Touchpad-Controlled
Parametric Equalizer
ECE 445
Group #24
Anthony Mangognia
Alexander Spektor
Farsheed Hamidi-Toosi
TA: Chad Carlson
Introduction
Goal: To create a real-time audio filtering
solution for musicians and sound engineers
Goal: To provide independent control over filter
parameters: center frequency, bandwidth, and
gain
Goal: To create a geometrically-intuitive input
device for the filter
Existing Alternatives
X-Band Equalizers
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Sliders or knobs
Limited control
Takes time to adjust
Discrete frequency bands
Digital Parametric Equalizers
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Pseudo-continuous frequency sweep
Cumbersome software-based control
Design Overview
Input Device: Pressure-sensitive touchpad.
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Horizontal position: center frequency
Vertical position: gain
Overall Pressure: bandwidth
Input-Filter Interface: RS-232 serial connection
Filter: DSP-implemented algorithm
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Second-order IIR filter (based on Mitra-Regalia
topology)
Filter coefficients update in real-time
Design Block Diagram
Audio In
DSP:
Parametric Equalizer Filter
Microcontroller:
Positioning Algorithm &
Touchpad DSP interface
Audio Out
Touchpad
1. Touchpad Process
Pressure Sensor Voltage Differential
Bias cancellation/Tuning Circuit
830x Amplifier
PIC A/D
Touchpad
Four corner-mounted pressure sensors on
height-adjustable shelves
Pressure sensor output voltage varies with finger
position
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Positioning algorithm
Surface much larger than typical commerciallyavailable touchpads
Touchpad Design
Sensor Mount
Slide-in touchpad
Pressure
Sensors
Bird’s Eye View
Height-adjustable
sensor mount
Touchpad
Surface
Sensor Mount
Pressure Sensors
Calibration: Voltage Vs. Pressure
2
0
-25
25
75
125
175
225
275
325
-2
Voltage (V)
Honeywell 125PC30G1
Pressure range: 0-30 psi
Sensitivity: 8.33mV/psi
Trial 1
Trial 2
-4
Trial 3
Trial 4
-6
Trial 5
Average
-8
-10
-12
Pressure (mm Hg)
Voltage
to Pressure Conversion
350
300
V-
GND
V+
+10
Pressure (mm Hg)
250
200
Average
150
Linear (Aver
100
50
y = -28.116x + 6.814
0
-12
Amplifier
-10
-8
-6
-4
-2
0
-50
Voltage (V)
2
Touchpad Signal Amplification
Instrumentation Amplifier: AD622AN
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Low-cost: $4.90
Gain: 2-1000x, external resistor control
56Ω  830x gain
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Easy integration: wide power supply voltage gain
(±2.6V-±15V)
Large gain  Large bias voltage
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Solved with 1MΩ pull-down resistors at inputs and
100k potentiometer (calibration)
2. Microcontroller-Enabled
Touchpad  DSP Interface
Four sensor A/D
Positioning Algorithm
Serial Transmission
Analog to Digital Conversion
8-bit A/D conversion for quicker calculations
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10-bit possible for PIC16F877A, but doubles
number of bytes for mathematical operations
Decreases resolution to 256 points maximum
More than enough to simulate continuous operation
Read 8 values for each sensor and average
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Functions as a digital LPF
Positioning Algorithm
Sensor 1: V1
(0, Ymax)
X2 = V2 Xmax / (V2 + V1)
Sensor 2: V2
(Xmax, Ymax)
Touchpad Surface
Y1 = V1 Ymax
/ (V1 + V0)
Sensor 0: V0
(0, 0)
(X,Y) – Finger Position
X1 = V3 Xmax / (V3 + V0)
Y2 = V2 Ymax
/ (V2 + V3)
Sensor 3: V3
(Xmax, 0)
Positioning Algorithm
Sensor 1: V1
(0, Ymax)
X2
Sensor 2: V2
(Xmax, Ymax)
Touchpad Surface
Y1
(Xavg,Yavg) – Average
Y2
Sensor 0: V0
(0, 0)
X1
Sensor 3: V3
(Xmax, 0)
Positioning Algorithm
Sensor 1: V1
(0, Ymax)
X’ = X1 (Ymax-Yavg) + X2 (Yavg)
Sensor 2: V2
(Xmax, Ymax)
Touchpad Surface
(X’,Y’) – Weighted Average
Y’ = Y1 (Xmax-Xavg) + Y2 (Xavg)
Sensor 0: V0
(0, 0)
Sensor 3: V3
(Xmax, 0)
Data Sent to DSP
Three-byte start sequence: “230” x3
Four sensor readings: S0, S1, S2, S3
Two one-byte positioning words (x3)
One three-byte stop sequence: “232” x3
230
230
230
S0
S1
S2
S3
X
X
X
Y
Y
Y
232
232
232
Serial Data Transmission
Data transmitted at 38400kbps
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Default rate for DSP
Data Format
1 START BIT
8 BIT WORD
Sent over standard serial cable
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DB-9 connector
1 STOP BIT
RS-232: Voltage Level Conversion
PIC output at TTL levels
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~ 0 - 5V
38400kbps serial data at
TTL from PIC
DSP input at RS-232 levels
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~ ±12V swing
Conversion with MAX232
line driver
MAX232
Line Driver
38400kpbs serial data at
RS-232 to DSP
3. DSP: Audio Filtering
