Method and apparatus for limiting band of moving-picture signal

A motion-compensated predictive error signal is generated in response to a moving-picture signal for every frame related to the moving-picture signal. Calculation is made as to an activity for each of pixels composing the frame in response to the motion-compensated predictive error signal. The activities for the respective pixels composing the frame are accumulated to calculate a 1-frame activity accumulation value. A band of the moving-picture signal in at least one of a spatial direction and a temporal direction is limited with a controllable band limiting characteristic. The controllable band limiting characteristic is controlled in response to the activity for each of the pixels and the 1-frame activity accumulation value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of limiting a band of a moving-picture 
signal. This invention also relates to an apparatus for limiting a band of 
a moving-picture signal. 
2. Description of the Prior Art 
Motion-compensated predictive encoding is one of highly-efficient encoding 
of a digital moving-picture signal. According to motion-compensated 
predictive encoding, every frame represented by a moving-picture signal is 
divided into blocks of a same size, and signal processing is executed 
block by block. Specifically, motion-compensated prediction is implemented 
by using a reference frame represented by picture data which results from 
decoding a previously-encoded frame. Calculation is given of a predictive 
error between a current block and a predicted block which results from the 
motion-compensated prediction. The predictive error is encoded. Thus, 
motion-compensated predictive encoding compresses a moving-picture signal 
by using a temporal correlation between successive frames represented by 
the moving-picture signal. 
According to a typical type of the encoding of a predictive error, the 
predictive error is subjected to orthogonal transform, and a signal which 
results from the orthogonal transform is quantized. Further, a 
quantization-resultant signal is subjected to an entropy encoding process. 
Thus, the typical type of the encoding of a predictive error compresses 
picture information by using a spatial correlation and a statistical 
correlation in a moving picture. 
In general, a temporal correlation, a spatial correlation, and a 
statistical correlation considerably vary from picture to picture. On the 
other hand, an amount of data (the number of bits of data) generated by 
encoding per unit time, that is, an encoding-resultant-data rate, is 
generally required to be a constant value independent of the contents of a 
moving picture. To meet such a requirement, the characteristics of 
quantization are changed in response to the characteristics of a moving 
picture. 
Specifically, in the case of pictures related to high temporal, spatial, 
and statistical correlations, fine quantization is executed to increase an 
encoding-resultant-data rate (the number of bits of data generated by 
encoding per unit time) to a desired rate. In the case of pictures related 
to low temporal, spatial, and statistical correlations, coarse 
quantization is executed to decrease an encoding-resultant-data rate (the 
number of bits of data generated by encoding per unit time) to the desired 
rate. 
In general, an encoding side and a decoding side are connected via a 
transmission line. The decoding side receives the output signal of the 
encoding side via the transmission line, and recovers an original 
moving-picture signal by decoding the output signal of the encoding side. 
Quantization at the encoding side causes a quantization distortion in 
every picture represented by the moving-picture signal recovered at the 
decoding side. The quantization distortion appears in the form of noise 
referred to as mosquito noise or block noise. 
According to a prior-art method designed to solve such a noise problem, 
temporal and spatial correlations in moving pictures are measured on the 
basis of an inter-frame difference (an inter-frame error) at a stage 
preceding a compressively encoding section, and signal bands in a temporal 
direction (a direction along a time base) and a spatial direction are 
limited in response to information of the measured correlations. The 
prior-art method disregards a temporal redundancy in moving pictures which 
is generally removed by motion-compensated prediction. Thus, in given 
signal conditions, the prior-art method needlessly limits the signal 
bands. 
SUMMARY OF THE INVENTION 
It is a first object of this invention to provide an improved method of 
limiting a band of a moving-picture signal. 
It is a second object of this invention to provide an improved apparatus 
for limiting a band of a moving-picture signal. 
A first aspect of this invention provides a method of limiting a band of a 
moving-picture signal which comprises the steps of generating a 
motion-compensated predictive error signal in response to the 
moving-picture signal for every frame related to the moving-picture 
signal; calculating an activity for each of pixels composing the frame in 
response to the motion-compensated predictive error signal; accumulating 
the activities for the respective pixels composing the frame to calculate 
a 1-frame activity accumulation value; limiting the band of the 
moving-picture signal in at least one of a spatial direction and a 
temporal direction with a controllable band limiting characteristic; and 
controlling the controllable band limiting characteristic in response to 
the activity for each of the pixels and the 1-frame activity accumulation 
value. 
A second aspect of this invention is based on the first aspect thereof, and 
provides a method wherein a frame related to the moving-picture signal 
which is currently band-limited is equal to a frame related to the 1-frame 
activity accumulation value currently used in said controlling the 
controllable band limiting characteristic. 
A third aspect of this invention is based on the first aspect thereof, and 
provides a method wherein a frame related to the moving-picture signal 
which is currently band-limited follows a frame related to the 1-frame 
activity accumulation value currently used in said controlling the 
controllable band limiting characteristic. 
