Control of a loudspeaker output
US008577047B2
(12) United States Patent
(10) Patent N0.:
(45) Date of Patent:
Gautama
(54)
CONTROL OF A LOUDSPEAKER OUTPUT
(75) Inventor:
Temujin Gautama, Boutersem (BE)
Notice:
2012/0179456 A1
Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 376 days.
TourWe
McIntosh et al.
Holman
Mihelich et a1.
12/2010
Magrath et al. .......... .. 381/71.11
7/2012 Ryu et al.
2415 816 A1
2 369 852 A1
10/1975
9/2011
Harma, A. et al. “Volume Control in Networked Audio Systems”,
IEEE Int’l. Workshop on Acoustic Signal Enhancement, IWAENC,
pp. 245-248 (2005).
Extended European Search Report for European Patent Appln. No.
Prior Publication Data
US 2011/0182435 A1
6/2007
6/2007
7/2008
11/2010
OTHER PUBLICATIONS
Jan. 24, 2011
(65)
Nov. 5, 2013
FOREIGN PATENT DOCUMENTS
DE
EP
(21) App1.No.: 13/012,598
(22) Filed:
A1
A1
A1
A1
2010/0310086 A1*
(73) Assignee: NXP B.V., Eindhoven (NL)
(*)
2007/0136050
2007/0140058
2008/0175397
2010/0290643
US 8,577,047 B2
Jul. 28, 2011
10151542.7(Jun. 1,2010).
(30)
Foreign Application Priority Data
* cited by examiner
Jan. 25, 2010
(51)
(EP) ................................... .. 10151542
Primary Examiner * Xu Mei
Assistant Examiner * William A JereZ Lora
Int. Cl.
(2006.01)
(2006.01)
H03G 11/00
H04R 29/00
(52)
US. Cl.
USPC .............. .. 381/55; 381/59; 381/104; 381/106
(58)
Field of Classi?cation Search
USPC ......... .. 381/55, 56, 58, 59, 98, 101, 102, 103,
381/104, 106; 333/14; 455/72
See application ?le for complete search history.
(56)
(57)
ABSTRACT
A loudspeaker drive circuit comprises a signal path compres
sor/limiter (12) for implementing a change to the peak-mean
amplitude ration in the time domain. A feedforWard control
loop measures an acoustic quality of the signal at the output of
a control loop compressor/limiter (20) or the output of the
signal path compressor/limiter (12), and also estimates a
loudspeaker excursion based on the signal at the output of the
References Cited
control loop compressor/limiter (20). The signal path com
U.S. PATENT DOCUMENTS
pressor/limiter is controlled based on the acoustic quality
5,600,718 A
2/1997 Dent et al.
6,201,873 B1*
7,372,966 B2*
8,019,088 B2*
3/2001
5/2008
9/2011
8,194,869 B2
8,340,307 B2
2004/0086140 A1
2005/0031131 A1
measurement and excursion estimation.
The invention provides a method for the maximisation of the
Dal Farra .................... .. 381/100
Bright .... ..
381/55
Holman ........................ .. 381/55
6/2012 Mihelich et al.
12/2012 Barr et al.
audio quality stays acceptable, and that the diaphragm dis
15 Claims, 1 Drawing Sheet
,
?ner
2
istics of a compressor/limiter, With the constraint that the
placement does not exceed a certain threshold.
5/2004 Fedigan et al.
2/ 2005 Browning et al.
impel
acoustic output of a loudspeaker by adjusting the character
“
US. Patent
Nov. 5, 2013
FIG. 1
FIG. 2
US 8,577,047 B2
US 8,577,047 B2
1
2
CONTROL OF A LOUDSPEAKER OUTPUT
to be fed back, and that the loudspeaker protection is instan
CROSS-REFERENCE TO REALTED
APPLICATIONS
taneous, as opposed to having a small delay due to the feed
back netWork.
