Audio signal energy level detection method and apparatus

Audio signal energy level detection method and apparatus
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USOO5l70437A
United States Patent [19]
[11] Patent Number:
Strahm
[45]
[54] AUDIO S‘IGNAL ENERGY LEVEL
Chris N. Strahm, Portland, Oreg.
[73] Assignee:
Audio Teknology, Inc., Portland,
Oreg.
Dec. 8, 1992
Primary Examiner—Jin F. Ng
DETECTION METHOD AND APPARATUS
[75] Inventor:
Date of Patent:
5,170,437
Assistant Examiner—Nina Tong
Attorney, Agent, or Firm-Kolisch Hartwell Dickinson
McCormack & Heuser
[57]
ABSTRACT
An audio signal energy level detector for use with audio
[21] Appl. No.: 599,295
signal processing is described. The energy level detec
[22]
tor includes a high~pass ?lter series-connected between
an audio input signal and a series-connected recti?er
Filed: 1
Oct‘ 17’ 1990
[51]
Int (15 _ ' _ _ _ _
[52]
Us. (:1. .................................. .. 381/106; 381/107;
_ _ _ ~ _ l ' _ _ _ _ _ " H036 7 m0
°°mr°1 input °f a variable gain device that processes the
381/102; 381/22
audio input signal. A characteristically —- l0 db/decadle
[58] Field of Search ............. .. 381/106, 107, 101, 102,
3810247 17
’
amplitude versus frequency Slope audio input signal 18
high-pass ?ltered, recti?ed and smoothed to produce a
constant-energy, DC gain control signal. When the
[56]
audio input signal is processed in accordance with the
References Cited
Us’ PATENT DOCUMENTS
113313;;
1
invention by the variable gain deviceto produce an
audio output signal sultable for dynam1c, downstream
gligkmer -1
1
and integrator the output of which is connectable to the
processing, the tonal balance and timbre of the audio
a.‘ 0‘ at a" '
input signal are preserved. Passive and active, half
3311:3106‘ 31' """""""" " 381/106
4’455’676 6/1984 Kaneda _ '
4,465,981 3/1984 Pike _
order, ?ve-stage high-pass ?lters are described that
provide the desired +10 db/decade amplitude versus
frequency slope over the audio frequency range.
4,602,218
7/1986
4,922,535
5/1990 Dolby ................................ .. 381/106
Vilmur et a]. .
A (db)
INPUT)
1 Claim, 1 Drawing Sheet
f
O/[\ ?Hz)
IN
12
A(d b)
VARIABLE GAIN
DEVICE
O‘\ W )
OUT _——jOZUTpUT
cm‘m.
/ ,22
1
l
l
gnaw
ENERGY LEVEL DETECTOR
FILT‘ESRS -> RECTlFlER
___
14
l
lNTEGRATOR-l
1
5,170,437
2
the audio inputs signal’s decreasing amplitude at higher
AUDIO SIGNAL ENERGY LEVEL DETECTION
METHOD AND APPARATUS
frequencies, is presented to the variable gain control
device. Illustrative high-pass ?lters include passive and
active, half-order, ?ve-stage, + lOdb/decade circuits.
BACKGROUND AND SUMMARY OF THE
5
BRIEF DESCRIPTION OF THE DRAWINGS
INVENTION
FIG.
1 is a' block diagram of a prior art circuit that
This application relates generally to audio level de
features amplitude level detection.
tection circuits for use in audio signal processing. More
FIG. 2 is a block diagram of the apparatus of the
particularly, it concerns energy level detection method
invention in its preferred embodiment, as used in audio
and apparatus that preserve the tonal balance of an
audio input signal by detecting the spectral. energy of
signal processing.
the audio input signal over its dynamic frequency range
and producing a gain control signal that is generally
equally responsive to the high- and low-frequency com
ponents of the audio input signal.
Prior art audio level detectors ignore the negative
amplitude versus frequency slope that is characteristic
FIG. 3 is a detailed schematic diagram of a passive
embodiment of the high-pass ?lter shown in FIG. 2.
