Modular Wireless Audio Mixer Final Report

ECE 445: Senior Design - Fall 2018

TA: Zhen Qin

Group 38 Members: Justin Rienton Jeff Woltman William Zhang

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Abstract The purpose of this project is to design a modular wireless audio mixer for mixing audio from electronic instruments. The system accepts an analog audio signal as the input and mixes the signal with inputs from the other channels to create an output signal. It also offers the ability to adjust the gain of the inputs and includes an equalizer to allow for user-controlled customization of the output signals. The output signal is sent to the end-user via Bluetooth. The following summarizes our design and verification results.

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Table of Contents

1Introduction................................................................................................................................................................................................1

1.1Objective................................................................................................................................................................................................................................1

1.2Background..........................................................................................................................................................................................................................1

1.3High-LevelRequirements...............................................................................................................................................................................................1

2Design............................................................................................................................................................................................................2

2.1BlockDiagram....................................................................................................................................................................................................................2

2.2PhysicalDesign...................................................................................................................................................................................................................2

2.3InstrumentModule...........................................................................................................................................................................................................32.3.1BluetoothTransmitter....................................................................................................................................................................................32.3.2BatteryModule..................................................................................................................................................................................................4

2.4BaseModule.........................................................................................................................................................................................................................52.4.1VoltageRegulator.............................................................................................................................................................................................52.4.2AudioMixer.........................................................................................................................................................................................................52.4.3Equalizer............................................................................................................................................................................................................10

3.Verification..............................................................................................................................................................................................13

3.1BluetoothModules..........................................................................................................................................................................................................133.1.1SamplingResolutionandFrequency.....................................................................................................................................................133.1.2TransmissionRange.....................................................................................................................................................................................13

3.2BatteryModule.................................................................................................................................................................................................................133.2.1VoltageRegulation........................................................................................................................................................................................133.2.2BatteryLife.......................................................................................................................................................................................................13

3.3AudioMixer........................................................................................................................................................................................................................14

3.4Equalizer.............................................................................................................................................................................................................................17

4Cost..............................................................................................................................................................................................................20

4.1CostAnalysis......................................................................................................................................................................................................................204.1.1LaborCosts.......................................................................................................................................................................................................204.1.2PartCosts...........................................................................................................................................................................................................20

5Conclusion................................................................................................................................................................................................21

5.1Accomplishments.............................................................................................................................................................................................................21

5.2ImprovementsandFurtherWork............................................................................................................................................................................21

5.4EthicsandSafety.............................................................................................................................................................................................................21

6References................................................................................................................................................................................................23

AppendixA:R&VTable............................................................................................................................................................................24

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1 Introduction

1.1 Objective

Sometimes it can be an inconvenience for amateur musicians to gather for a jam session. They must consider the noise level of their instruments and its effect on their immediate environment. In addition to this, necessary equipment such as cables and amplifiers can be difficult to transport. The premise of this project is to provide an affordable and portable way for bands with electric instruments to be able to practice or “jam” without causing distraction to their peers and neighbors. For electric instruments, setting up to play involves bringing their instrument, an amplifier, and the necessary cables to their gathering. This alone can be a burden. Our mixer will allow musicians to “jam” with others without being tethered to a cord. It will also allow for “quiet” playing because everything will be heard through wireless headphones. Our mixer design will also help solve the problem of portability since it will be wireless and compact.

1.2 Background

As consuming music has become increasingly wireless, it is surprising that producing music has not moved forward at the same rate. The market for wireless mixers is both small and expensive. Our initial searches have found other wireless analog mixers with MSRPs starting at $280 [1]. It also seems that even though these mixers are wireless, a cord must still be used to connect an electric instrument directly. On the instrument side, electric guitar transmitters are priced around $100 [2]]. With our solution, everything will be completely wireless, includes the equipment necessary for playing (i.e. instrument transmitters). This will eliminate the expense of buying transmitters exclusively. The concern for noise is becoming more relevant as people have started choosing apartments for their housing. For example, on the University’s campus it can be difficult for students to find a suitable location to play music without concern of a noise complaint. The campus facilities are not always available when needed. Coordinating a good time or place can be cumbersome for people in areas without access to facilities like this.

