Microphone filter system
US 20130114833A1
(19) United States
(12) Patent Application Publication (10) Pub. No.: US 2013/0114833 A1
Thomas et al.
54
MICROPHONE FILTER SYSTEM
(71)
Applicant: AKG Acoustics GmbH, Wien (AT)
(72)
Inventors: Umbauer Thomas, GablitZ (AT); Satra
(73)
(43) Pub. Date:
May 9, 2013
F orelgn
'
App l'lcatlon
'
P rlorlty
' '
D ata
30
Nov. 4, 2011
(EP) .............................. .. 11 450 137.2
Pubhcatlon Classl?catlon
Wolfgang, Wiener Neustadt (AT);
(51) Int CL
Pavlovlc Gino, W1en (AT)
H04R 3/00
(52) US. Cl.
Assignee; AKG Acoustics GmbH’ Wien (AT)
(200601)
USPC ........................................................ .. 381/122
(21) App1_ NO; 13/665,012
(57)
ABSTRACT
A microphone ?lter system for outputting an audio signal
independent of electrical impedance of downstream devices.
(22)
former that facilitate the outputting of the audio signal.
This system may include a ?lter section and an audio trans
Filed:
Oct. 31, 2012
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MICROPHONE FILTER SYSTEM
BACKGROUND OF THE INVENTION
[0001] 1. Priority Claim
[0002] This application claims the bene?t of priority from
European Patent Application No. 11 450 137.2, ?led Nov. 4,
2011, Which is incorporated by reference.
[0003] 2. Technical Field
[0004]
The invention relates to ?lter systems for micro
phones.
[0005]
3. Related Art
[0006]
In general, a distinction can be made betWeen pas
sive and active microphones, With dynamic microphones
belonging to the passive microphone group and condenser
and electret microphones belonging to the active microphone
group, for example, condenser microphones and electret
microphones, also called electrostatic microphones, may be
used in a recording area and may use a supply voltage that
may be provided by a connected device, such as a mixer or an
effects unit. In condenser microphones, a supply may provide
polarization voltage for electrodes of a microphone capsule
and an operating voltage for an associated microphone ampli
?er. In electret microphones, a supply may provide an oper
ating voltage for the microphone ampli?er, since the polar
iZation voltage may be provided by a charged Te?on coating.
[0007] In contrast, dynamic microphones may not use an
external poWer supply, because such microphones may use
direct conversion of sound vibrations into an electrical volt
age. Because of this direct conversion, dynamic microphones
may be useful for live concerts and on-stage use, for example.
[0008] Nevertheless, With this bene?t, there are tradeoffs.
For example, With dynamic microphones quality of sound
being placed upon illustrating the principles of the invention.
Moreover, in the ?gures, like referenced numerals designate
corresponding parts throughout the different vieWs.
[0013]
FIG. 1 depicts an example block diagram of an
example ?lter system.
[0014] FIG. 2 depicts an example illustration of an example
?lter section.
[0015] FIG. 3 depicts an example Waveform of three differ
ent example frequency ?lter blocks of an example active
?lter.
[0016] FIG. 4 depicts example interaction of example
phase transitions of three example ?lter blocks.
[0017] FIG. 5 depicts an example phase response of an
example resulting composite signal, Which may result from
the example ?lter system illustrated in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
[0018] In various situations, passive microphones, such as
dynamic microphones, may be used over active microphones.
Such situations may include instances When an external
poWer supply is optional.