FIG. 4 is a detailed schematic diagram of an active
15
embodiment of the high-pass ?lter shown in FIG. 2.
of audio signals, e.g. speech or musical program mate
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENT
rial. conventionally, an audio signal level detector sim
ply recti?es an audio input signal and integrates, or
appreciated that the block diagrams and circuits are
smooths, the recti?ed output to produce a direct cur
rent (DC) gain control signal-for use by a variable
gain control device-the amplitude of which is directly
proportional over the audio frequency spectrum to the
amplitude of the audio input signal. Because of the char
acteristic nature of an audio input signal, in which lower
frequencies tend to be of higher amplitude and higher
frequencies tend to be of lower amplitude (in accor
dance with the so-called “pink noise” curve), the output
'of the variable gain device that processes the audio
input signal is of constant amplitude, irrespective of the
frequency of the audio input signal. One problem with
variable gain devices so-controlied is a resultant shift of
the tonal balance and a reduction of the timbre of the
Referring collectively to FIGS. 1 through 4, it will be
appended by graphs that show the amplitude versus
frequency characteristics of various signals. The graphs
show (logarithmically scaled) frequency in Hz along
the abscissa and relative amplitude (A) in decibels (db)
along the ordinate. The origins (“O”) of the graphs are
labeled, and represent O-I-Iz and O-db. The graphs illus‘
trate the characteristic slopes of the signal/response
curves associated with various parts of the block dia
grams and circuits and, importantly, demonstrate the
differences between prior art audio signal processing
circuits and the present invention.
Referring ?rst to FIG. 1, a prior art audio signal
processing circuit is shown in block diagram form. It
audio input signal so-processed to produce a gain-con 35 includes an audio input signal connected to an INPUT
terminal 10 connected, in turn, to the input (IN) of a
trolled audio output signal.
It is a principal object of the present invention to
provide an audio signal energy, rather than amplitude,
level detection circuit that preserves the tonal balance
and timbre of the audio signal processed by a variable
gain device controlled by the level detection circuit
It is also an object of the invention to provide such a
VARIABLE GAIN DEVICE 12 the output (OUT) of
which is connected to an OUTPUT terminal 14. Those
skilled in the art will appreciate that VARIABLE
GAIN DEVICE 12 is conventional, and forms no part
of the present invention. It is suf?cient‘to say that
VARIABLE GAIN DEVICE 12 produces an output
circuit with a ?lter having a prede?ned, positive, ampli
(OUT) signal that is a function of its input (IN) signal,
tude versus frequency slope characteristic over the
with the gain, which may be positive or negative, con
audio frequency range.
45 trolled by its control (CNTRL) input. With a positive
control input, VARIABLE GAIN DEVICE 12 would
An important object of the invention is to provide
operate, for example, to expand the audio input signal at
such a ?lter circuit the amplitude versus frequency
INPUT terminal 10. With a negative control input,
slope in decibels per decade (db/decade) of which is
VARIABLE GAIN DEVICE 12 would operate, for
approximately equal in magnitude to that of a character
istic audio input signal and of opposite sign.
50 example, to compress the audio input signal at INPUT
terminal 10. By use of the circuit illustrated diagram
Another important object of the invention is to pro
matically in FIG. 1, along with downstream audio sig
vide such a circuit in a simple, easily manufactured
nal processing circuitry or equipment, a variety of dy
con?guration that contains a minimum of components.
Yet another object is to provide such an energy level
detection circuit that is straightforwardly incorporated
into existing audio signal processing circuits and sys
namic audio effects can be produced, e.g. signal com
pression, expansion, limiting, noise gating, de-essing,
automatic gain control (AGC), etc.
One circuit conventionally used to control a variable
gain device is an AMPLITUDE LEVEL DETEC
TOR 16. AMPLITUDE LEVEL DETECTOR 16
bodiment comprises an audio input signal terminal hav
ing series-connected thereto a high-pass ?lter having an 60 includes a RECTIFIER 18, which produces a DC out
terns.
