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CPP100
SECTION 1
CPP1OPS
25/10/06
www.snellwilcox.com
Version 1 Issue 6
1.7
Enhancer
The enhancer menu contains four different types of
filters: Enhance, MPEG De-enhance, Decoder
matching and Video mode. These filters are
exclusive and cannot be applied at the same time.
For example, it would be inappropriate to
simultaneously enhance and de-enhance the picture
and this option is excluded. Similarly, video modes
(which utilise the same adjacent fields and lines as
the de-enhancement filters) cannot be selected at the
same time as any of the other three filters.
The enhancer uses a combination of linear and non-
linear processes to generate edge correction and
peaking correction signals. The use of non-linear
processing ensures that high levels of correction are
possible without introducing edge distortion such as
overshoots and ringing normally associated with
traditional frequency boosting techniques.
Three settings of enhancement are provided for both
luminance and chrominance. Coring controls can be
used for noisy inputs to prevent enhancement of low
level noise. Three settings are provided for both
luminance and chrominance coring.
De-enhancement filtering can be used to suppress
high vertical\temporal frequencies that the MPEG
encoder may not be able to efficiently encode.
Material that originated on video with 60Hz or 50Hz
temporal sampling will encode less efficiently than
24Hz film originated material. For fixed or
multiplexed bit rate systems suppression of high
diagonal frequencies can reduce the peak bit rate
requirements.
A three dimensional vertical temporal filter is used to
provide suppression of moving and static diagonal
frequencies.
Three filter settings are provided for both luminance
and chrominance. They can be used to adjust the
degree of filtering from partial suppression of
diagonal frequencies at the low setting to full
suppression of diagonal frequencies over a wide
horizontal band at the high setting. Purely horizontal
frequencies remain unfiltered.
Three decoder matching settings are provided: Line
Comb, Field Comb and Other.. The purpose of each
of these filters is to provide complementary filtering
from a composite source which has been decoded
using a comb decoder such as NTSC/PAL line or
field comb.
A composite comb decoder will use a spatial or a
vertical/temporal aperture to separate luminance from
chrominance. With knowledge of the comb structure
the decoder matching filters in Prefix filter areas of
the spectrum which the composite decoder has not
previously filtered.
Cross-effects such as cross-colour (high frequency
luminance in chrominance) and cross luminance
(residual chrominance in luminance) are reduced in
amplitude.
The Line Comb setting is used for decoders which
utilise a line comb structure (vertical filter using
adjacent lines) for Y/C separation. The Field Comb
setting is used for decoder which utilise a field comb
structure based on 312H (PAL) or 263H (NTSC)
diagonal vertical/temporal filters such as the Prefix
internal field comb decoder.
For simple or notch decoders which do not utilise a
comb decoder strategy for Y/C separation or where
the Y/C separation strategy is unknown, the
Other
setting provides good suppression of high frequency
diagonal components which may have originated
from cross-effects.
For video based inputs two modes of video filtering
can be used to reduce the temporal bandwidth of the
video source to 30/25Hz from an original 60/50Hz.
These operate by filtering and subsampling the video
inputs so that the output rate has been temporally
reduced.
An MPEG encoder operating in video mode will be
able to encode the filtered output more efficiently by
utilising the high level of correlation between adjacent
fields.
The two video modes are filtered differently prior to
temporal subsampling.
Mode 1
provides a sharper
picture with a stronger 25Hz inter-frame component
than
Mode 2
which has no inter-frame component but
has less overall vertical/temporal bandwidth. The
choice will be dependent on the available MPEG bit
rate and subjective picture tests.