Video signal processing apparatus for reducing aliasing interference

In a video signal processing apparatus for restoring a signal which has been bandwidth-compressed by offset sub-sampling after restoring an image from sample points of a same field of a sub-sampled signal, a first adder performs an inter-field averaging process from inter-field signals before or after three adjacent fields. A second adder performs an inter-frame averaging process from inter-frame signals. One field difference and one frame difference are detected from the three adjacent inter-field signals thereby switching an output signal of the first adder and an output signal of the second adder in accordance with the magnitudes of the differences thus detected. In this way, aliasing interferences accompanied with bandwidth compression can be detected. By selecting an optimum eliminating filter, aliasing interference due to interfield sub-sampling and aliasing interference due to inter-frame sub-sampling can be eliminated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a video signal processing apparatus for restoring 
a high definition television signal which has been subjected to bandwidth 
compression using a sub-Nyquist sub-sampling method and transmitted. 
2. Description of the Prior Art 
A high definition television (HDTV) signal has a frequency bandwidth 
exceeding 20 MHz and must be subjected to bandwidth compression when 
transmitted using a satellite or the like. As an effective method for 
broadly compressing a frequency bandwidth of an HDTV signal, a sub-Nyquist 
sub-sampling method is known. For which, a multiple sub-Nyquist sampling 
encoding (MUSE) system, a high definition-multiple analog component 
(HD-MAC) system and the like have been proposed. (See Y. Ninomiya, et. 
al., "An HDTV Broadcasting System Utilizing a Bandwidth Compression 
Technique--MUSE", IEEE Trans. Vol.BC-33, No.4, pp.130, 1987, and F. W. P. 
Vreeswijk, et. al., "HD-MAC Coding for Compatible Broadcasting of HDTV 
Signals", Symposium record Broadcast Session 1989, pp.37 to 53.) 
With these methods, as shown in FIG. 11 , an offset sub-sampling is carried 
out in an inter-field and inter-frame manner to transmit a quarter of the 
sample points of an original signal and as a result, the perfect picture 
is transmitted in a four-field mode, so that the bandwidth of the original 
signal is compressed to 1/4 of its original bandwidth. On the receiving 
side, an untransmitted sample point (non-sample point) is interpolated 
from the transmitted sample point, thus restoring the original signal. For 
example, for the stationary area, interpolation is effected using all 
sample points of the present field and three past fields. On the other 
hand, for the moving area, interpolation is effected using only an 
intra-field sample point that has been transmitted. 
As explained above, the stationary area and moving area have different 
interpolating methods. As a result, on the receiving side, the motion of 
an image is detected and an image interpolated for the stationary area and 
an image interpolated for the moving area are mixed with each other in 
accordance with the motion of an image thus detected, thus restoring the 
image. 
Since such processings as explained above are effected on both the 
transmitting and receiving sides, one can enjoy high definition television 
broadcasts at home. However, the above-mentioned signal processing circuit 
(decoder) is extremely expensive and it is estimated that it will take a 
considerably long period of time for make it popular to home-use. Under 
such a circumstance, in order to make it possible to display and reproduce 
an image with any standard type television set and video tape recorder 
(VCR) currently popular to home-use, there is a known apparatus for 
converting that high definition television signals of the MUSE system into 
current standard type television signals using a TV signal standard 
converter, namely, MUSE-NTSC converter and, a video signal processing 
apparatus for performing interpolation in order to restore an image with a 
simple structure has been proposed. 
As a conventional video signal processing apparatus for this purpose, see, 
for example, "MUSE/NTSC Converter for EDTV", Technical Report of the 
Institute of Television Engineers of Japan 1990 BCS90-3 pp.13 to 18. 
With a conventional video signal processing apparatus as shown in FIG. 9, 
an inputted MUSE signal is fed to an interpolating circuit to interpolate 
a non-sample point from an intra-field sample point in order to restore a 
bandwidth-compressed signal. The signal subjected to an intra-field 
interpolating process, which is equivalent to the moving image processing 
of an MUSE decoder, in the interpolating circuit has the signal 
transmission characteristics as shown in a frequency characteristics chart 
of FIG. 12(a). As a result, for the stationary area, the high resolution 
component of a high definition television signal by offset sub-sampling of 
the MUSE system, as shown in a frequency characteristics chart of FIG. 