A fourth aspect of this invention provides an apparatus for limiting a band 
of a moving-picture signal which comprises first means for generating a 
motion-compensated predictive error signal in response to the 
moving-picture signal for every frame related to the moving-picture 
signal; second means connected to the first means for calculating an 
activity for each of pixels composing the frame in response to the 
motion-compensated predictive error signal; third means connected to the 
second means for accumulating the activities for the respective pixels 
composing the frame to calculate a 1-frame activity accumulation value: 
fourth means for limiting the band of the moving-picture signal in at 
least one of a spatial direction and a temporal direction with a 
controllable band limiting characteristic; and fifth means connected to 
the second means, the third means, and the fourth means for controlling 
the controllable band limiting characteristic in response to the activity 
for each of the pixels and the 1-frame activity accumulation value. 
A fifth aspect of this invention is based on the fourth aspect thereof, and 
provides an apparatus wherein a frame related to the moving-picture signal 
which is currently band-limited is equal to a frame related to the 1-frame 
activity accumulation value currently used in said controlling the 
controllable band limiting characteristic. 
A sixth aspect of this invention is based on the fourth aspect thereof, and 
provides an apparatus wherein a frame related to the moving-picture signal 
which is currently band-limited follows a frame related to the 1-frame 
activity accumulation value currently used in said controlling the 
controllable band limiting characteristic.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
With reference to FIG. 1, a band limiting apparatus includes an input 
terminal 1 subjected to an input digital signal sequentially representing 
moving pictures. The apparatus input terminal 1 is followed by a switch 2. 
The switch 2 is connected to memories 4 and 5. The switch 2 receives the 
input moving-picture signal from the apparatus input terminal 1, and 
transmits the input moving-picture signal to either the memory 4 or the 
memory 5. The switch 2 responds to a switch control signal fed from a 
suitable signal generator (not shown). The switch control signal changes 
between different states in synchronism with a frame related to the input 
moving-picture signal. Accordingly, the switch 2 alternately transmits the 
input moving-picture signal to one of the memories 4 and 5 at a given 
period corresponding to a frame period. The input moving-picture signal 
related to each of first alternate frames is stored into the memory 4 
while the input moving-picture signal related to each of second alternate 
frames is stored into the memory 5. The moving-picture signals are 
temporarily held in the memories 4 and 5 before being outputted or read 
out therefrom. 
A switch 3 follows the memories 4 and 5. The switch 3 transmits one of the 
output signals of the memories 4 and 5 to a later stage. The switch 3 
responds to the switch control signal. Accordingly, the switch 3 
alternately transmits one of the output signals of the memories 4 and 5 to 
a later stage at a given period corresponding to the frame period. 
Operation of the memories 4 and 5 will be further described. During each of 
first alternate 1-frame periods, the input moving-picture signal is 
written into the memory 4 via the switch 2 while the moving-picture signal 
is read out and transmitted from the memory 5 via the switch 3. During 
each of second alternate 1-frame periods, the input moving-picture signal 
is written into the memory 5 via the switch 2 while the moving-picture 
signal is read out and transmitted from the memory 4 via the switch 3. 
The moving-picture signal transmitted via the switch 3 travels to a memory 
6, a predictor 7, and a subtracter 8. The memory 6 serves as a 1-frame 
delay device. Specifically, the moving-picture signal transmitted via the 
switch 3 is stored into the memory 6 before being temporarily held 
therein. Then, the moving-picture signal is outputted from the memory 6 as 
a 1-frame-preceding moving-picture signal related to an 
immediately-preceding frame with respect to the current frame, that is, a 
frame represented by the moving-picture signal currently transmitted via 
the switch 3. 
The predictor 7 receives the moving-picture signal from the switch 3 as a 
current-fame moving-picture signal. The predictor 7 receives the output 
signal of the memory 6 as a 1-frame-preceding moving-picture signal. The 
predictor 7 uses the 1-frame-preceding moving-picture signal as a 
reference-frame moving-picture signal, and generates a motion-compensated 
predictive picture signal in response to the current-frame moving-picture 
signal and the reference-frame moving-picture signal. The predictor 7 
outputs the motion-compensated predictive picture signal to the subtracter 
8. 
The signal processing implemented by the predictor 7 is based on one of 
known motion-compensated prediction techniques such as a technique using 
motion vectors detected by block matching. 
The subtracter 8 receives the moving-picture signal via the switch 3. The 
subtracter 8 receives the motion-compensated predictive picture signal 
from the predictor 7. The subtracter 8 calculates a difference between the 
moving-picture signal received via the switch 3 and the motion-compensated 
predictive picture signal, thereby generating a first motion-compensated 
predictive error signal corresponding to the calculated difference. The 
subtracter 8 outputs the first motion-compensated predictive error signal 
to a memory 9. 
The first motion-compensated predictive error signal is stored into the 
memory 9 before being temporarily held therein and outputted therefrom as 
a second motion-compensated predictive error signal. In the case where the 
signal processing implemented by the predictor 7 is based on a 
motion-compensated prediction technique using motion vectors detected by 
block matching, every frame is divided into blocks of a given size, and a 
motion-compensated prediction error signal is processed block by block. In 
this case, to simplify signal processing executed by a later stage, the 
memory 9 is preferably used as a scan converter to enable the recovery of 
a signal-piece sequence equal to a normal scanning-line sequence. The 
memory 9 feeds the second motion-compensated predictive error signal to an 
activity calculator 10. 