[0019] Dynamic microphones may be independently con
nected to one or more doWnstream acoustic devices (ampli?er
or recording devices), While some dynamic microphones may
have a built-in passive ?lter. Dynamic microphones With a
passive ?lter may change the sound of the microphone and
adapt the microphone to a particular application ?eld Without
an external poWer supply. For example, a change in an audio
signal can be made through a passive ?lter built into a micro
phone housing. Such a passive ?lter may be designed With
sWitchable resistor-inductor-capacitor (RLC) elements and
output may depend on electrical impedance of doWnstream
devices.
may alloW for small changes in a transfer function or micro
SUMMARY
use, a voltage source for an active ?lter may not be available
[0009] A microphone ?lter system that can control quality
of sound output by outputting an audio signal independent of
electrical impedance of doWnstream devices. To output such
With dynamic microphones. Also, related to this trait, passive
microphones may be limited to providing frequency-depen
dent attenuation Without boost of microphone sound. Also,
operation of passive ?lters may be dependent on electrical
a signal, the system may use a transformer and ?lter section
that includes a signal converter, an active ?lter, a summing
unit, and an ampli?er.
[0010] The active ?lter may include ?lter blocks for modi
fying signal components of a microphone input signal, and
the summing unit may include one or more potentiometers for
further adjusting the modi?ed signal components. These parts
in conjunction With a transformer may modify frequencies or
phase characteristics of the microphone input signal as a
Whole or per signal component. Then, for example, the trans
former may output the audio signal independent of electrical
impedance of doWnstream devices.
[0011] Other systems, methods, features and advantages
Will be, or Will become, apparent to one With skill in the art
upon examination of the folloWing ?gures and detailed
description. It is intended that all such additional systems,
methods, features and advantages be included Within this
description, be Within the scope of the invention, and be
protected by the folloWing claims.
BRIEF DESCRIPTION OF THE DRAWINGS
phone sound.
[0020] Since passive ?lters may be designed for passive
impedance of doWnstream equipment (such as one or more
ampli?ers, mixers, or recording devices). Because of this
dependency, for example, operation of a dynamic micro
phone may result in tWo different ampli?ers providing tWo
different sounds.
[0021] To avoid unWanted and disturbing signal peaks,
electrical passive ?lters may be embedded in a microphone.
Such electrical passive ?lters can be permanently active or
may be activated or deactivated With sWitches. Typical ?lters
may include, for example, a 70 HZ high-pass ?lter, Whereby
loW-frequency impact and handling noises can be suppressed.
For condenser and electret microphones, such ?lters maybe
designed for an active poWer supply, Which may already be
present in such microphones. In contrast, dynamic micro
phones may use passive RLC ?lters Where changes to a fre
quency response may be carried out by RLC absorption or
anti-resonant circuits.
[0022] Passive ?lters may produce passively ?ltered sig
nals that have loWer poWer levels than respective input sig
nals. Also, because there may not be a poWer boost or con
trolled voltage, for example, dynamic microphones may
The system may be better understood With reference
provide an inconsistent output signal. Consistency may be
to the folloWing draWings and description. The components in
the ?gures are not necessarily to scale, emphasis instead
dependent on impedance of a connected device, such as a
mixer and/or an effects unit, and on an actual input source
[0012]
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(such as a microphone capsule). Both source impedance and
input impedance of passive ?lters have an in?uence on
response characteristics of a dynamic microphone. This can
cause microphones With the same presettings to produce dif
ferent sound, depending on connected equipment. To avoid
this inconsistency, equalizers may be used, Which may be
arranged betWeen a dynamic microphone and an ampli?er,
for example.
[0023] To achieve an audio signal independent of electrical
impedance of a doWnstream device, active ?ltering in some
cases may be used. Active ?ltering can be employed by com
ponents in condenser and electret microphones. Alterna
tively, ?ltering may be arranged for a dynamic microphone
that limits the variation in an audio signal that may be caused
as a result of varying impedances of doWnstream devices. For
example one or more ?lters or ?lter sections, Which may
include a signal converter, an active ?lter, a summing unit,
and an ampli?er or pole changer, arranged With an audio
transformer With tWo pairs of coils, may provide such func
tionality. Such functionality may be provided since this cir
cuit may have loW output impedance, regardless of existing
peripherals or the different impedances of individual doWn
stream devices.
