\FEEDBACK
United States Patent [191
[11]
4,113,983
Steel
[45]
Sep. 12, 1978
[54] INPUT FILTERING APPARATUS FOR
LOUDSPEAKERS
tus. The bass roll-off point of input ?ltering apparatus
for loudspeakers is automatically adjusted to an opti
[75] Inventor:
mum in accordance with the amplitude and spectral
content of the audio signal being received at the time,
Paul Franklin Steel, Luton, England
[73] 'Assignee: Teledyne Acoustic Research,
thereby attenuating components in the signal liable to
cause “bottoming” of the loudspeaker. The input ?lter
Norwood, Mass.
apparatus comprises a ?rst high-pass ?lter having a
[21] App1.No.: 678,461
breakpoint frequency at or near the lower threshold of
[22] Filed:
Apr. 20, 1976
[30]
Foreign Application Priority Data
Apr. 24, 1975 [GB]
the audio frequency range, a second variable band
United Kingdom ............. .. 16999/75
width, high-pass ?lter connected to receive the ?rst
high-pass ?lter output, a feedback circuit responsive to
the second ?lter output to provide a feedback signal as
[51]
Int. Cl.2 ............................................. .. H04B 1/64
a linear function thereof with a gain which is substan
[52]
US. Cl. ............................... .. 179/1 VL; 330/301;
tially inversely proportional to the square of the fre~
179/1 D; 179/1 P
quency, and circuit means including threshold means
[58]
Field of Search ......... .. 179/1 D, 1 F, 1 A, 1 VL;
[56]
333/18, 14; 330/109, 149
References Cited
connected with the second ?lter and responsive to the
U.S. PATENT DOCUMENTS
2,606,970
3,497,621
3,805,091
3,934,190
2/1970
8/1952
said feedback signal to relatively rapidly increase the
breakpoint frequency of the second ?lter in response to
values of the feedback signal which exceed a threshold
and to relatively showly reduce that breakpoint fre
333/18
Scott
Erath
.. 179/1 D
4/1974
Colin
.. 330/109
l/l976
Dolby .................................. .. 333/14
Primary Examiner—Kathleen H. Claffy
Assistant Examiner-—E. S. Kemeny
Attorney, Agent, or Firm—-Larson, Taylor and Hinds
quency in response to values of the feedback signal
which do not exceed the said threshold. Bass equaliza
tion circuit means having a frequency response at the
low end of the audio spectrum which is substantiallythe
inverse of the frequency response of said loudspeaker
means are provided between the output of the second
?lter and the loudspeaker drive circuit.
[57]
ABSTRACT
Improved input ?ltering apparatus for loudspeaker sys
tems, and loudspeaker systems including such appara
8 Claims, 10 Drawing Figures
BASS EQUALIZING
CIRCUIT
CONTROLLABLE
HP 7
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(FILTER CONTROL
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'CIRCUIT
U.S. Patent
Sept. 12,1978 I
CONTROLLABLE
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BASS EQUALIZING
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U.S. Patent
Sept. 12, 1978
Sheet 2 of4
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4,113,983
U.S. Patent
Sept. 12, 1978
Sheet 4 004
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1
4,113,983
2
thereof, described by way of example only with refer
INPUT FILTERING APPARATUS FOR
LOUDSPEAKERS
ence to the accompanying drawings wherein:
FIG. 1 is a block diagram of a loudspeaker system
FIELD'OF THE INVENTION
embodying input ?ltering apparatus in accordance with
the invention;
FIGS. 2, 3, 4 and 5 are simpli?ed diagrams of circuits
This invention relates to loudspeaker systems, and
more particularly to input ?ltering apparatus for loud
speaker systems. I
suitable for implementing the blocks of FIG. 1;
FIG. 6 is a graph illustrating the variation of peak
input voltage with frequency of a particular loud
The response of a loudspeaker is substantially linear
only when the amplitude of movement, that is to say the O speaker operated at the limit of its linear operating
distance of travel, of theloudspeaker diaphragm or
range;
cone is within a certain range characteristic of that
FIG. 7 is a graph illustrating the required frequency
loudspeaker, referred to herein as the linear operating
response of an equalising circuit for the loudspeaker to
range of the loudspeaker. If the loudspeaker is operated
which FIG. 6 relates;
so that the amplitude of movement of the cone or dia
phragm exceeds the linear operating range, distortion of
the output results (known as bottoming) which is sev
erly objectionable to a listener.
It is known that the amplitude of movement of a
loudspeaker diaphragm or cone for a given output is
inversely related to the frequency of the output. Ac
cordingly, “bottoming” is most likely to be caused by
low frequency components of the signal.
