# ps8 - MIT OpenCourseWare http/ocw.mit.edu 2.161 Signal...

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MIT OpenCourseWare http://ocw.mit.edu 2.161 Signal Processing: Continuous and Discrete Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .

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| F ( j W ) | 0 | F ( j ( W - W ) ) | s c r a m b l e r W W a W a s c r a m b l e d s p e e c h o r i g i n a l s p e e c h 0 W A 1 W - W A a 1 MASSACHUSETTS INSTITUTE OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING 2.161 Signal Processing - Continuous and Discrete Fall Term 2008 Problem Set 8: FIR Linear Filters Assigned: November 6, 2008 Due: November 18, 2008 Problem 1: A Mini-project: An Audio Scrambler Hint: We looked at this problem in Quiz 1. Look at the published solution. Text-messaging is out-of-date! You and I have been communicating by sending voice-messages to each other as “.wav” ﬁles. It’s not that I am paranoid, but everybody is out to get me! In particular, “they” have been intercepting all of our voice messages. But we are going to outwit “them” by using MATLAB to scramble our voice messages before sending them, making them unintelligible using a secret encoding scheme, and then we will descrambling the messages when we get them. your task is to design and implement the speech encoder and decoder. the requirement is that the scrambled message should occupy the same audio bandwidth (approximately 300 3000 Hz) as the original message, and the descrambled message should have good ﬁdelity. We will use a frequency domain “mirroring” scheme, where the frequency components are ﬂipped, so that high frequencies are translated to low frequencies and vice-versa. A spectral com- ponent with frequency Ω 1 will be translated to a new frequency Ω a Ω 1 , where Ω a is known to the sender and recipient. The resulting waveform is unintelligible until the frequency relationships have
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ps8 - MIT OpenCourseWare http/ocw.mit.edu 2.161 Signal...

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