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Transmission of Stereo Audio Signals with Lasers William Austin Curbow University of Arkansas, Fayetteville

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Transmission of Stereo Audio Signals with Lasers William Austin Curbow University of Arkansas, Fayetteville University of Arkansas, Fayetteville ScholarWorks@UARK Electrical Engineering Undergraduate Honors Electrical Engineering Theses 5-2014 Transmission of Stereo Audio Signals with Lasers William Austin Curbow University of Arkansas, Fayetteville Follow this and additional works at: http://scholarworks.uark.edu/eleguht Part of the Signal Processing Commons Recommended Citation Curbow, William Austin, "Transmission of Stereo Audio Signals with Lasers" (2014). Electrical Engineering Undergraduate Honors Theses. 31. http://scholarworks.uark.edu/eleguht/31 This Thesis is brought to you for free and open access by the Electrical Engineering at ScholarWorks@UARK. It has been accepted for inclusion in Electrical Engineering Undergraduate Honors Theses by an authorized administrator of ScholarWorks@UARK. For more information, please contact [email protected], [email protected]. Dr. Jingxian Wu Transmission of Stereo Audio Signals with Lasers An Undergraduate Honors College Thesis in the Department of Electrical Engineering College of Engineering University of Arkansas Fayetteville, AR by William Austin Curbow TABLE OF CONTENTS I. ABSTRACT ............................................................................................................................ 1 II. INTRODUCTION ............................................................................................................... 2 III. THEORETICAL BACKGROUND ..................................................................................... 3 IV. DESIGN PROCEDURE ...................................................................................................... 9 V. TESTING AND RESULTS ............................................................................................... 19 VI. CONCLUSION .................................................................................................................. 27 VII. REFERENCES .................................................................................................................. 28 LIST OF FIGURES Figure 1. Flow Diagram of Laser Transmission System ................................................................ 2 Figure 2. Basic PWM Waveform from Single Frequency Sinusoid .............................................. 4 Figure 3. Audio Amplifier Flow Diagram ..................................................................................... 5 Figure 4. BJT Differential Amplifier Circuit Topology ................................................................ 6 Figure 5. Class AB Output Stage with VBE Multiplier for Bias .................................................... 8 Figure 6. Simple Resistive Current Source Circuit ...................................................................... 14 Figure 7. Input Conditioning Circuit ........................................................................................... 15 Figure 8. Zobel Network .............................................................................................................. 16 Figure 9. Audio Amplifier Circuit Schematic.............................................................................. 17 Figure 10. Pulse-Width Modulation with 555 Precision Timer ................................................... 18 Figure 11. Pulse-width Modulator Circuit Schematic ................................................................. 19 Figure 12. Bias Point Simulation ................................................................................................. 20 Figure 13. Transient Response of Audio Amplifier..................................................................... 21 Figure 14. Frequency Response of Audio Amplifier ................................................................... 22 Figure 15. PCB Layout of Audio Amplifier Circuit .................................................................... 23 Figure 16. Input and Output Waveforms of the Audio Amplifier ............................................... 24 Figure 17. Pulse-width Modulation Waveform ............................................................................ 25 Figure 18. Rectification of Sinusoidal Signal from PWM Signal ............................................... 26 LIST OF TABLES Table 1. Designed Parameters of the Input Stage ........................................................................ 10 Table 2. Designed Parameters of the Voltage Amplification Stage ............................................ 11 I. ABSTRACT An alternative method for transmitting audio signals via the use of laser technology was presented here. The primary focus of this document is on the audio amplifier subsystem including the design, simulation, and performance results. Simulations using the PSPICE computer simulation software were utilized to investigate the theoretical designs of the audio amplifier subsystem. The audio amplifier was examined in three junctures including the noise- canceling input stage, the high voltage gain cell, and the low impedance driver. Through the design procedure using theoretical calculations, simulations using PSPICE computer software, and examination of the finished product with an oscilloscope, the audio amplifier was greatly investigated and verified for desired functionality. In addition, the pulse-width modulation technique was examined and implemented with precision 555 timers in order to transmit audio signals via laser. 1 II. INTRODUCTION In the modern world, audio is disseminated in a myriad of simple and complex enterprises. Examples include, but are not limited to, the basic transmission of analog signals through a copper wire, the more common transfer of FM or AM signals through the air, and a more innovative transfer of signals in a digital format realized by light through a fiber optic cable. This document presents an alternative method to transmitting a stereo audio signal: through lasers with pulse-width modulation. Pulse-width modulation (PWM) is a modulation technique comparable to the common methods of modulation such as frequency modulation (FM) or amplitude modulation (AM) and will be explained in greater depth later in this document. The transmission of audio via laser is not currently economically sensible as the other methods of transmission listed above are more affordable and offer more reliable modes of operation. The main drawback of the laser transmission method presented here is the requirement for the laser’s beam to be directly aimed at the receiver, and any slight misalignment at the transmitting side can cause the audio to be interrupted. However, lasers are still a verifiable method of audio transmission, and despite their drawbacks, can produce excellent quality sound reproduction. Passive Pulse Width Analog Music Source Lasers Crossover Modulator Source Power Supply Receiver Power Supply High Current Pulse Width Speakers Receiver Audio Amplifier Demodulator Figure 1. Flow Diagram of Laser Transmission System 2 Figure 1 on the previous page illustrates the subsystems of the laser transmission system from the origin at the analog audio source to the completion and realization of music at the speakers. The main subsystems designed in this project include the crossover, the pulse-width modulator, the source and receiver power supplies, the pulse-width demodulator, and the audio amplifier. This document will focus primarily on the design, the analysis, and the testing of the audio amplifier subsystem. III. THEORETICAL BACKGROUND The pulse-width modulation technique is a common method of modulation often used in voltage source inverters for variable frequency motor drives, in maximum power point trackers for photovoltaic battery chargers, and most notably in closed-loop DC-DC converters. PWM has recently made its way into the audio world with the improvement of the semiconductor devices used to implement the modulation such as the MOSFET and BJT. Accurate reproduction of sound requires an extremely high switching rate with high quality systems switching in the hundreds of megahertz [1]. PWM requires two signals: a high frequency triangular carrier signal and the desired audio or sinusoidal signal. The concept of PWM is straight forward and is illustrated in Figure 2 on the next page [2]. The top plot of Figure 2 shows the high frequency triangular waveform and the much lower frequency sinusoidal signal. The carrier frequency must be a steady triangular signal in order to accurately reconstruct the audio signal after rectifying the PWM signal. A comparator or operational amplifier is used to compare the magnitudes of the carrier signal and the audio signal. If the magnitude of the sinusoidal audio signal is less than the magnitude of the triangular carrier signal, then the PWM output is low or 0 V. 3 Figure 2. Basic PWM Waveform from Single Frequency Sinusoid On the other hand, the opposite is also true. If the magnitude of the sinusoidal audio signal is greater than the magnitude of the triangular carrier signal, then the PWM output is high or 5 V as shown in Figure 2. This previously analog signal can now be accurately transmitted with zeroes and ones or low and high voltages with any digital transmission process. With the original analog audio signal in a digital form, it may be transmitted with a laser. This process requires a fast-switching BJT or MOSFET driver that is capable of switching at the frequency of the carrier signal. The laser driver and laser beam pulse for different lengths of time as shown in the bottom plot of Figure 2. The following step consists of the acknowledgement
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