Receive/Decode Data from Touchpad
Update Filter Coefficients
Apply Filter to Audio Input and Send to Speakers
Filter Design
Based on Mitra-Regalia
second-order IIR
Design Equations:
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β = – cos(ωc)
k = 10(GAIN/20 dB)
α = (1 – tan (BW/2)
(1 + tan (BW/2)
Programmed in C for
TI-54x fixed-point DSP
A(z) = All-Pass Lattice
Mitra-Regalia Topology
Src: Montana University Web site. http://www.coe.montana.edu/ee/rmaher/ECEN4002/lab4_020226.pdf
Design Challenges: Touchpad
Pressure sensor noise
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Problem: 30mV peak-to-peak noise level
Solution: 8-point averaging filter after PIC A/D
PIC A/D crosstalk
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Problem: Changes in one pressure sensor affected values read
for other
Solution: Pull-down 0.1μF capacitors at A/D input pins
Serial communication pins
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Problem: PICPC communication and PICDSP
communication use different DB-9 transmit pins
Solution: Internal rewiring to accommodate both
Design Challenges: Filter
Filter type change
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Problem: Original algorithm (Chamberlin) produced
undesirable resonance frequencies
Solution: Switched to Mitra-Regalia topology
IIR Instability
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Problem: Direct form two implementation caused overflow
Solution: Implemented lattice structure to reduce overflow
Quantization
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Problem: Fixed-point quantization of coefficients
Solution: Lattice structure ensures pole-zero cancellation
Internal Component Test
Pressure Sensor + Amplifier
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Unwanted signal oscillation: 60mV peak to peak
Due to conflicting RC networks
Too high for 1V sampling range
Solution: Removed analog smoothing filter
Bandwidth Test
Center Frequency Test
Gain Test
GWN Input Boost Tests
GWN Input Cut Tests
Finished Product Test
Amplitude test performed with
oscilloscope
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Input: 11 kHz Sine Wave, 200 mV
peak-to-peak
Tests: gain = 2 and gain = .5 at
11kHz by measuring peak-to-peak
voltage of output using the scope
Amplitude resolution is .24 dB
from -6dB to 6dB
Exceeded design requirement
which stated 2-3 dB amplitude
resolution
Gain
1
2
0.5
Output
P-P
~200mV
~400mV
~100mV
Finished Product Test
Frequency tests performed with scope
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Fix bandwidth, set gain = 2
Inputs: Sine at set frequencies, Gaussian White Noise
Using FFT on scope, can see if frequencies boosted by 2x at
desired center frequency
Proposed frequency resolution: 1 Hz (not necessary)
Total of 200 center frequencies possible, distributed them
logarithmically since hearing is logarithmic
High resolution for low frequencies, less resolution for higher
frequencies
Results: Filter works for all frequencies within hearing range
(20Hz-20kHz)
Finished Product Test
Bandwidth Tests
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As total pressure increases, increase bandwidth
Tested using FFT on scope
Input signals: Gaussian white noise, music
Test to see if bandwidth varies from 50Hz to
22050Hz as pressure increases
Passed tests, including aural tests
Finished Product Test
Latency Tests
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The target of less than 100ms system latency was achieved
Delay due to pressure sensors negligible
Delay due to PIC negligible (assembly code minimized cycles)
DSP initially had some latency, but code was optimized by
eliminating FOR loops (less than 30000 cycles at ~MHz)
Usability tests confirm that system latency is not an issue
when using this system, negligible
Finished Product Test
Usability Tests and Conclusions
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Tested on music signals and white noise signals
Qualitative analysis:
Was the filtering audible?
Did the touchpad respond as desired?
How intuitive was it to “find” a desired frequency?
Is this design marketable? If so, why?
How much would this cost to manufacture?
Final Thoughts
Improvement: Better pressure sensors
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Higher output voltage  Less amplification
Improvement: Cascade feature
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A button to keep current settings and use new
settings in cascade
Other applications: Large touchpad has many
applications
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Computer input device for the disabled and kids
Acknowledgements
Chad Carlson
Marty & other ECE 445 TAs
Profs. Haken & Beauchamp
Machine Shop: Scott McDonald
Parts Shop
Questions?