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Unformatted text preview: 01/31/12 page 1 Rev. B ECEn 487 Digital Signal Processing Laboratory 2012 Lab 2 Finite Impulse Response Filtering Due Dates This is a three week lab. All TA check off must be completed by Tuesday February 21, or the lab will be marked late. Note that the 21st is a Monday instruction day, and no lab class will be held. Submit answers to the questions from the last page of this handout at the beginning of lab class on Tues. Feb. 7. Lab book write-up copy submission, beginning of lab class Tues. Mar. 28 Learning Objectives The purpose of this lab is for each student to design an FIR digital bandpass filter using MATLAB and use it to extract a desired Morse code signal from interfering signals at nearby audio frequencies. The extracted message will then be decoded. Students will become familiar with practical aspects of simple filter design techniques and gain an appreciation for its possible uses. Reading Assignment 1. Textbook, Oppenheim and Schafer, Sections 7.0 , 7.5, 7.6.1, and 8.7.3 . 2. MATLAB online help and Signal Processing Toolbox User's Guide tutorial and documentation on the signal processing toolbox (e.g. “help signal”) functions "freqz,", "fft," "conv," "filter," "fir1," "hamming," "bartlett,"blackman," "boxcar," "kaiser," and "chebwin." Introduction One of the most important and practical real-time DSP operations is digital filtering of sampled signals. Though IIR filters are sometimes used, we will concentrate on FIR filter implementations because they are always stable, can be designed to be exactly linear phase (no phase distortion or dispersion), and are easy to design to given filter specifications. Your goal in this lab will be to extract a particular Morse code signal from a hopelessly scrambled recording 01/31/12 page 2 Rev. B so that you can decode the message. This simulates a scenario that an amateur radio operator might encounter when trying to copy a weak overseas signal in a channel packed with local interference from other amateurs. Your only hope is that the interfering signals are centered on slightly different audio frequencies, so that a frequency selective filter may help reject them. We will use the windowed filter design technique from your textbook to design the filter. Morse code is an early binary on-off keying method used to manually communicate text characters. The transmitting operator operates a switch by hand to generate a series of longer "dash" tones, and shorter "dot" tones to form a character code. Characters are separated by longer spaces than the dots and dashes within a character. One ingenious aspect of the code is that more frequently used characters (in English) are assigned shorter codes. This anticipates the later practice of entropy coding for efficient digital communications. Today, the code is used primarily by amateur radio operators for fun and emergency communications. You can find a code table (which you will need to decode the message) at many websites, like http://www.qsl.net/4f5aww/module3c.htm. http://www....
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