Video bit rate control method

A video bit rate control method. SUBGOPs are defined as sets of two to four frames in a GOP. For example, if the GOP comprises IBBPBBPBBPBB frames, each of the SUBGOPs is defined as a set of the IBBP or BBP frames. Before bit assignment is performed with respect to frames to be coded, it is checked whether a scene change is present with respect to frames in successive ones of the SUBGOPs. If it is checked that the scene change is not present, a bit rate is controlled in a conventional manner. In the case where the scene change is present, a smaller bits than that in the conventional manner is assigned to frames with no scene change and a relatively large bit amount is assigned to a frame with the scene change. Therefore, according to the present invention, the video bit rate control method has the effect of preventing a degradation in a picture quality due to the scene change.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates in general to the control of a video bit 
rate, and more particularly to a video bit rate control method for 
controlling the video bit rate effectively even upon occurrence of a 
sudden scene change. 
2. Description of the Prior Art 
Generally, in the case of coding motion pictures, the generated bits for 
each frame must be varied according to a frame complexity to maintain a 
picture quality constant. However, in the case of coding the motion 
pictures to transmit them, a coded output bit rate is required to be 
maintained constant because coded data must be transmitted through 
constant channel bandwidth. 
In order to maintain the coded output bit rate constant, bits necessary to 
the coding must be controlled properly every frame. In this case, a 
degradation in the picture quality may become an issue. As a result, there 
has been proposed an effective video bit rate control method which is 
capable of avoiding an abrupt degradation in the picture quality and 
maintaining the coded output bit rate constant within the transmission 
channel band. 
Referring to FIG. 1, there is shown a block diagram of a general video bit 
rate control apparatus. As shown in this drawing, the video bit rate 
control apparatus comprises a video frame input unit 1 for inputting a 
video frame, a coder 2 for coding the video frame by the video frame input 
unit 1, and a buffer 3 for storing temporarily the coded data from the 
coder 2 and transmitting the stored data externally according to a 
constant transmission bit rate. The video bit rate control apparatus 
further comprises a video bit rate controller 4 for controlling an output 
bit rate of the coder 2 according to a storage level of the buffer 3 to 
prevent an overflow or an underflow of the buffer 3. 
Referring to FIG. 2, there is shown a flowchart illustrating a conventional 
video bit rate control method. constant bits are first assigned according 
to a limited transmission rate to each of a certain number of frames of a 
group of pictures (referred to hereinafter as GOP) to be coded. The 
subsequent bit assignment is performed in the frame coding order in 
consideration of characteristics of the frames and the remaining bits of 
every frame. 
Namely, at the step 201, assignment bits are calculated with respect to the 
frames in the GOP to be coded, in consideration of a bit redundancy of the 
GOP, the frame characteristics and the number of the frames remaining in 
the GOP, not coded. At the step 202, assignment bits are calculated in the 
unit of block according to assignment bits for frames and the storage 
level of the buffer 3 at the time point that blocks are coded. As a result 
of the calculation of the assignment bits at the step 202, quantization 
parameters are obtained at the step 203. Then at the step 203, the 
resultant assignment bits are transferred to the coder 2. 
It is checked at the step 204 whether a current block is a last one in the 
current frame to be coded. If it is checked at the step 204 that the 
current block is not the last one in the current frame to be coded, the 
assignment bits are calculated in the unit of block up to the last block 
in the current frame at the step 202 and the quantization parameters are 
obtained at the step 203. Namely, the steps 202 and 203 are repeatedly 
performed up to the last block in the current frame. On the contrary, if 
it is checked at the step 204 that the current block is the last one in 
the current frame, the above operation is repeatedly performed with 
respect to a new frame. 
By the way, in the above-mentioned conventional video bit rate control 
method, in the case where a sudden scene change occurs in the middle of 
coding the frames in the GOP, much bits are required to maintain the 
picture quality constant. In this case, the remaining bit amount is used 
to code a scene change frame and the subsequent frames because the 
constant bit amount was used for the frames previously coded. As a result, 
there is caused a degradation in the picture quality in any form. This 
degradation in the picture quality has a bad effect on a high definition 
television application. 
SUMMARY OF THE INVENTION 
Therefore, the present invention has been made in view of the above 
problem, and it is an object of the present invention to provide a video 
bit rate control method in which SUBGOPs are defined as sets of two to 
four frames in a GOP (for example, if the GOP comprises IBBPBBPBBPBB 
frames, each SUBGOP is defined as a set of the IBBP or BBP frames), it is 
checked, before bit assignment is performed with respect to frames to be 
coded, whether a scene change is present with respect to frames in 
successive ones of the SUBGOPs, a bit rate is controlled in a conventional 
manner if the scene change is not present, and, if the scene change is 
present, smaller bits than that in the conventional manner are assigned to 
frames with no scene change and a relatively large bits are assigned to a 
frame with the scene change, so that a degradation in a picture quality 
due to the scene change can be prevented. 