1.3 High-Level Requirements For this system to be successful, these high-level requirements must be satisfied:

1. The analog mixer must have a high Signal-to-Noise Ratio (SNR) at the output, with a noise floor near or below -75 dB ± 5%.

2. The equalizer must be able to adjust the amplitude of the audio signal centered at five distinct frequency: 80 Hz, 350 Hz, 2500 Hz, 6000 Hz, and 12000 Hz.

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3. The instrument module must be able to connect to its paired Base modules and transmit audio from a distance of at least eight meters with a battery life of at least 30 hours.

2 Design

2.1 Block Diagram The design consists of four modules: the instrument module, power unit module, master base module, and the slave base module. A maximum of four instrument modules connect each instrument to the base modules via Bluetooth connectivity. Each instrument sends a signal to the base units, where it handles the mixing and equalizing of the audio signals. The final output is sent to each musician’s Bluetooth-connected headphones. The slave unit works virtually the same as the master with the exception that the output of its onboard mixer is sent to the input of the master mixer. A physical connection is made between the slave and master using banana plugs. The high-level block diagram is shown in Figure 1.

Figure 1: Modular Mixer Block Diagram

2.2 Physical Design The physical hardware of the base modules can be seen in Figure 2. The left picture is a top view of the base module and the right picture shows a side view of the base module. The knobs in the center are potentiometers to control the input gain. The neighboring sliders are the

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potentiometers to control the levels of the equalization stage. The instrument modules are shown in Figure 3. The mono phone plug connects directly into the instrument.

Figure 2: Physical Design of Master and Slave Base Modules

Figure 3: Physical Design of Instrument Module

2.3 Instrument Module

2.3.1 Bluetooth Transmitter This block consists of a Bluetooth module that utilizes the Advanced Audio Distribution Profile (A2DP) to stream audio to the base module. We chose the RN-52 as our Bluetooth module due to its onboard Analog-to-Digital converter, Microcontroller, and antenna, shown in Figure 4. It

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requires 3.3 V ± 5% and 10 mA during transmission. The onboard MCU was programmed to accept the analog input from the instrument and enable transmission to the base module’s receiver. The transmitter was given a Bluetooth “friendly” name for use when pairing. The onboard ADC is then configured to convert the analog input with a 16-bit resolution sampled at 44.1 kHz.

Figure 4: Bluetooth Transmitter on PCB

2.3.2 Battery Module Three AA batteries were used to supply power to the instrument module for a total of 4.5 V. This module contains a voltage regulator to supply the correct voltage for the Bluetooth transmitter. The TI LM1117 regulator with a fixed 3.3 V output was used to keep a constant voltage for the Bluetooth Transmitter. This circuitry was also used in the mixer base units, but with a supply voltage of 12 V. A diagram of the battery module is shown below in Figure 5. The LM1117 supports an output current of up to 800 mA, which is far greater than the approximately 30 mA.

. Figure 5: Voltage Regulation Circuit

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2.4 Base Module

2.4.1 Voltage Regulator Because the Bluetooth modules need lower voltage to power them, a voltage regulator was used to step the voltage down from 12 V to 3.3 V. The circuit in this voltage regulator was identical to that of the battery module, shown in Figure 5 above.

2.4.2 Audio Mixer The audio mixer accepts two analog audio signals, mixes them, and outputs a mixed signal. Since our mixer has the option of mixing up to four instruments, the Slave base module sends a mixed audio signal to the master for further mixing. The mixer PCB outputs a mixed audio signal of up to four instruments. The final output is sent to the different equalizer modules for further customization. The chosen configuration was an active ‘virtual earth’ mixer implemented with the OPA134 op-amp chip. Since this was an active mixer design the voltage requirement was 11.5 -12.5 V. This design was chosen to reduce the amount of noise transmitted to the equalizer. The overall mixer module contains three stages. The first stage is a preamp which is a parallel configuration of non-inverting op-amps. This stage helps increase the signal-to-noise ratio of the audio signal after initial transmission. The second stage is a gain controller which allows the musicians to adjust the soundstage. In other words, this allows the musicians to increase the volume of individual instruments. The final stage is used to mix the altered audio signals and set the appropriate voltage level for the equalizer. A schematic of the entire mixer design is shown below.

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Figure 6: Complete Mixer Module

2.4.2.1 Preamp To prepare the audio signal for mixing, a preamp was used to increase the desired signal above the noise floor of the circuitry. One of the issues with amplification, is that the noise gets amplified along with the desired signal. Therefore, the configuration of op-amps needs to be considered carefully. To mitigate the amount of noise gain, three op-amps were placed in a parallel configuration with a single input audio signal going to the non-inverting side of each op-amp [6]. A useful calculation to characterize the benefits of using a parallel configuration is the signal-to-noise ratio. This will help understand the strength of the mixed signal with the noise floor of the op-amps and external noise. By using this configuration, the input signal gain increases by a multiple of two while the noise gain increases by a multiple of √2. This helps further separate the strength of the input signal from the noise floor thus reducing distortion in the output signal. To demonstrate the benefits of using a parallel configuration, the following equations were used: 𝑆$%&'& = 𝑆$% + 𝑁$% (1)

𝑆+,- = 𝑆$% ∗ 𝐺 (2)

𝑁012-+3 =45

(4789/;) (3)

where 𝑆$%&'& is the total rms power at the input stage of the op-amp, 𝑆$% is the rms power of the input signal, 𝑁$%is the rms power of the external noise, 𝑆+,- is the rms power of the output

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signal, 𝐺is the gain, 𝑁012-+3 is the noise gain multiple, 𝑅>$?is the equivalent resistance at the mixing bus, and 𝑁is the number of channels being mixed.

𝑆𝑁𝑅(1𝑜𝑝 − 𝑎𝑚𝑝) = GH'I&;'I&

JK= L H8M∗N

(;8M∗N)O;P7QRK (4)

where 𝑁1>Sis the internal noise associated with the active op-amp. With two op-amps in parallel the SNR becomes:

𝑆𝑁𝑅(2𝑜𝑝 − 𝑎𝑚𝑝𝑠) = (KH8M∗N)U

V(K;8M∗N)UO√KW;P7QXUO;P7QU

UYZ (5)

= (KH8M∗N)U

V(K;8M∗N)UO√KW;P7Q&'&UYZ

(6)

The schematic below shows what the preamp stage of the mixer looks like. The waveforms in Figure 8 show a simulation of the behavior of the preamp for a signal before and after the preamp stage.

Figure 7: Preamp Design for Single Signal [12]

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Figure 8: Waveforms of Amplified Audio Signal: Amplified Signal (Top); Pre-amplified Signal (Bottom)

2.4.2.2 Gain Controller The next step in the signal path was to adjust the soundstage. By doing so, the musicians have the ability to increase or decrease the volume of individual instruments. To achieve this, an inverting op-amp with a feedback loop was used to set the upper-bound for the possible gain an instrument signal could experience. At the output of this op-amp was a 10𝑘𝛺 turn potentiometer which was used to decrease the signal to zero or increase the signal to the maximum gain. Initially, the output from the potentiometer contained high frequency noise. To reduce this noise, an RC filter was placed at the output of the potentiometer. The value for resistance of the filter was set to be one-hundred times that of the potentiometer and was placed in parallel with a 10𝜇𝐹capacitor. In order to calculate the gain, we used a ‘virtual ground’ configuration for the mixer to simplify the math greatly.

𝐺𝑎𝑖𝑛 = b'I&b8M

(7)

Since the voltage on the non-inverting side of the amplifier is zero this implies a zero voltage on the inverting side of the amplifier. Thus, the Kirchhoff current law becomes

𝑖$% = 𝑖cd + 𝑖cK (8) 𝑖+,- =

b'I&45

(9)

𝑖+,-=𝑖$% (10) 𝑉𝑜𝑢𝑡𝑅𝑓

= 𝑖𝑠1 + 𝑖𝑠2 (11)

where 𝑖cdand 𝑖cK is the current of the first and second input signal, 𝑖$%is the current after mixing,𝑖+,-is the current at the output of the op-amp, and 𝑅0is the feedback resistance. The

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input current can then be expressed in terms of the voltage and resistance of each source and the gain is realized from some simple algebra.

𝑖$% =biX4iX

+ biU4iU

(12) 𝑉𝑜𝑢𝑡𝑅𝑓

= 𝑉𝑠1𝑅𝑠1+ 𝑉𝑠2

𝑅𝑠2 (13)

|𝑉𝑜𝑢𝑡| =𝑅𝑓𝑅𝑠1𝑉𝑠1 +

𝑅𝑓𝑅𝑆2𝑉𝑠2 (14)

Therefore, if 𝑅0 = 𝑅cd = 𝑅cKthe op-amp produces unity gain.

2.4.2.3 Final Mix The final stage of the mixer is the point where both the master and slave instrument signals mix together and go through another inverting op-amp before being sent to the equalizer. The gain at this stage was set to 20 to boost the mixed signal to the appropriate voltage level for the equalizer. The following schematic shows the final stage of the mixer design and a simulation of two instrument signals with the mixed output signal.

Figure 9: ‘Virtual Ground’ Summing Mixer Design [7]

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Figure 10: Waveforms of Mixed Audio Signals: Mixed Signal (Red); Random Voice Signal (Blue); Guitar

Signal (Green)

2.4.3 Equalizer The equalizers were designed to be uniquely customizable to each user. With this in mind, the equalizer was designed to be a five-band equalizer. It consists of a filter bank of band-pass filters to accommodate for each frequency band. Each band allows for a specific range of frequencies each centered at a key frequency in audio production: 80 Hz, 350 Hz, 2500 Hz, 6000 Hz, and 12000 Hz [3]. The output of each equalizer then goes to a Bluetooth module for transmitting the final audio signal. A schematic of the equalizer is shown below in Figure 11. The diagram depicts two of the five filters for the filter bank.

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Figure 11: Equalizer Schematic

Taking a closer look at the filters themselves, we used a multiple-feedback bandpass filter. One advantage of this type of filter is inductors were unnecessary to produce the band-pass shape. This also helped with costs because inductors can be expensive when operating with lower frequencies [4]. Since we used the OPA134 and OPA4134 chips for the op-amps, the equalizer required 11.5 -12.5 V. A schematic of the filter is shown below in Figure 12.

Figure 12: Multiple Feedback Bandpass Filter [4]

In order to get a desired center frequency for each filter, the resistor and capacitor components need to be calculated. For a straight-forward design of MFB filter, let C1 = C2 = C. Using the equations below, we can calculate the parameters for each MFB filter [4][5]:

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Assume 𝑓k= 80 Hz and C = 0.1𝜇𝐹,

𝑓k =d

KlmnXnU4X4U= d

Klnm4X4U= 80𝐻𝑧 (15)

𝑄 = 0stu

= dKv

4U4X= wk

dkkxyk= 2 (16)

𝑘 = 2𝜋𝑓k𝐶 = 2𝜋(80)(0.1 ∗ 10xy) = 5.03 ∗ 10x� (17) 𝑅d� =

���, 𝑤𝑎𝑛𝑡𝐻 = 1 → K

d(�.k�∗dk��)= 39.8𝑘𝛺 (18)

𝑅dt =�

(K�Ux�)�= K

(K∗KUxd)(�.k�∗dk��)= 5.7𝑘𝛺 (19)

𝑅d = 𝑅d�||𝑅dt =4X�4X�4X�O4X�

= ��.w(�.�)��.wO�.�

≈ 4.99𝑘𝛺 (20)

2 = dKv

4U4X→ 4 = d

�4U4X→ 16𝑅d = 𝑅K → 𝑅K = 79.8𝑘𝛺 (21)

where 𝑓k is the center frequency of the band-pass filter, BW is the bandwidth, Q is the quality factor, and H is the mid-band Gain. We want to keep the quality factor, Q, constant. Therefore, we chose 𝑓k and the bandwidth, BW, based on the quality factor equaling two. The parameters of the equalizer are shown below in Table 1.

Table 1: Equalizer Parameters

𝒇𝟎(Hz) 𝒇𝑳 (Hz) 𝒇𝑯 (Hz) C (𝛍F) 𝑹𝟏𝑨 (kΩ) 𝑹𝟏𝑩 (kΩ) 𝑹𝟐 (kΩ)

80 60 100 0.1 39.8 5.7 79.8

350 262.5 437.5 0.1 9.1 1.3 18.2

2500 1875 3125 0.047 2.7 0.387 5.4

6000 4500 7500 0.0022 24.1 3.4 48.2

12000 9000 15000 0.0022 12 1.7 24.1

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3. Verification

3.1 Bluetooth Modules

3.1.1 Sampling Resolution and Frequency The Bluetooth modules were configured according to the RN52’s command reference guide. We used a set command provided by the manufacturer to set the A2DP audio routing value to have a sample rate of 24 bits per sample and a sampling frequency of 44.1 KHz.

3.1.2 Transmission Range The Bluetooth module’s most important quality is the transmission range. In order to meet our high-level requirement, the transmitters must meet the minimum eight meters of range. To verify our results, we placed a transmitter and a Bluetooth receiver eight meters apart from each other. We attached a speaker to the output of the Bluetooth receiver and a waveform generator to the input of the transmitter. After pairing the two devices, we were able to hear a waveform transferred from the transmitter at the output of the speakers.

3.2 Battery Module

3.2.1 Voltage Regulation The voltage regulator was tested at its output using both a 12 V and a 4.5 V input. The output voltage was measured using a multimeter and verified to be within five percent of the required 3.3 V to power the Bluetooth modules. The multimeter output is shown below in Figure 13.

Figure 13: Multimeter output of voltage regulator

3.2.2 Battery Life To calculate the battery life of the instrument module, we connected a DC power supply to the voltage regulation circuitry to simulate battery power. We then monitored for the maximum

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current required, which was found to be 31 mA, as seen in Figure 13. Using the average capacity of a AA battery, we were able to determine the estimated worst-case battery life of the instrument module using the following formula:

𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦[𝑚𝐴ℎ]/𝐶𝑢𝑟𝑟𝑒𝑛𝑡[𝑚𝐴] = 𝐵𝑎𝑡𝑡𝑒𝑟𝑦𝐿𝑖𝑓𝑒[ℎ𝑜𝑢𝑟𝑠] (22) 1800𝑚𝐴ℎ/31𝑚𝐴 = 58.06ℎ𝑜𝑢𝑟𝑠 (23)

Figure 14: Maximum current draw of Bluetooth Module from power supply

3.3 Audio Mixer The audio mixer had two high-level requirements. It had to be able to take in two signals of different frequencies and output a summation of them. It also had to have a high signal-to-noise ratio with a noise floor near or below -75 dB with an error of ±5%. To test and verify that the mixer met these requirements, the response of the mixer was viewed in both the time and frequency domain. In the time domain, two signals of different frequencies were sent into the mixer and the voltage waveforms were viewed on an oscilloscope. In the frequency domain, the same signals were sent through the mixer and the frequency spectrum was viewed on a spectrum analyzer. The following figures show an example of what the waveform of a mixed signal would look like. In this case, the input signals were sine waves set to oscillate at 350 Hz and 2.5 kHz. The smaller peaks represent the 2.5 kHz signal which is superimposed with the 350 Hz signal. As the turn potentiometer of the 2.5 kHz approaches maximum gain, the resulting waveform has more prominent peaks from that signal, whereas the opposite is true for the 350 Hz signal.

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Output Signal: 𝑦(𝑡) = 𝑠𝑖𝑛(2𝜋(350)𝑡) + 𝑠𝑖𝑛(2𝜋(2500)𝑡)

Figure 15: Expected Waveform

Figure 16: Observed Waveform

The observed waveform yielded the expected results with the exception that it was not centered around 0 V. This led to the belief that there was DC offset present in the circuit. To combat this, a 10𝜇𝐹 coupling capacitor was placed at the output. This helped eliminate the offset. To verify that the mixer was performing correctly in the frequency domain, the same test was performed and output on a spectrum analyzer. The input signals used for this test were sine waves set to oscillate at 9 kHz and 15 kHz, both at 10 dBm. The reason for using different input frequencies was due to the limitations of the signal generators in lab. The lowest frequency signal that could be reproduced was a 9 kHz sine wave. The resulting frequency spectrum was as follows.

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Table 2: Frequency Spectrum of Mixer Output:

The bolded frequency and power are that of the summation frequency of the two input signals. Note that the power is the highest in the entire spectrum, and that the sum of the input power level is equal 14 dBm. This makes sense because the power of each signal is summed in units of watts. The calculation is as follows. 𝑃>u = 10«¬�7/dk (24)

𝑃­t> = 10log(𝑃>u) (25) 10𝑑𝐵𝑚 = 0.01𝑊 ⇒ 𝑝𝑜𝑤𝑒𝑟𝑜𝑓𝑜𝑛𝑒𝑖𝑛𝑝𝑢𝑡𝑠𝑖𝑔𝑛𝑎𝑙 (26) 0.01𝑊 + 0.01𝑊 = 0.02𝑊 ⇒ 13.01𝑑𝐵𝑚 ≈ 14𝑑𝐵𝑚 (27) The noise floor of the circuitry as well as the signal-to-noise ratio was found by viewing the output of one input sent through the mixer. The input signal was chosen to be 9 kHz, the lower limit of the signal generator. The result of this test can be seen below.

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Figure 17: Noise Floor of Mixer Output

To calculate the signal-to-noise ratio, the power level of the input signal was compared with the power level of the noise floor. The following calculation shows that the ratio was enough to recover a clean output signal.

𝑆𝑁𝑅u = H$¶%1·«+¸¹3;+$c¹«+¸¹3

= x�k­t>x�k­t>

= d×dk�»ud×dk�Xsu

= 1000 (28) 𝑆𝑁𝑅­t = 10𝑙𝑜𝑔dk(𝑆𝑁𝑅u) = 30𝑑𝐵 (29) Where 𝑆𝑁𝑅uis the signal-to-noise ratio with respect to watts and 𝑆𝑁𝑅­tis the signal-to-noise ratio with respect to decibels.

3.4 Equalizer In order to test and verify that the equalizer behaved as expected, a 2.5 kHz sine wave was sent to the input. We probed the output of the equalizer and displayed the waveform on an oscilloscope. The sine wave with no boosting or cutting is shown below in Figure 18.

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Figure 18: 2.5 kHz Sine Wave w/ no Boost or Cut

Then, we tuned the slide potentiometer connected to the filter with the 2.5 kHz center frequency. As seen by Figures 19 and 20 below, the amplitude of the waveform increased when “boosting” the frequency and decreased when “cutting” the frequency.

Figure 19: 2.5 kHz Sine Wave w/ Full Boost

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Figure 20: 2.5 kHz Sine Wave w/ Full Cut

Another way to test the equalizer’s functionality was to connect a small speaker to the output. First, we tested the equalizer with a 2.5 kHz sine wave. A clear tone was produced. Once we began boosting the frequency, the desired effect of increasing the sound occurred. Next, we connected the equalizer to the mixer, and inputted a mixed signal of a 350 Hz sine wave and 2.5 kHz sine wave. We kept the speaker connected to the output to listen to what would happen by boosting one of the signal’s frequencies. We chose these two frequencies because it allowed us to distinguish the difference in sound of each frequency. We boosted the slide potentiometer associated with the lower frequency and it could be heard that the lower frequency was increased. Then, we cut the lower frequency and the 2.5 kHz frequency became more prominent in the signal.

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4 Cost

4.1 Cost Analysis

4.1.1 Labor Costs Utilizing the average starting salary of an EE graduate at the University of Illinois [8], we calculated a reasonable labor rate.

($67,000 / (52 weeks * 40 hours)) = $32.21 per hour (30) $32.21 * 3 members * (15 hours/week) * 15 weeks = $21,741.75 (31)

4.1.2 Part Costs Table 3: Parts and Component Costs

Part Name Manufacturer Quantity Unit Price

Total Cost

RN52-I Bluetooth Module Microchip 9 $17.72 $159.48

OPA134 Low-Noise Op-Amp Texas Instruments 30 $2.42 $72.60

OPA4134 Low-Noise Op-Amps

Texas Instruments 10 $4.61 $41.60

LM1117 3.3V Linear Regulator Texas Instruments 9 $1.10 $9.90

PTA4543 Slide Potentiometer (10 kOhms)

Bourns 20 $1.10 $22.00

PC16S Knob Potentiometer (10 kOhms)

Piher 6 $4.69 $28.14

NYS224G ¼” Conductor Plug Neutrik 3 $1.67 $5.01

Miscellaneous Capacitors, Resistors, and Wires

N/A N/A $15 $15.00

PCB - Bluetooth Module PCBWay 10 $0.50 $5.00

PCB - Mixer PCBWay 5 $1.00 $5.00

PCB - Equalizer PCBWay 5 $7.60 $38.00

Total $401.73

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5 Conclusion

5.1 Accomplishments As far as individual modules were concerned, we were able to satisfy our high-level requirements. During the design process, we learned a lot about production starting from an idea, all the way through a final product.

5.2 Improvements and Further Work Although the individual modules were successful, we were unable to fully integrate our system to the original specifications. One of the main issues we ran into during the design was with the input signal from the instrument. We now know that further amplification was necessary to boost the signal to a proper level for the Bluetooth transmitters. The other issue we ran into involved the differential audio signal at the output of the Bluetooth receivers. We realized that we needed to either implement a difference op-amp to convert the signal to a single-channel signal or change the op-amps in the mixer and equalizer to one that accepts a differential input. Another aspect of our project we could improve on is the projected cost of our system. In order to help bring down the cost, we could have changed our design to utilize the op-amp chips with more than one op-amp (i.e. the OPA4134 chips). This would bring down the need for the single OPA134 chips and in turn, bring down the cost. The cost of the equalizer PCB’s could have been minimized by utilizing the OPA4134 chips because it would help shrink the size of the PCB. As for future updates, we would like to have had the time to add a digital output at the end to allow for the user to view the signal on a computer.

5.4 Ethics and Safety Batteries can potentially be dangerous and must be implemented in a way such that extended overheating and fires are not a concern for the instrument module. The mixer’s voltage regulator must also supply proper voltages to the various modules in order to avoid overheating and electrical failures. Proper listening volume is a concern for the end user. Users must take care to avoid listening to music with their headphones at maximum volumes (about 100 dBa) for over 15 minutes to avoid permanent hearing damage [9]. This information will be disclosed to the end user in accordance to the IEEE Code of Ethics [10]. Because our mixer is a Bluetooth device, there are several FCC regulations that must be adhered to in order to guarantee safety and legality. The Bluetooth module must be qualified with the Bluetooth SIG. We intend on using an unmodified, pre-qualified Bluetooth module to

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satisfy these requirements. This module will be using the approved A2DP profile during transmission of our audio signals.

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6 References

[1] Sweetwater.com. Peavy PV 6 BT Mixer with Bluetooth and Effects. [Online]. Available at: https://www.sweetwater.com/store/detail/PV6BT--peavey-pv-6-bt-mixer-with-bluetooth-and-effects [Accessed 9/20/2018]. [2] Guitarcenter.com. Line 6 Relay G10T Wireless Guitar Transmitter. [Online]. Available at: https://www.guitarcenter.com/Line-6/Relay-G10T-Wireless-Guitar-Transmitter.gc [Accessed 9/20/18] [3] “EQ tips Cheat Sheet”. Cheatography. [Online]. Available at: https://www.cheatography.com/fredv/cheat-sheets/eq-tips/ [Accessed 10/1/18] [4] “Multiple Feedback Band-pass Filter”. eCircuit Center. [Online]. Available at: http://www.ecircuitcenter.com/Circuits/MFB_bandpass/MFB_bandpass.htm [Accessed 10/1/18] [5] “Active Band-pass Filter”. ElectronicTutorials. [Online]. Available at: https://www.electronics-tutorials.ws/filter/filter_7.html [Accessed 10/1/18] [6] “Paralleling Amplifiers Improves Signal-to-Noise Performance”. Analog Devices. [Online]. Available at: http://www.analog.com/en/technical-articles/paralleling-amplifiers-improves-signal-to-noise-performance.html [Accessed 10/1/18] [7] Elliot, R. “Audio Signal Mixing”. ESP. [Online]. Available at: http://sound.whsites.net/articles/audio-mixing.htm [Accessed 10/1/18] [8] “Salary Averages”. University of Illinois. [Online]. Available at: https://ece.illinois.edu/admissions/why-ece/salary-averages.asp [Accessed 10/3/18] [9] “Noise Induced Hearing Loss (NIHL)” Dangerous Decibels. [Online]. Available at: http://dangerousdecibels.org/education/information-center/noise-induced-hearing-loss/ [Accessed 9/19/2018]. [10] IEEE.org. IEEE Code of Ethics. [Online]. Available at: https://www.ieee.org/about/corporate/governance/p7-8.html [Accessed 9/18/2018].

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Appendix A: R&V Table Requirement Verification Verification

Status Bluetooth Module 1. The onboard ADC must convert a microphone-level input to a digital audio signal with a 16-bit resolution and sampling frequency of 44.1kHz

1. Connect an audible dummy signal to the microphone inputs of the Bluetooth module.2. Enable the Probe the I2S output of the Bluetooth module with an oscilloscope.3. Using the oscilloscope, confirm that the frame is 16-bits wide.4. Using the oscilloscope, verify that the bit clock frequency is 44.1kHz * 16 * 2 = 1.4112 MHz for stereo and 44.1kHz * 16 = 705.6 kHz for mono.

Y

2. Transmitter must have a range of at least eight meters.

1. Pair the Bluetooth module with a compatible Bluetooth speaker.2. Using a function generator, create a 1 kHz sine wave and connect the output to the microphone input of the Bluetooth module.3. Verify that the audio can be heard while standing at a distance of eight meters.

Y

Voltage Regulator 1. Voltage regulator provide 3.3 V ± 5% from a 4.5 V source for the Bluetooth Transmitter module. It must provide current between 0 mA and 25 mA.

1. Attach a grounded resistive load to Vout of the voltage regulator of 132 Ohms.2. Measure the output voltage using an oscilloscope. Verify that the output voltage stays within 5% of 3.3 V.

Y

2. Battery Module should be able to provide power to the instrument module for 30 ± 5% hours.

1. Using the same setup, measure the amount of time required for the battery to drain while outputting 25 mA.

Y

Mixer Module 1. The output from the mixer must be the summation of the inputs to the mixer.

1. Using a function generator, create two sine waves, one with a 1 kHz frequency, and the other with a 800 Hz frequency as inputs to the mixer.2. Attach output of mixer to a speaker.

Y

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3. Verify that the output is a mix of both the input

2. Must have high Signal-to-Noise Ratio (SNR) with a noise floor near or below -75 dB ± 5%

1. Using a function generator, create two identical sine waves with a 1 kHz frequency as input.2. Using a spectrum analyzer, probe the output and verify the SNR.

Y

Equalizer 1. Each band-pass filter in the equalizer must provide ± 3 dB for the five frequency bands centered at 80 Hz, 350 Hz, 2500 Hz, 6000 Hz, and 12000 Hz.

1. For each frequency band, set the level to +3 dB and the rest to -3 dB.2. Connect a function generator to the input of the equalizer and run a frequency sweep across 20 Hz to 16 kHz.3. Connect a spectrum analyzer to the output and analyze the frequency response. The amplitude of the center frequency should be +3 dB relative to the input. The other frequency bands should be -3 dB.

Y