KR100203674B1 - Improved image coding system having functions for controlling generated amount of coded bit stream - Google Patents

Abstract

According to the present invention, in a coding system including MC-DCT and quantization, a complexity of an image to be currently encoded is calculated based on a quantization error value between a current frame input for encoding and a reconstructed previous frame, and the calculation result The present invention relates to a video encoding system having a bit generation amount adjusting function for adaptively adjusting a bit generation amount after encoding by selectively removing high frequency components of an input video signal using a two-dimensional discrete Fourier transform (DFT). To this end, the present invention calculates a quantization error value for each frame based on the pixel value difference signal between the current frame and the reconstructed previous frame, averages the calculated quantization error values of the corresponding frame and averages the plurality of preset frames. Quantization error calculation means for calculating a quantization error value; Refer to the calculated average quantization error values of the plurality of frames as the complexity of the frame to be currently encoded by the encoding means, and determine a plurality of preset bandwidths for adaptively limiting the frequency passband bandwidth of the current frame input for encoding Control means for generating a bandwidth determination signal corresponding to the calculated average quantization error value among the signals; And converting the video signal in the spatial domain with respect to the input current frame signal into two-dimensional DFT transform coefficients in the frequency domain in M × N block units using a two-dimensional discrete Fourier transform, and generating the generated bandwidth determination signal provided from the control means. Determine a high frequency pass band for the transformed 2D DFT transform coefficient blocks, limit the high pass band of each transformed 2D DFT transform coefficient block to the determined bandwidth, and limit the bandwidth of each quantized DFT block. Performing a inverse discrete Fourier transform on each to recover the original signal before encoding, and including a frequency selecting means for providing the encoding means as a current frame signal for motion estimation and compensation to the bandwidth limited frame signal restored to the original signal, In the quantization of the encoding means, the amount of bits generated after encoding without excessive increase of the step size is obtained. You can typically be adjusted.

Description

IMPROVED IMAGE CODING SYSTEM HAVING FUNCTIONS FOR CONTROLLING GENERATED AMOUNT OF CODED BIT STREEM

1 is a block diagram of a video encoding system having a bit generation amount adjusting function according to a preferred embodiment of the present invention.

2 is a view showing a crystal region for high frequency component limitation determined as an example, based on its complexity, for an 8x8 pixel block according to the present invention;

* Explanation of symbols for main parts of the drawings

100, 170: frame memory 110: subtractor

120: image coding block 130: entropy coding block

140: transmission buffer 150: video decoding block

160: adder 180: current frame prediction block

210: quantization error calculation block 220: control block

230: frequency selection block

The present invention relates to a video encoding system for compressing and encoding a video signal. More particularly, the present invention relates to a current frame and a reconstruction for encoding a video signal by using a motion compensation differential pulse code modulation (MC-DPCM) technique. The present invention relates to an image encoding system having a bit generation amount adjustment function suitable for adaptively adjusting a generation bit amount after encoding with reference to a variation of an input image signal predicted based on a quantization error value between previous frames.

As is well known in the art, the transmission of discrete video signals can maintain better image quality than analog signals. When a video signal consisting of a series of image frames is represented in digital form, a significant amount of data must be transmitted, especially for high-definition televisions (aka HDTVs). However, since the usable frequency range of the conventional transmission channel is limited, in order to transmit a large amount of digital data, it is necessary to compress the transmitted data and reduce the amount thereof. Among the various compression techniques for compressing data, hybrid coding schemes combining probabilistic coding techniques with temporal and spatial compression techniques are known to be the most efficient, and these techniques are already enacted by the World Standards Organization. It is widely disclosed in the recommendations of MPEG-1 and MPEG-2.

Most hybrid coding techniques use motion compensated DPCM (Differential Pulse Code Modulation), two-dimensional Discrete Cosine Transform (DCT), quantization of DCT coefficients, VLC (variable length coding), and the like. The motion compensation DPCM determines a motion of an object between a current frame and a previous frame, and predicts a current frame according to the motion of the object to generate a difference signal representing a difference between the current frame and a predicted value. This can be done for example by Staffan Ericsson's Fixed and Adaptive Predictors for Hybrid Predictive / Transform Coding, IEEE Transactions on Communication, COM-33, NO.12 (1985, December), or A motion Compensated Interframe Coding by Ninomiy and Ohtsuka. Scheme for Television Pictures, IEEE Transactions on Communication, COM-30, NO.1 (January, 1982).