The device 10 calculates the activity of every pixel-corresponding segment 
of the second motion-compensated predictive error signal. The activity 
calculator 10 outputs a signal representative of the calculated activity 
to a memory 11 and an accumulator (or an integrator) 12. 
FIG. 2 shows an example of the activity calculator 10. As shown in FIG. 2, 
the activity calculator 10 includes an input terminal 20, an 
absolute-value calculator 21, delay devices 22A, 22B, 22C, 23A, 23B, 23C, 
24, and 25, an adder 26, and an output terminal 27. 
In the activity calculator 10 of FIG. 2, the input terminal 20 is subjected 
to the second motion-compensated predictive error signal. The input 
terminal 20 is followed by the absolute-value calculator 21. The second 
motion-compensated predictive error signal is transmitted via the input 
terminal 20 to the absolute-value calculator 21. The device 21 calculates 
the absolute value of the value represented by every pixel-corresponding 
segment of the second motion-compensated predictive error signal. The 
absolute-value calculator 21 generates and outputs a signal "c" 
representing the calculated absolute-value. The output signal "c" of the 
absolute-value calculator 21 is applied to the delay devices 22A and 24 
and the adder 26. 
The device 22A delays the signal "c" by a period corresponding to one 
pixel, and thereby changes the signal "c" into a delay-resultant signal 
"b". The delay device 22A outputs the signal "b" to the delay device 23A 
and the adder 26. The device 23A delays the signal "b" by a period 
corresponding to one pixel, and thereby changes the signal "b" into a 
delay-resultant signal "a". The delay device 23A outputs the signal "a" to 
the adder 26. 
The device 24 delays the signal "c" by a period corresponding to one 
scanning line, and thereby changes the signal "c" into a delay-resultant 
signal "f". The delay device 24 outputs the signal "f" to the delay device 
22B, the delay device 25, and the adder 26. The device 22B delays the 
signal "f" by a period corresponding to one pixel, and thereby changes the 
signal "f" into a delay-resultant signal "e". The delay device 22B outputs 
the signal "e" to the delay device 23B and the adder 26. The device 23B 
delays the signal "e" by a period corresponding to one pixel, and thereby 
changes the signal "e" into a delay-resultant signal "d". The delay device 
23B outputs the signal "d" to the adder 26. 
The device 25 delays the signal "f" by a period corresponding to one 
scanning line, and thereby changes the signal "f" into a delay-resultant 
signal "i". The delay device 25 outputs the signal "i" to the delay device 
22C and the adder 26. The device 22C delays the signal "i" by a period 
corresponding to one pixel, and thereby changes the signal "i" into a 
delay-resultant signal "h". The delay device 22C outputs the signal "h" to 
the delay device 23C and the adder 26. The device 23C delays the signal 
"h" by a period corresponding to one pixel, and thereby changes the signal 
"h" into a delay-resultant signal "g". The delay device 23C outputs the 
signal "g" to the adder 26. 
With reference to FIG. 3, 3.times.3 neighboring pixels "a", "b", "c", "d", 
"e", "f", "g", "h", and "i" are defined in connection with the signals 
"a", "b", "c", "d", "e", "f", "g", "h", and "i" in the activity calculator 
10 of FIG. 2, respectively. Specifically, the signal "e" represents the 
calculated absolute-value of the value represented by the segment of the 
second motion-compensated predictive error signal which corresponds to the 
central pixel "e", that is, the pixel of interest or the pixel in 
question. The signal "a" represents the calculated absolute-value of the 
value represented by the segment of the second motion-compensated 
predictive error signal which corresponds to the left-upper pixel "a". The 
signal "b" represents the calculated absolute-value of the value 
represented by the segment of the second motion-compensated predictive 
error signal which corresponds to the mid-upper pixel "b". The signal "c" 
represents the calculated absolute-value of the value represented by the 
segment of the second motion-compensated predictive error signal which 
corresponds to the right-upper pixel "c". The signal "d" represents the 
calculated absolute-value of the value represented by the segment of the 
second motion-compensated predictive error signal which corresponds to the 
left pixel "d" in the intermediate line. The signal "f" represents the 
calculated absolute-value of the value represented by the segment of the 
second motion-compensated predictive error signal which corresponds to the 
right pixel "f" in the intermediate line. The signal "g" represents the 
calculated absolute-value of the value represented by the segment of the 
second motion-compensated predictive error signal which corresponds to the 
left-lower pixel "g". The signal "h" represents the calculated 
absolute-value of the value represented by the segment of the second 
motion-compensated predictive error signal which corresponds to the 
mid-lower pixel "h". The signal "i" represents the calculated 
absolute-value of the value represented by the segment of the second 
motion-compensated predictive error signal which corresponds to the 
right-lower pixel "i". 
In the activity calculator 10 of FIG. 2, the device 26 adds the absolute 
values represented by the signals "a", "b", "c", "d", "e", "f", "g", "h", 
and "i". In other words, the adder 26 calculates the sum of the absolute 
values represented by the signals "a", "b", "c", "d", "e", "f", "g", "h", 
and "i". The calculated sum of the absolute values is defined as an 
activity corresponding to the pixel "e" of interest. Accordingly, the 
adder 26 generates and outputs a signal representing the activity of every 
pixel-corresponding segment of the second motion-compensated predictive 
error signal. The activity-representing signal is transmitted from the 
adder 26 to the memory 11 (see FIG. 1) and the accumulator 12 (see FIG. 1) 
via the output terminal 27. 