The aim of this invention is to boost the loudspeaker output
signal, While keeping the loudspeaker output similar (but not
necessarily identical) to the digital input signal. Thus, the
This application claims priority under 35 U.S.C. §119 of
European patent application no. 101515427, ?led on Jan. 25,
2010,the contents of Which are incorporated by reference
invention is based on the recognition that the loudspeaker
protection should take into account audio quality, and not
only provide protection for excessive cone excursion.
According to the invention, there is provided a loudspeaker
herein.
This invention relates to the control of the output of a
loudspeaker.
drive circuit comprising:
a signal path compressor/limiter for implementing a
change to the peak-mean amplitude ratio, in the time
An important cause of loudspeaker failures is a mechanical
defect that arises When the loudspeaker diaphragm is dis
placed beyond a certain limit, Which is usually supplied by the
manufacturer. Going beyond this displacement limit either
damages the loudspeaker immediately, or can considerably
reduce its expected life-time.
domain;
a feedforWard control loop, comprising:
a control loop compressor/limiter corresponding to the
signal path compressor/ limiter;
a unit for computing a quality measure of the signal at
Furthermore, the combination of the use of small loud
the output of the control loop compressor/limiter;
speakers and the demand for high acoustic output, indicates
the need for loudness maximisation methods, but increases
20
the risk of exceeding the diaphragm displacement limit, also
speaker excursion based on the signal at the output of
the control loop compressor/limiter;
a controller for controlling the signal path compressor/
referred to as the ‘cone excursion’ limit.
There exist several methods to limit the displacement of the
diaphragm of a loudspeaker, and they can be divided into
three categories.
a cone excursion prediction unit for estimating a loud
limiter based on the quality measurement and cone
25
(i) The ?rst category consists of methods that process the
excursion estimation.
The invention provides a method for the maximisation of
input signal With variable cut-off ?lters (high-pass or other),
the acoustic output of a loudspeaker by adjusting the charac
the characteristics of Which are controlled via a feedback
teristics of a compressor/limiter, With the constraint that the
audio quality stays acceptable, and that the diaphragm dis
loop. The measured control signal is referred to as the dis
placement predictor.
30
sor/limiter varies the crest factor of the signal (the peak-mean
amplitude ratio) and thereby can increase the loudness With
out increasing the peak amplitude.
(ii) The second category consists of methods that also use
a displacement predictor, but Which feed it back into the input
signal.
(iii) The third category comprises methods that process the
signal With a bank of band-pass ?lters With adjustable gains in
placement does not exceed a certain threshold. The compres
The invention uses an adaptive compressor/limiter to
35
maximise the acoustic output of the loudspeaker and limit the
diaphragm displacement, rather than linear ?ltering. The
such a Way that excess excursion is prevented (by attenuating
only the frequency bands that cause most excursion).
invention is aimed at the maximisation of the acoustic output
This third category of methods uses a feedforWard strategy.
The use of a shelving ?lter has also been proposed in such a
only limitation of the diaphragm displacement.
feedforWard approach (in Us. Pat. No. 7,372,966), Which is
and the limitation of the diaphragm displacement rather than
40
The invention uses a compressor and limiter combination
(for example having control parameters including gain and
controlled via the displacement predictor in a feedforWard
manner, using a model of the loudspeaker to pre-process the
clipping level). The transfer function of this compressor/lim
signal before sending it to the ampli?er/ loudspeaker.
iter is taken into account in the estimation of the cone dis
U.S. Pat. No. 6,201,873 describes a setup for protecting
against excess audio distortion (mainly caused by cone excur
placement, so that this non-linear operation is taken into
account in the feedforWard control loop. The control loop
includes assessment of audio quality, computed on the basis
45
sion), or driving the loudspeaker to its maximum (to maxi
of the distortion caused by digital operations on the digital
signal, and not on the basis solely of excursion. The quality
mum cone excursion). It uses a cone excursion transfer func
tion (or a maximal voltage transfer function) module, the
output of Which is used to control a variable gain on the input.
U.S. Pat. No. 6,201,873 suggests the use ofa transfer function
measure may take into account expected or measured non
50
to give maximal voltage leading to just acceptable distortion,
Which is the maximal voltage that can be applied per fre
quency Without causing excess distortion. The distortion
being considered is that arising from excess cone displace
ment (no distortion in the converter and ampli?er is consid
linear effects that are present in the signal path (such as those
due to the ampli?er or the loudspeaker).