12(b), is reproduced as an aliasing interference, resulting in a 
degradation of the in image quality. Then, the signal thus which has been 
processed through the interpolating circuit is fed to a frame memory to 
delay it one frame period and then, the signal thus which has been 
one-frame delayed and the signal which has been interpolation processed 
are subjected to an inter-frame averaging process through an adder, 
thereby producing a signal in which aliasing interference due to 
inter-frame offset sub-sampling has been eliminated. In this case, 
however, with the moving image, the inter-frame averaging process makes it 
possible for a double image to occur, resulting in a degradation in image 
quality. As a result, a motion detection circuit detects a motion of an 
image, and the inter-frame averaged signal and the signal which has been 
only intra-field interpolated through the interpolating circuit are mixed 
with other in a mixing circuit in accordance with the motion of the thus 
detected image, so that even with the moving image, such a signal can be 
obtained which has no degradation in image quality as well as eliminating 
aliasing interference due to inter-frame offset sub-sampling. 
With the structure as above, however, by performing the inter-frame 
averaging process, aliasing interference due to inter-frame offset 
sub-sampling can be eliminated thereby causing a flicker component of 15 
Hz to disappear, but the bandwidth-compressed MUSE signal also contains 
aliasing interference due to inter-field offset sub-sampling. As a result, 
a high frequency band component exceeding a horizontal frequency of 12 MHz 
is reproduced as a flicker component of 30 Hz while being aliased to a 
vertical high frequency band, so that a problem arises in that the 
interference incompatibly visibly appears in a notched pattern in the 
vertical line area of an image so as to be attached thereto. 
In addition, another video signal processing apparatus is disclosed in, for 
example, Japanese Laid-Open Patent Application No.02-328707, "Video Signal 
Standard Converting Apparatus". 
With a conventional video signal processing apparatus as shown in FIG. 10, 
similar to the apparatus shown in FIG. 9, an inputted MUSE signal is 
subjected to interpolation of a non-sample point from an intra-field 
sample point in an interpolating circuit in order to restore a 
bandwidth-compressed signal. The thus interpolated signal is fed field 
memories connected in series thereto, and then, a median-value signal of 
three adjacent inter-field signals is output by a median-value signal 
selector. An output signal from the median-value signal selector and 
output signals from the field memories are averaged by an adder to obtain 
an average-value signal of the signals which are in an inter-field 
correlation with each other, thus making it possible to obtain a signal in 
which the aliasing interference due to inter-field offset sub-sampling is 
eliminated. Here, the averaging process of highly correlated inter-field 
signals between three adjacent signals is carried out adaptively, so that 
the aliasing interference due to inter-field offset sub-sampling of the 
MUSE system can be eliminated without giving any effect to the moving 
image. 
With the structure as above, however, by performing an optimum inter-field 
averaging process between three adjacent fields, the aliasing interference 
due to inter-field offset sub-sampling can be eliminated and as a result, 
the flicker component of 30 Hz which visibly appears in a notched pattern 
in the vertical line area of an image so as to be attached thereto can be 
eliminated. However, the bandwidth-compressed MUSE signal still contains 
the aliasing interference due to inter-frame offset sub-sampling thus 
creating a problem in that a flicker component of 15 Hz remains in the 
oblique component of the image. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a video signal processing 
apparatus capable of eliminating both aliasing interference due to 
inter-frame offset sub-sampling and aliasing interference due to 
inter-field offset sub-sampling. 
In order to attain the above-mentioned object, a video signal processing 
apparatus of this invention comprises an interpolating circuit for 
interpolating a non-sample point from intra-field sample points in order 
to restore a high definition television signal which has been 
bandwidth-compressed by offset sub-sampling, a first memory for delaying a 
signal thus interpolated for one field period, a second memory for 
delaying an output signal of the first memory for a further one field 
period, a first adder for performing an adjacent inter-field averaging 
process from input and output signals of the first and second memories, a 
second adder for performing an inter-frame averaging process from an input 
signal of the first memory and an output signal of the second memory, a 
correlation discriminating circuit for detecting one field difference and 
one frame difference from input and output signals of the first and second 
memories and for outputting a switching signal in accordance with 
magnitudes of the differences thus detected, and a signal switching 
circuit for switching an output signal of the first adder and an output 
signal of the second adder in accordance with the output switching signal 
from the correlation discriminating circuit. Preferably, this apparatus 
may further comprise a motion detection circuit for detecting a motion of 
an image from an input signal of the first memory and an output signal of 
the second memory, and a mixing circuit for mixing an output signal of the 
signal switching circuit and an input/output signal of the first memory in 
accordance with the motion of an image thus detected. 