[0029] The ?lter section 11 may include a signal converter
2 and an active ?lter 5 (such as a level ?lter). The ?lter section
11 may also include one or more ?lter blocks for one or more
respective frequency ranges, and an ampli?er and/or pole
changer (such as an ampli?er 7). The microphone 1 may
feature a balanced audio output, including an in-phase output
(+) and an out-phase output (—).
[0030] The audio output may be an original input signal la
of the ?lter system and may be transmitted to the audio
transformer 3. The audio transformer may include tWo pairs
of coils 3a and 3b; and the coils may have the same trans
former core. Also, the audio output may be transmitted to the
signal converter 2. The illustrated coil pairs 3a and 3b in this
case may have a shared secondary Winding, and/or, for
example, a continuous secondary Winding can be used.
[0031] The signal converter 2 may convert a symmetrical
signal to an asymmetrical signal and pass it on to the active
?lter 5. The active ?lter 5 may perform desired changes. For
example, the active ?lter 5 may include three ?lter blocks for
three different frequency ranges (such as signal components
511, 5b, and 5c of an asymmetrical signal). The output of the
active ?lter 5 may be passed on to an ampli?er and/or pole
changer such as the ampli?er 7. Also, the output of the active
[0024] The poWer supply voltage used for the active parts of
the ?lter(s), may be provided, for example, by a connected
mixer. Frequency or phase characteristics of an input signal
?lter may be passed on to an input of the audio transformer 3.
In one example, the input of the audio transformer 3 may
may be passed via a ?lter section and added or subtracted With
phantom poWering or a poWer supply via an accumulator, a
battery, or a mains adapter) may be connected to the signal
the original input signal by a transformer, depending on phase
shift of the original input signal.
[0025] The ?lter section may include at least one ?lter
block for a speci?c frequency range. And, the at least one
?ltering block may be operated by touch, rotary, and/or tilting
include a loWer pair of coils 3b. A voltage supply 4 (such as
converter 2, the active ?lter 5, and/ or the ampli?er 7; and may
provide poWer to these components.
[0032]
An output of audio transformer 3 may be a connec
tor, such as a standardiZed XLR connector. The connector
elements external to a microphone’s housing, for example.
may provide, for example, a connection to a mixer 8. The
[0026] Phantom poWering may be used in order to drive an
impedance converter and a doWnstream preampli?er con
facilitate electrical coupling betWeen the mixer and the trans
former 3. Also, a ?ltered output signal 12 may be transmitted
tained in a condenser and/or an electret microphone, as Well
as polariZation in a condenser capsule. In audio engineering,
phantom poWering may represent poWer supply of micro
phones With a DC voltage betWeen 9 and 48 V, for example. In
practice, a supply voltage of 48 V14 V (P 48 phantom poWer)
may be more Widespread. Alternatively, using the ?lter sec
tion, a microphone may be operable When phantom poWering
is lacking.
[0027] With phantom poWering connected, different audio
mixer 8 may be poWered by the poWer supply 4, Which may
via such a connection.
[0033]
Where the mixer 8 is not provided, or a poWer sup
ply for active ?ltering is not provided, for example, the micro
phone 1 can be operated Without ?ltering, such as in a passive
mode. In the passive mode, for example, an input signal la
may be communicated un?ltered via the audio transformer 3
to the mixer 8.
[0034] FIG. 2 depicts an example illustration of an example
?lter section. Such as the ?lter section 11 depicted in FIG. 1.
signal characteristics can be generated by changing a fre
In the ?gure, for example, the input signal la may arrive from
quency response. The ?lter section may have an advantage in
the signal converter 2 to the active ?lter 5. In the active ?lter
that it may be passively operated Without poWer supply and
5, for example, included may be three ?lter blocks for three
different frequency ranges, such as the signal components 511,
Without active in?uence of a frequency response, like a
dynamic microphone. HoWever, in response to the micro
phone being in an active mode, and so being operated With a
poWer supply, the frequency response can be changed. Due
mainly to loW output impedance of the ?lter section, the same
result can alWays be obtained With different connected
devices. These in?uences of the microphone sound can be
differentiated With respect to a quality of a ?lter curve, and a
level and a frequency of an input signal.
