Data compressing system

In a code compressing system for determining a quantizing step of a image compressing code, a second DCT calculating portion performs a DCT calculation for each element reduced and divided by a picture dividing portion. A second quantizing portion quantizes the calculated result with the maximum value of quantizing steps of an applicable system. A third quantizing portion quantizes the calculated result with the minimum value of the quantizing steps of the applicable system. A code amount controlling portion determines the difference between the code amounts generated by the second quantizing portion and the third quantizing portion for each element divided by the picture dividing portion and obtains an optimum quantizing step corresponding to the determined result, the target compressing ratio, and the target encoding bit rate. A first quantizing portion quantizes the calculated result of a first DCT calculating portion with the optimum quantizing step determined by the code amount controlling portion.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a data compressing system for use with a 
picture compressing apparatus of a still picture, and a picture 
decompressing apparatus for continuously decompressing a compressed still 
picture and displaying as a moving picture, in particular, a code amount 
controlling system corresponding to a data compressing system, and a 
compressed moving picture encoding apparatus, 
2. Description of the Related Art 
As data compressing systems for maintaining a compressing ratio and 
encoding bit rate constant (in other words, a data compressing system with 
a target compressing ratio and a target encoding bit rate), the following 
related art reference is known. 
As a first related art reference "a data compressing system for repeating a 
compression encoding process until a target compressing ratio and a target 
encoding bit rate is accomplished, determining optimum parameters such as 
quantizing steps, and compressing data corresponding to the parameters" is 
known. 
FIG. 4 is a block diagram showing a structure of a picture data recording 
system (as disclosed in Japanese Patent Laid-Open Publication No. 
Hei4-48874) corresponding to the data compressing system of the first 
related art reference. This picture data recording system comprises a 
frame memory 41, a discrete cosine converting circuit 42, a quantizing 
circuit 43, a zigzag scanning circuit 44, a Huffman encoding circuit 45, a 
code amount measuring circuit 48, a scaling factor controlling circuit 46, 
and a quantizing matrix circuit 47, A quantizing step is defined 
corresponding to a value based on a scaling factor received from the 
scaling factor controlling circuit 46. 
As a second related art reference, "a data compressing system for dividing 
a picture into elements and designating a quantizing level of each element 
as optimum parameters of a target compressing ratio and a target encoding 
bit rate" is known. 
The data compressing system of the second related art reference corresponds 
to a moving picture encoding apparatus as disclosed in Japanese Patent 
Laid-Open Publication No. Hei4-321069 and a television telephone picture 
signal compressing apparatus as disclosed in Japanese Patent Laid-Open 
Publication No. Hei4-266286. 
FIG. 5(a) is a schematic diagram showing the conception of a block division 
corresponding to the data compressing system as disclosed in the Japanese 
Patent Laid-Open Publication No. Hei4-329089. In this case, an original 
picture (of a person) is divided into rectangular blocks (block divided 
regions) and optimum quantizing steps thereof are determined. 
FIG. 5(b) is a schematic diagram showing the conception of a block (zone) 
division corresponding to the data compressing system as disclosed in the 
Japanese Patent Laid-Open Publication No. Hei4-266286. In this system, an 
application for a television telephone system is supposed. Referring to 
FIG. 5(b), a person is disposed at the center portion of the screen. The 
center portion 59 (zone 2) is finely quantized, whereas the periphery 
(zone 1) thereof is coarsely quantized. In such a manner, the entire code 
amount of the picture is controlled. 
As a third related art referenced, FIG. 6 is a block diagram (functional 
block diagram) of a moving picture encoding apparatus as disclosed in 
Japanese Patent Laid-Open Publication No. Hei4-331592 that provides a more 
detail structure than the above-described related art reference as 
disclosed in the Japanese Patent Laid-Open Publication No. Hei4-329089. 
This moving picture encoding apparatus as disclosed in Japanese Patent 
Laid-Open Publication No. Hei4-331592 comprises a picture input means 21, 
a picture selecting means 22, an orthogonal transforming means 23, 
quantizing means 24, a variable length encoding means 25, an 
intra/inter-frame determining means 26, a current frame picture storing 
means 27, a preceding frame picture storing means 28, a decoded picture 
calculating means 29, a determined result storing means 30, and a 
quantizing step determining means 31. In this moving picture encoding 
apparatus, when data amount (code amount) that is encoded in a particular 
time period is larger than a predetermined reference code amount (target 
code amount), the quantizing step is increased so as to suppress the 
generated code amount. On the other hand, when a particular code amount is 
smaller than the predetermined reference code amount, the quantizing step 
is decreased so an to increase the generated code amount. 