In brief summary, the invention in its preferred em
approximately +l0db/decade slope substantially over
put signal that is proportional to the amplitude of the
the audio frequency range and a (preferably signal
smoothing) recti?er the output of which is connectable
to the control input of a variable gain device through
which the audio signal may be processed. The bene?cial
alternating audio input signal impressed on INPUT
result of high-pass ?ltering the audio input signal before
terminal 10. Connected in series with RECTIFIER 18
is an INTEGRATOR 20, which integrates, or smooths,
the DC output signal from RECTIFIER 18 to produce
a relatively smooth DC control signal.
I recti?cation (and smoothing) is that a control signal
AMPLITUDE LEVEL DETECTOR 16 is so called
having an amplitude that is generally constant, despite
because (as illustrated by the graph nearest its output
3
5,170,437
is as an audio signal energy level detector for use with
a variable gain device having input and control termi
nals. The energy level detector includes ?lter means, or
shown nearest INPUT terminal 10 in FIG. 2), which
audio input signal may be seen to have the
- lOdb/decade amplitude versus frequency slope that is
HIGH-PASS FILTER 24, responsive to an audio input
signal impressed upon INPUT terminal 10, the ?lter
means having a predetermined response (e. g. that
shown graphically nearest the output of ENERGY
LEVEL DETECTOR 22 in FIG. 2) that, as may be
seen by reference to FIG. 2, attenuates low-frequency
characteristic of audio signals over the audio frequency
range, i.e. from approximately 20-I-Iz to approximately
20,000-Hz. Notably, the output (OUT) signal from
VARIABLE GAIN DEVICE 12 under control of
spectral components while passing high-frequency
spectral components, thereby producing a generally
AMPLITUDE LEVEL DETECTOR 16 has a con
stant amplitude over the audio frequency range (as
shown graphically nearest the output of VARIABLE
GAIN DEVICE 12 in FIG. 2). Thus, with prior art,
amplitude level detectors, the output signal from a vari
able gain device controlled thereby exhibits a shift of
constant output signal in response to a characteristically
low-frequency/high-amplitude, high-frequency/low
amplitude audio signal (e.g. that shown graphically
nearest INPUT signal terminal 10 in FIG. 2). The en
ergy level detector also includes recti?er means, or at
the tonal balance and a reduction of the timbre of an
least RECTIFIER 18, and optionally and preferably
also INTEGRATOR 20, responsive to the generally
audio signal processed therethrough, i.e. the audio out
put signal amplitude is generally constant, irrespective
of large frequency-sensitive differences in the amplitude
of the audio input signal.
20 level output signal of the ?lter means to produce a con
trol signal for connection with a control terminal of
VARIABLE GAIN DEVICE 12, for processing the
audio input signal at the input terminal of VARIABLE
GAIN DEVICE 12, thereby to produce a gain-con
trolled audio output signal at OUTPUT signal‘terminal
14 for further downstream processing.
Those skilled in the art will appreciate that the inven
Turning now to FIG. 2, the invention in its preferred
embodiment is described. It will be understood that
functional blocks of FIG. 2 that are identical with those
of FIG. 1 are identically designated, i.e. INPUT termi
nal l0, VARIABLE GAIN DEVICE 12, OUTPUT
terminal 14, RECTIFIER 18 and INTEGRATOR 20
are conventional. Importantly, the invention in its pre
ferred embodiment includes an audio input signal EN
ERGY LEVEL DETECTOR 22 interposed between
INPUT terminal 10 and the control (CNTRL) input of
tion represents a signi?cant improvement over conven
tional audio input signal level detectors. Conventional
audio signal processing circuits for processing an audio
input signal having a prede?ned amplitude versus fre
quency slope, e.g. the so-called “pink noise” curve,
VARIABLE GAIN DEVICE 12. Audio input signal
include a variable gain control device controlled by a
ENERGY LEVEL DETECTOR 22 includes a HIGH
control signal produced by a series-connected recti?er
an input of which is connected to an audio input signal
terminal and an integrator an input of which is the recti
?ed output of the recti?er. Such a circuit con?guration,
e.g. that shown in FIG. 1, achieves amplitude level
PASS input FILTER 24-i.e. a ?lter having a prede
?ned characteristic of being dynamically more respon
sive to higher audio frequencies than to lower audio
frequencies——that is directly operatively connected to
an audio input signal impressed on INPUT terminal 10
and is upstream from RECTIFIER 18. In response to
detection and suffers an often undesirable shift in the
the impressed audio input signal, HIGH-PASS FIL
TER 24 produces a ?ltered audio signal that is generally
constant over the audio frequency range. ENERGY
LEVEL DETECTOR 22 also includes level detection
means—including RECTIFIER 18, which preferably
45
includes signal smoothing means, or INTEGRATOR,
20, operatively connected to input FILTER 24 and
which is responsive to the ?ltered audio signal—for
detecting the energy level in the ?ltered audio signal to
produce a control signal proportional thereto, thereby
to control an audio signal processor downstream from
such audio input signal.