In practice, the'incidence of bottoming in the loud
speaker is to some extent reduced by the natural bass
roll-off of the loudspeaker; this has the effect of attenu
FIG. 8 is a block diagram of a practical embodiment
of the invention;
FIG. 9 is a circuit diagram of part of the embodiment
of FIG. 8; and
FIG. 10 is a graph illustrating different frequency
response characteristics of the input stages of the em
bodiment of FIG. 8.
A list of the symbols used in the drawings and in the
following description appears at the end of the descrip
tion.
FIG. 1 is a block diagram of a loudspeaker system
comprising simpli?ed input ?ltering apparatus embody
ating the amplitude of movement of the cone at low .
ing the invention. The system comprises a controllable
frequencies. A further reduction can be achieved by
?lter 11, a bass equalizing circuit 13, feedback circuit 15,
providing a high-pass ?lter in the input channel to the 30 ?lter control circuit 17 and loudspeaker 19, the audio
loudspeaker to attenuate low frequency components of
input signal to the system being provided on line 20.
the input signal to the loudspeaker. However, to pro
The travel of a loudspeaker cone is inversely propor
vide high ?delity reproduction of, for example, music, it
tional to the square of the frequency for a constant
is desirable to avoid introducing permanent attenuation
sound pressure output. The bass equalizing circuit 13 is
of the low audio frequency components. Indeed, it is
arranged so that the output sound pressure from the
often desirable to eliminate the effect of the bass roll-off
loudspeaker 19 is proportional to the input to the equal
of the loudspeaker in attenuating these components, for
izing circuit on line 22, the response of the circuit being
example by providing an equalizing circuit in the input
of the form
channel to the loudspeaker. Provision of such an equal
izing circuit, or other means to extend the bass response
of a loudspeaker system, aggravates the problem of
bottoming.
SUMMARY OF THE INVENTION
The feedback circuit 15 comprises a linear network
with
a frequency response which is inversely propor
It is an object of the present invention to provide 45
tional to the square of the frequency, and thus responds
input ?ltering apparatus for a loudspeaker, the ?ltering
to the signal on line 22 to provide a feedback signal on
characteristics of the apparatus being controlled so as to
line 21 which is an analog of estimated cone travel. The
limit the amplitude of movement of the loudspeaker
gain factor of circuit 15 is selected so that its output
cone or diaphragm to within the linear operating range
of the loudspeaker.
The invention provides input filtering apparatus for a
loudspeaker, the apparatus including a variable band
' width high-pass ?lter and control means therefor to
vary the breakpoint frequency thereof in accordance
with the amplitude and spectral content of the audio
signal being received at the time; the breakpoint fre
50 equals VT when the loudspeaker cone is estimated to
reach its travel limit L. Filter control circuit 17 controls
the breakpoint frequency w,, of the ?lter 11 such that if
the feedback signal on line 21 is greater than V], we
rapidly increases, so reducing the amplitude of low
frequency components of the signal supplied to the
quency is varied so as to attenuate components of the
loudspeaker. If the feedback signal on line 21 is less than
V1, w, is gradually returned to a low value, thereby
appearfrom the following description of embodiments
composite resistive part,
permitting progressively lower frequency components
signal which would otherwise cause the loudspeaker to
of signals to pass through the loudspeaker.
be driven outside its linear operating range. By varying
the breakpoint frequency of the ?lter, as distinct from, 60 FIG. 2 shows a simple con?guration for a controlled
?lter circuit 11 in which the breakpoint frequency w, is
for example, the Q of the ?lter, there is minimal effect
varied by varying the gate voltage VG applied to the
on components of the signal other than those likely to
?eld effect transistor T1. The drain-source resistance
cause the non-linear operation.
RTof transistor T1 is dependant on VG, such that RTis
DESCRIPTION OF THE PREFERRED
65 small for low gate voltages and large for large negative
EMBODIMENT
gate voltages. The combination of resistors R1, R2 and
RTform the resistive part of a CR network. If R Nis the
Further features and advantages of the invention will
4,113,983
Vout
Vin
1
_
4
This is the desired response for the feedback circuit 15
of FIG. 1.
A practical embodiment of the invention will now be
1
1 + SCIRN
described, the embodiment having been speci?cally
tailored to suit speaker type AR7.
The graphs of FIGS. 6 and 7 were derived experi
mentally. FIG. 6 illustrates the variation of peak input
voltage with frequency of a speaker type AR 7 when
operated at the limit of its linear operating range. FIG.
7 illustrates the inverse of the frequency response of the
loudspeaker at the low frequency end of the audio spec
and R Nis variable between the limits:
This provides the quantities A and w0 with the values
1 and l/(C1RN) respectively.
