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CPP100
SECTION 1
CPP1OPS
25/10/06
www.snellwilcox.com
Version 1 Issue 6
1.6
Median level
Six settings are provided for the median filter level
control. The low setting provides modest filtering and
has high rejection of false alarms caused by picture
movement and texture etc. The medium and high
settings are biased increasingly towards removal of
larger dropouts and dirt but consequently may have a
higher false alarm rate resulting in a general
softening of the picture.
Spatial Filter
Spatial filtering typically involves filtering using an
aperture that comprises adjacent pixels from the
same field period. Spatial median filters can be
effective at suppressing impulse noise originating
from film dust or small dropouts. However they are
also effective as Gaussian noise reduction filters.
Y and C Spatial Levels
The spatial filter operates by median filtering a small
kernel of adjacent pixels and then comparing the
median filtered pixel level with the current pixel. The
spatial filter has three level settings that are used to
vary the comparison threshold and effectively set the
balance between the level of noise suppression and
detail preservation. Typically used in conjunction with
other temporal based filters such as the recursive
and transversal filters, spatial noise reduction can
increase the overall noise reduction level.
Linear Filters
A suite of linear filters allows fine control of the
horizontal bandwidth of the luminance signal.
Brickwall low-pass filters ranging from 2.5MHz to
4.2MHz provide good band-limiting facilities for
MPEG encoders that use half resolution processing.
These filters also provide variable peaking or
boosting at each of the selected cut-off frequencies.
Boosting prior to brickwall filtering can raise the
overall perception of picture sharpness. In addition to
the above filters there is a set of extra low pass filters
where greater band limiting is required. These filters
have a cut-off ranging from 2.4MHz to 0.9MHz.
The ten sets of Gaussian low-pass filters gently
attenuate high frequencies and can be used to
correct material which has previously been boosted
or enhanced as well as reducing high frequency
noise.
Similarly, five sets of Gaussian high-pass filters
provide
variable
correction
of
high-frequency
luminance that may have been attenuated from faulty
distribution links or analogue VTR processes.
Scratch Filter
This filter has been designed to detect and repair
vertical scratches, of variable contrast, and length, be
they black, white or both, while maintaining picture
quality where there are no scratches. To maximise
the benefit obtained from this filter, a suite of filter
strengths has been provided.