Note: Descriptions are shown in the official language in which they were submitted.
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-1- RCA 80, 9~4
FIELD COMB FOR LUMINANCE SEPARATION OF NTSC SIGNALS
This invention relates to circuitry for
separating chrominance and luminance components from
composite video signal, and more particularly to a comb
filter utilizing a one field delay and incorporating
circuitry to compensate for motion induced artifacts.
The use of comb filters for separating luminance
and chrominance is well known by those skilled in the art
of video processing. Their cost, however, has generally
limited their use to relatively simple interline comb
filters. A description of an interline comb filter may be
found in RCA Review, Vol. 41, No. l, March 1980, pp. 3-28,
in the article by DO Pritchard, entitled "A COD Comb
Filter for Color TV Receiver Picture Enhancement."
Interline comb filters effectively separate
chrominance and luminance components but may produce
several undesirable effects. These effects include: loss
of diagonal resolution; reduction of vertical resolution;
and the production of "hanging dots" along vertical
transitions.
Inter frame comb filters which operate on signals
separated by integral video frame intervals separate
chrominance and luminance components without any of the
foregoing undesirable effects. However, inter frame comb
filters may generate extremely annoying artifacts around
the edges of moving image objects. To overcome this
shortcoming it has been proposed to utilize adaptive comb
filters. One such adaptive filter combines a line comb
filter with a frame comb filter. In this arrangement,
separated luminance and chrominance is acquired from the
line comb filter during intervals of inter image motion and
from the frame comb filter in the absence of motion.
Adaptive systems, though producing generally improved
signals, are relatively expensive and may have compromised
performance.
The present invention includes a comb filter
system which is generally superior to the interline comb
filter or the adaptive inter frame comb filter and requires
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significantly less hardware than the inter frame comb
filter.
The present invention consists of apparatus for
separating luminance signal from composite video by using
an interfold comb filter and an interline comb filter. A
luminance motion compensation signal may be derived by
subtracting in phase composite signals separated by e.g.
262 lines. This compensation signal is combined with comb
filtered luminance signal to produce a luminance signal
without significant motion induced artifacts and only
marginally noticeable hanging dots.
FIGURES lo and lo are schematic representations
of a reproduced image and which indicates particular
horizontal lines involved in line comb filters and field
comb filters, respectively;
FIGURE 2 is a block diagram of an interfold
luminance comb filter embodying the present invention.
Referring to FIGURE lay the center portion of
the drawing represents the scan lines of an interlaced
20 raster scanned image. The solid horizontal lines are the
scan lines of the odd fields and the intervening broken
lines represent the scan lines of even fields. At the
left edge of the drawing are three columns of numbers
designating the scan lines for three successive fields.
25 The (~) sins associated with the line numbers indicate
the relative phase of the chrominance signal associated
with each line. It will be readily appreciated by those
skilled in the art of video systems that one field of
conventional NTSC signal includes 262 1~2 lines. Every
30 second field begins with a half line (latter half) while
intervening fields end with a half line (beginning half).
Signals delayed by one field period plus or minus a half
line period correspond to vertically aligned samples, and
are located on adjacent interlaced lines. In the NTSC
- 35 system, signal samples separated by 262 lines have an in
phase chrominance sub carrier relationship while signal
samples separated by 263 lines have a chrominance
sub carrier relationship that is 1~0 degrees out of phase.
TV
I RCA 80,984
To the right of the drawing is a waveform
depicting a change in chrominance amplitude meant to
illustrate a vertical transition which spans the picture
width, i.e. a horizontal color edge in the displayed
image. The transition represents the fastest chrominance
signal change that can occur due to the effective sampling
rate (in the vertical direction of conventional raster
scan video cameras.
The curved arrows along the vertical column
designated "a" indicate the horizontal lines of signal
that are additively combined to produce line comb filtered
luminance signal. The curved arrows to the left of column
"a" designate the lines in the odd fields that are
combined, and the curved arrows to the right of column "a"
indicate the lines that are combined in even fields. For
example, in the odd fields, line 1 is added to line 2 to
produce luminance signal corresponding to displayed line
2. Line 2 is added to line 3 to produce luminance signal
corresponding to displayed line 3, etc.
During vertical chrominance transitions the
chrominance signal is not completely canceled from the
line comb filtered luminance signal. The residual
chrominance signal in the comb filtered luminance
generates what is known as "hanging dots" along horizontal
edges. Note from FIGURE lo if lines 264 and 265 are
averaged the result is YOKE where Y represents luminance
signal and I is the change in chrominance signal between
lines 26~ and 265. The term ~C/2 is undesired residual
chrominance signal. Similarly, if lines 2 and 3 are added
and the sum divided by two, the resultant signal is YOKE
where the term I is undesired residual chrominance
signal. The residual chrominance values are indicated
graphically by the horizontal arrows along the vertical
dashed line b. The length of the arrow indicates the
I relative amplitude of the residual chrominance associated
with the respective line of comb filtered signal.