[0028] FIG. 1 depicts a block diagram of an example ?lter
system. The ?lter system may be constructed in the form of a
controller. An input signal coming from a microphone 1 may
be applied to an audio transformer 3 and a ?lter section 11.
The audio transformer 3 may be a loW frequency (LF) trans
former. The output signal of the ?lter section 11 may be fed
back to the audio transformer 3.
5b, and 5c of an asymmetrical signal. In such an example, an
increase for the signal component 5a and a decrease for the
signal components 5b and 50 may occur, and such settings
may occur from a doWnstream summing unit 6. The doWn
stream summing unit 6 may include potentiometers, such as
three respective potentiometers for the signal components 511,
5b, and 50. A doWnstream ampli?er and/ or pole changer
(such as ampli?er 7) may combine, amplify, pole change,
and/or attenuate, phase sections, such as combining pro
cessed signal components 5a", 5b", 50" into a signal 9 (as
discussed With respect to FIG. 4).
[0035] FIG. 3 depicts an example Waveform of three differ
ent example frequency ?lter blocks of an example active
?lter. For example, this ?gure depicts phase changes per
formed by the ampli?er and/or pole changer, such as ampli?er
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7. The phase changes in this ?gure are represented by the
signal components 511, 5b, and 5c of the asymmetrical signal
(depicted in the upper roW) and the processed signal compo
nents 5a‘, 5b‘, 5c‘ (depicted in the loWer roW). In this case, the
signal components 511, 5b, and 50 have been changed to the
processed signal components 5a‘, 5b‘, and 50'. Such changes
to the signals, by phase shifting or another signal processing
function, may depend on ?lter settings through potentiom
On the secondary Winding, the signal may be added or sub
tracted With or from, respectively, the original input signal la,
depending on the phasing of the original input signal la.
[0041]
FIG. 5 depicts an example phase response of an
example resulting composite signal, Which may result from
the example ?lter system illustrated in FIG. 1. Also, depicted
is the audio transformer 3 connected as an adder. In a similar
manner, it may be connected as a subtractor. In such an
eters of the summing unit 6. For example, for a frequency
example, Where a pure tone arrives With same phasing at
increase at an output of the ?lter system, a signal may be
inputs of the audio transformer 3, the pure tone may be
emitted ampli?ed at the output. This may be modeled by the
passed Without phase change; While for a frequency decrease
at the output, the signal may be shifted by a predetermined
folloWing formula (1).
number of degrees, such as 180°.
[0036] In one example, there may be respective ?lter blocks
for individual signal components, such as the signal compo
nents 5a, 5b, and 50. These component frequencies may be
adjustable With one or more potentiometers in summing unit
6. Also, they may be adjustable With one or more ?lter blocks,
such as the ?lter blocks used by the active ?lter 5. For
example, the active ?lter 5 may be composed of three ?lter
blocks. For example, the signal component 5a of a corre
sponding ?lter block has a setting of a ?rst frequency (such as
40 HZ). The signal component 5b of a corresponding ?lter
block has a setting of a second frequency (such as 700 HZ).
The signal component 50 of a corresponding ?lter block has
a setting of a third frequency (such as 2700 HZ). These fre
quencies may be selected and/ or adjusted by a control mecha
nism.
[0037] In FIG. 3, in the ?rst column, for the signal compo
nent 5a, a frequency increase occurs. In the second and third
columns, for the signal components 5b and 50, a frequency
decrease occurs. Whether a frequency increase or a frequency
decrease occurs for a signal component 5a, 5b or 50, such an
increase or decrease may be adjustable using a respective
potentiometer in the summing unit 6.
[0038] FIG. 4 depicts example interaction of example
phase transitions of three example ?lter blocks. Speci?cally,
FIG. 4 depicts the phase response of the combined signal 9
from FIGS. 2 and 3, Where single phase sections 5a", 5b" and
50'' result from the signal components 511, 5b, and 5c and the
respective processed signal components 511', 5b‘, and 5c’.
