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Multi-Disciplinary Engineering Design Conference
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: 06520
DESIGN OF AN INTELLIGENT TEMPO METER
J.E.Gifford (Electrical Engineering, Sponsor)
Scott DVileskis (Electrical Engineering)
Frank Gill (Mechanical Engineering)
Jeremy LaDuke (Industrial Engineering)
ABSTRACT
musicians is playing and report this information back
to the user in real time. This feedback allows the user
to control their own tempos accordingly, even if the
music slows down or speeds up intentionally.
Outfitting the drummer in a small ensemble or the bass
drummer in a marching band with such a device,
provides them with a practical and reliable method of
regulating the whole band’s tempos during a
performance.
Although many types of instrumental music groups
such as orchestras and concert bands typically utilize a
conductor to set and monitor their performance
tempos, several other types of groups such as
marching bands, rock bands, and jazz bands typically
depend on the drummer or drum section to perform
this crucial function. This design project explored the
feasibility of creating a device to be used as a drum
accessory to provide real-time feedback to the
drummer about the tempo at which they are playing, in
order that they can maintain an appropriate tempo for
the entire band during a performance. The “tempo
meter” device resulting from this exercise can, in fact,
be an extremely effective tool due to its ability to
accurately determine performance tempos despite
varying rhythms in the drum part being performed.
INTRODUCTION
The sponsor for this project is a bass drum player in a
marching band and was looking for an unobtrusive
but effective means of monitoring the band’s average
speed (or tempo) during parades.
The device
traditionally employed by musicians to maintain
consistent tempos is a metronome.
However,
metronomes are of limited usefulness in performance
situations because they are only capable of providing a
constant reference tempo; when the tempo of the
performance drifts or shifts intentionally (which is a
normal and common occurrence) the metronome is by
definition incapable of following along and therefore
becomes useless. What is required in order to solve
this problem is a tempo meter – a device capable of
monitoring the tempo at which a musician or group of
There is already one tempo meter device which is
commercially available: the Tempo Ref [1]. However
the implementation of this particular device severely
limits its usefulness. It simply displays the duration
between each two consecutive notes as an average
tempo. If all of the notes being played are of the same
duration, then the value displayed should be fairly
consistent and therefore somewhat useful. However,
this sort of music wouldn’t be very interesting to listen
to. Typically a drummer is playing various different
rhythms according to the music, in which the durations
between notes are always changing. Therefore, it was
the sponsor’s desire to investigate the feasibility of
designing a better tempo meter, one capable of
determining the tempo of the music with reasonable
accuracy despite its various rhythms.
The primary goal of the project team was to design
and construct one working unit to be used by the
sponsor on various bass drums during parades. A
secondary goal of the team was to attempt to design
the device in such a way that it could be used for other
types of drums (snare drums) and for other
applications, such as a rock concert or a for a practice
aid. The device resulting from this project addresses
each of those goals.
© 2006 Gifford, DVileskis, Gill, LaDuke
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Proceedings of the Multi-Disciplinary Engineering Design Conference
NOMENCLATURE
Design Requirements: The first step in designing the
tempo meter device was to determine the sponsor’s
requirements for the device from a user’s perspective.
The most important design parameters for this project
were that the device reliably and accurately report the
tempo over a wide variety of rhythms and tempos and
that the displayed value be easy to read in various light
conditions. However, there were several other key
parameters which were important with respect to
making sure the final product would be useful for its
intended purpose. Specifically, the device had to be
unobtrusive, which meant being relatively small (less
than 3”x4”x1”) and lightweight (less than one pound).
This also meant that the device must be mounted to
the drum in a manner that wouldn’t interfere with
playing the drum, i.e. not attached to the heads, the
rims, or the mallets/sticks. Nor was it acceptable for
the device mounting to require any permanent
modifications to the drum.
There was also a
requirement that the device have at least ten hours of
battery backup as well as requirements that it be both
rugged, rain resistant, and esthetically pleasing.
Concept Generation: The next step in the design
process was to take the initial requirements and start
brainstorming ideas for an implementation to meet
those requirements as well as ideas for enhancements
or improvements on the original sponsor’s concept. In
varying degrees, this process lasted the entire duration
of the project, modifying the design concept in order
to (hopefully) improve or add value to the final
product. Two of the most significant enhancements to
the original concept were the addition of a metronome
mode and the addition of a time-of-day clock. Neither
of these features directly enhance the tempo meter
function of the device, but both add significant value
to the overall package without adding much
complexity or cost to the design of the device itself.
