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© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Sampling and Aliasing:Sampling and Aliasing:An Interactive and On-Line An Interactive and On-Line
Virtual ExperimentVirtual Experiment
""What We See is Not What It Is!What We See is Not What It Is!""
Prof. M. KosticProf. M. Kostic Mechanical EngineeringMechanical Engineering
NORTHERN ILLINOIS UNIVERSITYNORTHERN ILLINOIS UNIVERSITY
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
What are “What are “samplingsampling” ” and “and “aliasingaliasing” ?” ?
Figure 2: The aliasing examples: “wagon wheel illusion,” appearance that a spoke wheel rotates backward while it moves forward (left); a stroboscope experiment to “freeze” rotating fan blades or to make them appear to move slowly forward or backward (center); and appearance of staircase steps along edges (jaggies) in an image (right).
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
W.W.S.W.W.S.N.N.W.I.I.W.I.I. ""What We See What We See is Notis Not What It Is! What It Is!""
It’s all about ...It’s all about ...… … Sampling and AliasingSampling and Aliasing!!
Even ...Even ...W.Y.S.W.Y.G.W.Y.S.W.Y.G. ""What You See is What You Get!What You See is What You Get!””
……is REALLY NOT true (the same)is REALLY NOT true (the same) on different Monitors and Printerson different Monitors and Printers
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Signal/Sampling ResolutionSignal/Sampling Resolution
SamplingSampling//Time Resolution Time Resolution ((SpeedSpeed))
Sig
nal
Sig
nal
// Mag
nit
ud
e M
agn
itu
de
Res
olu
tion
Res
olu
tion
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
There is NOThere is NOsuch thing as...such thing as...……too large hard drive disk, too large hard drive disk, or ...or ...……too large sampling resolutiontoo large sampling resolution!!
Well praised ... Well praised ... high-definition digital graphicshigh-definition digital graphicsand CD-quality musicand CD-quality music
……are NOT good (true) enoughare NOT good (true) enough!!
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
• Under-sampling Under-sampling looseslooses (important) wave (important) wave detailsdetails !!
• But it also But it also may changemay change the wave the wave form/shapeform/shape due to Aliasing due to Aliasing !!
((UnderUnder))Sampling & Aliasing ...Sampling & Aliasing ...
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Real measurements Real measurements of a simple sine-harmonicof a simple sine-harmonic
How come this if the measured signalis a simple sine-harmonic wave?
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Sampling-Beat WaveSampling-Beat Wave
Sampled signal
Real signal
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Good SamplingGood Sampling
Sampled signal
Real signal
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Sampling with AliasingSampling with Aliasing
Sampled signal
Real signal
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Stroboscope light Rotating disk(RPM)
Sampling RPM with StroboscopeSampling RPM with Stroboscope
If RPM and Strobe speeds(frequencies) are the same
the reference mark willappear stationary (ZERO aliasing)
If the Strobe speed is fasterIf the Strobe speed is slower
If the Strobe speeds ishalf of RPM, the disk willturn twice and the mark
will appear stationary again!
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Folding DiagramFolding Diagram
NO
Ali
as. Aliasing
ZEROAliasing
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Sampling and Aliasing…Sampling and Aliasing…
Sampling is a discrete observation or
measurement, while Aliasing is an illusion, appearance of something that is NOT, due to shortcomings of sampling,
…thus everything we perceive or measure is, to a lesser or grater degree, an ILLUSION, responsible for many discoveries and re-discoveries in invention history.
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Sampling and Aliasing…Sampling and Aliasing…
Even if we are not interested in those high-frequency signal components, the problem is that they may “fold-back” and falsely appear as low-frequency components that do not exist in reality at all, thus trick us. Such phenomena are well-known as aliasing, which is actually an illusion.
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Interactive Experiments ...Interactive Experiments ...
LabVIEW Virtual InstrumentsLabVIEW Virtual Instruments
Internet Interactive ExperimentInternet Interactive Experiment
Animated MovieAnimated Movie
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
The Art of Sampling and AliasingThe Art of Sampling and Aliasing
Real signal
Sampled signal
MAGNITUDE Ratio
PHASE Shift
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Conclusion…Conclusion…
This interactive module will stimulate users’ curiosity and motivate them to conveniently “check out” different options and possibilities, and thus accelerate experience and by active, what-if inquiry and “experimentations” enhance their comprehension and interest for deeper thinking and understanding…
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Conclusion…Conclusion…
These new learning tools are not and cannot replace the traditional learning by cognitive thinking, but, if designed well, the new simulation tools may qualitatively enhance learning environment by stimulating inquiry and building confidence and motivation – in short, may become a “virtual eye and mind opener”!.
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
Who does the future belongs to ?Who does the future belongs to ?
??
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
No Limits …No Limits …
The Future The Future Belongs Belongs
To…To…
… … Whoever Whoever Gets There Gets There
FirstFirst
NOSPEED
LIMIT
© MMIII* Prof. M. Kosticwww.kostic.niu.edu
You may contact Prof. Kostic at:You may contact Prof. Kostic at:
mailto:mailto:[email protected]@niu.eduor on the Web:or on the Web:
www.kostic.niu.eduwww.kostic.niu.edu
http://http://prof.mkostic.comprof.mkostic.comProf. M. KosticProf. M. Kostic
Mechanical EngineeringMechanical Engineering
NORTHERN ILLINOIS UNIVERSITYNORTHERN ILLINOIS UNIVERSITY