In general, two-dimensional DCT converts digital image data blocks, for example, 8 x 8 blocks, into DCT conversion coefficients by using or removing spatial redundancy between image data. The technique is described in Chen and Pratt's Scene Adaptive Coder, IEEE Transactions on Communication, COM-32, NO.3 (1984, March). The DCT conversion coefficient may be processed through a quantizer, a zigzag scan, a VLC, or the like to effectively reduce (or compress) the amount of data to be transmitted.

More specifically, the motion compensation DPCM predicts the current frame from the previous frame according to the motion of the object estimated between the current frame and the previous frame. The estimated motion may be represented by a two-dimensional motion vector representing the displacement between the previous frame and the current frame.

Typically, there are several approaches to estimating the displacement of an object. These are generally classified into two types, one of which is a block-based motion estimation method using a block matching algorithm, and the other is a pixel-based motion estimation method using a pixel circulation algorithm.

In the motion estimation method for estimating the displacement of an object as described above, the displacement is obtained for each pixel by using the pixel-based motion estimation method. This method has the advantage of being able to estimate pixel values more accurately and easily handle scale changes (e.g., zooming, which is a movement perpendicular to the image plane), while the motion vectors for each pixel Since it is determined, it is impossible to transmit substantially all the motion vectors to the receiver as large amounts of motion vectors occur.

In addition, in block-based motion estimation, a block having a predetermined size of the current frame is moved by one pixel in a search range of a previous frame and compared with corresponding blocks to determine an optimal matching block having a minimum error value. From this, the interframe displacement vector (the extent to which the block has moved between frames) for the entire block is estimated for the current frame to be transmitted. Here, in determining the similarity between two corresponding blocks between the current frame and the previous frame, the average absolute difference, the mean square difference, etc. are mainly used, as is well known in the art.

On the other hand, the image bit stream encoded by the encoding technique as described above, that is, encoding techniques such as motion compensation DPCM, two-dimensional DCT, DCT coefficient quantization, and VLC (or entropy encoding) is transmitted to the output side of the image encoding system. The next transmission point stored in the buffer is sent to the transmitter for transmission to the remote destination. At this time, the time of transmission here is related to the size (that is, capacity) and the transmission rate of the transmission buffer, and is controlled so that a malfunction (data overflow or data underflow) does not occur in the transmission buffer.

More specifically, the amount of bits generated for each frame at the time of encoding is changed due to various factors (e.g., the complexity of the image). In view of this, in the image encoding system, the average data rate can be kept constant. Control output buffer. That is, the video encoding system checks the bit generation amount up to the frame currently encoded based on the data fullness state information of the output transmission buffer and adjusts the bit amount to be allocated in the current frame. In other words, in the conventional typical video encoding system, the amount of bits generated in the encoding system is adjusted by controlling the quantization step size (QP) substantially based on the data full state information of the output transmission buffer, that is, if the amount of bits generated before has been large, The bit generation amount is controlled by reducing the bit generation amount by adjusting the step size largely, and in the opposite case, by adjusting the quantization step size small to increase the bit generation amount.

However, as described above, the conventional method of adjusting the bit generation amount by adjusting the quantization step size based on the data fullness state information of the output side transmission buffer is performed when encoding and transmitting video data corresponding to each frame at the same data rate. In the case where the image to be encoded is complex (a large amount of high frequency components are generated), a large amount of bits is generated, which causes a problem that the quantization step size becomes large, resulting in severe image quality degradation in the reproduced image. The high frequency component generated here is a component that is substantially insensitive to human visual characteristics (a component that hardly affects the image quality of a reproduced video).

Accordingly, the present invention is to solve the above-mentioned problems of the prior art, in the coding system including MC-DCT, quantization, based on the quantization error value between the current frame and the reconstructed previous frame input for encoding Calculate the complexity of the video to be encoded, and selectively remove the high frequency components of the input video signal using the two-dimensional Discrete Fourier Transform (DFT) according to the calculation result, thereby adjusting the bit generation amount after encoding It is an object of the present invention to provide an image encoding system having a function of generating amount.