Microphone Sensor
Voltage vs. TIme
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0
Voltage (volts)
DESIGN PROCEDURE
The first sub-system implementation to be determined
was the impulse sensor, or “trigger”, mechanism.
Many different sensor technologies were investigated,
including accelerometers, contact microphones, audio
microphones, and others. Figures 1 and 2 contrast the
responses of a contact microphone and a Piezoelement sensor to a single strike of a bass drum. The
Piezo trigger generates a signal with a greater
amplitude and a much quicker decay, both of which
make detecting distinct impulses easier. Therefore, it
was determined that the best solution was to use
industry-standard drum triggers or drum pads, which
utilize Piezo elements to generate an analog signal
where a sharp spike indicates that the drum or pad has
been struck. It was also decided, after much debate,
that the device would have a phono jack connection
for an external trigger, but no built-in sensor device.
These two decisions, taken together, allow for greater
flexibility of the tempo meter, in that it can be used
with a standard drum pad or any of many different
styles of drum triggers depending on the application.
It also allows the device to be used as a practice aid by
connecting a trigger attached to a snare drum via a
standard patch cable to the tempo meter which can be
placed on a nearby music stand. This approach also
reduces the cost of manufacturing the device itself.
0
0.2
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0.6
0.8
1
1.2
1.4
1.6
1.8
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0.18
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-1
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Time (seconds)
Fig. 1 Contact Microphone
Piezo Sensor
Voltage vs. Time
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2
1
Voltage (volts)
ADC: analog-to-digital converter
BPM: beats per minute (standard unit of tempo)
LED: light emitting diode
LCD: liquid crystal display
PCB: printed circuit board
RTC: real time clock
beat: the "pulse" of the music, evenly spaced at a
constant rate; when marching, each footstep is
one beat
impulse: a single strike of the drum; one note in the
music; a single trigger event; impulses may or may
not coincide with beats
rhythm: the pattern of notes or impulses; typically
more complex than a straight beat
tempo: the rate of the beats, in BPM
0
0
0.02
0.04
0.06
0.08
0.1
0.12
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-1
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Paper Number 06520
Time (seconds)
Fig. 2 Piezo Sensor
Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference
The next sub-system to be addressed was the display.
Numeric LED displays were discussed but rejected
due primarily to the large resulting current draw on the
battery-based power supply. The discussion then
turned to LCDs. Different types of LCD screens were
looked at, e.g. reflective vs. transflective vs.
transmissive, color vs. monochrome, numeric vs.
graphic. A monochromatic reflective-type display was
chosen, again primarily due to current draw concerns.
The selected LCD draws under 5mA, whereas a
similar backlit LCD draws over 100mA. A graphic
LCD (128x64 pixels) was chosen in order to facilitate
displaying both the tempo value and time-of-day
simultaneously on the same screen in different font
sizes. This also allowed for greater freedom later in
the development with respect to the appearance of the
display and the operation of the various modes.
Another sub-system which was the subject of much
deliberation was the power supply. The original idea
was to use a single 9 volt cell, but there were concerns
about the inefficiency of generating 5 volts (or 3 volts)
from 9 volts via a linear regulator. The next idea was
to use a battery of four AA 1.5 volt alkaline cells with
a low-dropout regulator to generate the 5 volts. In the
end, this was replaced by a battery of four AAA 1.5
volt alkaline cells with a 5 volt LDO. This reduces the
overall battery life of the device, but the resulting
reduction in physical size of the device was deemed to
be a worthwhile tradeoff. There were also discussions
of supporting AAA 1.2 volt NiMH rechargeable cells,
but the resulting 4.8 volt supply wouldn’t be sufficient
to power the various 5 volt components, so this idea
was dropped.
Component Selection: The first component selected
was the LCD panel.
These tend to be more
customized and less standardized than most of the
other components in the system, so it became
necessary to find an LCD that was actually available
that met the criteria stated above and then work around
this particular component. For example, the LCD
panel that was selected has a built-in LCD controller
chip but doesn’t have a built in negative supply to
drive the LCD screen. Therefore, it was necessary to
also source a +5/-5 volt dual output switching power
supply. The LCD screen is driven off the –5 volt
supply, and all of the other components
(microcontroller, filter op-amp, RTC, LCD controller)
are driven off of the +5 volt supply. The backup
supply for the RTC is a single 1225 3 volt Lithium
cell, which will maintain proper RTC backup for ten
years.