Another video signal processing apparatus of this invention comprises a 
first memory for delaying the bandwidth-compressed high definition 
television signal for one field period, a second memory for delaying an 
output signal of the first memory for a further one field period, first 
and second and third interpolating circuits for interpolating non-sample 
points from intra-field sample points in order to restore a sub-sampled 
signal in accordance with input and output signals of the first and second 
memories, a first adder for performing an adjacent inter-field averaging 
process from output signals of the first, second and third interpolating 
circuits, a second adder for performing an inter-frame averaging process 
from an output signal of the first interpolating circuit and an output 
signal of the third interpolating circuit, a correlation discriminating 
circuit for detecting one field difference and one frame difference from 
output signals of the first, second and third interpolating circuits and 
for outputting a switching signal in accordance with magnitudes of the 
differences thus detected, and a signal switching circuit for switching an 
output signal of the first adder and an output signal of the second adder 
in accordance with the switching signal from the correlation 
discriminating circuit. This apparatus may further comprise a motion 
detection circuit for detecting a motion of an image from an output signal 
of the first interpolating circuit and an output signal of the third 
interpolating circuit or from an input signal of the first memory and an 
output signal of the second memory, and a mixing circuit for mixing an 
output signal of the signal switching circuit and an output signal of the 
first or second interpolating circuit in accordance with the motion of an 
image thus detected. 
As arranged as above, a television signal subjected to intra-field 
interpolation is supplied to two memories. Then, an inter-field averaging 
process is performed between the present field and correlative adjacent 
field before or after for eliminating aliasing interference due to 
inter-field offset sub-sampling, and an inter-frame averaging process is 
performed for eliminating aliasing interference due to inter-frame offset 
sub-sampling. The larger of these aliasing interferences is discriminated 
in accordance with the magnitudes of an inter-field difference and an 
inter-frame difference to be switched. Accordingly, not only the aliasing 
interference due to inter-field offset sub-sampling but also the aliasing 
interference due to inter-frame offset sub-sampling can be eliminated. In 
addition, the motion of an image is detected, and the signal subjected to 
the inter-field averaging process between the present field and 
correlative adjacent field before or after, the signal subjected to the 
inter-frame averaging process, and the signal subjected to the intra-field 
interpolation are mixed with each other. Accordingly, even in the case of 
a moving image, degradations including the occurrence of double image can 
be prevented. 
Furthermore, as arranged as above, such aliasing interferences that are 
caused by inter-field and inter-frame offset sub-samplings can be 
eliminated. At the same time, a memory for storing signals before 
interpolation is provided thereby making a reduction in the memory 
capacity possible.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram of a video signal processing apparatus for 
reducing aliasing interference according to a first embodiment of this 
invention, which comprises an input terminal 11 for receiving a high 
definition television signal which has been bandwidth-compressed by offset 
sub-sampling (hereinafter typically expressed as a MUSE signal), an 
interpolating circuit 12 for interpolating a non-sample point from an 
inter-field sample point in order to restore a bandwidth-compressed 
signal, field memories 13 and 14, adders 16 and 17 each for performing an 
averaging process, a one field difference detection circuit 18 for 
detecting aliasing interference due to inter-field offset sub-sampling, a 
one frame difference detection circuit 19 for detecting aliasing 
interference due to inter-frame offset sub-sampling, a correlation 
discriminating circuit 20 for determing the larger of the inter-field and 
inter-frame aliasing interferences, and a signal switching circuit 21, and 
further comprising a motion detection circuit 22, a mixing circuit 23, and 
an output terminal 24 for outputting a high definition television signal 
having aliasing interferences eliminated therefrom. 
With the apparatus structured as above, the operation will be explained 
below. 