In order to achieve the above object, the present invention provides a discrete cosine transform, quantization, and the like on an error signal between an input current frame and a prediction frame obtained through motion estimation and compensation in macroblock units using the current frame and the reconstructed previous frame. Compression encoding is performed through encoding means including entropy encoding to generate an encoded bit stream, and the quantization has a bit generation amount adjusting function of adjusting a step size based on the fullness information of the bit stream stored in an output buffer. In an image encoding system, a quantization error value is calculated for each frame based on a pixel value difference signal between the current frame and a reconstructed previous frame, and the calculated quantization error values of the corresponding frame are averaged for a plurality of preset frames. Calculate the mean quantization error Quantization error calculating means; The preset plural number of the plurality of frames is referred to as a complexity of the frame to be currently encoded by the encoding means, and a preset plurality for adaptively limiting the frequency passband bandwidth of the current frame input for encoding. Control means for generating a bandwidth determination signal corresponding to the calculated average quantization error value among the bandwidth determination signals of? And converting the image signal in the spatial domain with respect to the input current frame signal into two-dimensional DFT transform coefficients in the frequency domain in M × N block units using a two-dimensional discrete Fourier transform, and generating the bandwidth provided from the control means. Determine a high frequency pass band for the transformed 2D DFT transform coefficient blocks based on a determination signal, limit the high pass band of each transformed 2D DFT transform coefficient block to the determined bandwidth, and limit the bandwidth Inverse discrete Fourier transform is performed on each of the quantized DFT blocks to restore the original signal before encoding, and the bandwidth limited frame signal reconstructed as the original signal is transmitted to the encoding means as a current frame signal for the motion estimation and compensation. With a bit generation amount adjustment function, characterized in that it further comprises a frequency selection means for providing Provided is an image encoding system.

The above and other objects and various advantages of the present invention will become more apparent from the preferred embodiments of the present invention described below with reference to the accompanying drawings by those skilled in the art.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a block diagram of a video encoding system having a bit generation amount adjusting function according to a preferred embodiment of the present invention. As shown in the figure, the image encoding system of the present invention includes a first frame memory 100, a subtractor 110, an image encoding block 120, an entropy encoding block 130, a transmission buffer 140, and image decoding. Block 150, adder 160, second frame memory 170, current frame prediction block 180, quantization error calculation block 210, control block 220, and frequency selection block 230.

Referring to FIG. 1, the input current frame signal is input to the next frequency selection block 230 stored in the first frame memory 100, and the frequency selection block 230 is provided from a control block 220 to be described later. Adaptively limiting the frequency of the input frame signal according to a control signal (bandwidth determining signal for frequency domain classification) calculated based on the complexity of the image according to the quantization error value between the current frame and the previous frame reconstructed and reconstructed, that is, 2 The high-frequency components (which are insensitive to comparative human vision) of the input image are removed using a dimensional discrete Fourier transform (DFT). See FIG. 2 for a detailed operation of the frequency selection block 230. Will be described later in detail. Then, the current frame signal in which the high frequency component is adaptively removed is provided to the subtractor 110, the current frame prediction block 180, and the quantization error calculation block 210 that characterizes the present invention through the line L11. do.

First, the subtractor 110 performs motion compensated predicted motion on the moving object provided from the current frame prediction block 180 described later through the line L19 from the current frame signal provided in the first frame memory 100 through the line L11. The current frame signal is subtracted, and as a result, the data, that is, the error signal representing the differential pixel value, is obtained by using discrete cosine transform (DCT) and one of quantization methods well known in the art through the image coding block 120. By encoding a series of quantized DCT transform coefficients. At this time, the quantization of the error signal in the image encoding block 120 is based on the step size based on the quantization parameter QP determined according to the data fullness state information provided from the output side transmission buffer 140 described later through the line L21. Is adjusted.

Further, according to the present invention, the error signal on the line L11 is provided to the quantization error calculation block 210 to be described later, the quantization error calculation block 210 is described later through the input image signal for the current frame on the line L11 and the line L16. The quantization error value is calculated by using the image signal of the reconstructed previous frame provided from the second frame memory 170, that is, it appears as a difference signal between the input image before encoding (DCT, quantization) and the previous image reconstructed after encoding. A quantization error value is calculated, and the calculated quantization error value is referred to as the complexity of the current frame to be encoded. In the present invention, the frequency selection block 230 adaptively (or selectively) removes (ie, filters) high frequency components that are relatively insensitive to human visual characteristics using the two-dimensional DFT based on the complexity calculated as described above.