The compressor/ limiter has a non-linear transfer function
that operates in the time domain (Which cannot be imple
mented using only ?ltering). The adaptivity of this function
55
limits the amount of distortion and prevents damage to the
loudspeaker.
ered).
This is another example of a feedforWard system Which
In this Way, the acoustic output of the loudspeaker can be
uses a ?lter Which has a frequency response matched to the
maximised While maintaining acceptable audio quality, and
excursion versus frequency relationship of the loudspeaker.
This also therefore implements a cone displacement predic
60
The signal path compressor/limiter is preferably adapted to
tor. A compressor is controlled to provide the desired amount
of attenuation based on the frequency analysis made by a
apply a variable gain and implement a controllable dynamic
range limiting function. For example, the path compressor/
?lter, such that the gain is controlled in a frequency-depen
dent manner. The control of the gain for a given frequency is
a linear function.
This invention relates to a feedforWard method of process
ing, Which gives the advantage that no measured signal needs
at the same time limiting the diaphragm displacement to a
safe limit.
limiter can comprise a variable gain unit and an amplitude
65
clipping unit. Both can be varied, possibly With different
response times, based on the quality analysis and on the
excursion analysis.
US 8,577,047 B2
4
3
The circuit can further comprise an input ?lter, for example
loudspeaker 16 is connected. The system has a feedforWard
for implementing one or more of:
control loop, Which includes a control loop compressor/lim
removing frequency components corresponding to reso
nance peaks in the acoustic output of the loudspeaker;
iter 20 corresponding to the signal path compressor/limiter
12. By this is meant that the compressor/limiter 20 imple
high pass ?ltering;
boosting loWer frequencies;
ments a non-linear function Which can be used to determine
correcting for the loW ef?ciency With Which the loW fre
may be identical components, but this is not essential. For
the effect of the function of the compressor/limiter 12. They
quencies are reproduced by the loudspeaker.
example, the effect of the signal path compressor/limiter 12
The input ?lter can be controlled by the controller to pro
on the parameters being monitored in the feedforWard path
(audio quality and cone excursion) may be predictable With a
simpli?ed version of the compressor/limiter 12.
The compressor/limiter combines tWo functions,
vide adaptive ?ltering based on the feedforWard control loop.
The unit for computing a quality measure can comprise
means for comparing the signal before the control loop com
pressor/ limiter With the signal after the control loop compres
explained beloW.
sor/limiter. It can also comprise means for comparing the
Compression relates to limiting the dynamic range using a
signal before the signal path compressor/ limiter With the sig
variable gain (this can be a sloW effect), Which can be con
sidered as analogous to an automatic volume control.
nal after the signal path compressor/limiter. It can include a
module to model possible non-linear effects that are present
in the signal path (such as those due to the ampli?er or the
Limiting the dynamic range relates to instantaneous (or
very fast) limiting of the amplitude, e.g., by hard or soft
loudspeaker).
The invention also provides a portable device comprising a
loudspeaker and control circuit of the invention. The portable
device can for example comprise a mobile telephone.
20
The difference betWeen a compressor and a limiter is some
What vague, but relates to With hoW quick and hoW severe the
effect is.
The compressor/limiter thus can comprise a variable gain
The invention also provides a method of processing an
audio input signal to derive a loudspeaker drive signal, com
prising:
25
using a signal path compressor/limiter to implement a
change to the peak-mean amplitude ratio, or crest-factor,
of the audio input signal in the time domain;
Within a feedforWard control loop:
using a control loop compressor/limiter corresponding
to the signal path compressor/limiter;
30
computing a quality measure of the signal at the output
of the control loop compressor/ limiter or of the signal
With reference to the accompanying draWings, in Which:
FIG. 1 shoWs a loudspeaker system of the invention; and
FIG. 2 shoWs a mobile telephone in Which the loudspeaker
system can be implemented.
