Digital Signal Processing Reference
In-Depth Information
4
INTRODUCTION TO DIGITAL
FILTERING
CHAPTER OUTLINE
4.1 Median Filtering
19
4.2 FIR Filters 20
4.3 FIR Filter Construction 20
4.4 Computing Frequency Response
23
Digital filtering is used in different aspects of video signal
processing. One application is reducing random noise effects or
corrupted pixels in images or video. Finite Impulse Response
(FIR) filters can be used to smooth out an image, and to reduce
random noise, but the drawback is that it will soften or blur sharp
edges in the image.
4.1 Median Filtering
Another type of filter often better suited to reducing noise in
an image is the median filter. A median filter is a non-linear
filter, which does not use multipliers. It analyzes the image
pixel by pixel, and replaces each pixel with the median of
neighboring entries. The pattern of neighboring pixels repre-
sents a window, which slides, entry by entry, over the entire
image. The window is usually a box or cross pattern, centered
on the pixel being analyzed. Since the window has an odd
number of entries, then the median is simple to define: it is just
the middle value after all the entries in the window are sorted
numerically.
Median filtering is used as a smoothing technique, which is
effective at removing noise in smooth patches or smooth regions
of a signal. Unlike low-pass FIR filters, the median filter tends to
preserve the edges in an image. Because of this, median filtering
is very widely used in digital image processing.