Meanwhile, although the detailed illustration in FIG. 1 is omitted, in the frequency selection block 230, the two-dimensional DFT is based on the bandwidth determination signal based on the calculated quantization error value information provided from the control block 220 described later according to the present invention. Determining (ie, filtering) a high frequency component which is relatively insensitive to a human visual characteristic is determined using. Accordingly, the present invention can appropriately adjust the quantization step size in the quantization step in the image encoding block 120 that causes deterioration of image quality in the reproduced video even in a complex image accompanied by an increase in the encoded bit generation amount. A detailed process of adaptively limiting the passband of the input frame using the bandwidth determination signal set based on the calculated quantization error value information will be described later in detail.

Next, the quantized DCT transform coefficients on line L13 are sent to entropy coding block 130 and image decoding block 150, respectively. Here, the quantized DCT transform coefficients provided to the entropy coding block 130 are encoded, for example, through a variable length coding scheme, and are provided to the transmission buffer 140 on the output side. It is delivered to the transmitter not shown for the transmission of.

Meanwhile, the quantized DCT transform coefficients on the line L13 provided from the image coding block 120 to the image decoding block 150 are converted into a frame signal reconstructed again through inverse quantization and inverse discrete cosine transform, and then adder 160. The adder 160 adds the reconstructed frame signal from the image decoding block 150 and the predicted current frame signal provided from the current frame prediction block 180 described later through line L19 to reconstruct the previous image. A frame signal is generated, and the previous frame signal reconstructed as described above is stored in the second frame memory 170. Accordingly, the immediately previous frame signal for every frame encoded through such a path is continuously updated, and the reconstructed previous frame signal thus updated is provided to the current frame prediction block 180 described later for motion estimation and compensation. do.

In addition, the reconstructed reconstructed previous frame signal stored in the second frame memory 170 is provided to the quantization error calculation block 210 described below through line L16 for calculating the complexity of the frame according to the present invention.

On the other hand, in the current frame prediction block 180, the current frame signal in which the high frequency component on the line L11 provided from the frequency selection block 230 according to the present invention is selectively removed or the high frequency component is not removed and the second frame described above are removed. Based on the reconstructed previous frame signal on the line L15 provided from the frame memory 170, a predetermined search range (for example, 16 × 16 or 32 × 32 search range) of the previous frame reconstructed using the block matching algorithm is predetermined. Predicts the current frame in units of blocks of (e.g., 8x8 or 16x16 DCT blocks) and then generates a predicted current frame signal on line L19 to the subtractor 110 and adder 160, respectively. to provide. At this time, the switch SW on the line L19 is turned on / off according to the control signal CS from the system controller (not shown). When the switch SW is turned on, the current encoding mode is the inter mode. In contrast, when off, the current encoding mode is intra mode. Accordingly, the subtractor 110 provides an error signal between the current frame signal and the predicted frame signal to the image encoding block 120 during inter-mode encoding, and transmits the current frame signal itself to the image encoding block 120 during intra-mode encoding. to provide.

The current frame prediction block 180 also generates a set of motion vectors for each selected block (8x8 or 16x16 blocks) on line L17 and provides it to the entropy coding block 130 described above. Here, the sets of detected motion vectors are predicted in a block (8x8 or 16x16 block) of the current frame and a preset search area (e.g., 16x16 or 32x32 search range) in the previous frame. It is the displacement between the most similar blocks. Therefore, in the above-described entropy coding block 130, the quantized DCT transform coefficients on the line L17 are encoded through, for example, a variable length coding technique, to generate an encoded bit stream.

On the other hand, in the quantization error calculation block 210 of the present invention, the quantization error between the current frame on the line L11 inputted after encoding and the previous frame on the line L16 reconstructed and reconstructed after encoding is calculated, that is, before encoding (DCT, quantization). A quantization error value is calculated for each frame, which may appear as a difference signal between the input image and the previous image reconstructed after encoding, and the calculated quantization error value is referred to as the complexity of the next frame to be encoded.

For example, assuming that one frame has a size of M × N, Ip is a picture of a previous frame reconstructed and reconstructed after encoding, and Ic is a picture of a current frame input for encoding. Each pixel value at position x, y) corresponds to Ip (x, y) and Ic (x, y), and the quantization error QE (Quantization Error) at this time is calculated by the following equation (1).