The invention provides a loudspeaker drive circuit using a
compressor/limiter. A feedforWard control loop has a control
There are tWo criteria that are monitored to adapt the char
acteristics of the compressor/limiter, both of Which are com
puted in a feedforWard manner (i.e., they are derived from the
(i) the audio quality (Q) should be acceptable;
35
signal at the output of the control loop compressor/
limiter; and
controlling the signal path compressor/ limiter based on
the quality measurement and excursion estimation.
Examples of the invention Will noW be described in detail
element (possibly preceded by a linear ?lter), folloWed by a
fast or instantaneous limiter for limiting of the signal ampli
tude. The limiter implements a non-linear operation in the
time domain (such as clipping).
input signal):
path compressor/limiter;
estimating a loudspeaker cone excursion based on the
clipping.
(ii) the loudspeaker diaphragm displacement, or cone
excursion Qi) shouldnot exceed the maximally alloWed level.
The compressor/limiter characteristics (and possibly the
?ltering operation) may be adjusted on the basis of a sloW
adaptation to ensure su?icient audio quality and a user-de
40
45
?ned average cone excursion, in combination With a fast
adaptation to ensure that the maximally alloWable diaphragm
displacement is not exceeded.
The ?ltering operation of ?lter 11 is for removing undes
ired resonance peaks in the acoustic output of the loud
speaker, although this pre-?ltering is not essential. The trans
fer function of a loudspeaker (from input signal to acoustic
loop compressor/limiter corresponding to the signal path
output as a function of frequency) can exhibit one or multiple
compressor/limiter. A quality measure of the signal at the
magnitude peaks due to resonance frequencies of the loud
speaker and/or enclosure. Reducing these resonance peaks
output of the control loop or signal path compressor/limiter,
possibly folloWed by a module for modelling non-linear
50
the loudspeaker, is determined as Well as an estimated loud
speaker excursion. The signal path compressor/limiter is con
to remove frequencies that are reproduced by the loudspeaker
With very loW ef?ciency.
trolled based on the acoustic quality measurement and cone
excursion estimation.
The system of the invention adjusts the characteristics of a
55
due to the loW e?iciency With Which the loW frequencies are
ment.
60
reproduced by the loudspeaker. Indeed, in a typical loud
speaker, the acoustic output for frequencies beloW the loud
speaker resonance frequency are loWer than for frequencies
above resonance. For a closed-box speaker con?guration,
e.g., the acoustic output has a loW-frequency roll-off that
folloWs a second-order high-pass ?lter characteristic for fre
loudspeaker, While maintaining acceptable audio quality, and
Without damaging the loudspeaker.
The system of the invention is shoWn in FIG. 1. The (digi
tal) input signal 10 is ?ltered by ?lter 11 and sent to a com
pressor/limiter 12. This signal (after conversion to the ana
logue domain) is sent to an ampli?er 14 to Which the
The ?ltering operation may include a boost or correction of
the loWer frequencies to compensate for the high-pass char
acteristic of the acoustic output of the loudspeaker, Which is
compressor/limiter, and possibly of the ?ltering operation, on
the basis of the audio quality and the diaphragm displace
The objective of the system can be considered as obtaining
as much sound pressure level (SPL) at the output of the
can ‘?atten’ the frequency response and create headroom that
may be used for boosting the input signal.
The ?ltering operation may also include a high-pass ?lter
effects in the signal path, such as those due to the ampli?er or
65
quencies beloW resonance. This can be corrected doWn to a
user-de?ned loWer frequency limit, at the cost of additional
diaphragm displacement.
US 8,577,047 B2
6
5
The operation that is required for the correction (not lim
ited to the closed-box con?guration), can be formalised as a
signal, such that the control can be performed in a feedfor
Ward manner and Without delay. The computation of the
?ltering operation:
excursion can be an iterative process (as a result of the non
5(1) = 16(1) * (Emmi-MU)
= 16(1) * (1 + Vadd 'haddm),
linearity) and the control loop compressor limiter is used for
this iterative process so that the signal path is not disturbed by
this process.
(1)
(Z)
The control mechanism can combine a sloW adaptation and
a fast adaptation.
