Field motion suppression in interlaced video displays

A field motion suppression technique in interlaced video displays produces a motion suppressed frame without the use of a fixed threshold for motion detection. One of two interlaced fields is selected to remain unchanged, and the other field is examined on a pixel by pixel basis using vertical pixels both above and below from both fields to determine motion for each pixel. When motion is detected for a pixel, the pixel value is replaced with an interpolated value from the unchanged field, otherwise the pixel value is left unchanged.

BACKGROUND OF THE INVENTION 
The present invention relates to motion suppression techniques in video 
systems, and more particularly to a field motion suppression technique in 
interlaced video displays which preserves vertical resolution of a 
resulting motion suppressed video frame. 
In commercial television systems, such as NTSC and , each video frame 
has two interlaced fields. If the lines within a frame are numbered 
sequentially from top to bottom, then the odd numbered lines are from the 
odd field and the even numbered lines are from the even field. If an 
object is moving rapidly, it may appear in different locations within each 
field. Certain devices, such as video cassette recorders, still stores, 
etc, are capable of "freezing" a single frame of a video signal. An object 
in motion within the picture represented by the single frame appears to 
flicker within two areas of the frame as the two interlaced fields are 
alternately displayed. An extreme illustration of this effect is shown in 
FIG. 1 where a stationary rectangular object and a moving ball are 
pictured. As the two fields are continuously redisplayed each of the 
images of the ball appears to flicker. What is desired is a display with 
the ball either in one position or the other. 
A simple prior art solution is to simply pick one of the two fields and 
continuously redisplay it for both fields. For example if the odd field is 
chosen, then line 1 is displayed in alternate fields as lines 1 and 2, 
line 3 is displayed in alternate fields as lines 3 and 4, etc. This 
approach fills in the missing lines, but produces rather odd looking 
images because it tends to make the most well defined edges look jagged. 
A better approach involves averaging. Assuming that the odd field is once 
again chosen for display, lines in the even field are produced by 
averaging. A pixel to be displayed on line 2 is obtained by averaging the 
pixel above on line 1 with the pixel below on line 3. This produces a more 
acceptable looking picture than the simple previous approach, but there is 
still a visible loss of vertical resolution. 
In both of the above approaches vertical resolution is sacrificed within 
the entire picture image, even though most of the image may be stationary 
between the two fields. Ideally stationary objects within the image should 
not suffer a loss of vertical resolution along with those objects that are 
in motion. 
Therefore what is desired is a method of detecting motion between the two 
consecutive fields selected to produce a video frame and change pixel 
values only where motion is detected to preserve vertical resolution for 
stationary objects. 
SUMMARY OF THE INVENTION 
Accordingly the present invention provides a field motion suppression 
technique in an interlaced video display which replaces pixels where 
motion is detected with an interpolated value. The pixels of the field to 
be replaced are each compared with the pixels immediately above and below 
in the same field to find a minimum difference within that field, and the 
pixels in the other field are compared to find a difference and an 
interpolated value. A difference between fields is obtained from the pixel 
value and the interpolated value. If the difference between fields is 
greater than both the internal field differences, then motion between 
fields is determined and the interpolated value is substituted for the 
pixel value. Otherwise the pixel is determined to be stationary and the 
pixel value is retained. 
The objects, advantages and other novel features of the present invention 
are apparent from the following detailed description when read in 
conjunction with the appended claims and attached drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 2(a)-2(g) it is assumed that the even field is 
retained and the odd field is interpolated where there is motion. FIG. 
2(a) illustrates the moving object of FIG. 1 where a determination is to 
be made for the value of the pixel from a middle line L3 from values for 
the corresponding pixel on lines above and below from the same and 
opposite fields, L1, L5 and L2, L4, and FIG. 2(b) illustrates the 
stationary object of FIG. 1 where a determination is to be made for the 
value of the pixel from a middle line L3 as for the moving object. With 
respect to the moving object lines 1, 3 and 5 are white (reverse image) 
while lines 2 and 4 are black, while for the stationary object lines 1 and 
2 are white and lines 3-5 are black. 
The first step of the motion suppression process is illustrated in FIG. 
2(c) where the absolute value of the differences between the pixel from L3 
and the pixels above and below from the same field, L1 and L5, are 
determined. The lesser of the absolute values is selected and identified 
as the odd difference value OD. For the moving ball both differences are 
zero and the value for OD is zero. For the rectangle one difference is 
zero while the other is 100, where white is 100 and black is zero in 
value, but the value for OD is still zero as being the lesser value. The 
next steps shown in FIGS. 2(d) and 2(e) use the pixels immediately above 
and below the pixel from L3 in the opposite field, i.e., corresponding 
pixels from lines L2 and L4. The absolute value of the difference between 
the pixels of L2 and L4 is determined and identified as the even 
difference value ED, and the average of the pixels from lines L2 and 14 is 
determined and identified as an interpolated alternative pixel value AP. 
For the ball AP is black, or zero, while for the rectangle AP is 
intermediate between white and black, or 50. 