Here, the quantization error QE value calculated using Equation (1) may be a value that substantially reflects the complexity of the image data. Accordingly, the present invention uses the quantization error QE value calculated using the current frame and the previous frame reconstructed after reconstruction after encoding as the complexity of the next video signal input for encoding. In this case, the calculated quantization error QE value may be considered to be related to the information amount (bit generation amount) of the image. If the current encoded image is complex, the calculated quantization error QE value will be relatively large, and vice versa. The calculated quantization error QE value will be small.

In view of this aspect, the quantization error QE value between the current frame and the reconstructed previous frame calculated at the time of encoding may be interpreted as a value related to the amount of information to be transmitted. In the calculation of the average error AE value of each frame according to the present invention, since the reconstructed previous frame signal required in the process of performing the motion estimation and compensation is stored in the frame memory, such an encoding system as described above. Using a part of, it is easy to implement the desired quantization error QE value which can be represented as the difference signal between the current frame and the reconstructed previous frame described above without any further calculation. Then, the quantization error QE value calculated in the quantization error calculation block 210 as described above is provided to the next control block 220.

On the other hand, the control block 220 generates a frequency bandwidth determination signal B on the line L23 value for limiting the frequency of the input image based on the quantization error QE value provided from the quantization error calculation block 210. 230). Here, the bandwidth determination signal B for region division generated and provided to the frequency selection block 230 limits the frequency band of the input frame signal. In this case, the bandwidth determination signal B necessary for classifying the frequency domain is calculated by the following method, and the process of setting the frequency domain of the input image to be restricted according to the present invention using the bandwidth determination signal B is described in the appended Article. It will be described later in detail with reference to FIG. 2.

More specifically, the process of outputting the bandwidth determination signal B for frequency domain division using the quantization error QE value output from the quantization error calculation block 210 in the control block 220 is as follows.

The process of obtaining MQE and SQE in Equation (2) is as follows.

That is, when the frame rate of the video signal is 30, the average of 30 QE values calculated for 1 second is MQE, and the standard deviation of the value is SQE. Therefore, the area separation value B can be obtained by comparing the obtained MAE and SQE values with the QE values generated in each frame. That is, the control block 220 determines the area division value B by using the average and standard deviation of the QE values generated during the previous 30 frames. As a result, the currently generated QE value obtained through this process is again used to calculate the mean value and standard deviation of 30 frames. Accordingly, the control block 220 discards the first obtained QE value (the oldest QE value in time) among the previously generated QE values of the 30 frames and averages using the newly input QE value from the motion error calculation block 210. To find the standard deviation. Of course, the currently generated QE value is also not used to calculate the mean and standard deviation after the 32nd frame.

As is apparent from the above Equation (2), the area division value B has an integer value between 1 and 4, which is an error signal output from the subtractor 110 described above (the current frame on the line L11 and the prediction frame on the line L19). This is to adaptively adjust the bandwidth according to the QE value in the two-dimensional low pass filtering before quantization of the DCT transform coefficients of the discrete cosine transform of the difference signal).

On the other hand, the frequency selection block 230, based on the bandwidth determination signal B for the frequency domain classification provided from the above-described control block 220 limits the high frequency components relatively insensitive to the time in the input image, the process is Substantially, it can be divided into two-dimensional frequency conversion process and frequency selection process. In this case, the two-dimensional frequency conversion process uses a two-dimensional DFT, and in the frequency selection process based on the bandwidth determination signal B provided from the control block 220 described above. The pass band of the 2D DFT transformed video signal is determined.

Next, in the frequency selection block 230 as described above, the input image is two-dimensional DFT transformed, and the frequency of the two-dimensional DFT-converted video signal is selected based on the bandwidth determination signal B for frequency domain classification. This will be described in detail with reference to FIG. 2.

First, the frequency selection block 230 uses the similarity of the spatial domain of the input image signal. The frequency selection block 230 uses the Fourier function to convert the image signal (pixel data) of the spatial domain using a Fourier function according to Equation (3) below. For example, two-dimensional DFT transform coefficients of a frequency domain of 8x8 units are converted.