The sloW adaptation can increase the level of compression
if the audio quality is above a certain threshold, and decrease
otherwise. The fast adaptation can quickly alter the compres
sion function if the expected cone excursion exceeds the
Where x(t) is the input signal, * denotes convolution,
hcomc?on?) is the impulse response of the ?lter required for
the correction, yadd is a gain controlling the amount of cor
rection. hadd(t) is the impulse response of the ?lter that yields
the ‘correction’ signal, i.e., the signal that is added to the
maximally alloWed limit. Determining the desired compres
sion characteristics to limit the desired cone excursion can
original signal, x(t), to obtain the corrected signal, s(t). The
consist of an iterative procedure, since a compressor/limiter
correction operation can be controlled by means of hadd(t)
and yadd, the characteristics of the correction, e.g., doWn to
implements a non-linear operation. This iterative procedure is
represented by the dashed line 24 from the control module 28
to the compressor/ limiter 20 preceding the excursion predic
Which frequency the correction is performed, and its gain.
The ?ltering operation may implement a combination of
any of the functions outlined above.
The term compressor/limiter is used to refer to any module
20
The control mechanism can optionally control the ?ltering
operation as also shoWn in FIG. 1 (dashed line 26). This may
be to alloW for a larger bandWidth by changing the cut-off
frequency of the high-pass ?lter. In that case, a trade-off
that changes the crest factor of a signal, i.e., the peak-to-mean
amplitude ratio. An example is an ampli?cation folloWed by
a saturating nonlinearity (e.g., a sigmoid function or a hard
limiter). This can result in an increase of signal poWer Without
tor 22.
25
betWeen increased bandWidth and increased level of com
increasing the maximal signal amplitude, Which in turn, can
pression is chosen.
result in an increased sound pressure level at the output of the
The control mechanism can optionally control the loW
frequency correction in such a Way that the bandWidth is
loudspeaker.
The characteristics of the compressor/limiter are adaptive.
The audio quality of the compressed signal, at the output of
the control loop compressor/limiter 20 or of the signal path
compressor/limiter 12 in FIG. 1, can be estimated by com
paring it to the uncompressed signal. Thus, as shoWn in FIG.
1, the uncompressed audio signal s is providing to the quality
monitoring block 18. In FIG. 1, the quality monitoring block
is has as input the output of the control loop compressor/
limiter, but the output of the signal path compressor limiter
extended or decreased by modifying hadd(t).
30
35
The control mechanism can optionally control the gain of
the (?xed) loW-frequency correction, yadd. In that case, it is
possible to limit the loW-frequency correction in a ?rst step,
and, if this is insuf?cient, the level of compression.
The system further comprises a digital to analogue con
verter, Which is not shoWn in FIG. 1, as part of the loudspeaker
drive system. The main processing can be implemented on a
DSP or micro-controller.
can instead be used. Both arrangements are still feedforWard
The source signal at the input 10 can be retrieved from a
approaches in that the compressed signal is used for control
ling the adaptation of the output and not for computing the
memory, or can be input to the DSP or microcontroller via an
40
sound reproduction systems, While protecting the loudspeak
The quality measures can be based on psycho-acoustic
models, or simple distortion-based models. It can include a
module to model possible non-linear effects that are present
in the signal path (such as those due to the ampli?er or the
ers. An important application is in mobile phones, Where
loWer-quality loudspeakers are often employed, but high
45
loudspeaker). The various possibilities Will be knoWn to those
skilled in the art.
An example of a quality measure is the mean-square-error
betWeen the original signal, scaled With the gain expected
from the compressor/limiter, and the compressed signal.
An audio quality threshold should be de?ned, depending
on the loudspeaker. Indeed, loWer-quality loudspeakers may
the art.
The invention claimed is:
becomes unacceptable.
55
can be determined from a model of the loudspeaker, the
parameters of Which can be estimated in an on-line or off-line
manner, or the parameters can be knoWn from design.
signal path compressor/ limiter;
60
output of one of the control loop compressor/limiter
and the signal path compressor/limiter based on an
input signal of the feedforWard control loop;
ule.