The circuit in FIG. 3 is suitable for controlling the
trum, and thus forms the response characteristic re
?lter of FIG. 2. The transistor T2 acts as a switch. If the
quired for the equalizing circuit 13. From these experi
ments, the following values for the loudspeaker parame
feedback signal voltage goes suf?ciently negative with
respect to earth, the transistor switches on and the volt
age across the capacitor C; rises fast towards earth po
tential. When the travel feedback signal the transistor
switched off, the charge on the capacitor is drained
ters were derived:
Wr = 470 rad s'1
d = 0.46
L = 0.007m
through the resistor R3 causing the voltage to decay
k = 4700 V/m
towards the negative ‘rail. VT in this case is approxi 20 FIG. 8 is a block diagram of the practical embodi
mately 0.7V if T; is a silicon transistor.
ment, those parts of the system which are common to
The required form of response for the bass equalizing
that of FIG. 1 being indicated by corresponding refer
circuit 13 is (a + (b/s) + (c/s2)) and the circuit of FIG.
ence numerals.
‘ ‘
4 provides such a response as can be shown as follows:
Assgme I.C. performs as an ideal operational ampli?er
SO I at
— Vout =
Since low frequencies produce large movement of
25 the cone, rumble in a sound source can be responsible
for large movement, so a high-pass ?lter 23 is incorpoé
rated for a sharp roll off below 30 Hz when combined
with the controlled ?lter 11. The range of variation of
the controlled ?lter breakpoint frequency is from 190 to
30 6200 radians per second. The graph of FIG. 10 shows
the combined response of circuits 23 and 11 for various
R6
if R8 is much larger than Rs,
breakpoint frequencies of the controllable ?lter cover
ing thefull range of variation available. To make the
system compatible with the sound source employed, an
35 attenuator is incorporated with a volume control for the
sound level block 25. A power ampli?er 27 having a
gain of 32 precedes the AR 7 speaker 19.
‘
and if R5 is much larger than R4,
To clarify further the control circuitry, that used for
blocks 25, 23, 17 and 11 is shown in FIG. 9. The circuit
for the controlled ?lter 11 is similar to that of FIG; 2 but
the input impedance of the buffer 29 provides the upper
limit of the combined resistance value RN in the C-R
network. The output of this ?lter is provided at point A
which is connected to the feedback circuit 15 and the
bass equalization circuit 13. The ?lter controller 17 is
If C4 (R5 + R7) and C3 R4 are sufficiently large time
constants, then
similar to that of FIG. 3 but its negative rail has the
1
drain voltage of transistor T4 imposed upon it. To keep
the resistance offered by T4 reasonably linear half the
drain voltage is applied to the DC. level controlling the
50
E
(R, + R,) cum,
1_
+ 81
______.._
1:81am, + R,)c,c,
gate. So the control voltage returned to the gate of T4 is
supplied via a potential divider consisting of two 470K
resistors 31 and 33. This arrangement allows woto decay
at a rate of about 1 octave per second at the end of a
A suitable choice of values for the components will
large input signal.
produce the desired response.
To produce a response inversely proportional to the‘ 55 In a modi?cation (not illustrated) of the embodiment
of FIG. 8, implementation of the bass equalizing ‘circuit
square of the frequency for the‘feedback circuit 15, tWO.
13 and feedback circuit 15 is facilitated and stability is
R-_C networks can be cascaded as shown in FIG. 5. In
th1s case:
Vout
Vin
_
1
(1 + scsRio) (1 + ~"C(>R11)
if C5R|O and C6 Ru are large time constants
Vout
Vin
improved by modifying the response of the input ?lter
23 by the factor (1 + (K/s))2, where the constant K, in
one example, takes the value of 1300, and modifying the
responses of the equalizing circuit 13 and feedback cir
cuit 15 by the inverse of that factor, i.e. (l + (K/s))*2.
It will be appreciated that this modi?cation does not
affect the frequency response of the apparatus as a
65 whole, but the signal on line 22 is not in this case di
rectly proportional to output sound pressure from the
loudspeaker. The responses of the principal blocks in
one example of such a modi?cation are as follows:
4,113,983
6
second ?lter being a variable bandwidth, high-pass
?lter;
s)2
A(l + 1300
Input ?lter corres
pondln to locks
1,23, 5ofFlG.8)} (1+ 2:0 + 3651200 )(1 + (120 )
5
(b) a feedback circuit connected to receive the output
from the series connected ?rst ‘and second ?lters to
provide a feedback signal as a function thereof, the
said function being of the form l/(s + k)2;
- (c) circuit means including threshold means con
Feedback circuit
ifgéifs s°3iii’='fo'°s)
'
S
(1 + 1300 )
nected with said second ?lter and responsive to the
2
said feedback signal to relatively rapidly increase
the breakpoint frequency of said second ?lter in
response to values of the said feedback signal
1O
Bass
equalizing circuit
corres onding to
800 + ""52 + ‘F5000 )
—
locks 3, 27 of FIG. 8)
which exceed a threshold and to relatively slowly
(1 + 5_ )2
1300
reduce the said breakpoint frequency in response to
values of the said feedback signal which do not
exceed the said threshold; and
where A is equal to or less than 20 depending on the
(d) bass equalization circuit means connected to re
setting of the volume control.
