U.S. patent number 3,814,856 [Application Number 05/334,947] was granted by the patent office on 1974-06-04 for control apparatus for sound reinforcement systems.
Invention is credited to Daniel E. Dugan.
United States Patent |
3,814,856 |
Dugan |
June 4, 1974 |
CONTROL APPARATUS FOR SOUND REINFORCEMENT SYSTEMS
Abstract
A control apparatus for a sound reinforcement system having
plural audio input program microphones installed in a theater or
like environment. The ambient sound or noise level in the
environment is sensed and is employed to control each microphone
individually so that a microphone is activated only if the audio
material impinging thereon exceeds the ambient noise level. The
audio output of the entire system is continuously and automatically
adjusted so that the gain or amplification to which audio program
signals are subjected is proportional to the number of program
microphones that are active, thereby optimizing the operation of
the system.
Inventors: |
Dugan; Daniel E. (San
Francisco, CA) |
Family
ID: |
23309569 |
Appl.
No.: |
05/334,947 |
Filed: |
February 22, 1973 |
Current U.S.
Class: |
381/57;
381/73.1 |
Current CPC
Class: |
H04R
27/00 (20130101) |
Current International
Class: |
H04R
27/00 (20060101); H04r 027/00 () |
Field of
Search: |
;179/1VC,1VL,1AT,1B,1P,1CN,1H,1A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Olms; Douglas W.
Attorney, Agent or Firm: Caplan; Julian
Claims
What is claimed is:
1. Sound reinforcement apparatus for a system that includes a
plurality of program microphones and at least one loudspeaker and
is adapted to operate in the presence of ambient noise, comprising
in combination:
means for generating a direct current noise reference signal having
a level proportional to the ambient noise level;
a plurality of audio signal processors, each of said processors
being associated with a respective one of said program microphones,
each said processor being adapted to transmit the output of the
associated program microphone only when such output exceeds the
ambient noise level and including
means for generating a direct current program envelope signal
having a level proportional to the level of the associated program
microphone output;
means for comparing said program envelope signal with the noise
reference signal to develop a gating signal only when said program
envelope signal exceeds the noise reference signal by a
predetermined ratio;
an audio signal controller responsive to said gating signal for
transmitting the audio output of the associated program microphone
only when said gating signal is present; and
feedback means for providing hysteresis to maintain said gating
signal so long as the audio output of the associated program
microphone remains within a given increment of said predetermined
ratio; and means for mixing outputs of all said processors and
feeding them to said loud speaker.
2. Apparatus according to claim 1 wherein said output mixing means
includes a summing network having an individual input associated
with and connected to each respective audio signal controller, said
network having a single output, a variable gain amplifier having an
input connected to said single output, and means for varying the
gain of said variable gain amplifier in proportion to the number of
audio signal controllers that are transmitting.
3. Apparatus according to claim 2 wherein said gain varying means
includes means for adding the DC output signals from all said
comparing means in said processors to derive a DC voltage having a
magnitude proportional to the number of said signal controllers
that are transmitting, and means connecting said adding means to
said variable gain amplifier so that said variable gain amplifier
has a gain directly proportional to the quantity .sqroot.n/N in
which n represents the total number of program microphones in the
system and N represents the number of program microphones that are
transmitting.
4. Apparatus according to claim 1 wherein said noise reference
signal generating means comprises a noise microphone disposed in
the same environment as said program microphones and said
loudspeaker, means for rectifying the output of said noise
microphone, and filter means connected to said rectifying means for
generating a DC signal that varies as the envelope of the ambient
noise impinging on said noise microphone, said rectifying means
being connected to said comparing means in all said audio signal
processors.
5. Apparatus according to claim 4 in combination with a logarithmic
amplifier connected in circuit between said noise microphone and
said rectifying means, said logarithmic amplifier being adapted to
amplify low level noise signals by an amount greater than high
level noise signals so as to compress the voltage range over which
the ambient noise signal varies.
6. Apparatus according to claim 1 wherein said direct current
program envelope generating means in each said audio signal
processor comprises a logarithmic amplifier connected to the output
of each said program microphone, said logarithmic amplifier being
adapted to amplify low level program signals by an amount greater
than high level program signals so as to compress the voltage range
over which the audio signal varies, means for rectifying the output
of said logarithmic amplifier, and filter means connected to said
rectifying means for generating a DC signal that varies as the
envelope of the audio program signal, said rectifying means being
connected to said comparing means.