The signal processing executed by the activity calculator 10 includes a 
step of subjecting the second motion-compensated predictive error signal 
of the pixel "e" of interest to a low pass filtering process in spatial 
directions. Accordingly, the activity calculated by the device 10 results 
from the low pass filtering process. The low pass filtering process 
suppresses a local increase in the second motion-compensated predictive 
error signal which would be caused by, for example, noise in the input 
moving-picture signal. 
With reference back to FIG. 1, the activity-representing signal 
corresponding to every pixel is stored into the memory 11 from the 
activity calculator 10. In addition, the activity-representing signal 
corresponding to every pixel is inputted into the accumulator 12. The 
generation of the first motion-compensated predictive error signal and the 
calculation of the pixel-corresponding activity continue to be repeated 
until the activity-representing signals corresponding to respective pixels 
composing one frame (the currently processed frame) are stored into the 
memory 11 and are inputted into the accumulator 12. 
The accumulator 12 is informed of the activities represented by the 
activity-representing signals. The accumulator 12 adds or accumulates the 
activities into the accumulation value. The accumulator 12 responds to a 
frame sync signal fed from a suitable signal generator (not shown). The 
frame sync signal serves as a reset signal. Each time the signal 
processing for one frame has been completed, the accumulation value 
calculated by the accumulator 12 is reset to "0". Specifically, the 
accumulator 12 calculates the sum of the activities corresponding to the 
respective pixels composing one frame (the currently processed frame). The 
calculated sum of the activities is referred to as the 1-frame activity 
accumulation value ACTf. The accumulator 12 generates a signal 
representing the 1-frame activity accumulation value ACTf. The accumulator 
12 outputs the generated signal to a parameter deciding device 15. 
The activity-representing signals corresponding to respective pixels 
composing one frame (the currently processed frame) are sequentially read 
out from the memory 11 and are then fed to the parameter deciding device 
15. The pixel-corresponding activity represented by every 
activity-representing signal read out from the memory 11 is denoted by the 
character "ACTp". The activity ACTp represented by the output signal of 
the memory 11 relates to a pixel within a frame corresponding to the 
1-frame activity accumulation value ACTf represented by the output signal 
of the accumulator 12. 
The parameter deciding device 15 is informed of the 1-frame activity 
accumulation value ACTf represented by the output signal of the 
accumulator 12. The parameter deciding device 15 is informed of the 
pixel-corresponding activity ACTp represented by the activity-representing 
signal fed from the memory 11. The parameter deciding device 15 determines 
a band limit control parameter Pt for a temporal-direction low pass filter 
13 and a band limit control parameter Ps for a spatial-direction low pass 
filter 14 in response to the 1-frame activity accumulation value ACTf and 
the pixel-corresponding activity ACTp. 
The temporal-direction low pass filter 13 and the spatial-direction low 
pass filter 14 are of variable types. The temporal-direction low pass 
filter 13, the spatial-direction low pass filter 14, and the band limit 
control parameters Pt and Ps are designed to provide the following 
functions. When the band limit control parameter Pt is equal to "0", the 
temporal-direction low pass filter 13 assumes a through state and hence 
does not execute any low pass filtering function. When the band limit 
control parameter Pt is equal to "1", the temporal-direction low pass 
filter 13 fully executes a given low pass filtering function. When the 
band limit control parameter Pt is equal to a value between "0" and "1", 
the temporal-direction low pass filter 13 executes the low pass filtering 
function at a degree corresponding to the value of the band limit control 
parameter Pt. When the band limit control parameter Ps is equal to "0", 
the spatial-direction low pass filter 14 assumes a through state and hence 
does not execute any low pass filtering function. When the band limit 
control parameter Ps is equal to "1", the spatial-direction low pass 
filter 14 fully executes a given low pass filtering function. When the 
band limit control parameter Ps is equal to a value between "0" and "1", 
the spatial-direction low pass filter 14 executes the low pass filtering 
function at a degree corresponding to the value of the band limit control 
parameter Ps. 
FIG. 4 shows an example of the parameter deciding device 15. As shown in 
FIG. 4, the parameter deciding device 15 includes input terminals 31 and 
32, ROM's 34 and 35, calculators 38 and 39, limiters 40 and 42, and output 
terminals 41 and 43. 
In the parameter deciding device 15 of FIG. 4, the input terminal 31 is 
subjected to the output signal of the accumulator 12 (see FIG. 1) which 
represents the 1-frame activity accumulation value ACTf. The signal 
representing the 1-frame activity accumulation value ACTf is applied via 
the input terminal 31 to an address input terminal of the ROM 34, an 
address input terminal of the ROM 35, and the calculators 38 and 39. 
