Thursday, May 31, 2012

Aliasing Distortion


Aliasing Distortion

Summary: This module brings out a clear understanding of the aliasing effect when signals are not sampled at the Nyquist Sampling Frequency.





Aliasing Distortion

A signal composed of components of different frequencies ,when sampled at a sampling frequency less than twice the value of the highest frequency component( the Nyquist sampling frequency)present in the sampled signal, results in the under sampled component posing itself as a low frequency component. This component passes through the system as an alias causing distortion. This is known as aliasing distortion.
Illustration of aliasing using Matlab/Simulink:
Simulink Block Diagram:
    Total simulation time: 0.05seconds
    
Figure 1
Figure 1 (graphics1.png)
The Sine wave consists of components of 250Hz,1000Hz,3500Hz
This wave is sampled at 4000Hz .The component which is under sampled is 3500Hz.
This component passes through posing as a low freq component of 500Hz
Simulink model :
The Sine Wave block is picked from the Signal Processing Sources Blockset , using the Simulink Library Browser .
1.The Sine wave block is given the following block parameter values :
Amplitude:[1 4 5]
Frequency(Hz): [250 1000 3500]
Phase offset(radians):0
Sample mode: Discrete
Output Complexity :Real
Computation method:Trigonometric fcn
Sample time:1/4000
Samples per frame:1
2.The FFT block:
Default parameters
3.The Spectrum scope block:
Buffer Input
Buffer size:128
Buffer overlap : 64
Window type: Hann
Window sampling: periodic
Specify FFT length:
FFT length:128
No.of spectral averages:2
The Frequency Spectrum indicating 3500Hz signal posing as 500Hz due to aliasing
Figure 2
Figure 2 (Picture 1.jpg)
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