In the data compressing system of the first related art reference, data 
should be fed back several times so that the generated code amount is 
smaller than the target code amount. Thus, this system is effective for an 
apparatus that deals with still pictures that are not required to be 
processed on real time base in a predetermined time period (an example of 
this apparatus is an electronic still picture camera). However, when this 
system is used for an apparatus that continuously displays still pictures 
as moving pictures or an apparatus that compresses pictures on real time 
basis so as to record pictures, it is very difficult to "repeatedly 
control the feed-back process". 
In addition, in the data compressing system of the second related art 
reference, there are the following problems. In "the system for dividing a 
frame into zones and encoding each zone with a quantizing steps 
corresponding thereto" as with the data compressing system disclosed in 
the Japanese Patent Laid-Open Publication No. Hei4-266286 titled "Picture 
signal Compressing Apparatus for Use with Television Telephone", this 
system is effective for a picture of which a person is disposed at the 
center portion of a screen with a high probability as in a television 
telephone. However, in this system, when a large object is not disposed at 
the center portion, picture quality and code amount cannot be optimally 
obtained and effectively controlled. 
And, in the data comressing system of the third related art reference, 
there are the following problems. 
In "the system for dividing a picture into blocks, increasing the 
quantizing step in the case that the code amount of a divided block is 
larger than a predetermined target code amount, and decreasing the 
quantizing stop in the case that the code amount of a divided block is 
smaller than the predetermined target code amount", unlike with the 
problem (1), when a person is disposed at the center portion of a picture, 
the following problem takes place. Since the code amount of the background 
of a picture is smaller than the target code amount and thereby the 
quantizing stop is decreased, the picture quality is improved. However, 
since the code amount at the center portion of the picture is larger than 
the target code amount and thereby the quantizing stop is increased, the 
picture quality is deteriorated. Thus the picture quality of the 
background portion is high, whereas the picture quality of the center 
portion is low. 
In other words, in the conventional data compressing systems, when a 
particular target compressing ratio and a particular target encoding bit 
rate should be required, the picture quality is deteriorated. 
It is very important in the control field of an image data to "control a 
code amount while keeping the picture quality in a system that compresses 
and stores data or a system that transmits data at a predetermined 
constant encoding bit rate". 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a data compressing system 
for preventing the picture quality from deteriorating even if a 
predetermine constant compressing ratio or a predetermined constant 
encoding bit rate are required. 
The present invention is a data compressing system, comprising a first DCT 
calculating portion for performing a DCT calculation for an original 
picture, a first quantizing portion for quantizing a calculated result of 
the first DCT calculating portion with an optimum quantizing step 
determined by a code amount controlling portion, a variable length 
encoding portion for counting a valid coefficient and a run number of "0" 
values corresponding to the quantized result of the first quantizing 
portion and outputting a variable length code corresponding to the counted 
result, a picture dividing portion for reducing the original picture and 
dividing it into several elements, a second DCT calculating portion for 
performing a DCT calculation for the elements divided by the picture 
dividing portion, a second quantizing portion for quantizing a calculated 
result of the second DCT calculating portion with the maximum value of 
quantizing steps of an applicable system, a third quantizing portion for 
quantizing the calculated result of the second DCT calculating portion 
with the minimum value of the quantizing steps of the applicable system, 
and a code amount controlling portion for controlling a quantizing stop of 
the first quantizing portion for each of the elements corresponding to the 
difference of the code amount generated by the second quantizing portion 
and the code amount generated by the third quantizing portion. 
In the data compressing system according to the present invention, a first 
DCT calculating portion performs a DCT calculation for an original 
picture. A first quantizing portion quantizes the calculated result of the 
first DCT calculating portion at an optimum quantizing step determined by 
a code amount controlling portion. A variable length encoding portion 
counts a valid coefficient and a run number of "0" values corresponding to 
the quantized result of the first quantizing portion and outputs a 
variable length code corresponding to the counted result. A picture 
dividing portion reduces and divides the original picture into several 
small elements. A second DCT calculating portion performs a DCT 
calculation for the elements divided by the picture dividing portion. A 
second quantizing portion quantizes the calculated result of the second 
DCT calculating portion with the maximum value of the quantizing steps of 
an applicable system. A third quantizing portion quantizes the calculated 
result of the second DCT calculating portion with the minimum value of the 
quantizing steps of the applicable system. The code amount controlling 
portion determines the difference between the code amount generated by the 
second quantizing portion and the code amount generated by the third 
quantizing portion for each element divided by the picture dividing 
portion and determines an optimum quantizing step for each element 
corresponding to the result, the target compressing ratio, and the target 
encoding bit rate. 