As shown in FIG. 2(f) a field difference value FD is determined from the 
absolute difference between the pixel value from L3 and the alternate 
value AP. For the ball FD is 100, while for the rectangle FD is 50. With 
the respective values for OD, ED and FD motion with respect to the pixel 
L3 is determined. If both OD and ED are less than FD, then motion is 
detected for the pixel of L3 and the alternate pixel value AP is used in 
lieu of the value of the L3 pixel. For the ball the values of OD and ED 
are zero which is less than the value of FD which is 100. Therefore the 
value of the pixel of L3 is replaced with the value of AP which 
corresponds to black, filling in the ball. For the rectangle the value of 
OD is less than the value of FD, but the value of ED is greater than the 
value of FD. Therefore there is no motion and the value of the pixel from 
L3 is retained. 
A significant feature of the above-described technique is that, contrary to 
previous motion detection schemes, no fixed thresholds are used. The data 
effectively sets its own thresholds for motion detection on a pixel by 
pixel basis. The technique is equally applicable to chrominance values as 
it is to luminance values. In systems having color components, such as YIQ 
and YUV systems common in NTSC and pAL television systems, the technique 
is applied independently to the luminance and chrominance of each pixel 
and, where motion is detected in either or both luminance and chrominance 
channels, both luminance and chrominance AP values are substituted. For 
RGB both the luminance and chrominance may be considered simultaneously by 
processing the green channel which contains most of the luminance 
information, substituting the AP values in all three RGB signals when 
motion is detected in the green signal. 
The technique may be implemented in computer software, in digital hardware 
for real time processing or in analog hardware. For example as shown in 
FIG. 3 an input video data signal is stored in a still, or frame, store 10 
under control of a microprocessor 12. The even field of the video data 
signal is output to a display processor 14. The display processor 14 
combines the even field with the odd field as processed by a motion 
detector 16 for display on an appropriate monitor 18. For each pixel L3 of 
the odd field to be processed, pixels L1, L5 and L2, L4 immediately above 
and below that pixel from both fields are also output from the still store 
10. These five pixels are input to the motion detector circuit 16. Pixels 
L1, L3, L5 are input to an odd difference circuit 20 while pixels L2, L4 
are input to an even difference circuit 22 and an alternative pixel 
circuit 24. A field difference circuit 26 has as inputs the pixel L3 and 
the output of the alternative pixel circuit 24. The outputs of the odd, 
even and field difference circuits 20, 22, 26 are input to a logic circuit 
28 which outputs a select signal depending upon whether motion is detected 
for the pixel L3. The select signal from the logic circuit 28 is input to 
a multiplexer 30 together with the pixel L3 and the output of the 
alternative pixel circuit 24. The output of the multiplexer 30 is a series 
of pixel values that make up the modified odd field for input to the 
display processor 14. 
The motion detector 16 is shown in greater detail in FIG. 4. The pixels L1, 
L3, L5 from the odd field are input to summing circuits 32, 34 so that the 
differences L3-LI and L3-L5 are produced. These differences are input to 
respective absolute value circuits 36, 38 to obtain an absolute value for 
each difference. The outputs of the absolute value circuits 36, 38 are 
input to a comparator 40 and to a select multiplexer 42. If 
.vertline.L3-L1.vertline. is greater than .vertline.L3-L5.vertline., than 
the output of the comparator 40 provides a select signal to the select 
multiplexer 42 which outputs .vertline.L3-L5.vertline. as the odd 
difference value OD, and vice versa. The pixels L2, L4 from the even field 
are input to respective summing circuits 44, 46 to obtain, respectively, 
the sum and difference of the pixels. The sum of the pixels (L2+L4) is 
input to a divider 48 where the sum is divided by two to obtain the 
average of the two pixels AP. The difference of the pixels (L2-L4) is 
input to an absolute value circuit 50 to obtain the absolute value of the 
difference as the even field difference value ED. The AP and L3 values are 
input to another summing circuit 52 which obtains the difference (L3-AP), 
which difference is input to an absolute value circuit 54 to produce the 
difference between the even and odd fields FD. The OD, ED and FD outputs 
are input to comparators 56, 58 so that, if OD or ED is greater than or 
equal to FD, a "1" is output for that comparator. The outputs of the 
comparators 56, 58 are input to an OR gate 60 to provide the select signal 
for the motion multiplexer 30 such that if either or both outputs are "1" 
the original pixel value L3 is selected over the interpolated value AP. 
The present technique is useful to convert pairs of fields into motion 
suppressed frames. Either the odd or even field is selected as being 
subject to change in order to eliminate motion and corresponding flicker 
from the resulting frame. Continuous processing of video in this manner is 
especially useful when transferring images to film since each pair of 
fields must be converted into a motion suppressed frame for exposure onto 
a film frame. Other applications include video frame rate doubling, 
generally accomplished within a television receiver. In this situation the 
current video field is always displayed unchanged, and the missing lines 
are filled in with pixels from the previous field or interpolated values 
from the current field based on motion detection according to this 
technique. Finally other applications which require accurate motion 
detection may use the present technique, such as for video noise 
reduction. 
Thus the present invention provides a field motion suppression technique in 
interlaced video displays which preserves vertical resolution for 
stationary objects while detecting motion on a pixel by pixel basis 
without the use of a fixed threshold.