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.
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 2
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. DSP INPUT Balanced XLR or ¼ Jack Balanced to Line Level DC Offset 1 KHz LPF Mixing Amplifier Parametric Equalization OUTPUT ¼ Jack 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. 3
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 4
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 Prepare Audio for FFT (windowing, etc) Quadratic Interpolation to Improve Frequency Resolution Take FFT Compare Frequency with LUT to find in tune note Find Fundamental Frequencies Output Desired Harmony Figure 2 Software Flowchart 5
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 2 nd 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. 6
Revised Notch Filter Design Figure 4 below shows the revised equalization circuitry Figure 4 Revised circuit Configuration utilizing Notch filters 7
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 8
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 9
Design of the 2 PCB boards for the analog circuitry Figure 8 Gantt chart 10
Figure 9 XLR Balanced to Line Mic Preamp Figure 10 Housing Figure 11 5 Band Equalizer 11
Figure 12 DSP 12