7. Apparatus according to claim 1 in combination with means
connected to said comparing means for affording a visual indication
when said gating signal is developed and means for selectively
adjusting the predetermined ratio between said program envelope
signal and said noise reference signal.
8. Apparatus according to claim 7 wherein said adjusting means
includes a variable attenuator in circuit between said noise
reference signal generating means and said comparing means.
9. Output control apparatus for an audio reinforcement system that
has a plurality of audio program inputs and a single output, said
control apparatus comprising a variable gain amplifier having an
output terminal connected to said single output, an input terminal,
and a control terminal, said variable gain amplifier being adapted
to boost a signal applied at said input terminal by an amount
proportional to the magnitude of a control signal applied to said
control terminal, means for summing all said audio program inputs
into said input terminal, and means connected to said control
terminal for producing said control signal that automatically and
continuously varies in proportion to the number of audio program
inputs that are active so that the gain of said variable gain
amplifier bears an inverse relation to the number of active audio
program inputs.
10. Apparatus according to claim 9 in combination with means for
generating a DC noise reference signal having a level proportional
to the noise level in the audio program environment, means
associated with each of said audio program inputs for generating a
DC program envelope signal having a level proportional to the level
of said audio program input, a plurality of comparators one of
which is associated with each said audio program input and is
adapted to produce a gating signal when the level of the program
envelope signal exceeds the noise reference signal by a
predetermined ratio, a network for adding all said gating signals,
and means connecting said adding means to said variable gain
amplifier so that said variable gain amplifier has a gain directly
proportional to the quantity .sqroot.n/N in which n represents the
total number of audio program inputs in the system and N represents
the number of gating signals present.
Description
This invention relates to the art of sound reinforcement in meeting
halls, churches, legislatures, theaters, concert halls and other
places where sound is reinforced by the use of plural microphones,
amplifiers and loudspeakers. More particularly, it relates to an
improved control apparatus which eliminates some of the
disadvantages of multiple-microphone systems. More particularly
still, this invention relates to a microphone mixer system that can
switch multiple microphones on and off automatically in response to
wanted and unwanted sounds, and also adjust the gain of all the
microphones to achieve the maximum gain possible for the number of
microphones in use at any moment.
Increasing dependence upon sound reinforcement systems in places of
assembly has necessitated the use of greater numbers of microphones
in these systems. As more microphones are used in systems, two
problems characteristic of multiple-microphone systems have become
more prominent. First, such systems are subject to objectionable
pickup of ambient noise and reverberation. For example, if a person
speaks near one microphone, all the other open microphones in the
system pick up ambient noise and the reverberations of both the
speaker's voice and the loudspeakers. Second, the amount of gain
obtainable from any microphone in a system before acoustical
feedback occurs is reduced in proportion to the number of
microphones open, to the point where a system having plural
microphones must have an operator fading microphones up and down in
order to provide any useful amount of gain. In most applications of
reinforcement systems it is not practical to have an operator
present. When reinforcement is provided for a panel discussion, for
example, it is necessary to have a live microphone in front of each
panelist, and the number of open microphones reduces the gain of
the system severely. In many musical and theatrical applications,
the action is too fast for even a skilled operator to follow, so
that even with an operator, several microphones must be open much
of the time. In legislatures and meetings, the necessity for a
sound operator to react to the need for a microphone to be on,
identify the microphone, and turn it up before a person can speak
is often an obstruction to the efficiency of proceedings. Churches
often need to have three or more microphones at different
locations, and maximum gain is needed at each location.
Clearly, there is a need for a sound reinforcement system which
automatically keeps microphone channels attenuated except when they
are needed and which automatically adjusts gain to maximize gain
under differing conditions. The present invention provides such a
system.
A principal object of this invention is to provide a means by which
multiple microphones can be utilized in a sound reinforcement
system without incurring the disadvantages of increased ambient
noise pickup and reduced gain.