[0039] Active ?ltering, by the active ?lter 5, for example,
may be based on the audio transformer 3, because the micro
phone 1 may be connected to a primary Winding of the audio
transformer 3. In FIGS. 1 and 5, the audio transformer 3
includes tWo pairs of coils 3a and 3b, With tWo primary
Windings and tWo secondary Windings. The secondary Wind
ings may be connected in series and serve as a summer. The
?rst primary Winding of the audio transformer 3 may be
directly connected to the microphone 1 and the second pri
mary Winding to the ?lter section 11.
[0040] Where the poWer supply 4 is not connected, the
active ?lter 5 is deactivated or not functional and an original
input signal la may be transformed directly via the ?rst pair of
coils 3a onto the secondary Winding and played back by an
ampli?er, speaker, or recording device. Where the poWer
supply 4 is connected, the original input signal la may be
passed to the ?lter section 11 and may be processed by the
active ?lter 5. Individual ?lter blocks of the active ?lter 5 may
be constructed for different frequency ranges from active
elements With active electronic elements, such as transistors
and operational ampli?ers. The signal modi?ed by the active
?lter 5 may be fed to the second part of the primary Winding
of the audio transformer 3, and to the second pair of coils 3b.
Where U01” is output voltage of the transformer.
Where Um is input voltage of the transformer.
And, Where Udlfis differential voltage of the transformer.
[0042] Where phasing of one of the inputs is shifted by 0°
(such as Where 0:180°), the pure tone may be attenuated at
the output. This may be modeled by the folloWing formula
(2)
[0043] From the audio transformer 3 the output signal 12 of
the active ?lter system results, Which may include aspects of
the signal 9, the signal components 5a‘, 5b‘, and 5c‘, and the
original input signal 1a.
[0044] The audio transformer 3 may be designed for a
range of output impedance (such as an output impedance of
50-150 Ohms, Where the transmission behavior reaches from
about 10 HZ to 20 kHZ, for example).
[0045] The active ?lter 5 can be any number of ?lter blocks
and can be designed for any number of frequency bands.
Depending on the setting of the individual potentiometers and
the con?guration of the ampli?er and/ or pole changer, as an
adder or a subtractor, either an increase or a decrease in the
individual phase sections 5a", 5b" and 50" or of the output
signal 12 may be obtained.
[0046]
An example bene?t of the microphone 1 With the
audio transformer 3 compared to microphones With a poWer
supply and built-in active ?lters, is a fully balanced retrans
mission of the audio signal to the next stage, such as forWard
ing the output to a mixer. Contrary to past microphones, the
microphone 1 may be usable With the poWer supply 4 discon
nected, and at the same time, a condenser or electret micro
phone. Or an external signal source can also be connected to
the microphone 1 Without unWanted distortions or artifacts in
the outputted sound. In using the condenser and electret
microphone, such devices may be fed With a poWer supply
(such as poWer supply 4). Such feeding of poWer may be
sourced by the ?lter system itself, Which is illustrated by a
poWer supply line 10 shoWn by a dashed line in FIG. 1.
[0047] While various embodiments of the invention have
been described, it Will be apparent to those of ordinary skill in
the art that many more embodiments and implementations are
possible Within the scope of the invention. Accordingly, the
invention is not to be restricted except in light of the attached
claims and their equivalents.
We claim:
1. A system, comprising:
a microphone;
an audio transformer; and
a ?lter section that includes a signal converter, an active
?lter, a summing unit, and an ampli?er,
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the active ?lter including one or more ?lter blocks for one
or more respective signal components,
the summing unit including one or more respective poten
tiometers for the one or more respective signal compo
nents, and
the microphone, the ?lter section, and the audio trans
former being operatively coupled to produce an audio
signal independent of electrical impedance of doWn
stream devices.
2. The system according to claim 1, Where the one or more
Where U01” is output voltage of the transformer,
Where Um is input voltage of the transformer,
and Where Udlfis differential voltage of the transformer.