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choice was restricted to microcontrollers that the team
were already familiar with, specifically the TI
MSP430 and the Microchip PIC families of
processors. The final decision was in favor of a
PIC18F series part over the MSP430, because the
MSP430 is only available as a 3 volt surface mount
package. Using the PIC avoided the need for 3V/5V
buffers between the microcontroller and the LCD
controller logic. There was also an attempt to select
all through-hole components rather than surface mount
components for ease of prototyping and assembly
(even though a commercialization strategy for this
device would obviously include using surface mount
components). The particular PIC selected was the
PIC18F2515 (a 28-pin part), because it had the
required number of I/O pins (20+) and more than
enough Flash (48kB) and RAM (4kB) to implement
this application. It also has a built-in I2C bus interface
(for the RTC) and a 10-bit ADC (for the impulse
detection).
The remaining component selection was relatively
straightforward. Experimenting with various Piezoelement drum triggers showed that the dominant
frequency is around 3kHz. Therefore the values on the
first-order active filter input stage were selected to
provide a cutoff frequency of 5kHz, and the ADC was
configured to sample at 10kHz (twice the cutoff
frequency). Likewise, a 25MHz crystal oscillator was
chosen for the PIC in order to guarantee enough
processing power to process ADC interrupts at 10kHz.
Pushbuttons were selected for the power and function
controls from a manufacturer who produces both
alternate-action switches (for the power switch) and
momentary switches (for the three function switches)
in the same form factor and also produces matching
caps, so that they would be guaranteed to fit properly.
Prototyping: Once components were selected and the
schematic was created, components were ordered and
two perf-board prototypes were built. Also, two
PICkit II in-circuit programmers were ordered. The
corresponding development environment and ‘c’
compiler are freely downloadable from Microchip, so
that was all that was needed to get development
platforms up and running. A 5-pin header and a
couple of extra resistors are all that is required on the
target boards in order to use the programmer.
Therefore, the final product will also include the incircuit programming header, allowing for further
enhancements in the future if desired. Once the
electrical design and component selection were
verified through testing of the prototypes, a custom
PCB was laid out and ordered.
Another critical component selection was the
microcontroller. There are countless microcontroller
families available to choose from, many of which
could have been successfully utilized in this design.
So in order to make the decision manageable, the
© 2006 Gifford, DVileskis, Gill, LaDuke
Proceedings of the Multi-Disciplinary Engineering Design Conference
SOFTWARE DESIGN
Impulse Detection: The PIC’s Timer0 module is
configured to timeout every 100us. The high priority
interrupt handler (which services Timer0 timeouts)
thus initiates an A-to-D conversion every 100us unless
one is still in progress. It also increments a global tick
counter every hundredth timeout (every 10ms) which
provides a mechanism for the rest of the system to
keep track of time accurately. The A-to-D acquisition
and conversion process normally takes around 20us to
complete, at which point the low priority interrupt
handler queues the result. Once every 10ms tick, the
low level handler also inspects all of the samples
collected since the previous tick. All of the signal’s
peaks are rectified and averaged. If the peak-average
for one tick’s worth of samples is more than twice that
of the previous tick’s samples, then it is considered an
impulse and the duration since the previous tick is
queued up for the tempo determination algorithm to
process at a later time. Back-to-back impulses are
disallowed in order to avoid double registering
impulses.
Tempo Determination: The real heart of this device is
the tempo determination algorithm. This starts with
the impulse detector, which cues up the durations
between impulses. Any durations over 2.5 seconds are
ignored (not queued) in order to provide a sort of
holdover function. Whenever the application runs and
sees that a new impulse has been detected, it adds the
new duration to a running tally that it keeps. When the
tally surpasses 3.5 beats worth of time at the current
target tempo, the application runs the tempo algorithm
to determine the new target tempo and then displays
the new value. This effectively means that the tempo
display will be updated roughly every four beats,
which seems to be a reasonable compromise between
too often (every beat?) and not often enough. Also,
the algorithm inspects the last eight beats worth of
impulse timings (rather than just four) in order to
improve the stability and accuracy of the calculated
value.