4
Yet another way of describing the present invention
signal) it produces an output signal over the audio fre
quency range that is directly proportional to the ampli
tude of the audio input signal at INPUT terminal 10 (as
50
tonal balance of the audio input signal. Timbre, which
gives the spoken word and music their richness, is ad
versely affected because of the way conventional, am
plitude level detection is accomplished.
The present invention, which constitutes an improve
ment over conventional audio signal processing cir
cuits, accomplishes energy level detection for purposes
of controlling the conventional variable gain device by
the interposition of HIGH-PASS FILTER 24 between
the audio input signal INPUT terminal 10 and the input
of RECTIFIER 18. Preferably, HIGH-PASS FILTER
24 provides an approximately +l0-db/decade ampli
tude versus frequency slope, which is seen to be of equal
magnitude and opposite sign of the amplitude versus
Another way of describing the present invention is as
an audio signal level detection circuit. The level detec
frequency slope that characterizes the audio input signal
tion circuit includes audio INPUT signal terminal 10, a 55 being processed by such audio signal'processing cir
detector 22 including means for rectifying a signal input
thereto via INPUT terminal 10 and smoothing the recti
?ed output thereof, or RECTIFIER 18 and INTE
GRATOR 20, to produce a smoothed DC output signal
representing the approximate amplitude of the signal
cuits. Thus, the present improvement preserves the
tonal balance and timbre of the audio input signal for
further downstream processing.
The preferred audio signal processing method of the
60 invention may now be understood in view of the above
input thereto. The level detection circuit also includes
description of the preferred embodiment. The preferred
HIGH-PASS FILTER 24 operatively connected be
method includes the steps of l) impressing on an input
tween audio INPUT signal terminal 10 and detector 22.
terminal, e.g. INPUT terminal 10, an audio input signal
The audio signal level detection circuit thereby de
characterized by relatively high-amplitude, low-fre
creases the amplitude of the lower frequency compo 65 quency and low-amplitude, high-frequency components
nents relative to the higher frequency components of
(as shown graphically nearest INPUT terminal 10 in
the audio input signal impressed on INPUT signal ter
FIG. 2); 2) high-pass ?ltering, e.g. via HIGH-PASS
minal 10.
FILTER 24, the audio input signal impressed on the
5
5,170,437
input terminal, thereby producing a ?ltered audio signal
6
the desired+ IOdb/decade slope over the desired audio
having approximately equal-amplitude, low-frequency
frequency range, while providing ampli?cation of the
and high-frequency components; and 3) rectifying, e.g.
high-frequency components as well as attenuation of
via RECTIFIER 18, the ?ltered audio signal to pro
duce a DC control signal directly proportional to the
?ltered audio signal (as shown graphically nearest the
output of ENERGY LEVEL DETECTOR 22 in FIG.
2), the control signal being connectable to a control
input of a variable gain device, e. g. VARIABLE GAIN
the low-frequency components (as shown graphically
on the right side of FIG. 4, wherein ampli?cation/at
tenuation occurs over the audio frequency range ex
tending from fL to M).