The quantities to which the symbols used above and
ceive the output of the series connected ?rst and
second ?lters and having a frequency response
in the drawings relate are as follows:
A = Controlled-Filter Gain at High Frequency
w, = Controlled Filter Breakpoint Frequency
s = Natural Frequency
G = Overall High Frequency Voltage Gain
w, = Resonant Frequency of Loudspeaker in Cabinet
(Radians/sec)
25 the output of the bass equalization means being con
nected for supply to said loudspeaker means; wherein K
d = Damping Factor of Loudspeaker in Cabinet
is a constant, s is the signal frequency, F(s) corresponds
L = Travel limit of the Loudspeaker (Meters)
generally with the frequency response of a high-pass
k = Low Frequency Volts per meter movement of
?lter with a breakpoint frequency at or near the lower
Loudspeaker cone.
I claim:
30 audio threshold frequency, and G(s) corresponds gener
ally with the inverse of the frequency response of the
1. Loudspeaker means in combination with input
loudspeaker at the low end of the audio-frequency spec
?ltering apparatus therefore, said input ?ltering appara
trum.
tus comprising:
3. In a loudspeaker system, input ?ltering apparatus
for ?ltering an input audio signal and supplying the
(a) a ?rst high-pass ?lter connected to receive an
input audio signal and having a breakpoint fre
?ltered signal to loudspeaker means, the apparatus com
quency at or near the lower threshold of the audio
prising:
frequency range;
(b) a second, variable bandwidth, high-pass ?lter
(i) a variable bandwidth high-pass ?lter connected to
?lter the input audio signal and to supply the ?l
tered signal to the loudspeaker means; and
(ii) control means, connected with said variable band
connected to receive the ?rst high-pass ?lter out
Put;
(0) a feedback circuit responsive to the second high
width high-pass ?lter and selectively responsive to
pass ?lter output to provide a feedback signal as a
increases and decreases in the amplitude of low
linear function thereof with a gain which is sub
stantially inversely proportional to the square of
the frequency;
audio-frequency components of the audio signal,
for respectively increasing and decreasing the
45
breakpoint frequency of said ?lter so as to limit the
(d) circuit means including threshold means con
nected with said second high-pass ?lter and respon
sive to the said feedback signal to relatively rapidly
increase the breakpoint frequency of said second
high-pass ?lter in response to values of the said
feedback signal which exceed a threshold and to
relatively slowly reduce the said breakpoint fre
quency in response to values of the said feedback
signal which do not exceed the said threshold; and
(e) bass equalization circuit means connected to re
ceive said second high-pass ?lter output and having
a frequency response at the low end of the audio
spectrum which is substantially the inverse of the
frequency response of said loudspeaker means, the
output of the bass equalization means being con
nected for supply to said loudspeaker means.
2. Loudspeaker means in combination with input
?ltering apparatus therefore, said input ?ltering appara
tus comprising: _
(a) ?rst and second ?lters connected in series to re
ceive an input audio signal, the ?rst ?lter having a
frequency response of (l + (K/S)) 2F(s) and the
amplitude of the low audio-frequency components
of the signal supplied to the loudspeaker means.
4. Apparatus as claimed in claim 3 comprising a fur
ther high-pass ?lter connected in series with said vari
able bandwidth high-pass ?lter and having a breakpoint
frequency near the lower audio threshold frequency.
5. Apparatus as claimed in claim 3 wherein said con
trol means comprises feedback means, responsive to the
output of said variable bandwidth high-pass ?lter, for
providing a feedback signal as a substantially linear
function thereof with a frequency response characteris
tic which decreases with increasing frequency, and
circuit means, responsive to said feedback signal, for
varying the break-point frequency of said ?lter accord
ingly.
6. Apparatus as claimed in claim 5 wherein said feed
back means comprises a feedback circuit, having a fre
quency response characteristic which varies substan
65 tially inversely with the square of the frequency and
connected to receive the output of said variable band
width high-pass ?lter, for providing said feedback sig
nal.
7
4,113,983
8
said feedback signal which do not exceed said thresh
7. Apparatus as claimed in claim 5 wherein said cir
old.
cuit means includes threshold means, connected with
8. Apparatus as claimed in claim 3 further comprising
bass equalization circuit means connected to receive the
said variable bandwidth high-pass ?lter, for relatively
rapidly increasing the said breakpoint frequency thereof
output of said variable bandwidth high-pass ?lter and
in response to values of said feedback signal which
exceed a threshold, and for relatively slowly reducing
the said breakpoint frequency in response to values of
having a frequency response at the low end of the audio
spectrum which decreases with increasing frequency.
8
15
20
25
35
45
50
55
65
t
i
t
i
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