The ROM 34 serves as a signal converter or a function generator. The ROM 34 
outputs a signal representative of a value F1 in response to the signal 
representing the 1-frame activity accumulation value ACTf. Specifically, 
the ROM 34 stores preset signals representative of values F1 at storage 
segments having different addresses respectively. The signal representing 
the 1-frame activity accumulation value ACTf serves as an address signal 
applied to the ROM 34. One of the storage segments of the ROM 34 is 
accessed in response to the address signal, that is, the signal 
representing the 1-frame activity accumulation value ACTf. A signal 
representative of a value F1 is read out or outputted from the accessed 
storage segment of the ROM 34. It is shown in FIG. 5 that the value F1 
represented by the output signal of the ROM 34 linearly increases from "0" 
to "1" as the 1-frame activity accumulation value ACTf increases from a 
first given value L1a to a second given value L1b. The value F1 is "0" 
when the 1-frame activity accumulation value ACTf is smaller than the 
first given value L1a. The value F1 is "1" when the 1-frame activity 
accumulation value ACTf is greater than the second given value L1b. The 
output signal of the ROM 34 is applied to the calculator 38. 
In the parameter deciding device 15 of FIG. 4, the input terminal 32 is 
subjected to the output signal of the memory 11 (see FIG. 1) which 
represents the pixel-corresponding activity ACTp. The signal representing 
the pixel-corresponding activity ACTp is applied via the input terminal 32 
to the calculators 38 and 39. 
The calculator 38 determines a basic band limit control parameter Pt' in 
response to the value F1, a preset value K1, the 1-frame activity 
accumulation value ACTf, and the pixel-corresponding activity ACTp 
according to the following equation. 
##EQU1## 
Here, the preset value K1 is greater than "1". When the 
pixel-corresponding activity ACTp is equal to "0", the basic band limit 
control parameter Pt' is equal to the value F1. When the 
pixel-corresponding activity ACTp is infinite, the basic band limit 
control parameter Pt' is equal to the value "K1.multidot.K1.multidot.F1". 
In the case where the value F1 is greater than "0", the basic band limit 
control parameter Pt' increases as the pixel-corresponding activity ACTp 
increases. Further, in the case where the 1-frame activity accumulation 
value ACTf is smaller than the first given value L1a, the value F1 is "0" 
as previously described so that the basic band limit control parameter Pt' 
is also "0". On the other hand, in the case where the 1-frame activity 
accumulation value ACTf is greater than the second given value L1b, the 
value F1 is "1" as previously described so that the basic band limit 
control parameter Pt' is equal to or greater than "1". The calculator 38 
informs the limiter 40 of the basic band limit control parameter Pt'. 
The signal conversion executed by the ROM 34 is designed to disregard a 
variation in the value F1 which would be caused by a variation in the 
value ACTf due to small noise. In addition, the signal conversion executed 
by the ROM 34 is designed to prevent an excessive increase in the width of 
data outputted from the ROM 34 which would be caused by an increase in the 
value F1 above "1" due to an increase in the value ACTf. 
The limiter 40 changes the basic band limit control parameter Pt' into the 
final band limit control parameter Pt by a limiting process. Specifically, 
when the basic band limit control parameter Pt' is smaller than or equal 
to "1", the limiter 40 sets the final band limit control parameter Pt 
equal to the basic band limit control parameter Pt'. When the basic band 
limit control parameter Pt' is greater than "1", the limiter 40 sets the 
final band limit control parameter Pt equal to "1". Operation of the 
limiter 40 is designed to meet a requirement for the variable range of the 
final band limit control parameter Pt and also to prevent an excessive 
increase in the width of data outputted from the limiter 40. The limiter 
40 outputs a signal representing the final band limit control parameter 
Pt. The signal representing the final band limit control parameter Pt is 
transmitted from the limiter 40 to the temporal-direction low pass filter 
13 (see FIG. 1) via the output terminal 41. 
The first given value L1a, the second given value L1b, and the preset value 
K1 determine the sensitivity of the final band limit control parameter Pt 
with respect to the input activity. 
The ROM 35 serves as a signal converter or a function generator. The ROM 35 
outputs a signal representative of a value F2 in response to the signal 
representing the 1-frame activity accumulation value ACTf. Specifically, 
the ROM 35 stores preset signals representative of values F2 at storage 
segments having different addresses respectively. The signal representing 
the 1-frame activity accumulation value ACTf serves as an address signal 
applied to the ROM 35. One of the storage segments of the ROM 35 is 
accessed in response to the address signal, that is, the signal 
representing the 1-frame activity accumulation value ACTf. A signal 
representative of a value F2 is read out or outputted from the accessed 
storage segment of the ROM 35. It is shown in FIG. 6 that the value F2 
represented by the output signal of the ROM 35 linearly increases from "0" 
to "1" as the 1-frame activity accumulation value ACTf increases from a 
first given value L2a to a second given value L2b. The value F2 is "0" 
when the 1-frame activity accumulation value ACTf is smaller than the 
first given value L2a. The value F2 is "1" when the 1-frame activity 
accumulation value ACTf is greater than the second given value L2b. The 
output signal of the ROM 35 is applied to the calculator 39. 
The calculator 39 determines a basic band limit control parameter Ps' in 
response to the value F2, a preset value K2, the 1-frame activity 
accumulation value ACTf, and the pixel-corresponding activity ACTp 
according to the following equation. 