When it runs, the algorithm inspects all of the impulse
durations within the current sample set and attempts to
categorize each of the durations as a whole note, half
note, quarter note, eighth note, sixteenth note, dotted
half note, dotted quarter note, dotted eighth note, or
dotted sixteenth note based on the current target
tempo. Anything shorter than a sixteenth note is
ignored. It also has to allow for a certain amount of
slop in its categorization of the notes (20% under the
reference or 25% over the reference). If the algorithm
doesn’t find a majority of the durations in the current
sample set that are categorizeable based on the current
target tempo, then it assumes that the tempo has
changed and attempts to re-categorize each of the
delays assuming that the tempo is somewhere in the
Page 4
range of 75-149BPM. Unfortunately, there has to be a
default range because of aliasing, e.g. quarter notes at
120BPM are by definition identical to eighth notes at
60BPM. Also because of aliasing, it is possible for the
algorithm to get “confused” for short periods of time,
but it generally corrects itself quickly.
Overall Functionality: The device has three different
operational modes: metronome mode, tempo meter
mode, and clock set mode. The device defaults to
metronome mode at 120BPM upon power-up. The
target tempo of the metronome can be adjust up or
down within a range of 40-180BPM by tapping or
holding the UP or DOWN buttons. These buttons
increment or decrement the target tempo 1BPM each
time they are tapped and scroll 10BPM per second
after being held down for 1 second. In this mode, the
target tempo value is displayed on the LCD, but
flashes at the specified rate (with a 50% duty cycle).
Any impulse detected from the trigger input will
automatically toggle the device into tempo meter
mode. The TAP button emulates an impulse when
tapped, so tapping this button will also toggle the
device from metronome mode to tempo meter mode.
Once in tempo meter mode, the current metronome
target tempo becomes the starting target tempo for the
tempo meter. It is still displayed on the LCD, but no
longer flashes. As long as there are impulses being
detected (less than 2.5 seconds apart) the target tempo
value will continue to be updated and re-displayed
every four beats. Tapping either the UP or DOWN
buttons in this mode toggles the device back to the
metronome mode. In all modes, the mode is displayed
across the bottom of the screen, and the time-of-day is
displayed in the upper-right corner of the display.
Simultaneously pressing both UP and DOWN toggles
the device into clock set mode. In this mode,
instructions are displayed explaining what each of the
buttons does. The UP button increments the minutes
field, and the DOWN button increments the hours
field. Holding either of these buttons for over 1
second will scroll the appropriate field. The tap button
returns the device to the metronome mode, but the
most recent target tempo (from either mode) is
maintained. The clock stops advancing while in the
clock set mode, and the new time value (with seconds
set to zero) is written back to the RTC upon exit from
this mode.
There was an overall design goal to keep the user
interface simple and for it to require as little
interaction as possible. In most cases, the procedure is
to turn on the power and start playing, which is about
as simple as it can be. Various parameters that could
have been user adjustable (such as the number of beats
between updates) were hard-coded in order to
maintain this ease-of-use.
Paper Number 06520
Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference
MECHANICAL DESIGN
Material selection: Due to the limited time frame for
this project and the sponsor’s limited budget for this
project, it was decided to mill the case out of
aluminum billet. The aluminum stock was donated to
the team, and the milling was performed on a CNC
machine at the RIT campus. Aluminum would be an
unlikely choice for the case material if this device
were to be mass produced; plastic would be a much
more likely choice. However, for a single working
unit (which doubles as a proof-of-concept) aluminum
is a very attractive option. It is relatively lightweight,
yet extremely strong, thus providing the necessary
ruggedness. It also makes for a very sano finished
product.
Case design: The case itself is designed in two halves:
a top (front) half and a bottom (back) half. The two
halves are located to each other by way of matching
stepped edges around their mating surfaces. There are
four #6-32 flathead stainless screws with sealing Orings that connect the two halves of the case together.
The mounting holes for these screws are arranged in a
non-symmetric pattern in order to provide proper
alignment of the two case halves. The PCB is
mounted to four mounting bosses located in the top
half of the case by way of four 4-40 screws. Two of
these bosses are located directly between pairs of
pushbuttons, in order to provide as much support for
the buttons as possible. Most of the components are
mounted to the front of the PCB, but the batteries are
all mounted on the back of the PCB for easy
replacement. The potentiometer for adjusting the
contrast on the LCD and the in-circuit programming
header are also located on the back of the PCB. All of
the machine screws inside the case (PCB and LCD
mounts) are Loctited in place to protect against
vibration. The case screws should be held in place by
the O-rings. The case also has holes for each of the
four buttons, and an opening for the LCD screen. The
LCD opening is filled with a piece of Lexan to protect
the surface of the LCD, and the Lexan is slightly
recessed into the aluminum case to protect itself. In
order to improve the rain resistance of the device, the
Lexan window is sealed to the aluminum with an
appropriate silicone sealant.