The MUSE signal inputted from the input terminal 11 is fed to the 
interpolating circuit 12 to interpolate a non-sample point from 
inter-field sample points in order to restore a bandwidth-compressed. 
Next, the signal interpolated signal is fed to the field memories 13 and 
14 connected in series. An output signal of the interpolating circuit 12 
and an output signal of the field memory 13 are subjected to inter-field 
averaging process through the adder 16 to obtain a signal having aliasing 
interference due to inter-field offset sub-sampling eliminated. On the 
other hand, the output signal of the interpolating circuit 12 and an 
output signal of the memory 14 are subjected to inter-frame averaging 
process through the adder 17 to obtain a signal having aliasing 
interference due to inter-frame offset sub-sampling eliminated. In 
addition, input and output signals of the field memories 13 and 14 are 
respectively fed to the one field difference detection circuit 18 and one 
frame difference detection circuit 19 to detect aliasing interference due 
to inter-field offset sub-sampling and aliasing interference due to 
inter-frame offset sub-sampling, and fed to the correlation discriminating 
circuit 20 to discrimine the larger one of the inter-field and inter-frame 
aliasing interferences. Thus, the signal switching circuit 21 controls the 
signal having no aliasing interference due to inter-field offset 
sub-sampling outputted from the adder 16 and the signal having no aliasing 
interference due to inter-frame offset sub-sampling outputted from the 
adder 17 so as to transmit a signal having the larger of the inter-field 
and inter-frame aliasing interferences eliminated. 
In addition, the motion detection circuit 22 detects the motion of an image 
from the input signal of the field memory 13 and the output signal of the 
field memory 14. The signal outputted from the signal switching circuit 
21, which is obtained by switching through the circuit 21 the signal 
subjected to inter-field averaging process before or after and the signal 
subjected to inter-frame averaging process, and the signal subjected to 
intra-field processing only outputted from the interpolating circuit 12 
are mixed with each other through the mixing circuit 23 in accordance with 
the motion of the thus detected image. Accordingly, even in the case of a 
moving image, no degradation (occurrence of a double image) results, so 
that a signal that has aliasing interferences substantially eliminated can 
be obtained from the output terminal 24. 
As explained above, according to this embodiment, an intra-field 
interpolation equivalent to the moving image process of an MUSE decoder is 
carried out in the interpolating circuit 12. Then, for the aliasing 
interference to be induced the case of a stationary image, the inter-field 
averaging process of the output signal of the interpolating circuit 12 and 
the output of the field memory 13 is carried out through the adder 16 to 
obtain such a signal that has no aliasing interference due to inter-field 
offset sub-sampling. At the same time, the inter-frame averaging process 
of the output signal of the interpolating circuit 12 and the output signal 
of the field memory 14 is carried out through the adder 17 to obtain a 
signal that has no aliasing interference due to inter-frame offset 
sub-sampling. Then, with these two signals, in order to detect the state 
of inter-field and inter-frame aliasing interferences from the signals of 
three adjacent fields, the one field difference detection circuit 18 for 
detecting the absolute value of an inter-field difference from these 
signals of the three adjacent fields, the one frame difference detection 
circuit 19 for detecting the absolute value of an inter-frame difference 
therefrom and the correlation discriminating circuit 20 for comparing 
these differences to detect the larger of them in magnitude are disposed 
as shown in FIG. 2. As shown in FIG. 3, in accordance with the value of 
the inter-field aliasing interference becoming a flicker component of 30 
Hz and the value of the inter-frame aliasing interference becoming a 
flicker component of 15 Hz, the signal subjected to processing which has a 
larger in aliasing interference is switched by the switching circuit 21 to 
be outputted. 
Accordingly, a signal that has no aliasing interferences due to inter-field 
and inter-frame offset sub-samplings can be obtained from the switching 
circuit 21. 
FIG. 4 is a block diagram of a video signal processing apparatus for 
reducing aliasing interference according to a second embodiment of this 
invention. A median-value signal selector 15 for extracting a median-value 
signal of three adjacent inter-field signals is additionally added to the 
apparatus of the first embodiment so as to be disposed before the adder 
16, and other components are the same as those in the first embodiment and 
indicated by the same referential designations as above. 