According to the present invention, a generation code amount is predicted 
beforehand for a reduced picture of an original picture. In addition, an 
optimum quantizing step for each element of the reduced and divided 
picture is determined corresponding to "the difference between the code 
amount generated with the maximum quantizing step and the code amount 
generated with the minimum quantizing step", a large (high) quantizing 
step can be designated to an element with a mono-chrome pattern, whereas a 
small (low) quantizing step can be designated to an element with a fine 
pattern. Thus, even if the predetermined compressing ratio and the 
predetermined encoding bit rate are required, the picture quality can be 
prevented from deteriorating. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the following detailed description 
of a best mode embodiment thereof, as illustrated in the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will be described in detail, with 
reference to the accompanying drawings. 
FIG. 1 is a block diagram showing a data compressing system according to an 
embodiment of the present invention. In the system, an original picture 
contains a luminance signal (Y) and color difference signals (C.sub.R) and 
(C.sub.B) at a predetermined ratio. 
The data compressing system according to the embodiment comprises a first 
DCT calculating portion 10, a first quantizing portion 11, a variable 
length encoding portion 12, a picture dividing portion 13, a second DCT 
calculating portion 14, a second quantizing portion 15, a third quantizing 
portion 16, and a code amount controlling portion 17. 
FIGS. 2(a) to 2(c) are schematic diagrams showing the conception of a 
process of the data compressing system according to the embodiment of the 
present invention. 
FIG. 3 is a flow chart showing an optimum quantizing step determining 
process (for determining "an optimum quantizing step" used in the first 
quantizing portion 11) of the data compressing system according to the 
embodiment. This process comprises an original picture reducing step 301, 
a picture dividing step 302, a DCT calculating step 303, a maximum value 
quantizing stop 304, a minimum value quantizing step 305, a code amount 
calculating step 306, a generated code amount difference determining step 
307, an original picture generated code amount estimating step 308, an 
optimum quantizing step determining step 309, and an optimum quantizing 
step providing step 310. 
Next, the operation of the data compressing system according to the 
embodiment will be described. 
An original picture is supplied to the first DCT calculating portion 10. 
The first DCT calculating portion 10 performs an DCT calculation for the 
original picture and converts the original picture into data of frequency 
components. 
Thereafter, the first quantizing portion 11 quantizes the calculated result 
(data of frequency components) of the DCT calculation by the first DCT 
calculating portion 10 with "an optimum quantizing step determined by the 
code amount controlling portion 17" (that will be described later). 
The variable length encoding portion 12 successively zigzag-scans each DCT 
coefficient Fij (8.times.8 blocks) quantized by the first quantizing 
portion 11 from low order coefficients to high order coefficients and 
designates variable length codes corresponding to the counted result of a 
valid coefficient (without "1" values) and a run number of "0" values and 
converts the resultant data into two-dimensional data. Thereafter, the 
resultant data is encoded into Huffman code by a Huffman encoding circuit. 
Thus, the variable length encoding portion 12 outputs a code. This code is 
recorded On recording mediums such as an IC card, a VCR, and a DVD and 
also output to a wireless/wired transmission system. 
On the other hand, the original picture is also supplied to the picture 
dividing portion 13. In the process shown in FIG. 3 (optimum quantizing 
step determining process), an "optimum quantizing step" used in the first 
quantizing portion 11 is determined. Next, the process shown in FIG. 3 
will be described step by step. 
When the original picture is supplied to the picture dividing portion 13, 
it reduces the original picture (at step 301). Thereafter, the picture 
dividing portion 13 divides the reduced picture into several elements (at 
step 302). 
The second DCT calculating portion 14 performs a DCT calculation for the 
picture elements reduced and divided by the picture dividing portion 13 
(at step 303). 
The result of the DCT calculation performed by the second DCT calculating 
portion 14 is supplied to the second quantizing portion 15 and the third 
quantizing portion 16. 