A further object of this invention is to provide means by which
sound reinforcement systems having any number of microphones can be
operated automatically with maximum gain available at each
microphone.
A still further object of this invention is to provide a means for
reducing the ambient noise and reverberation pickup of a sound
reinforcement system by automatically and silently gating
microphones on and off in response to the signal-to-noise ratio
existing at each microphone.
A still further object of this invention is to provide a means by
which the gain of a multiple-microphone reinforcement system
utilizing switching of microphones can be maximized automatically
in accordance with the number of active microphones.
Another object of this invention is to provide the above-mentioned
features in a device which is easy to install and operate,
requiring no special wiring and usable with all other standard
sound system components.
Yet another object of the present invention is to provide a sound
reinforcement system for a plurality of microphone inputs which
affords automatic feedback suppression. This object is achieved by
a system made according to the present invention; should
self-oscillation occur, it will be sensed as noise by the system
and the offending microphone or microphones will be shut off
automatically.
The embodiment of this invention that is described in detail
hereinbelow includes a microphone mixer system that has an
individual input channel for each of the microphones in the system.
The mixer system also has an input for a reference or
noise-sampling microphone. There is a channel associated with the
reference or noise sampling microphone input which establishes a
threshold signal that is supplied to each of the program channels.
The apparatus is arranged so that a given program channel will not
transmit until the level of the program material therein exceeds
the noise threshold. For this purpose, each channel has a DC
comparator that compares the envelope of the program material in
that channel with the envelope of the noise level and produces a
gating signal only when the level of the program material exceeds
the level of noise.
Each of the program channel outputs is connected to a summing
amplifier which sums the audio signals from whichever channels are
gated on into a single mixed signal channel which then goes into a
voltage-controlled amplifier for gain adjustment.
To effect the adjustment of gain relative to the number of
microphone channels open, a summing amplifier sums the outputs of
the channel comparators. This results in a voltage which increases
step-wise according to the number of microphones open. This voltage
is processed by a DC function generator to produce an appropriate
control voltage for the voltage-controlled amplifier in the audio
output channel.
The mixer is installed as though it were a conventional mixer,
except for the addition of the reference microphone, which is
positioned to pick up a general balance of sound in the area of the
program microphones. A sensitivity control is provided to adjust
the switching threshold of each program microphone channel. This is
set just below the point at which ambient noise would cause the
microphone to switch on. Each microphone will then automatically
switch on instantaneously whenever it is in the direct sound field
of a sound source, but will remain off to distant or off-mike
sources, as switching is actuated by the difference between the
envelopes of the signal and reference microphone channels. To aid
the operator in adjusting the sensitivity controls, an indicator
light for each channel shows when the channel is in the on
condition. Another special control on the mixer controls the
overall "boost" function and determines the amount of system gain
increase that will automatically occur when less than the maximum
number of microphones is open at any time.
The audience listening to reinforced sound hears a mixture of the
output of the sound system, reverberation and ambient noise. The
same reverberation or ambient noise which causes a microphone to
switch off also masks the perception of that switching by the
audience. If it is desired to use the output of this mixer for
broadcast or recording, it is necessary to mix in some ambient
noise signal.
The automatic functions and operating advantages of this invention
can effect a significant improvement in the performance of a sound
reinforcement system in most applications. Any number of persons
may participate in a panel discussion, with each having the benefit
of the gain of a single-microphone sound system. When several
people talk at once, gain will automatically be reduced just enough
to prevent feedback. When this invention is used with a rock band,
it is possible to have high-gain microphones available for the
amplification of soft acoustical instruments without overloading
the sound system when loud amplified instruments start to play, as
unused microphones will be attenuated automatically. In a
legislative chamber or convention hall, this invention makes it
possible to have any number of microphones available, one for every
person who may need to address the assembly, without the need for a
signalling system to get the attention of a sound operator. The
only wiring necessary for the installation of this type of sound
system is a standard microphone cable for each microphone. When
this invention is used in a church, it is possible to have
microphones with maximum gain available at as many locations as are
needed, with significant relief from the feedback problems usually
encountered when several microphones are used in a highly
reverberant space.
The principles of the invention together with additional specific
objects and features thereof will be fully apprehended from the
following detailed description of an illustrative embodiment and
from the drawing, in which
The FIGURE is a block diagram of a microphone control system
incorporating the present invention.
The system has a plurality of identical program microphone input
channels, of which only two are shown in the drawing for clarity
and are identified by reference numerals 12 and 12'. In this
embodiment one microphone channel 14 serves to develop the
reference or noise signal by which the automatic attenuation
threshold of all the program microphone channels is determined.
Because program microphone input channels 12 and 12' are
substantially identical, a detailed description of only channel 12
will be given, it being understood that corresponding reference
numerals primed designate equivalent elements in channel 12'.
Microphone channel 12 includes a preamplifier 16 to the input of
which a microphone 18 is connected. Preamplifier 16 is of
conventional design and in a typical system is a preamplifier
including feedback gain adjustment and a switchable 150Hz high pass
filter with a slope of 12 dB per octave. The audio output from
preamplifier 16 is connected through a channel gain control
attenuator 20 to the input of an audio signal controller 22. Audio
signal controller 22 includes a gate or control input terminal 24,
and can take the form of a variable attenuator, a variable gain
amplifier, or a gate, its function being to produce an audio output
signal that is proportional to the input at gate terminal 24. The
audio output of signal controller 22 is connected to a mixer and
output controller 26 in which it is combined with other signals,
e.g., that from signal controller 22'.
A direct current signal proportional in level to the program
material picked up by microphone 18 is compared to a corresponding
signal produced by noise channel 14, and for this purpose, program
channel 12 includes, connected to the output of preamplifier 16, an
input high pass filter 28, a logarithmic amplifier 30, a full wave
rectifier 32, and a smoothing filter 34, which is connected to one
input 36a of a comparator 36. Elements 28, 30, 32 and 34 cooperate
to form a signal generator that produces at the input of comparator
36 a fluctuating DC voltage or direct current program envelope that
has a level analogous to the logarithum of the amplitude envelope
of the audio signal generated by microphone 18. Comparator 36 has a
second input 36b, and is arranged to produce at its output a gating
signal only when the level of the signal at input 36a exceeds the
level of the signal at input 36b. Input 36b is the non-inverting
input of the comparator and to it is connected a DC signal having a
magnitude proportional to the noise level in the environment in
which the system is operative.
High-pass filter 28 reduces the effect of low-frequency signals on
the control voltage, since acoustical standing waves at low
frequencies will tend to mask directional information carried by
the mid and high frequencies. It is typically a simple capacitive
rolloff, down 3 dB at 300 Hz and decreasing at a rate of 6 dB
octave. Logarithmic amplifier 30 compresses the voltage range of
the audio signal to simplify subsequent processing. Its output is
full-wave rectified at 32 and averaged by filter 34, which
typically has a time consultant of 7 milliseconds. Comparator 36
can be satisfactorily embodied in a high-gain DC amplifier with
differential inputs and an output that swings between two levels
according to which of the inputs is greater.
For producing a DC signal having a magnitude proportional to the
ambient noise, a noise microphone 38 supplies the input for noise
channel 14. Noise channel 14 is substantially identical to the
portion of program channel 12 by which a DC signal is generated
that has the magnitude proportional to the magnitude of the audio
program signal. More specifically, noise channel 14 includes a
preamplifier 40 that is substantially identical to preamplifier 16,
a high pass filter 42 that is substantially identical to filter 28,
a logarithmic amplifier 44 that is substantially identical to
amplifier 30, a full wave rectifier 46 that is substantially
identical to rectifier 32, and a smoothing filter 48 that is
substantially identical to filter 34. The output of smoothing
filter 48 is connected to a buffer amplifier 50 which supplies to a
conductor 52 a DC signal that varies in magnitude in accordance
with the level of noise sensed by microphone 38. Conductor 52 is
connected to input terminal 36b through a threshold control 54 and
is similarly connected to the input 36b' of comparator 36' through
a threshold control 54'. Conductor 52 is likewise connected to all
other program channels to the end that the audio output of each
microphone 18 is transmitted only if the program material impinging
on the microphone exceeds by an appropriate amount the level of
ambient noise. Threshold control 54 affords adjustment with respect
to each program channel of the level at which comparator 36 will
activate signal controller 22 to transmit the audio material
through the program channel.
Between the output of comparator 36 and input 36b is a resistor 56
which provides positive feedback and thus affords hysteresis.
Hysteresis is essential at this stage due to the inevitable
inconsistencies in acoustically originated signals; in this
context, the presence of hysteresis provides a difference in the
level of the signal supplied to input 36a that will activate
comparator 36 and the level of the signal at input 36a that will
deactivate the comparator. A typical difference in the level to
turn the comparator on and the level to turn the comparator off is
6 dB. It is essential that the amount of hysteresis be the same for
both large and small signals, and this requirement is satisfied in
the present invention because of the effect of logarithmic
amplifiers 30 and 44 in the DC signal generators. The output of
comparator 36 is connected through a non-linear filter 58 to
control terminal 24 of signal controller 22. Filter 58 functions to
condition the output signal of comparator 36 to a proper value to
control whatever specific circuitry is employed in signal
controller 22. Non-linear filter 38 typically includes a series
diode and a parallel capacitor and functions to limit the voltage
swing between one polarity and zero, and to delay the reaction time
of signal controller 22 with respect to the occurrence of a gating
signal from comparator 36. More particularly, it is desirable to
slow the turn-on time slightly, e.g., 5 milliseconds, to prevent
the generation of an audible switching transient, and to slow the
turn-off of signal controller 22 for about 100 milliseconds to fade
out the audio signal gradually rather than abruptly, a more
desirable condition from the listener's standpoint. For providing a
visual indication when comparator 36 applies an "on" signal to
control terminal 24 of signal controller 22, a visual indicator 60
is connected in parallel with the control terminal. Visual
indicator 60 can be a light producing device, such as a light
emitting diode or an incandescent lamp, and assists the operator in
adjusting each threshold control 54, 54' during the installation
and/or service of the system.
Program channel 12 has two outputs. The first is an audio output
from signal controller 22 at conductor 62 which transmits the
program material picked up by microphone 18 when the level of such
signal is at a sufficiently high level to cause signal controller
22 to transmit the audio signal there-through. The second output of
program channel 12 is a DC gating signal at conductor 64 which
swings between two voltage levels, one indicating that the audio
program material is at a lower level than the noise and the other
indicating that the audio program material level exceeds the noise.
The audio signals from each program channel are connected to output
controller 26 where they are summed in a mixing network 66 which
has an input for each of the respective conductors 62, 62', etc. of
the program channels 12, 12', etc. The output of mixing or summing
network 66 is connected through an amplifier 68 to the input of a
variable gain amplifier 70. Variable gain amplifier 70 includes a
control terminal 72; the amplifier is arranged to amplify the
signals supplied thereto by an amount proportional to the magnitude
of a control signal supplied to control terminal 72. The audio
signal is thence connected through a final amplifier 74 to a
loudspeaker system schematically identified at 76.
The gain or variable gain amplifier 70 is controlled in accordance
with the number of microphones 18 that are transmitting audio
signals to network 66 so as to prevent self oscillation or
feed-back from occurring. For generating a signal for application
to control terminal 72 to achieve this mode of operation an output
control signal generator 78 is provided. Conductors 64, 64', etc.
are connected to an input summing resistor network 80 which
constitutes the input of generator 78. The summed signal is
amplified by amplifier 82 which drives a DC function generator 84,
a conventional curve shaping circuit. The input to DC function
generator 84 is a DC signal that has a discrete level that is
dependent upon the number of program microphones 18 that are made
active through the respective program channels 12 so that the
output of amplifier 82 is analogous to the number of program input
channels active at any time. The function generator is configured
to produce an output appropriate for driving the variable gain
amplifier 70 in whatever form it is actually embodied. Amplifier 70
and function generator 84 are adapted so that the overall
electro-acoustical system power gain is constant irrespective of
the number of program channels that are active. The desired
condition is expressed by the formula:
E.sub.out = E.sub.in .sqroot.n/N
or
gain = E.sub.out /E.sub.in = .sqroot.n/N
in which E.sub.out is the output voltage at the output of amplifier
70, E.sub.in is the input voltage to amplifier 70, n is the number
of microphones in the entire system and N is the number of
microphones or program channels that are active at a given time.
Function generator 84 includes a boost control, identified
schematically at 86, which is arranged when set at a minimum to
assure that the gain of amplifier 70 remains constant at a minimum
value without being affected by the number of channels that are
active. When boost control 86 is at a maximum, the gain adjusting
function afforded by function generator 84 is fully effective.
Boost control 86 is adjusted when the system is installed and when
the nature of the program material supplied to the system is
changed in order that feedback is avoided while maximum overall
gain is achieved.
One system designed according to the present invention has six
microphones 18 and six associated program channels 12. In such
system the coaction of function generator 84 and variable gain
amplifier 70 can be appreciated from the following table:
Number of Micro- Boost in dB by Gain of phones 18 Activated
Amplifier 70 Amplifier 70 1 + 7.8 2.45 2 + 4.8 1.73 3 + 3.0 1.41 4
+ 1.8 1.23 5 + 0.8 1.10 6 + 0.0 1.00
The operation of the present invention is as follows: The
microphones 18 are installed at appropriate locations in the area
wherein sound reinforcement is desired, and conventional cables are
employed to connect the microphones to the apparatus that houses
program channels 12, 12', etc., noise channel 14, controller 26,
and signal generator 78. Noise microphone 38 is similarly connected
to the apparatus. Noise microphone 38 is located at a suitable site
so that the noise picked up thereby is typical or exemplary of the
background noise impinging on the program microphones 18. Each
individual channel is then adjusted by manipulating threshold
control 54 so that visual indicator 60 turns on when the desired
program material is picked up by microphone 18 and turns off at all
other times. This adjustment is repeated for each channel, e.g.,
12'. After all the individual channels are adjusted, boost control
86 is adjusted so that the overall system gain will be such as to
avoid self-oscillation or feedback irrespective of the number of
program channels that are active.
With the system adjusted as set forth above, and in the absence of
any program input to microphones 18, 18', etc., all comparators 36,
36', etc. will be off because the ambient noise picked up by
microphone 38 will cause input 36b of the comparator to exceed that
at input 36a. This condition of comparator 36 conditions signal
controller 22 to block transmission of signal to conductor 62. When
an audio signal, e.g., voice, is directed toward a microphone, for
example, microphone 18, the DC signal which is developed by the
generator composed of elements 28, 30, 32, and 34 imposes a signal
on input 36a of the comparator that is greater than the noise level
at input 36b, whereupon the comparator will produce a gate signal
which is applied to control terminal 24 so as to cause signal
controller 22 to transmit the audio signal to conductor 62 and
output controller 26. The amount of amplification or gain to which
the audio signal is subjected is determined by the signal applied
to control terminal 72 of variable gain amplifier 70 which in turn
is determined by the number of program channels that are active at
a given time. Thus, the output of amplifier 82 is proportional to
the number of active program channels and this voltage level is
converted by function generator 84 to a signal that is of suitable
magnitude and polarity to effect the desired degree of control of
variable gain amplifier 70. For example, if one program channel is
active, the gain of amplifier 70 will be relatively high and the
material transmitted by such channel will be clearly heard by the
audience. If, on the other hand, several program channels are
active the audience will hear approximately the same overall power,
but each individual voice, instrument, or like source, will be at a
somewhat lower level. Accordingly, irrespective of the number of
microphone channels that are active, there will be insufficient
output power to cause feedback or self oscillation of the
system.
Should self oscillation or feedback occur, the resultant noise
produced thereby will impinge equally on noise microphone 38 and
program microphones 18, 18', etc. Consequently, each comparator 36
will receive a noise input at terminal 36b that causes the
comparator to turn off the associated signal controller 22. The
system thus affords self suppression of feedback because
loudspeaker 76, noise microphone 38 and program microphones 18,
18', etc. reside in a common environment.
It is to be understood that the embodiment described herein is
merely illustrative of the principles of the invention. For certain
applications it will be advantageous to embody the invention in a
form in which a noise microphone is provided for the control of
each program microphone, and in others it will be possible to
derive the ambient noise reference signal from the sum of all
program microphone signals. Various modifications may be effected
by persons skilled in the art without departing from the spirit of
the invention.
* * * * *