In order to obtain a signal that has aliasing interference due to 
inter-field offset sub-sampling, the median-value signal selector 15 
extracts a median-value signal of three adjacent inter-field signals from 
input/output signals of the field memories 13 and 14. Then, the 
median-value signal and the output signal of the memory 13 are subjected 
to averaging process through the adder 16 to obtain a signal having 
aliasing interference eliminated. 
In addition, the median-value signal selector 15 compares, as shown in FIG. 
7, the magnitudes of the two respective signals of three signals inputted 
with each other to detect a median-value signal on an amplitude selection 
basis therefrom. The selection means is switched in accordance with the 
thus detected result to output the median-value signal 
In addition, the median-value signal selector 15 is not limited to that 
shown in FIG. 7, but any circuit can be used for this purpose if it can 
extract a median-value signal. Theoretical values of median-value 
selection in the median-value signal selector 15 are tabulated as shown 
below. 
______________________________________ 
Magnitude relation- 
ship of signals Selected 
(ascending order) 
Outputs of comparator 
signals 
A B C A &gt; B A &gt; C B &gt; C MID 
______________________________________ 
2 1 3 0 1 1 A 
2 3 1 1 0 0 A 
1 2 3 1 1 1 B 
3 2 1 0 0 0 B 
1 3 2 1 1 0 C 
3 1 2 0 0 1 C 
1 1 2 1 1 1 B 
2 2 1 1 0 0 A 
1 2 1 1 1 0 C 
2 1 2 0 1 1 A 
2 1 1 0 0 1 C 
1 2 2 1 1 1 B 
1 1 1 1 1 1 B 
______________________________________ 
Next, the operation for eliminating aliasing interference will be explained 
in detail while referring to FIGS. 8(a)-8(c) which are waveform diagrams 
for explaining the operation of eliminating aliasing interference as an 
example. 
In a stationary area or at a changing point of motion, a median-value 
signal X (shown by marks in FIGS. 8(a)-8(c)) output through the 
median-value signal selector 15 becomes a signal before or after a signal 
Y of the present field (shown by .circleincircle. marks in FIGS. 8(a)-8(c) 
as shown in FIGS. 8(a) and 8(b)-. Accordingly, by being subjected to the 
inter-field averaging process on the median-value signal X and the signal 
Y of the present field through the adder 16, the aliasing component due to 
inter-field offset sub-sampling can be eliminated. On the other hand, in a 
moving area where an inputted television signal has motion, as shown in 
FIG. 8(c), the median-value signal X output through the median-value 
signal selector 15 becomes the signal Y of the present field. This means 
that the signal Y of the present signal itself can be obtained, resulting 
in obtaining no degradation in image quality. Accordingly, the averaging 
process is performed adaptively on inter-field signals high in correlation 
between adjacent three fields, so that the aliasing interference due to 
inter-field offset sub-sampling of the MUSE system can be eliminated 
without giving any effect to the moving image itself. Further, even if the 
motion detection circuit 22 fails to discriminate the motion of an image, 
the effect of the inter-field averaging process on the moving image can be 
made small. 
FIG. 5 is a block diagram of a video signal processing apparatus for 
reducing aliasing interference according to a third embodiment of this 
invention, which comprises an input terminal 31 for receiving an MUSE 
signal, field memories 32 and 33, intra-field interpolating circuits 34, 
35 and 36 each for interpolating a non-sample point from intra-field 
sample points in order to restore a bandwidth-compressed MUSE signal, 
adders 38 and 39 for performing averaging processes, a one field 
difference detection circuit 40 for detecting aliasing interference due to 
inter-field offset sub-sampling, a one frame difference detection circuit 
41 for detecting aliasing interference due to inter-frame offset 
sub-sampling, a correlation discriminating circuit 42 for determining the 
larger one of the inter-field and inter-frame aliasing interferences, and 
a signal switching circuit 43, further comprising a motion detection 
circuit 44, a mixing circuit 45 and an output terminal 46 for outputting a 
high definition television signal having aliasing interferences 
eliminated. 