Ruptured pointing arrows designate residual chrominance
having the same relative polarity as the chrominance
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transition, and left ward pointing arrows are of opposite
polarity.
Vertical line b' occurs at a point in space
corresponding to a half cycle of the chrominance
sub carrier from point b. The chrominance signal at point
b' is 1~0 degrees out of phase with the chrominance signal
at point b. As a consequence, the polarity of residual
chrominance in the comb filter luminance signal (for like
chrominance transitions) along vertical line b' is
opposite to that along dashed line b. Since the residual
chrominance linearly adds/subtracts from the luminance
signal, it is seen that along a horizontal color image
edge, the residual chrominance tends to drive the
luminance amplitude alternately toward white and black
levels at alternate half cycles of the chrominance signal.
It can also be seen that for a color transition occurring
between two lines, residual chrominance is found in -three
lines of the line comb filtered luminance signal.
Next consider FIGURE lo which is similar to
FIGURE lo except that it depicts using adjacent interlaced
lines in a comb filter system. The curved arrows to the
left of the dashed vertical "C" indicate the horizontal
lines of signal combined during a first field of a field
comb filtered luminance signal and the curved arrows to
the right of vertical "C" indicate the horizontal lines
used in the next successive field of comb filtered
signals. The luminance comb filtered samples are the
average values of samples from two successive fields.
Spatially the samples are one-half line apart,
i.e. they are separated by one-half -the vertical distance
that samples combined in a line comb filter are separated.
Due to the inherent bandwidth limitations, the amplitude
of chrominance changes are necessarily less than the
amplitude of a chrominance transition between lines in a
single field. This results in the maximum amplitude of
residual chrominance contaminating the comb filtered
luminance signal being less for interfold comb filtered
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-5- RCA 80,984
signals than interline comb filtered signals for the same
chrominance transition.
The arrows along vertical lines "d" and "d"'
represent the residual chrominance signal in a field comb
filtered luminance signal for the chrominance signal
illustrated at tune right side of the drawing. The
occurrence of residual chrominance is restricted to two
lines and the maximum amplitude is one-half the maximum
amplitude exhibited in the line combed signals. In
addition, the polarity of the residual chrominance signal
is opposite on adjacent lines. These three features
dramatically reduce the observability of "hanging dots" in
field combed luminance signals. The one polarity residual
chrominance on one line tends to brighten the picture
while the opposite polarity residual chrominance on the
adjacent interlaced line tends to darken the picture.
The two points are close enough that the eye integrates
the brightness over the picture area -tending not to
recognize the hanging dot.
As in line comb filters, there is an inherent
loss in vertical detail in the field combed luminance
signal. That is, for any comb filtered line the luminance
signal is the averaged luminance signal over e.g. two
lines. Thus, if from line 2 in field l to line 265 in
field 2 the luminance signal, Y, changes by an amount MY,
the effective change in the field combed luminance signal
is JOY. To restore the comb filtered signal a value YO-YO
must be added back to produce the true luminance
transition.
Observing the left side of FIGURE lo, and
assuming the comb filtered luminance signal is derived
from adding lines 2 and 265, over which lines a luminance
transition is presumed to have occurred in the vertical
direction, it will be noted that the same transition
occurs between lines 265 and 527. Lines 265 and 527 have
the same phase chrominance signal. Thus, subtracting one
from the other will generate a signal corresponding to the
luminance transition. Halving this signal produces the
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-6- RCA 80,984
desired vertical detail for addition back into the comb
filtered luminance signal. Note however, that a system
operating simultaneously on lines 2, 265 and 527 to
produce a comb filtered signal requires two fields of
storage.
In accordance with the principles of the present
invention, only one field of storage is required. This is
accomplished by combining the closest in-phase lines
adjacent the lines utilized in the comb filter. Where
lines 2 and 265 are averaged to produce comb filtered
luminance, the appropriate in-phase lines for developing a
difference signal are lines 265 and 3. If the luminance
transition spans several lines and is generally monotonic,
this difference signal will be exact, and this is
generally the case. Note also that if image motion exists
between fields, this motion information will be included
in the difference signal. Adding this motion information
back into the combed luminance signal tends to correct
artifacts occurring in field combed images.