##EQU2## 
Here, the preset value K2 is greater than "1". It is preferable that the 
preset value K2 differs from the preset value K1. The preset value K2 may 
be equal to the preset value K1. When the pixel-corresponding activity 
ACTp is equal to "0", the basic band limit control parameter Ps' is equal 
to the value F2. When the pixel-corresponding activity ACTp is infinite, 
the basic band limit control parameter Ps' is equal to the value 
"K2.multidot.K2.multidot.F2". In the case where the value F2 is greater 
than "0", the basic band limit control parameter Ps' increases as the 
pixel-corresponding activity ACTp increases. Further, in the case where 
the 1-frame activity accumulation value ACTf is smaller than the first 
given value L2a, the value F2 is "0" as previously described so that the 
basic band limit control parameter Ps' is also "0". On the other hand, in 
the case where the 1-frame activity accumulation value ACTf is greater 
than the second given value L2b, the value F2 is "1" as previously 
described so that the basic band limit control parameter Ps' is equal to 
or greater than "1". The calculator 39 informs the limiter 42 of the basic 
band limit control parameter Ps'. 
The signal conversion executed by the ROM 35 is designed to disregard a 
variation in the value F2 which would be caused by a variation in the 
value ACTf due to small noise. In addition, the signal conversion executed 
by the ROM 35 is designed to prevent an excessive increase in the width of 
data outputted from the ROM 35 which would be caused by an increase in the 
value F2 above "1" due to an increase in the value ACTf. 
The limiter 42 changes the basic band limit control parameter Ps' into the 
final band limit control parameter Ps by a limiting process. Specifically, 
when the basic band limit control parameter Ps' is smaller than or equal 
to "1", the limiter 42 sets the final band limit control parameter Ps 
equal to the basic band limit control parameter Ps'. When the basic band 
limit control parameter Ps' is greater than "1", the limiter 42 sets the 
final band limit control parameter Ps equal to "1". Operation of the 
limiter 42 is designed to meet a requirement for the variable range of the 
final band limit control parameter Ps and also to prevent an excessive 
increase in the width of data outputted from the limiter 42. The limiter 
42 outputs a signal representing the final band limit control parameter 
Ps. The signal representing the final band limit control parameter Ps is 
transmitted from the limiter 42 to the spatial-direction low pass filter 
14 (see FIG. 1) via the output terminal 43. 
The first given value L2a, the second given value L2b, and the preset value 
K2 determine the sensitivity of the final band limit control parameter Ps 
with respect to the input activity. 
With reference back to FIG. 1, the temporal-direction low pass filter 13 
receives the moving-picture signal via the switch 3. The device 13 
subjects the received moving-picture signal to a temporal-direction low 
pass filtering process responsive to the band limit control parameter Pt 
represented by the output signal of the parameter deciding device 15. 
FIG. 7 shows an example of the temporal-direction low pass filter 13. As 
shown in FIG. 7, the temporal-direction low pass filter 13 includes an 
input terminal 50, a delay device 51, a calculator 52, and an output 
terminal 53. 
In the temporal-direction low pass filter 13 of FIG. 7, the input terminal 
50 is subjected to the moving-picture signal transmitted via the switch 3 
(see FIG. 1). The input terminal 50 is connected to the delay device 51 
and the calculator 52. The moving-picture signal is fed from the input 
terminal 50 to the delay device 51 and the calculator 52. The device 51 
delays the moving-picture signal by a period corresponding to one frame. 
The delay device 51 outputs the delay-resultant signal to the calculator 
52. The moving-picture signal fed from the input terminal 50 to the 
calculator 52 represents a value "a" at a pixel in the current frame. The 
output signal of the delay device 51 represents a value "b" at the same 
pixel in the immediately preceding frame. The calculator 52 is informed of 
the values "a" and "b". Further, the calculator 52 is informed of the band 
limit control parameter Pt. The calculator 52 determines a 
filtering-result value St in response to the value "a", the value "b", and 
the band limit control parameter Pt according to the following equation. 
##EQU3## 
This equation corresponds to a low pass filtering process in a temporal 
direction. The degree of the low pass filtering process depends on the 
band limit control parameter Pt. The calculator 52 generates a signal 
representing the filtering-result value St. The signal representing the 
filtering-result value St is transmitted from the calculator 52 to the 
spatial-direction low pass filter 14 (see FIG. 1) via the output terminal 
53 as a first filtering-resultant moving-picture signal. 
When the band limit control parameter Pt is equal to "0", the 
filtering-result value St is equal to the value "a". In this case, the 
temporal-direction low pass filter 13 is in a through state so that the 
moving-picture signal passes through the temporal-direction low pass 
filter 13 without undergoing any filtering process. When the band limit 
control parameter Pt is equal to "1", the filtering-result value St is 
equal to the value "(a+b)/2" which agrees with a mean between the pixel 
values "a" and "b" related to the current frame and the immediately 
preceding frame. In this case, the temporal-direction low pass filter 13 
fully executes a given low pass filtering process in a temporal direction. 
When the band limit control parameter Pt is equal to a value between "0" 
and "1", the filtering-result value St is equal to a weighted mean between 
the values "a" and "b". In this case, weighting coefficients for the 
values "a" and "b" depend on the band limit control parameter Pt, and the 
temporal-direction low pass filter 13 executes the temporal-direction low 
pass filtering process at a degree depending on the band limit control 
parameter Pt. 