The operation of this apparatus will be explained below. A MUSE signal 
inputted from the input terminal 31 is stored in the field memories 32 and 
33 connected in series. Then, subjected to interpolation of a non-sample 
point from an intra-field sample point for a bandwidth-compressed signal 
of each of three adjacent fields. Next, an output signal of the 
interpolating circuit 34 and an output signal of the interpolating circuit 
35 are subjected to an inter-field averaging process through the adder 38 
to obtain a signal having aliasing interference due to inter-field offset 
sub-sampling eliminated. On the other hand, an output signal of the 
interpolating circuit 34 and an output signal of the interpolating circuit 
36 are subjected to inter-frame averaging process through the adder 39 to 
obtain a signal having aliasing interference due to inter-frame offset 
sub-sampling eliminated. In addition, interpolated signals of three 
adjacent fields of the interpolating circuits 34, 35 and 36 are fed to the 
one field difference detection circuit 40 and one frame difference 
detection circuit 41 to respectively detect the aliasing interference due 
to inter-field offset sub-sampling and the aliasing interference ,due to 
inter-frame offset sub-sampling. And then, the larger of inter-field and 
inter-frame aliasing interferences is determined by through the 
correlation discriminating circuit. Then, the switching circuit 43 
controls the signal outputted from the adder 38 and having no the aliasing 
interference due to inter-field sub-sampling by performing inter-field 
averaging process between the present field and correlative adjacent one 
before or after and the signal outputted from the adder 39 and having no 
aliasing interference due to inter-frame offset sub-sampling, so that the 
signal having eliminated the larger of these inter-field and inter-frame 
aliasing interferences can be passed therethrough. 
In addition, the motion detection circuit 44 detects the motion of an image 
according to the output signal of the interpolating circuit 34 and that of 
the interpolating circuit 36. Then, the signal outputted from the signal 
switching circuit 43 and subjected to inter-field or inter-frame averaging 
process and the signal outputted from the interpolating circuit 34 and 
subjected to the inter-field process only are mixed with each other 
through the mixing circuit 45 in accordance with the motion of an image 
thus detected. Accordingly, even in the case of moving image, a signal 
having no degradation (occurrence of double image or the like) results and 
having aliasing interferences substantially eliminated can be obtained 
from the output terminal 46. 
As explained above, according to this embodiment, by disposing field 
memories for obtaining three adjacent inter-field signals so as to store a 
television signal before interpolation, aliasing interferences due to 
inter-field and inter-frame offset sub-samplings of the MUSE, system can 
be almost perfectly eliminated and the capacity of a field memory to be 
used can be reduced in half. 
FIG. 6 is a block diagram of a video signal processing apparatus for 
reducing aliasing interference according to a fourth embodiment of this 
invention. The apparatus of this embodiment has a median-value signal 
selector 37 for extracting a median-value signal of three adjacent 
inter-field signals disposed before the adder 38 of the third embodiment. 
Other components are the same as those of the third embodiment and 
indicated by the same reference designations as above. 
In order to obtain a signal having no aliasing interference due to 
inter-field offset subsampling, the median-value signal selector 37 
extracts a median-value signal of the three adjacent interfield signals 
from the output signals of the interpolating circuits 34, 35 and 36. The 
median-value signal is subjected to averaging process with an output 
signal of the interpolating circuit 35 through the adder 38 to obtain a 
signal having an no aliasing interference As a result, in a fashion 
similar to the second embodiment, even when the motion detection circuit 
44 fails to discriminate the motion of an image, the effect on the moving 
image in the inter-field averaging process can be reduced. 
In addition, the one field difference detection circuit 18 in the first and 
second embodiments is arranged so as to use all of the three adjacent 
inter-field signals. However, it may be arranged so as to detect an 
inter-field difference either before or after the present field. Also, the 
motion detection circuit 22 detects the motion of an image from one 
inter-frame signal. However, the circuit may detect the motion from two 
inter-frame signals by further outputting one additional inter-frame 
signal. In the third and fourth embodiments, the one field difference 
detection circuit 40 is also arranged so as to use all of the three 
adjacent inter-field signals. However, it may be arranged so as to detect 
an inter-field difference either before or after the present field. The 
motion detection circuit 44 detects the motion of an image from one 
inter-frame signal which is subjected to interpolating process. However, 
it may detect the motion from one inter-frame signal which is before 
interpolating process to be performed. In a fashion similar to the first 
and second embodiments, the motion may be detected from two inter-frame 
signals.