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CPP100
SECTION 9
CPP1OPS
25/10/06
www.snellwilcox.com
Version 1 Issue 6
9.3
Recursive Filtering cont.
Noise Floor Measurement
Accurate noise floor measurement is required to set the threshold above which picture differences are
perceived as motion. Luminance and chrominance picture differences are processed using a combination of
full-wave rectifiers, linear filters and non-linear mapping tables to generate the control value K which
determines the amount of noise reduction which can be applied on a pixel-by-pixel basis. Luminance
differences are low pass filtered before being rectified and summed with the rectified chrominance difference
signal. Subsequent non-linear mapping tables are used to amplify the combined picture differences. Finally a
2D spatial filter is used which has contributions from adjacent pixels and lines. The overall effect of the rectifier
and spatial filter is to form the mean modulus of the picture difference signal. This is similar to measuring the
r.m.s. value of the difference signal but is computationally easier and in the absence of motion is a good
representation of the r.m.s. value of the noise (which forms the only contribution to the picture differences).
A side chain is used to integrate the processed differences and a software algorithm evaluates the noise floor
based on a large history of previous picture difference measurements. Integrated picture differences will have
a minimum value when there is no motion. The noise floor measurement produces a control value which
determines the sensitivity of the motion detector (shown as a non-linear transfer function below).
The control parameter for this function can be configured manually through the THRESHOLD adjustment or
normally it can be left in the AUTO setting in which case the microprocessor will automatically control the
sensitivity.
Semi-Transversal
The semi-transversal filter operates in conjunction with the recursive filter to increase its effectiveness. Quite
unlike the traditional transversal filters described in the preceding section it operates by selecting the most
appropriate output from a chain of picture stores at the output of the recursive filter.
An algorithm is used to determine which of the stores contains the highest level of noise reduced picture. The
overal effect is to increase the amount of noise reduction in a typical picture. For example, moving objects
cause the recursive filter to turn off at the edge of the moving object. This leads to a recurrence of noise which
takes a number of frames to reduce to the defined user level. The semi-transversal filter is able to monitor the
recurrence of noise and delay the output of the recursive filter up to a maximum of three frames. Operating on
a pixel-by-pixel basis, the overall level of noise reduction in a typical picture is maintained at a more uniform
level and is less dependent on movement. As the semi-transversal filter complements the recursive filter, it
cannot be used without the recursive filter. Effective at all recursive filter settings its operation can be seen
as a reduction in the level of revealed noise trail following moving objects.
The semi-transversal filter operates in a fully automatic mode - there are no user adjustments required.