For this purpose, the feedforWard path includes the com
an excursion prediction unit for estimating a loud
speaker excursion based on the signal at the output of
pressor/ limiter 20 corresponding to the signal path compres
sor/limiter, in front of the cone excursion predictor unit 22. In
this Way, the in?uence of the compressor/ limiter is taken into
1. A loudspeaker drive circuit comprising:
a signal path compressor/limiter con?gured to change a
peak-mean amplitude ratio in a time domain;
a feedforWard control loop, comprising:
a control loop compressor/limiter corresponding to the
a unit for computing a quality measure of the signal at an
The excursion is predicted for the signal after the compres
sor/limiter using characteristics supplied by the control mod
acoustic output is desired.
FIG. 2 shoWs a mobile phone 30 including the loudspeaker
system 32 of the invention.
Various modi?cations Will be apparent to those skilled in
50
alloW for a higher degree of distortion before the audio quality
The diaphragm displacement, also called cone excursion,
analogue-to-digital converter (ADC).
The invention can be used for maximising the loudness in
output at that time.
65
the control loop compressor/limiter; and
a controller for controlling the signal path compressor/
account in the cone excursion prediction. Thus, the cone
limiter based on the quality measurement and excur
excursion is predicted for the current (compressed) input
sion estimation.
US 8,577,047 B2
8
7
2. A circuit as claimed in claim 1, wherein the signal path
compressor/limiter is adapted to apply a variable gain and
implement a controllable dynamic range limiting function.
3. A circuit as claimed in claim 1, Wherein the signal path
compressor/limiter comprises a variable gain unit and an
12. A method of processing an audio input signal to derive
a loudspeaker drive signal, comprising:
using a signal path compressor/limiter to change a peak
mean amplitude ratio of an audio input signal in a time
5
using a control loop compressor/limiter corresponding
to the signal path compressor/limiter;
4. A circuit as claimed in claim 1, further comprising an
input ?lter.
computing a quality measure of the signal at an output of
one of the control loop compressor/limiter and the
5. A circuit as claimed in claim 4 Wherein the input ?lter
implements at least one of:
signal path compressor/ limiter based on an input sig
nal of the feedforWard control loop;
estimating a loudspeaker excursion based on the signal
at the output of the control loop compressor/limiter;
and
removing frequency components corresponding to at least
one resonance peak in the acoustic output of the loud
speaker;
high pass ?ltering; boosting loWer frequencies; and cor
recting the expected loW ef?ciency With Which loW fre
quencies are reproduced by the loudspeaker.
controlling the signal path compressor/ limiter based on
the quality measurement and excursion estimation.
13. A method as claimed in claim 12, Wherein using the
6. A circuit as claimed in claim 4, Wherein the input ?lter is
signal path compressor/limiter comprises applying a variable
controlled by the controller.
7. A circuit as claimed in claim 1, Wherein the unit for
computing a quality measure comprises means for comparing
gain and implementing a controllable dynamic range limiting
20
compressor/limiter.
25
speaker, and/or
9. A loudspeaker system comprising a circuit as claimed
claim 1 and a loudspeaker.
11. The portable device of claim 10, Wherein the portable
device is a mobile telephone.
remove frequency components corresponding to at least
one resonance peak in the acoustic output of the loud
perform high pass ?ltering, and/ or
boost loWer frequencies, and/or
correct the expected loW ef?ciency With Which loW fre
effects that are present in the signal path.
10. A portable device comprising a loudspeaker system as
claimed in claim 9.
function.
14. A method as claimed in claim 12, further comprising
?ltering the audio input signal before application to the signal
path compressor/ limiter to:
the signal before the control loop or signal path compressor/
limiter With the signal after the control loop or signal path
8. A circuit as claimed in claim 1, Wherein the unit for
computing comprises a module to model the non-linear
domain;
Within a feedforWard control loop:
amplitude clipping unit.
30
quencies are reproduced by the loudspeaker.
15. A method as claimed in claim 14, Wherein the ?ltering
is controlled by the feedforWard control loop.
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