In accordance with the present invention, the above and other objects can 
be accomplished by a provision of a video bit rate control method for a 
video bit rate control apparatus which comprises video frame input means 
for inputting a video frame, coding means for coding the video frame 
inputted by the video frame input means, buffering means for storing 
temporarily the coded result from the coding means; and video bit rate 
control means fox controlling an output bit rate of the coding means 
according to a storage level of the buffering means to prevent an overflow 
or an underflow of the buffering means. The video bit rate control method 
comprises the first step of defining a plurality of SUBGOPs in a GOP and 
checking whether a scene change is present in successive two of the 
SUBGOPs, the successive two SUBGOPs being current and subsequent SUBGOPs; 
the second step of checking whether last frames in the current and 
subsequent SUBGOPs are predicted frames, if it is checked at the first 
step that the scene change is present in the subsequent SUBGOP, and 
calculating assignment bits with respect to frames to be coded or applying 
weights to the assignment bits in accordance with the checked result; the 
third step of checking whether the last frame in the current GOP and a 
last frame in a previous SUBGOP are the predicted frames, if it is checked 
at the first step that the scene change is not present in the subsequent 
SUBGOP, and calculating the assignment bits with respect to the frames to 
be coded or assigning extra bits additionally to the assignment bits in 
accordance with the checked result; and the fourth step of calculating 
assignment bits in the unit of block according to the calculated 
assignment bits for frames and the storage level of the buffering means at 
the time point that blocks are coded, obtaining quantization parameters as 
a result of the calculation of the assignment bits in the unit of block, 
transferring the resultant assignment bits to the coding means and 
performing repeatedly the above operation up to a last one of the blocks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 3, there is shown a flowchart illustrating a video bit 
rate control method in accordance with the present invention. In order to 
increase a coding efficiency of motion pictures, as in an MPEG coding 
method, a GOP is classified into intra (referred to hereinafter as I) 
frames, predicted (referred to hereinafter as P) frames and 
bidirectionally predicted (referred to hereinafter as B) frames for 
coding. The GOP comprises a plurality of SUBGOPs, each of which is defined 
as a set of the IBBP(or BBP) frames, a set of the IBP(or BP) frames or a 
set of the IP(or PP) frames in the frame order at the step 301. 
It is checked at the step 302 whether a scene change is present in 
successive two of the SUBGOPs. The successive two SUBGOPs are current and 
subsequent SUBGOPs. If it is discriminated at the step 303 that the scene 
change is present in the subsequent SUBGOP, it is checked at the step 304 
whether a last frame in the current SUBGOP is the P frame. If it is 
checked at the step 304 that the last frame in the current SUBGOP is not 
the P frame, weights are applied to assignment bits to save the bits 
number at the step 305. 
On the contrary, if the last frame in the current SUBGOP is the P frame at 
the step 304, it is checked at the step 306 whether a last frame in the 
subsequent SUBGOP is the P frame. In the case where it is checked at the 
step 306 that the last frame in the subsequent SUBGOP is the P frame, the 
weights are applied to the assignment bits to save the bits number at the 
step 307. If it is checked at the step 306 that the last frame in the 
subsequent SUBGOP is not the P frame, the assignment bits are calculated 
with respect to frames to be coded, in a conventional manner, at the step 
308. 
In the case where the scene change is not present in the subsequent SUBGOP 
at the step 303, it is checked at the step 309 whether the scene change is 
present in the current SUBGOP. If it is checked at the step 309 that the 
scene change is not present in the current SUBGOP, the assignment bits are 
calculated with respect to the frames to be coded, in the conventional 
manner, at the step 308. On the contrary, if the scene change is present 
in the current SUBGOP at the step 309, it is checked at the step 310 
whether the last frame in the current SUBGOP is the P frame. 
If the last frame in the current SUBGOP is not the P frame at the step 310, 
the assignment bits are calculated with respect to the frames to be coded, 
at the step 311. On the contrary, if it is checked at the step 310 that 
the last frame in the current SUBGOP is the P frame, it is checked at the 
step 312 whether a last frame in a previous SUBGOP is the P frame. 
When the last frame in the previous SUBGOP is not the P frame at the step 
312, bits saved in frame coding of the previous SUBGOP are additionally 
assigned to the P frame at the step 313. On the contrary, if the last 
frame in the previous SUBGOP is the P frame at the step 312, the bits 
saved in the frame coding of the previous SUBGOP are additionally assigned 
to the P frame at the step 314. 
At the step 315, assignment bits are calculated in the unit of block 
according to the assignment bits by frames and the storage level of the 
buffer 3 at the time point that blocks are coded. As a result of the 
calculation of the assignment bits at the step 315, quantization 
parameters are obtained at the step 316. Then at the step 316, the 
resultant assignment bits are transferred to the coder 2. It is checked at 
the step 317 whether a current block is a last one in a current frame to 
be coded. If it is checked at the step 317 that the current block is not 
the last one in the current frame to be coded, the assignment bits are 
calculated in the unit of block up to the last block in the current frame 
at the step 315 and the quantization parameters are obtained at the step 
316. Namely, the steps 315 and 316 are repeatedly performed up to the last 
block in the current frame. On the contrary, if the current block is the 
last one in the current frame at the step 317, the above operation is 
repeatedly performed with respect to a new frame. 
