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Main Menu Session 2220

Using Real Signals with Simulated Systems

Joseph P. Hoffbeck University of Portland

Abstract

Students often find real systems more interesting than simulations, but using real systems in a course can be impractical. One compromise between using real systems and pure simulation is to capture signals from real systems and have the students process them using simulated systems. The advantages of this approach include exposing the students to deviations from the ideal such as noise and timing imperfections, and allowing them to experiment with different solutions in software. An example of this method is presented where a caller identification signal is captured from the telephone system, and is demodulated using the numeric computation package MATLAB.

Introduction

It is often necessary to rely on simulations of complex systems in order to demonstrate their behavior to a class since access to real systems can be limited due to cost, space, and time constraints. While simulations are sometimes the only practical approach, they can be too far removed from real systems to be convincing to the students or to really capture the imagination of the students. Furthermore, simulations often produce results that are too good in that they often do not include the imperfections associated with real systems such as noise, distortion, and timing imperfections.

Capturing signals from real systems and processing them using simulated systems has the advantage of using real world signals recorded directly from actual systems, and at the same time retaining the flexibility and convenience of using simulated systems. Only the instructor needs to have access to the actual system to record the signals, and the students can process the results using appropriate software, experimenting with different methods simply by making changes in software.

To demonstrate this teaching method, a project is described that captures the caller identification (CID) signal that is used to transmit the name and number of a telephone caller, and demodulates the signal using the numeric computation package MATLAB. A recording of a CID signal can be made with a telephone coupler that converts the telephone line signals to line level, or alternatively a recording can be obtained from the author (by sending email to hoffbeck@up.edu). The CID signal is demodulated with the routines in the Communications Toolbox, which is an optional package for MATLAB. Decoding the signal allows the students to discover the name and number that is encoded in the CID signal, gives the students exposure to a real communications protocol, and gives them an interesting introduction to the simulation capabilities of MATLAB. The CID signal is a good candidate for this exercise because it is

Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright ©2002, American Society for Engineering Education

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Hoffbeck, J. (2002, June), Using Real Signals With Simulated Systems Paper presented at 2002 Annual Conference, Montreal, Canada. 10.18260/1-2--10546