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Session # 3659

Sampling and Aliasing: An Interactive and On-Line Virtual Experiment “What we ‘see’ is not what it is!”

M. Kostic1 Northern Illinois University

Abstract

An interactive virtual instrument is further developed for use in class and on- line over the Internet, to simulate sampling and aliasing phenomena, an essential concept in many science and engineering courses, including physics, dynamics, control, measurement methods, and digital signal processing. A stroboscope- mechanical system is used to further visualize concept and show striking similarity with electronic data gathering and to demonstrate aliasing. While students learn procedures to calculate the system frequency response, many do not fully understand how and why a real-time signal output is distorted. As the aliasing phenomena are added to the measurement challenge, the students become confused and discouraged to get involved and fully comprehend these important physical processes. The virtual instrument allows users to easily experiment (or play) with different signals and instrument characteristics, and interactively see the relevant outcomes, the system frequency response and aliasing if present, along with input and output signals, both qualitatively (visually/graphically) and quantitatively (numerically). The interactive simulator should stimulate users’ curiosity and accelerate learning by active, “what- if” inquiry and experimentations, and thus, enhance their experience and comprehension. Each and every one of these new tools, when designed well and used creatively, may qualitatively enhance the learning environment.

Introduction

What are “sampling” and “aliasing” and why they are important? In science and engineering, sampling means “discrete observing” or measuring, or probing with a certain “sampling” rate or frequency, like with a stroboscope blinking light, see Figure 1. Aliasing happens when an analog object, signal or data is represented (measured or “seen”) by a discrete system, i.e. in a discrete domain or a grid. Used in the Figure 1: Discrete sampling of the motion of a bouncing ball by stroboscope light on a single picture frame context of processing digitized waveform (“supper-fast” photography). signals (e.g. audio) and images (e.g. video), aliasing describes the effect of under- sampling during discretization which can generate a false (apparent) low frequency for signals,

1 Prof. M. Kostic, Northern Illinois University, Department of Mechanical Engineering, DeKalb, IL 60115; kostic@niu.edu; Web: www.kostic.niu.edu

Proceedings of the 2003 American Society for Engineering Education Annual Conference & Exposition Copyright © 2003, American Society for Engineering Education

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Kostic, M. (2003, June), Sampling And Aliasing: An Interactive And Online Virtual Experiment Paper presented at 2003 Annual Conference, Nashville, Tennessee. 10.18260/1-2--11848