It will be appreciated by those skilled in the
art of video comb filters that the difference signal will
contain not only luminance vertical detail and luminance
motion detail, but also chrominance motion and vertical
detail information. It would therefore appear to be
desirable to low pass filter the difference signal to
eliminate the chrominance components before adding it to
the combed luminance signal. In fact, there is a trade
off to be made. If the chrominance component is filtered
out of the difference signal, the "hanging dots" are less
apparent but the edges of objects moving rapidly
horizontally are less well defined. Thus, there is a
choice between hanging dots or horizontal motion detail.
Referring again to FIGURE lo, the arrows along the
vertical line "e" designate the residual chrominance in
the comb filtered luminance signal with a wide band
difference signal added back. This residual chrominance
tends to produce "hanging dots" but they are significantly
less apparent than for interline comb filtered signal.
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The residual chrominance is worse only for sharp
chrominance transitions. For transitions that span
several lines the residual chrominance is in general
improved.
FIGURE 2 is a block diagram of circuitry for
performing the above-described field comb filtering to
produce motion corrected separated luminance signal from
composite video. The circuitry may use digital or analog
devices depending on the signal to be processed.
In FIGURE 2, base band composite video is applied
to input terminal 10 from which it is routed to the input
ports of a 262 line delay element 20, a signal adder 40
and a signal subtracter 50. Delay element 20 delays
signal applied thereto by 262 horizontal video line
periods. The delayed signal from delay element 20 is
applied to a further delay element 30 which delays signal
by one horizontal video line period. The delayed signal
from element 20 is also applied to respective input ports
of signal subtracter 50 and a second subtracter 60.
20 Delayed signal from delay element 30 is applied to
respective input ports of signal adder 40 and signal
subtracter 60. Finally the signal sums from adder 40 and
the signal differences from subtracter 50 are applied to
respective input ports of a signal adder 70. The output,
80, of adder 70 is comb filtered, motion corrected
luminance signal. The OlltpUt, 90, of subtracter 60 is
line combed chrominance signal.
Consider that signal from horizontal line n is
present a-t the output of delay element 30. The signal at
the input to element 30 therefore corresponds to line nil.
Lines n and nil being successive lines, their chrominance
components are 180 degrees out of phase. wine n being
subtracted from line nil in subtracter 60 cancels the
luminance component of the applied video signal and
produces a chrominance signal of twice amplitude which
will be subsequently halved for further chrominance
processing.
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If the output signal from delay element 20 is
line no then the input to element 20 is line n+263.
Lines n from delay element 30 and n+263 are summed in
adder 40. Note that if n is an odd number, then n~263 is
5 an even number. For NTSC signal, odd numbered lines all
have the same phase chrominance sub carrier and even
numbered lines have the opposite phase sub carrier. Lines
n and n+263 are separated by 263 lines or one field period
plus a half line period (for NTSC signal). Adding the
10 even and odd numbered lines in adder 40 cancels the
chrominance component and produces separated luminance
signal. This luminance signal contains residual
chrominance at chrominance transitions, lacks a portion of
vertical luminance detail, and includes artifacts due to
15 - interfold motion.
Lines no from delay element 20 and n+263 are
applied to subtracter 50. These lines are either both odd
or both even, thus they have the same chrominance phase.
These lines are separated by a full field period less
20 half line period. Line no is subtracted from line n-l-263
to generate a difference signal which includes vertical
detail and motion information. This difference signal is
applied to one input of adder 70 where it is combined with
the separated luminance signal from adder 40 to reinsert
US the vertical detail and motion information therein.
With respect to the luminance signal the
algorithm performed by the FIGURE 2 circuit may be
described by the equation:
y = Sn,~,263-~Sn+ Sn+263-sn+l ) ( 1 )
30 where is the signal produced at output port 80 and Sun is
signal from line n where "n" is an integer.
As indicated before, it may be preferred to low
pass filter the difference signal before reinsertion into
the luminance signal. This is indicated by the element
US 100 in phantom coupled between elements 50 and 70. If
such an element 100 is incorporated in the circuitry, then
it may be required to insert a delay element between
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elements 40 and 70 to compensate for inherent signal
delays in element 100.
For convenience, the interline chrominance comb
filter is shown utilizing the one line delay element 30.
An alternative chrominance comb filter is shown in phantom
including the subtracter 110. In this alternative, signal
delayed by one line is tapped from delay element 20 and
subtracted in subtracter 110 from signal applied at input
terminal 10. Note that when this alternative chrominance
kamikaze used, the comb filtered luminance, the difference
signal and the comb filtered chrominance all have
contributions from line n+263.
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