 is virtually flat. As the
signal amplitude decreases, there is a gradual and con-
tinuous increase in bass boost. Specifically, for eachtwo dB decrease in signal amplitude, there is a corre-
sponding one dB (approximate) incréase in ]ow frequency
bass response. This relationship is maintaiied for up to
a maximum of 16 dB boost. The ultimate boost is limited
to prevent audible hum. This circuitry is embodied in the
commercially available Zenith Sound by Bose digital tele-
vision systems.
There has been described novel apparatus and
techniques for providing the electroacoustical benefits
of the invention in the aforesaid patent that may be used
without gaining access to the volume control and thus
allow these electroacoustical advantages to be obtained
when energized by conventional audio signal sources
having conventional volume controls. It is evident that
those skilled in the art may now make numerous uses and
modifications of and departures from the specific embodi-
ments described herein without departing from the inven-
tive concepts. Consequently, the invention is to be con-
strued as embracing each and every novel feature and
novel combination of features present in or possessed by
the apparatus and techniques herein disclosed and limited
solely by the spirit and scope of the appended claims.
What is claimed is:
