ME 224 Final Project
AUDIO CONTROL
DECEMBER 2001
ME 224
PROFESSOR ESPINOSA
NORTHWESTERN UNIVERSITY
RACHEL FINE
GARETH HAYES
JOSHUA HIGGINS
JEHANA RAY
TABLE OF CONTENTS
TABLE OF CONTENTS .................................................................................................... 1
INTRODUCTION .............................................................................................................. 2
BACKGROUND INFORMATION.......................................................................... 2
EXPERIMENTAL PROCEDURE ........................................................................... 7
TESTING RESULTS AND ANALYSIS ......................................................................... 12
SUMMARY ...................................................................................................................... 16
BIOGRAPHIES ................................................................................................................ 17
REFERENCES ................................................................................................................. 20
APPENDIX A: BILL OF MATERIALS .......................................................................... 21
1
INTRODUCTION
This project hopes to explore the mechanisms used to manipulate sound waves.
Specifically we will show how a limiting circuit can be used to create a sort of
“Smart Sound” system commonly found in televisions. This technology can be
achieved through fairly basic circuitry. The results can be very useful in
amplifying quieter, or lower amplitude, outputs and minimizing louder, or higher
amplitude, outputs. Both types of outputs are corrected by the circuitry to fit
within a set voltage range set by the user. In this lab we will use LabView to
control the voltage range and also to gather the necessary data to show how our
circuit is working. We will show that regardless of how high or low the input
frequency is, the output voltage will not go beyond a certain range specified by
the user. We will also test the upper and lower ranges of the limiter to see if
performance is affected at these extremes. We will show that a limiter can be
utilized to control output and test the effectiveness of our limiter in this capacity.
BACKGROUND INFORMATION
The “Smart Sound” system we have based our project upon is a fairly common
and widely used application, especially in televisions. The desire to limit the
volume of loud television shows and commercials resulted in the creation of this
technology. It is made possible by the fairly basic premise of limiting circuitry.
This is a simplified explanation of the “Smart Sound” system but is valid.
Limiting circuit:
Limiting circuits can be very simple, as shown in Figure 1,
or fairly complex, as shown in Figure 2, depending on the needs called for by the
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user. Due to the needs for audio limiting we required we used the circuit shown
in Figure 2. Both circuits do show the basic properties of a limiting circuit,
however.
Figure 1
The circuit above limits the output voltage through the use of a pair of diodes,
placed in parallel and in reverse polarity, and a resister in series with these. The
nature of the diodes results in the output voltage never getting higher than ±0.7V.
In many cases this sort of circuit is used to protect amplification circuits. In the
case of our lab, we are using the more complex circuit of Figure 2 to actually limit
the output voltage based on an input from a microphone circuit.
The result of
the more complex circuit is similar to that of the simple circuit shown in Figure 1
in that both limit the output voltage over a certain range. A major difference is
that while the output voltage of the first circuit is set by the diodes and cannot be
changed, the range allowed by the second circuit can be manipulated. This allows
the limiting circuit we have utilized to incorporate a LabView initiated volume
knob to control the amplitude of the output voltage
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Figure 2
The circuit uses an FET to control the voltage and two TL072 op-amps. We
replaced these op-amps with a single TL074 op-amp as it allows for virtually the
same results. R14 and C5 determine the attack time, about 5ms and R12 and C5
determine the recovery time, approximately 1 second. R11, C3 C4 and R13 form
the distortion cancelling circuit that is necessary to avoid compromise of the
circuit. Ideally the system should be used above 500 Hz, as the distortion rises
with decreasing frequency. This circuit does limit distortion very well within the
threshold voltage. The threshold voltage is the variable limit to which the voltage
is to be limited to. All of these values work well with audio signals but we were
at liberty to change them to see how each helped to control the system. Some
approximate specifications of the limiting circuit we used are shown in Figure 3
4
Limiter Specifications
Maximum Attenuation
Noise Level (unweighted)
Typical Max. Output Level
Gain
FET Voltage (at max. o/p)
Distortion
40dB
-80dB (ref. 1.65V RMS output)
1.65V RMS
6.8 (16dB)
< 45mV typical)
< 0.5% typical
Figure 3
This circuit that we utilized is a comparably cheap model of what is used in more
complex systems. It allows us to show the basic results found in the more
expensive and elaborate systems in an easy to visualize and manipulate circuit.
We decided to focus on the limiting circuit as opposed to other methods for
electronic control we investigated because we felt it most closely resembled the
results that are gained in an actual commercial level “Smart Sound” systems.
While other methods, such as a band-pass filtering systems, clipping circuits,
compression devices, and simple RC or RLC filters, would have also given us
insight into the workings of electronic control systems, we felt the limting circuit
would give us more to work with than these other alternatives.
Microphone:
We also built a condenser microphone onto our bread
board. Two views of the microphone, both the actual microphone component and
the correlating circuit that we used to utilize it are shown in Figure 4 & 5. The
condenser microphone works by the movement of a diaphragm in relation to a
rigid backplate. This movement creates a change in an electric field between the
two. This, in turn affects the current that goes to an amplifier and thus, the output
is amplified or attentuated. We are using the microphone to pick up an input
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signal, anything from a human speaking voice to a tap onto the actual microphone
and then to translate it to our limiting circuit.
Figure 4
Figure 5
This particular microphone creates a 2.2 KΩ impedence. This allows for a
lowering of the necessary voltage bias. The fairly simple microphone circuit also
utilizes an FET.
6
EXPERIMENTAL PROCEDURE
The experimental apparatus for this experiment is quite simple. The main step in
setting up this experiment is building the circuits, shown in Figures 2 & 4, on a
breadboard. Once the circuit has been built, it needs to be connected to the DAQ
so that the computer can control the experiment and compile the results in a more
convenient manner. An oscilloscope need also be attached to the outputs so that
the results can be viewed in real time. The final piece of equipment needed is a
speaker that will produce sound into the microphone. The easiest way to provide
a controlled sound is to output audio files through the computer’s speakers, which
must be placed in close proximity to the microphone to eliminate outside noise.
Building the Circuit:
The microphone circuit (Figure 4) as well as the
limiter (Figure 2) should be built on the breadboard and the outputs connected to
the oscilloscope. The breadboard with the complete circuit can be seen in Figure
6 below.
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Figure 6
LabVIEW Construction:
There are two main purposes of the LabVIEW
program:
1. Control the system by producing exact, user-defined voltage inputs to the circuit
2. Compile the resultant data in a clear, convenient fashion.
To execute purpose (1), the program must utilize the DAC stg function to output a
value through the DAQ and into the circuit. Then a knob, designated as a control,
should be placed in the Front Panel of LabVIEW to allow the user to select a
desired voltage by “turning” the knob.
To execute purpose (2), the program must first go through the above step. Thus, a
sequence function with two frames should be used to ensure that the voltage input
occurs before the output is generated. In the second step of the sequence, an ADC
stg function is employed to retrieve data from the DAQ’s input port. This data is
then outputted to a Waveform Graph for real-time viewing of the results, as well
as to a text file for later manipulation.
An example program can be seen in Figure 7 on the next page.
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9
a)
b)
c)
Figure 7
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Connecting the DAQ:
The DAQ should be connected such that its input
voltage comes from Channel 2 (if using the LabVIEW function in Figure 7 on the
output side of the DAQ, and the resultant output comes from Channel 0 on the
input side of the DAQ. The input and output sides are shown in Figure 8 below.
Ground
Output from circuit
Ground
Input voltage
Figure 8
Running the Experiment:
Once the experiment is set up, performing
the actual experiment is quite simple. The LabVIEW program is run
continuously, and the user controls the voltage input using the knob on the Front
Panel. The resultant output can be seen in real time on the oscilloscope as well as
the Waveform chart. The results can also be viewed in spreadsheet format and
the data can be manipulated as chosen. The important factor to observe is the
amplitude. We will manipulate the system to not only see how the limiter works
with ideal inputs, but also with very high and very low values of input.
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TESTING RESULTS AND ANALYSIS
As described in the previous sections, the purpose of this device is to actively
limit an audio signal within a specified amplitude based on a set input voltage.
Software interfaces through the LabVIEW program were used to provide an
interface with the electronics in an effort to illustrate how in-software signal
control can be accomplished. This general objective has been accomplished
through the course of the experiment, and the circuit - as was originally intended limits signal output actively based on the manipulation of a graphical volume
knob.
Throughout the experiment, we ran into a number of hurdles due to the fact that
we had substituted a few unavailable components with similar model parts. The
main concern occurred during initial conditioning of the circuit prior to LabVIEW
implementation. The fact was that the part substitution had produced a
considerably larger amplification of the initial signal than expected. This and
other less significant problems were accounted for, and both the microphone
conditioning circuit and the audio limiter were found to be in operating condition.
The only process matter that remained was to facilitate an active control of the
circuit. In initial testing, it was revealed that a control voltage range of
approximately zero to 2V at a current on the range of 1.5A was enough to make
the circuit transition from its maximum amplification to a behavior where the
circuit was overloaded. The overload was such that the signal reached a point of
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producing near constant voltage, which in effect masked the microphone output
signal.
Eventually the circuit voltage ranges were found and applied to the programming,
which allowed the group to produce constantly controlled amplitude for our given
signals. Figure 9 illustrates the effect of our circuit on the waveform information.
Figure 9
The top signal [ch1] is the primary amplified signal output from the microphone
conditioning circuit, while the lower signal [ch2] represents the limited output.
Both signals are on the same voltage/div scale on the oscilloscope window, to
eliminate any confusion about scaling of the signals. By setting the scales as
such, the direct amplitude relation between the two is representative of the
amount of signal limiting, and is therefore of primary importance in this diagram.
The control voltage at this point is barely above 1V, and its result can be seen as
the relative compression of [ch2] as compared to the initial signal.
It should be noted that the limited signal has fully maintained its original
frequency data in the path of signal processing. This information is not
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significantly affected by the limiter circuit in any way, as can be clearly
demonstrated by a comparison of the two signals, principally when one looks at
the corresponding peaks of each signal. Peaks of the two waves are coincident
through the oscilloscope view, which was given a slight DC offset to separate the
signals enough to represent them without interference of the two graphical
representations. Given this association, it is possible to set the oscilloscope’s
math function to give a direct visual representation of the amount of limiting
applied to the signal as opposed to the direct comparison method just discussed.
The concept is as follows: When the second signal has been unprocessed, it is an
exact copy of the original signal, and by applying the function [ch1]-[ch2] a
related function is graphed. When the limiter circuit is not active, the function
produces a graph of near zero value – essentially a flat line aside from a given
amount of background noise. As the limiter is turned to an active state, the graph
begins to increase its signal and reaches a maximum output as the limiter
approaches its maximum capability. In essence this resultant signal appears as a
modified version of the original microphone conditioner output, albeit with a
slight offset.
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CONCLUSIONS
Given a second look at the conditioning process, it would be possible to improve
upon one major aspect in our limiter software interface concerning the
relationship between voltage and audible volume. This is due to the fact that
audio signals are not linearly related to input voltage, but instead are associated by
means of a logarithmic relation. LabVIEW has the ability to create outputs based
on mathematic relations, which can ameliorate this discrepancy. Given enough
time to condition the circuit in this regard, the volume control could be improved
to provide more practical control over audible volume. In this case, the visual
method used in the experiment is characterized by a linear relationship between
voltages and therefore must also undergo the same transformation to be a
completely accurate representation of the audible signal. Be that as it may, the
current setup has certainly served its purpose throughout this experiment.
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SUMMARY
A microphone is a type of transducer that takes a mechanical input and converts it
to give an electrical output. To eliminate surrounding interference, a limiter is
needed. A limiter attenuates a high amplitude output by limiting the current that
goes into the amplifier set by a particular voltage. In the case of our project, the
voltage is manipulated and stabilized using a designed program in LabView.
Physically speaking, the microphone and limiter work together to clarify the
direct input without creating a drastic damper on the amplitude, i.e. the loudness.
This is a basic version of commercially available “Smart Sound” systems that are
commonly found in televisions. We were able to implement a version of this and
test it. Our circuits gave us insight into the workings of the limiting circuit and
the microphone. LabVIEW was able to act as both a data collector and a
threshold controller for the limiter. The threshold is basically the voltage set by
the user that the output voltage is maintained within. Ideally the limiter does not
affect the shape of the input waveform, just the amplitude. This would result in
volume control without sacrificing sound quality through distortion. Our results
showed that the circuit we used does a good job of completing this desired task.
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BIOGRAPHIES
Fine, Rachel
Presently a fourth-year mechanical engineering student at Northwestern
University, Ms. Fine plans to graduate June 2002. Past experience includes
satellite design for the Space Dynamics Laboratory (Logan, UT), energy
management for Goodyear Tire and Rubber Company (Akron, OH), and
computer consulting for Academic Technologies (Evanston, IL). She plans to
further her education by obtaining a Master of Science degree in Aerospace
Engineering. Potential career vocations include astronautics and
systems/spacecraft engineering.
Hayes, Gareth
Gareth is currently a junior mechanical engineering student in the McCormick
School of Engineering and Applied Science at Northwestern University. He
graduated from Watertown High School in Watertown, NY in 1999. He is
currently involved in Northwestern’s co-op program as a mechanical design
engineer for Cummins-Allison Corp. located in Mount Prospect, IL. He plans on
specializing in design at Northwestern and graduating in June of 2004. After
college, Gareth plans on obtaining employment in the field of mechanical
engineering.
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Higgins, Joshua
Joshua Higgins is a Junior Mechanical Engineering major in McCormick. His
focus is on design engineering. Joshua is from Rockdale, IL and graduated from
Joliet Central High School in 1999. Past projects at Northwestern University
include website development for Norris University Center through the first-year
Engineering Design and Communication (EDC) course, much of which was
subsequently implemented into their actual site improvement the following year.
He has also worked with United Parcel Service through the EDC program.
Involvement in the latter project was centered at their Midwest ground-based
package processing facility, and was focused on an effort to increase working life
of pulley machinery in their conveyor belt systems. Joshua involves himself in a
number of design projects on his own time; currently he is in the process of
designing and building an alternate scale-length electric guitar. This coming year
he will compete in Design Competition for the first time.
Ray, Jehana
Jehana Ray was born in Silver Spring, Maryland and raised in DeSoto, Texas.
Today, Jehana is a junior in the McCormick School of Engineering at
Northwestern University in Evanston, Illinois. Her focus is in mechanical
engineering with a specialization in design and a theme in economics and art.
Post graduation, Jehana wants to work for a well-known medical company in their
diagnostic division. Her goals are to design surgical instruments for animals and
human beings that will facilitate surgery, and possibly design prosthetics, as well,
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with hopes of bettering mankind. After two to four years working as an engineer,
Jehana would like to enroll in a graduate school to become a veterinarian, her
dream career goal. As a veterinarian, she hopes to be able to use her skills as an
engineer to improve veterinary medicine and veterinary instrumentation, in order
to keep all animals safer, healthier, and happier.
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REFERENCES
[1] Allison, Phil. “Fast Audio Peak Limiter.” 2000.
http://sound.westhost.com/project67.htm
[2] Alspector, Professor J. “Limiting Circuits.”
http://eceweb.uccs.edu/alspector/Limiting%20Circuits%20web/limiting%20ci
rcuits.htm
[3] Elliot, Rod. “Musical Instrument (Expandable) Graphic Equaliser.”
http://sound.westhost.com/project64.htm
[4] "LabVIEW Student Edition 6i", Robert H. Bishop, Prentice Hall, Inc., 2001.
[5] Plonus, Martin. Electronics and Communications for Scientists and
Engineers. Harcourt/Academic Press, Burlington, Mass; 2001.
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APPENDIX: BILL OF MATERIALS
Hardware
Proto-Board 203A (“Bread
Board”)
TDS 200 Series Digital
Oscilloscope
Servo to Go DAQ
Personal computer
Computer Speakers
Circuitry as in Table I
Software
National Instruments LabVIEW
Microsoft Excel
STG driver
Table I : Elements of the circuit
Description
Type
Quantity
Microphone
9.7MM Diameter/18MM Leads
1
2.2 Kohm
10K
2K2
39K
1M
4K7
39M
3K
220K
100K
2K7
1.0 micro F
10 micro F
250 micro F
.05 micro F
33 micro F
100 micro F
47 micro F
10 pico F
15 nino F
33 micro F
LM386, 2xTL072
1
2
1
1
1
3
1
1
1
2
1
3
2
1
1
1
1
1
1
1
1
3
10 K ohms
1
2
Resistors
Capacitors
OP AMP
Potentiometer
Diode
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