With reference back to FIG. 1, the spatial-direction low pass filter 14 
receives the first filtering-resultant moving-picture signal from the 
temporal-direction low pass filter 13. The device 14 subjects the first 
filtering-resultant moving-picture signal to a spatial-direction low pass 
filtering process responsive to the band limit control parameter Ps 
represented by the output signal of the parameter deciding device 15. 
FIG. 8 shows an example of the spatial-direction low pass filter 14. As 
shown in FIG. 8, the spatial-direction low pass filter 14 includes an 
input terminal 120, delay devices 122A, 122B, 122C, 123A, 123B, 123C, 124, 
125, and 128, and a calculator 126. 
In the spatial-direction low pass filter 14 of FIG. 8, the input terminal 
120 receives the first filtering-resultant moving-picture signal from the 
temporal-direction low pass filter 13 (see FIG. 1). The first 
filtering-resultant moving-picture signal agrees with a signal "c" 
representing a value at the pixel "c" of FIG. 3. The signal "c" is applied 
to the delay devices 122A and 124 and the calculator 126. 
The device 122A delays the signal "c" by a period corresponding to one 
pixel, and thereby changes the signal "c" into a delay-resultant signal 
"b". The signal "b" represents a value at the pixel "b" of FIG. 3. The 
delay device 122A outputs the signal "b" to the delay device 123A and the 
calculator 126. The device 123A delays the signal "b" by a period 
corresponding to one pixel, and thereby changes the signal "b" into a 
delay-resultant signal "a". The signal "a" represents a value at the pixel 
"a" of FIG. 3. The delay device 123A outputs the signal "a" to the 
calculator 126. 
The device 124 delays the signal "c" by a period corresponding to one 
scanning line, and thereby changes the signal "c" into a delay-resultant 
signal "f". The signal "f" represents a value at the pixel "f" of FIG. 3. 
The delay device 124 outputs the signal "f" to the delay device 122B, the 
delay device 125, and the calculator 126. The device 122B delays the 
signal "f" by a period corresponding to one pixel, and thereby changes the 
signal "f" into a delay-resultant signal "e". The signal "e" represents a 
value at the pixel "e" of FIG. 3. The delay device 122B outputs the signal 
"e" to the delay device 123B and the calculator 126. The device 123B 
delays the signal "e" by a period corresponding to one pixel, and thereby 
changes the signal "e" into a delay-resultant signal "d". The signal "d" 
represents a value at the pixel "d" of FIG. 3. The delay device 123B 
outputs the signal "d" to the calculator 126. 
The device 125 delays the signal "f" by a period corresponding to one 
scanning line, and thereby changes the signal "f" into a delay-resultant 
signal "i". The signal "i" represents a value at the pixel "i" of FIG. 3. 
The delay device 125 outputs the signal "i" to the delay device 122C and 
the calculator 126. The device 122C delays the signal "i" by a period 
corresponding to one pixel, and thereby changes the signal "i" into a 
delay-resultant signal "h". The signal "h" represents a value at the pixel 
"h" of FIG. 3. The delay device 122C outputs the signal "h" to the delay 
device 123C and the calculator 126. The device 123C delays the signal "h" 
by a period corresponding to one pixel, and thereby changes the signal "h" 
into a delay-resultant signal "g". The signal "g" represents a value at 
the pixel "g" of FIG. 3. The delay device 123C outputs the signal "g" to 
the calculator 126. 
As shown in FIG. 3, the 3.times.3 neighboring pixels "a", "b", "c", "d", 
"e", "f", "g", "h", and "i" are defined in connection with the signals 
"a", "b", "c", "d", "e", "f", "g", "h", and "i" in the spatial-direction 
low pass filter 14 of FIG. 8, respectively. The central pixel "e" agrees 
with the pixel of interest or the pixel in question. 
In the spatial-direction low pass filter 14 of FIG. 8, the delay device 128 
receives the output signal of the parameter deciding device 15 which 
represents the band limit control parameter Ps. The device 128 delays the 
received signal by a period corresponding to the resultant of one scanning 
line and one pixel, and thereby changes the received signal into a 
delay-resultant signal which corresponds to the pixel "e" of interest. The 
delay device 128 outputs the delay resultant signal to the calculator 126. 
The band limit control parameter represented by the output signal of the 
delay device 128 is denoted by the character. "Psd". 
The calculator 126 determines a filtering-result value Ss in response to 
the values "a", "b", "c", "d", "e", "f", "g", "h", and "i", and the band 
limit control parameter Psd according to the following equation. 
##EQU4## 
This equation corresponds to a low pass filtering process in spatial 
directions. The degree of the low pass filtering process depends on the 
band limit control parameter Psd. The calculator 126 generates a signal 
representing the filtering-result value Ss. The signal representing the 
filtering-result value Ss is transmitted from the calculator 126 to the 
output terminal 16 as a second filtering-resultant moving-picture signal. 
When the band limit control parameter Psd is equal to "0", the 
filtering-result value Ss is equal to the value "e". In this case, the 
spatial-direction low pass filter 14 is in a through state so that the 
first filtering-resultant moving-picture signal passes through the 
spatial-direction low pass filter 14 without undergoing any filtering 
process. When the band limit control parameter Psd is equal to "1", the 
filtering-result value Ss is equal to the value "(a+b+c+d+e+f+g+h+i)/9" 
which agrees with a mean among the values "a", "b", "c", "d", "e", "f", 
"g", "h", and "i". In this case, the spatial-direction low pass filter 14 
fully executes a given low pass filtering process in spatial directions. 