The second quantizing portion 15 quantizes the calculated result with the 
maximum value of the quantizing steps in an applicable system 
corresponding to the data compressing system of the embodiment (at stop 
304). 
The third quantizing portion 16 quantizes the calculated result with the 
minimum value of the quantizing steps of the applicable system (at step 
305). 
Example of the above-described "applicable system" are a system according 
to the MPEG (Moving Picture Experts Group) standard, a system according to 
the JPEG (Joint Photographic Experts Group) standard, and so forth. 
The code amount controlling portion 17 calculates code amounts of data 
(generated code amount) corresponding to the quantized results of the 
second quantizing portion 15 and the third quantizing portion 16 (at step 
306). 
The code amount controlling portion 17 compares the code amount generated 
by the second quantizing portion 15 and the code amount generated by the 
third quantizing portion 16 for each divided element (region of the 
element) and determines whether there is a large difference between the 
generated code amounts (at step 307). 
The code amount controlling portion 17 predicts the generated code amount 
of the original picture corresponding to the code amount of the picture 
reduced by the picture dividing portion 13 (at step 308). In addition, the 
code amount controlling portion 17 determines an optimum quantizing step 
for each element corresponding to the target compressing ratio, the target 
encoding bit rate, and the determined result of step 307 (at step 309). 
The code amount controlling portion 17 supplies the value of the optimum 
quantizing step determined at step 309 to the first quantizing portion 11 
(at step 310). 
Next, with reference to FIGS. 2(a) to 2(c), the conception of the process 
in the data compressing system according to the embodiment of the present 
invention will be described. 
FIG. 2(a) is a schematic diagram showing an original picture. Referring to 
FIG. 2(a), a person in disposed at the center portion of the picture, 
whereas the background of the person is a wall. 
FIG. 2(b) is a schematic diagram showing a reduced picture of the original 
picture shown in FIG. 2(a). The reduced picture is further divided into 
several elements (16 elements). 
FIG. 2(c) is a schematic diagram showing a result of the code amount 
comparing and determining step (at step 307 of FIG. 3). The calculated 
result of the DCT calculation (performed by the second DCT calculating 
portion 14) for each reduced and divided picture element is supplied to 
the second quantizing portion 15 and the third quantizing portion 16. 
Thereafter, the code amounts generated by the second quantizing portion 15 
and the third quantizing portion 16 are compared for each element by the 
code amount controlling portion 17. In FIG, 2(c), an element with "X" 
represents that "the difference between generated code amounts is small", 
whereas an element with tryst represents that "the difference between 
generated code amounts is large". 
The code amount controlling portion 17 predicts all the generated code 
amount of the reduced picture shown in FIG. 2(b). For example, the code 
amount controlling portion 17 predicts a current element of a current 
frame according to compare a current element of the current frame with a 
preceding element of a preceding frame, In addition, the code amount 
controlling portion 17 determines an optimum quantizing step for each 
element corresponding to the target compressing ratio, the target encoding 
bit rate, and information representing whether or not each element is X or 
Y (see FIG. 2(c ). 
In the case of an X element, even if the quantizing step is varied, the 
code amount does not vary. In other words, even if the quantizing step is 
not decreased, a proper picture quality can be accomplished. Thus, the 
quantizing step of the X element is designated to a large value of the 
quantizing steps used for the applicable system. 
In contrast, in the case of a Y element, when the quantizing step is 
varied, the generated code amount largely varies. In other words, when the 
quantizing step is decreased to a small step, the picture quality is 
improved. Thus, the quantizing step of the Y element is designated to a 
small value of quantizing steps used for the applicable system. 
For example, suppose a picture of which a person is disposed at the center 
portion and the background thereof is a wall. In this case, an element 
corresponding to the wall is X in FIG. 2(c), whereas an element 
corresponding to a person is Y in FIG. 2(c). The generated code amount of 
the wall is small due to the picture feature. Thus, the code amount 
controlling portion 17 increases the quantizing step of the region of the 
wall as the X element and correspondingly decreases the quantizing step of 
the region of the wall as the Y element so as to increase the generated 
code amount and improve the picture quality, 
Although the present invention has been shown and described with respect to 
a best mode embodiment thereof, it should be understood by those skilled 
in the art that the foregoing and various other changes, omissions, and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the present invention.