Skilled persons will appreciate that, within the spirit
DEVICE 12, for processing the audio input signal,
of the invention, HIGH-PASS FILTER 24 may have
an amplitude versus frequency slope that is only nomi'
thereby to produce on an output terminal, e.g. OUT
nally, rather than precisely, l0db/decade; may operate
PUT terminal 14, for further downstream audio signal
processing a modi?ed audio output signal in such man
over a different frequency range; may utilize different
circuit elements; and/or may be of an different circuit
ner that at least one of (and preferably both of) the
relatively high-amplitude, low-frequency and low
topology than that illustrated herein. Those of skill also
will appreciate that ENERGY LEVEL DETECTOR
amplitude, high-frequency characteristics of the audio
'22 may use an alternative detection scheme and/or
input signal is (are) preserved (as shown graphically
circuit topology, so long as it has a high-pass input ?lter
nearest the output of VARIABLE GAIN DEVICE 12
in FIG. 2). To those skilled in the art it will be under
capable of advantageously modifying the amplitude
stood that the rectifying step preferably includes
versus frequency characteristics of the audio input sig
20 nal, prior to detection or recti?cation. It is this high
smoothing the recti?ed output, e.g. via INTEGRA
TOR 20 (which may be integral with RECTIFIER is)
to produce the control signal that is connectable to the
control input of the variable gain device.
Referring next to FIG. 3, a schematic diagram of 25
pass ?ltration that results in energy level detection
rather than amplitude level detection, and the attendant
preservation of the tonal balance and timbre of the
audio input signal for downstream audio signal process~
mg.
HIGH-PASS FILTER 24 in a preferred embodiment is
Accordingly, although preferred and alternative pre
shown, along with an amplitude versus frequency graph
of its frequency response. HIGH-PASS FILTER 24 in
this embodiment is a passive, half-order, ?ve-stage high
pass ?lter comprising a network of resistors and capaci
tors in the con?guration and with the 32-kQ/O.l6-p.F,
ferred embodiments of the invention, as well as a pre
ferred method of practicing it, have been described, it
should be appreciated that modi?cations may be made
thereto without departing from the scope of the inven
tion as de?ned in the appended claims.
It is claimed and desired to secure by Letters Patent:
1. In an audio signal processing circuit for processing
IO-kQ/QOS-uF, 3.2-kQ/0.016-p.F and l-kQ/QOOSpF
values shown. Those skilled in the art will appreciate
that the series-connected resistor/capacitor combina
tion in each parallel-connected stage provides a fre
quency corner that, in combination with the other
an audio input signal having a prede?ned amplitude
versus frequency slope, the processing circuit including
a variable gain control device controlled by a control
stages including the lOO-kfl resistor-only stage (which
signal, the improvement comprising:
passes a low-level signal of any frequency) provides a
stepped linear approximation of the desired+ IOdb
/decade slope over the audio frequency range from
approximately ZO-Hz to approximately 20,000-I-Iz (as
shown graphically on the right side of FIG. 3, wherein
an energy level detection circuit operable over the
40
is generally constant and proportional to the en
ergy of the audio input signal, said energy level
detection circuit including
?ltering occurs over the audio frequency range extend
ing from f[_ to fH).
Referring ?nally to FIG. 4, a schematic diagram of
audio frequency spectrum for monitoring the audio
input signal and producing a control signal which
a half-order high~pass ?lter characterized by an am
45
HIGH-PASS FILTER 24 in an alternative preferred
embodiment is shown, along with an amplitude versus
frequency graph of its frequency response. HIGH
PASS FILTER 24 in this embodiment is an active,
half-order, ?ve-stage high-pass ?lter comprising the
plitude versus frequency slope that is of substan
tially the same magnitude as that of the audio input
signal and of opposite sign, wherein said ?lter is
characterized by a slope of approximately +10
decibels per decade, and
a series-connected recti?er and integrator, each hav
very same resistor-capacitor network as that of the
ing an input and output, wherein said input of said
recti?er is operatively connected to said half-order
passive ?lter, but, instead of having a 100-11 resistor at
the output, has an ampli?er the gain of which is deter
mined by a IO-kflfeedback resistor. Those skilled in the
art will appreciate that HIGH-PASS FILTER 24 in
?er, and wherein said output from said integrator is
this alternative preferred embodiment similarly pro
used as a control signal to control such variable
vides a frequency corner that, in combination with the
gain control device.
high-pass ?lter and said input of said integrator is
operatively connected to said output of said recti
other stages, provides a stepped linear approximation of
‘I
65
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