When the band limit control parameter Psd is equal to a value between "0" 
and "1", the filtering-result value Ss is equal to a weighted mean among 
the values "a", "b", "c", "d", "e", "f", "g", "h", and "i". In this case, 
weighting coefficients for the values "a", "b", "c", "d", "e", "f", "g", 
"h", and "i" depend on the band limit control parameter Psd, and the 
spatial-direction low pass filter 14 executes the spatial-direction low 
pass filtering process at a degree depending on the band limit control 
parameter Psd. 
A timing adjustment arrangement (not shown) including at least one delay 
device is provided so that the pixel corresponding to the moving-picture 
signal currently fed to the temporal-direction low pass filter 13 via the 
switch 3 will agree with the pixel corresponding to the signal currently 
fed to the parameter deciding device 15 from the memory 11. Further, the 
pixel corresponding to the first filtering-resultant moving-picture signal 
currently fed to the spatial low pass filter 14 from the 
temporal-direction low pass filter 13 agrees with the pixel corresponding 
to the signal currently fed to the parameter deciding device 15 from the 
memory 11. 
In general, the band limiting apparatus of FIG. 1 is used as a pre-filter 
followed by a highly efficient encoding apparatus or a compressively 
encoding apparatus. As previously described, in the band limiting 
apparatus of FIG. 1, the motion-compensated predictive error signal is 
generated for every frame. The pixel-corresponding activity ACTp is 
calculated on the basis of the motion-compensated predictive error signal. 
The 1-frame activity accumulation value ACTf is calculated by summing the 
pixel-corresponding activities. Thus, detection is made as to temporal and 
spatial correlations in moving pictures for every local area within a 
frame region. The band limit control parameters Pt and Ps are determined 
in response to the pixel-corresponding activity ACTp and the 1-frame 
activity accumulation value ACTf. In other words, the band limit control 
parameters Pt and Ps are determined in accordance with the detected 
temporal and spatial correlations. Thus, the band limit control parameters 
Pt and Ps are controlled pixel by pixel as well as frame by frame in 
response to the detected temporal and spatial correlations. Accordingly, 
the temporal-direction low pass filtering process and the 
spatial-direction low pass filtering process on an input moving-picture 
signal by the temporal-direction low pass filter 13 and the 
spatial-direction low pass filter 14 are controlled pixel by pixel as well 
as frame by frame in response to the detected temporal and spatial 
correlations. In other words, the characteristics of limiting the band of 
the input moving-picture signal in temporal and spatial directions are 
controlled pixel by pixel as well as frame by frame in response to the 
detected temporal and spatial correlations. This operation of the band 
limiting apparatus of FIG. 1 enables a reduction in a picture-quality 
deterioration which would be caused by highly efficient encoding or 
compressively encoding. 
In the band limiting apparatus of FIG. 1, the predictor 7 generates the 
motion-compensated predictive picture signal. Accordingly, a temporal 
redundancy in moving pictures which is generally removed by highly 
efficient encoding or compressively encoding is considered in determining 
the band limit control parameters Pt and Ps. Thus, it is possible to 
prevent the band of the input moving-picture signal from being needlessly 
limited under given signal conditions. 
Second Embodiment 
FIG. 9 shows a second embodiment of this invention which is similar to the 
embodiment of FIG. 1 except for design changes indicated hereinafter. The 
embodiment of FIG. 9 dispenses with the memory 11 of FIG. 1. In the 
embodiment of FIG. 9, a parameter deciding device 15 is directly connected 
to an activity calculator 10. Further, in the embodiment of FIG. 9, a 
latch 17 is interposed between an accumulator 12 and the parameter 
deciding device 15. 
In the embodiment of FIG. 9, the device 10 calculates the activity of every 
pixel-corresponding segment of a second motion-compensated predictive 
error signal outputted from a memory 9. The activity calculator 10 outputs 
a signal representative of the calculated activity to the accumulator 12 
and the parameter deciding device 15. The activity represented by the 
output signal of the activity calculator 10 is used by the parameter 
deciding device 15 as a pixel-corresponding activity ACTp. 
In the embodiment of FIG. 9, the accumulator 12 outputs a signal 
representative of a 1-frame activity accumulation value ACTf to the latch 
17. The device 17 periodically latches the output signal of the 
accumulator 12 at a timing determined by a frame sync signal. 
Specifically, the device 17 latches the output signal of the accumulator 
12 immediately before the accumulation value calculated by the accumulator 
12 is reset to "0". The latch 17 outputs the latched signal to the 
parameter deciding device 15 as a signal representing the 1-frame activity 
accumulation value ACTf. The 1-frame activity accumulation value ACTf 
represented by the output signal of the latch 17 is used by the parameter 
deciding device 15 in the calculation of band limit control parameters Pt 
and Ps for a frame immediately following the frame related to the 1-frame 
activity accumulation value ACTf (that is, the frame related to the output 
signal of the latch 17). 
The embodiment of FIG. 9 is advantageous in that the memory 11 of FIG. 1 
can be omitted.