In accordance with the preferred embodiment of the present invention, in 
the case where the GOP is in the form of the IBBP(or BBP) frames, a 
relatively small bits are assigned to the frames preceding to three to 
five before the frame with the scene change. In the case where the GOP is 
in the form of the IBP(or BP) and IP(or PP) frames, a relatively small 
bits are assigned to the frames preceding to two to three before the frame 
with the scene change. 
The steps 308 and 311 of calculating the assignment bits with respect to 
the frames to be coded are performed in consideration of a bit redundancy 
of the GOP, the frame type (I, P and B) and the number of the frames 
remaining in the GOP, not coded, in a similar manner to that in a 
conventional video bit rate control method ISO(International Organization 
for Standardization) IEC (International Electrotechnical Commission)/JTC1 
(Joint Technical Committee 1) SC29 (Sub-Committee 29)/WG11 (Working Group 
11), MPEG(Moving Picture Experts Group), TM(Test Model). 
FIG. 4 is a flowchart illustrating the steps 305 and 307 of applying the 
weights to the assignment bits to assign smaller bits than that of the 
steps 308 and 311 to the frames to be coded. First, it is checked at the 
step 401 whether the last frame in the current SUBGOP is the P frame. If 
it is checked at the step 401 that the last frame in the current SUBGOP is 
not the P frame, the weights are applied to the assignment bits on the 
basis of the following expressions to make the assignment bits relatively 
small, at the step 402: 
EQU T.sub.I "=T.sub.I *C1 
EQU RT.sub.I =(1-C1)T.sub.I 
EQU T.sub.B "=T.sub.B *C2 
EQU RT.sub.B =(1-C2)T.sub.B 
On the contrary, if it is checked at the step 401 that the last frame in 
the current SUBGOP is the P frame, it is checked at the step 403 whether 
the last frame in the subsequent SUBGOP is the P frame. If it is checked 
at the step 403 that the last frame in the subsequent SUBGOP is the P 
frame, the weights are applied to the assignment bits on the basis of the 
following expressions to make the assignment bits relatively small, at the 
step 404: 
EQU T.sub.P "=T.sub.P *C1 
EQU RT.sub.P =(1-C1)T.sub.P 
EQU T.sub.B "=T.sub.B *C2 
EQU RT.sub.B =(1-C2)T.sub.B 
FIG. 5 is a flowchart illustrating the steps 313 and 314 of applying the 
extra bits additionally to the assignment bits to assign larger bits than 
that of the steps 308 and 311 to the frames to be coded. First, it is 
checked at the step 501 whether the last frame in the previous SUBGOP is 
the P frame. If it is checked at the step 501 that the last frame in the 
previous SUBGOP is the P frame, the bits saved in the previous SUBGOP are 
additionally assigned at the step 502 on the basis of the following 
expressions: 
EQU T.sub.P "=T.sub.P +RT.sub.P +2RT.sub.B 
EQU T.sub.B "=T.sub.B 
On the contrary, if it is checked at the step 501 that the last frame in 
the previous SUBGOP is not the P frame, the bits saved in the previous 
SUBGOP are additionally assigned at the step 503 on the basis of the 
following expressions: 
EQU T.sub.P "=T.sub.P +RT.sub.I +2RT.sub.B 
EQU T.sub.B "=T.sub.B 
In FIGS. 4 and 5, T.sub.I, T.sub.P and T.sub.B are target bits of the I, P 
and B frames, respectively, which are defined in a similar manner to 
target bit rates of an MPEG system T.sub.I ", T.sub.P " and T.sub.B " are 
modified target bits in I, P and B modes, respectively. C1 and C2 are 
weights smaller than 1. RT.sub.I, RT.sub.P and RT.sub.B are the number of 
bits saved in the I, P and B frames of the previous SUBGOP to code the 
scene change frame, respectively. 
As apparent from the above description, according to the present invention, 
the video bit rate control method can control the video bit rate 
effectively even upon the occurrence of the sudden scene change. 
Therefore, the video bit rate control method of the present invention can 
effectively be applied to a broadcasting field in which a degradation in a 
picture quality due to the scene change becomes an issue, or a digital 
storage medium edit field in which a high efficient compression is 
required. Further, the video bit rate control method of the present 
invention may be applied to a video communication video codec such as a 
video phone, a storage medium video codec and a digital broadcasting video 
codec utilizing a digital video compression technique. 
Although the preferred embodiments of the present invention have been 
disclosed for illustrative purposes, those skilled in the art will 
appreciate that various modifications, additions and substitutions are 
possible, without departing from the scope and spirit of the invention as 
disclosed in the accompanying claims.