Input jack: The input is a 2-conductor (mono) 1/8”
female phono jack which mounts in the side of the top
case half. The design team originally planned on a
1/4” jack, which would have allowed for the use of a
standard patch cable to connect the trigger to the
tempo meter device. However, the 1/4” jack was
replaced with an 1/8” jack in order to save valuable
space inside the case. This allowed the case to be
significantly smaller than would otherwise have been
possible. At worst, this would require the use of a
simple 1/4”-to-1/8” adapter. In the sponsor’s case, the
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trigger will be customized to have the appropriate
male jack connected directly to it. Therefore, this is a
minor issue.
Also, this device can be used with more than one drum
at a time, such as a snare drum and bass drum of a
drumset, by simply connecting two triggers together in
parallel with a simple Y-adapter. This particular
arrangement would significantly improve the
performance of the tempo determination algorithm
when used with a drumset, because it would be able to
see (hear?) more of the rhythm being played.
Mounting bracket: A bracket was designed specifically
for attaching the tempo meter device to the shell of a
marching bass drum. The bracket is a U-shaped metal
piece with a pivot hole in each side which locate on
pivot pins milled into the case. The bracket snaps onto
these pins, and a thumbscrew is used to hold the
device in place relative to the bracket. A curved slot
for the thumbscrew allows for a wide range of
mounting angles to accommodate various sized bass
drums as well as various sized bass drummers. The
bracket itself is mounted to the bass drum by way of
two strips of 3M DualLock recloseable fastener, which
is plenty stout enough to hold the device steady on top
of the drum during a parade.
SPECIFICATIONS
Default tempo: 120BPM
Metronome tempo range: 40-180BPM
Tempo meter fallback tempo range: 75-149BPM
Tempo precision: +/-1BPM
Input filter cutoff frequency: 5kHz
Sampling rate: 10kHz
Input noise threshold: 0.25Vp-p
Display size: 128x64
Tempo size: 24x48
Clock size: 8x16
Battery life: ~15hrs
Clock backup: 10 years (@ 25ºC)
Overall Dimensions: 100x77x40mm
RESULTS
The primary result of this project is that the concept
has been proven to be feasible. The sponsor is
satisfied with the overall result, both in performance
and appearance, and expects to use the device to
monitor parade tempos for many years. The physical
thickness of the device exceeds the original
requirements, but in all other aspects, the design met
the requirements set forth by the sponsor.
The sponsor was also interested in the
commercialization possibilities for this type of device.
© 2006 Gifford, DVileskis, Gill, LaDuke
Proceedings of the Multi-Disciplinary Engineering Design Conference
Therefore, manufacturability became a secondary
design goal for the design team. However, the
realities of a twenty week project schedule and a
limited sponsor budget severely limited their options.
One example of this is the choice of case material.
While plastic would be a much more cost-effective
solution for mass production of a device such as this,
the $20,000 cost of having a mold created made
aluminum the material of choice for a single unit. If
the sponsor was to pursue a commercialization
strategy for this device, a major part of the ensuing
effort would be to rehash 90% of the electrical and
mechanical design of the device. This is because
many of the components selected for this project were
necessarily commercial-off-the-shelf items, but would
be better implemented as custom pieces designed
specifically for this application. This includes the
LCD, phono jack, pushbuttons, and battery holders.
Obviously, the PCB, case, and bracket would also
require a redesign. The primary goals of these
changes would be to reduce the overall size of the
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device and to reduce the per-unit cost in a mass
production scenario.
ACKNOWLEDGEMENTS
The design team would like to extend its thanks to Dr.
Daniel Phillips (mentor) for his assistance and
guidance and to David Krauter and Amy Kennicutt for
their research into the marketability of the concept.
Also, thanks go to John C. Gifford and to REDCOM
Laboratories, Inc. for their generous donations of
materials that helped make this project a reality.
REFERENCES
[1] Parsons, M., 2003, “LT Lug Lock Tempo Ref”,
Modern Drummer, 27(12), pp. 48-49.
Paper Number 06520