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CPP100
SECTION 9
CPP1OPS
25/10/06
www.snellwilcox.com
Version 1 Issue 6
9.10
Linear Filters
Introduction
There are three types of linear filter which are available:
1. Brickwall Low Pass Filter with optional Boost
2. Gaussian Low Pass Filter.
3. Gaussian High Pass Filter.
The function of these filters is to perform various levels of noise reduction or compensation on the luminance
component of the video signal.
The requirement to filter the luminance component can be desirable since its high bandwidth means that it is
susceptible to high frequency noise. Also as the human eye is more susceptible to the luminance signal (as it
is this signal which controls picture contrast) filtering of the signal can be extremely important.
Brickwall Filter
This filter is employed to reduce noise resident in the high frequency region of the luminance spectrum.
The filter has two ranges: Low pass and Extra Low Pass
Low Pass.
There are 18 user selectable cut-off frequencies (specified at -6dB) ranging from 4.2 MHz down to 2.5 MHz in
0.1 MHz steps. The stopband performance is -34 dB or better with no boost.
In addition to the selectable cut-off frequencies the user can also select a given boost at each cut-off:
0dB (no boost), 1dB, 2dB, 3dB, 4.5dB, 6dB.
The boost value selected is the amount of gain applied to the filter response at the chosen cut-off frequency.
The selectable boost facility is incorporated to allow the user to increase the perception of sharpness in the
picture by boosting the contrast. The sharpness of a picture can sometimes be significantly reduced by filtering
the high luminance frequencies so the inclusion of the extra boost helps to restore some of the sharpness to
the picture. A boost of 6dB will result in the cut-off frequency being increased by up to 0.4 MHz.
The actual amount of boost selected will be a trade-off between cut-off and picture sharpness.
Extra Low Pass.
There are 6 user selectable cut-off frequencies (specified at -6dB) ranging from 2.4 MHz down to 0.9 MHz in
0.3 MHz steps. These filters can be used where the luminance component of the video signal needs to be
significantly reduced in bandwidth. This set of filters would only normally be used in exceptional circumstances.