Auto Harmonizer

EEL 4924 Electrical Engineering Design (Senior Design)

Final Design Report

26 April 2012

Team Name: Slubberdegullions

Team Members: Josh Elliott and Henry Hatton, Jr.

Project Abstract:

Our project consists of creating an Auto Harmonizer which will capture an audio

signal and generate a harmonized output in real time. This project will rely extensively on

digital signal processing to accomplish the harmonization, but will also utilize analog

circuitry to allow for the inclusion of adjustable equalization. The device will be able to take

an analog input from an XLR port or a ¼ inch audio jack, pre-amplify the input signal,

condition it for the DSC, and mix the original signal and the harmonized signal before

sending them to the equalizer. Additionally, users will be able to customize the response of

the device through the use of knobs which will adjust the equalization of the output signal.

The two most important technical components of this project are the programming of the

DSC and the design of the analog equalization circuitry.

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Table of Contents Introduction..………………………………………………………………..............................3

Objectives………………………………………………………………...................................3

Design Aspect Digital / DSP………………………………………………..............................4

Design Aspect Analog…………………………………………………………….……….......6

Analog Equalization Technology……………...........................................................................9

Cost Estimate…………………..…………...............................................................................9

Division of Labor…………..…………….................................................................................9

List of Tables 1. Center Frequency Bands…………………………………………………....................6

List of Figures 1. System Flowchart….....…………………………………………………......................3

2. Software Flowchart ………….....………......................................................................5

3. Initial Band-Pass Filter Configuration…..…….............................................................6

4. Revised circuit Configuration utilizing Notch filters…....…………...……………….7

5. Balanced to Line Mic Preamp………………………………...……………………….8

6. DC Offset and 1k Hz LPF……………………………………………………………..8

7. Mixer Amplifier……………………………………………………………………….8

8. Gantt Chart…….………….....……….........................................................................10

9. XLR Balanced to Line Mic Preamp…………….……………………………………10

10. Housing……………...……………………………………………………………….11

11. 5 Band Equalizer………………………………………………………………..……11

12. DSP……………...……………………………………………………………………12

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Introduction

The heart of this project lies in the development of a system that will allow the user to

harmonize an input signal with at least one harmonized output. Furthermore, the user will

need to be able to modify the quality of the out through an adjustable, analog equalization

circuit. Figure 1 below shows the System Flowchart.

INPUT Balanced XLR

or¼“ Jack

DSP

Mixing Amplifier

OUTPUT¼ “ Jack

Balanced to Line Level

DC OffsetParametric

Equalization1 KHz LPF

Figure 1 System Flowchart

Project Objectives – Digital / DSP

Real-time harmonization of an input signal with a sample from memory.

The input is recorded for a short period of time; data capture occurs at the behest of an

external trigger. Data output also happens in response to a hardware trigger.

The input signal can be any periodic audio signal.

FFTs are used to create proper harmonies.

Project Objectives – Analog

Proving a balanced XLR to line level or ¼” input jack.

Pre-amplifying the microphone input signal.

Providing a DC offset for an optimal input to the DSP

Providing 5 bands of equalization to include the following center frequency channels

shown in Table 1.

Utilizing active low-pass and notch filters.

Minimize noise in the system.

Reduce RF susceptibility.

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Design Aspect – Digital / DSP

Overview

This portion of the project relies extensively on the use of software algorithms

implemented on a TI DSC to produce the required digital signal processing necessary to

accomplish harmonization. Some key factors that guided the design process are as follows:

Utilization of TMS320F28335 DSC (Used in EEL4744)

Chosen because of familiarity and processing capabilities

Not a heavy hitting DSP, but its use greatly simplified potential design problems

that would have been salient with less powerful microprocessors

Immediate availability of dev-board for early testing and development

Extensive amount of resources and support readily available on campus

Real-time Harmonization

Required much software optimization, but proved to be possible

Delayed output of harmony is very small

Assumption of Periodic Input

Made discerning the fundamental frequency more attainable

A signal with a primary frequency component caused frequency domain operations

to be simpler

Fast Fourier Transforms

Difficult algorithm to get properly functioning on a small DSC

Utilizing existing TI libraries made the process easier

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Software Flowchart

The flowchart in Figure 2 below describes the basic structure of the algorithm that is

used to accomplish the harmonization of the signal. The software flow depicted below

occurs at every instance in which an external trigger is detected.

Receive Audio from ADC

Take FFT

Prepare Audio for FFT

(windowing, etc)

Find Fundamental Frequencies

Quadratic Interpolation to

Improve FrequencyResolution

Compare Frequency with LUT to find “in

tune” note

Output Desired Harmony

Figure 2 Software Flowchart

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Design Aspect – Analog

Provide a multi-band active parametric equalizer to fine tune the harmonized output

signal. Active filters are favored for the low cost, light weight, small size and gain

availability. Utilizing 2nd

order filters consisting of 2 capacitors and 2 resistors. The center

frequency (fₒ) bands are shown below in Table 1.

Table 1

Center Frequency Bands

Band fₒ (Hz)

1 32

2 125

3 500

4 2,000

5 16,000

We experimented with various circuit configurations for the active band-pass filters.

The initial circuit configuration for one of the band-pass filters is shown below in Figure 3.

Figure 3 Initial Band-Pass Filter Circuit Configuration

Design Challenges

The difficulty with the initial circuit configuration was too much gain for the audio

operational amplifiers. After adjusting the filters to unity gain a cleaner, less distorted audio

signal passed through the filter stages. Additional problems with the analog circuitry were

with the PCB. Narrow trace separation width increased noise in the system.

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Revised Notch Filter Design

Figure 4 below shows the revised equalization circuitry

Figure 4 Revised circuit Configuration utilizing Notch filters

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Figure 5 shows the design for the Balanced XLR input to line level input microphone

preamplifier.

Figure 5 Balanced to Line Mic Preamp

Figure 6 shows the design for the DC offset to provide the DSP input with a level of

approximately 1.5 Volts and a 1 kHz LPF to make the signal processing easier.

Figure 6 DC Offset and 1 kHz LPF

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Figure 7 shows the mixer amplifier design.

Figure 7 Mixer Amplifier

Analog Equalization Technology

Equalization filter circuitry is used in applications such as program enhancement,

sound reinforcement, telecommunications and data acquisition. There are many different

types of equalizers. A passive equalizer consists of inductors, capacitors and resistors and

does not require power to operate. The advantages for the passive equalizer are low noise

performance, good reliability, low RFI interference susceptibility and high dynamic range.

The disadvantages for passive equalizer are large size, weight, cost and the need for

shielding. The active equalizer features operational amplifiers and various other components

and require power to operate. The advantages for active equalizers are small size, light

weight, low cost and gain availability. The disadvantages for the active equalizer are

increased noise, small dynamic range and RFI susceptibility.

Division of Labor

Josh Elliott

Interfacing of DSC with peripherals

DSP software algorithms for DSC

PCB for DSC and digital components

Henry Hatton, Jr.

Design of the Balanced to Line Mic Preamplifier

Design of the DC Offset circuitry and 1 kHz LPF

Design of the equalization circuitry

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Design of the 2 PCB boards for the analog circuitry

Figure 8 Gantt chart

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Figure 9 – XLR Balanced to Line Mic Preamp

Figure 10 – Housing

Figure 11 – 5 Band Equalizer

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Figure 12 – DSP