1 1. The method according to claim 8, Where the active ?lter
is embedded in a housing of the microphone.
12. The method according to claim 8, Where the one or
more ?lter blocks receive operational input from one or more
touch, rotary, or tilting elements attached to the microphone.
13. The method according to claim 8, Where the micro
?lter blocks include one or more transistors or operational
phone is a dynamic microphone.
ampli?ers.
14. The method according to claim 8, Where the active ?lter
receives poWer regulated by a mixer.
3. The system according to claim 1, Where the active ?lter
is embedded in a housing of the microphone.
4. The system according to claim 1, Where the active ?lter
is embedded in a housing external to the microphone.
5. The system according to claim 1, Where the one or more
?lter blocks are operable by touch, rotary, or tilting elements.
6. The system according to claim 1, Where the microphone
is a dynamic microphone.
7. The system according to claim 1, Where the active ?lter
is operable to receive poWer regulated by a mixer.
8. A method comprising:
receiving an input signal at a microphone;
processing frequencies or phase characteristics of the input
signal at a ?lter section communicatively coupled to the
microphone, the ?lter section including a signal con
verter, an active ?lter, a summing unit, and an ampli?er,
the active ?lter including one or more ?lter blocks for
one or more respective signal components of the input
signal, the summing unit including one or more respec
tive potentiometers for the one or more respective signal
components of the input signal;
adding or subtracting the phase characteristics of the input
signal at a transformer; and
outputting a processed audio signal independent of electri
cal impedance of doWnstream devices, in response to the
processing of the frequencies or the phase characteris
tics of the input signal and the adding or the subtracting
of the phase characteristics of the input signal at the
transformer.
9. The method according to claim 8, in response to the input
signal at the transformer including a pure tone, the adding or
the subtracting of the phase characteristics of the input signal
at the transformer further includes adding or subtracting the
phase characteristics according to:
15. A dynamic microphone, comprising:
an audio transformer that includes tWo pairs of coils; and
a ?lter section that includes a signal converter, an active
?lter, a summing unit, and an ampli?er,
the active ?lter including one or more ?lter blocks for one
or more respective signal components,
the summing unit including one or more respective poten
tiometers for the one or more respective signal compo
nents, and
the ?lter section and the audio transformer being opera
tively coupled to produce an audio signal independent of
electrical impedance of doWnstream devices.
1 6. The microphone according to claim 15, Where the audio
transformer includes a ?rst and a second primary Windings
and a ?rst and a second secondary Windings.
17. The microphone according to claim 16, Where the
active ?lter is connected to the ?rst primary Winding, and
Where a microphone input is connected to the second primary
Winding.
18. The microphone according to claim 17, Where the ?rst
and the second secondary Windings are connected in series
and are operable as summer.
19. The microphone according to claim 15, Where:
the microphone is operable to receive an input signal;
the ?lter section is operable to process frequencies or phase
characteristics of the input signal that produce a pro
cessed audio signal;
the transformer is operable to add or subtract the phase
characteristics of the input signal that further enhance
the processed audio signal; and
the microphone is further operable to output the enhanced
processed audio signal independent of electrical imped
ance of doWnstream devices.
20. The microphone according to claim 19, Where:
the signal converter is operable to: convert one or more
Where U01” is output voltage of the transformer,
Where U1.” is input voltage of the transformer, and Where
Udlfis differential voltage of the transformer.
10. The method according to claim 8, in response to the
input signal at the transformer including a shifted tone of 6°,
the adding or the subtracting of the phase characteristics of
the input signal at the transformer further includes adding or
subtracting the phase characteristics according to:
symmetrical aspects of the input signal to one or more
asymmetrical aspects, and pass the one or more asym
metrical aspects to the active ?lter; and
the active ?lter is further operable to process the frequen
cies or the phase characteristics of the input signal that
produce the processed audio signal according to the one
or more asymmetrical aspects.
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