In Equation (3), f (u, v) represents the value of each pixel, u represents the position in the horizontal direction, and v represents the position of the pixel in the vertical direction. Therefore, the value for each pixel of the N × N block has the following value. That is, when N = 8 u and v have a value between 0 and 7. At this time, each value has an integer value between 0 and 255. In addition, in Equation (3), Z (k, 1) means the converted signal, and k, 1 means frequency components in the horizontal and vertical directions, respectively. Therefore, when N = 8, an 8x8 DFT block, that is, a signal in the spatial domain is converted into a signal in the frequency domain.

More specifically, in the frequency selection block 230, the bandwidth determination signal B for frequency domain division provided from the control block 220 of FIG. 1 through the line L23, for the DFT conversion coefficients obtained through the above-described process. Based on this, the frequency band that is passed is determined. As described above, since the bandwidth determination signal B for frequency domain division is an integer value between 1 and 4, the frequency selected according to this is as follows.

That is, the frequency selection block 230 selects a specific frequency from the converted frequency Z (k, 1). Here, k, 1 is an integer value between 0 and N-1. Therefore, the value output from the frequency selection block 230 is a signal from which a specific frequency component (ie, a high frequency component) is removed. For example, when N = 8, the pass frequency band will be determined as shown in FIG. 2 as an example.

As shown in FIG. 2, when the bandwidth determination signal B value for the frequency domain division provided from the control block 220 of FIG. 1 to the frequency selection block 2330 through the line L23 is 1, the conversion frequency Z (k, 1) are all selected, and when the B value is 2, 3, or 4, as shown in FIG. That is, when the value of B in FIG. 2 is 4, frequencies below the dotted line such as Z (1,7), Z (2,6), etc. are all mapped to 0.

Next, as described above, the DFT transform coefficients (signals in the frequency domain) from which the frequency (high frequency component) of the specific region is removed according to the bandwidth determination signal B value determined based on the complexity of the image are shown in (4). Is converted back to the original spatial signal.

In Equation (4), f (u, v) denotes the value of each pixel, u and v denote the position of the pixel in the horizontal and vertical directions, and Z (k, 1) denotes the converted signal. , k, 1 means frequency components in the horizontal and vertical directions, respectively. Therefore, when N = 8, 8x8 DFT blocks in the frequency domain are converted into signals in the spatial domain.

As a result, in the frequency selection block 230, the frequency of a specific region according to the complexity of the image is obtained by the subtractor 110 of FIG. 1, the current frame prediction block 180, and the quantization error calculation block 210 of the present invention through the line L11. Is a video signal from which the high frequency component of the image is selectively (or adaptively) removed according to the bandwidth determination signal B calculated based on the complexity of the image. Signal).

Accordingly, in the image encoding block 120 of FIG. 1, in the case of a complex image, encoding (quantization) is performed in a state in which a high frequency component of an image relatively insensitive to human vision is selectively (or adaptively) removed as described above. Therefore, the low frequency signal, which is a visually important component, can be encoded with less quantization error. If the low pass bandwidth of the frequency is not limited as in the present invention despite the complicated image, as a result, the amount of bits generated after encoding increases and the quantization step size becomes large. A large number of quantization errors are generated, and as a result, image quality deterioration due to quantization will be caused in the playback image of the receiver.

As described above, according to the present invention, the current frame to be encoded using the current frame input for encoding and the quantization error value information calculated for each frame using the previous frame reconstructed and reconstructed after encoding for motion estimation and compensation If the current input image is a complex image based on the result of the calculation, and the current input image is a complex image, a high frequency component of the image insensitive to the human vision is firstly given by corresponding weight, and then the MC-DCT, quantization, etc. encoding is performed. By performing, the amount of bits generated after encoding can be effectively adjusted without excessive step size increase in the quantization step. Therefore, according to the present invention, it is possible to effectively reduce image quality deterioration due to quantization error inevitably present in a reproduced image when reconstructing and displaying an encoded image.

Claims (5)

Compression means including discrete cosine transform, quantization, and entropy encoding for the error signal between the input current frame and the prediction frame obtained through motion estimation and compensation in units of macroblocks using the current frame and the reconstructed previous frame. A video encoding system having a bit generation amount adjustment function of encoding and generating a coded bit stream, wherein the quantization is adjusted based on the full state information of the bit stream stored in an output buffer. A quantization error calculation is performed to calculate a quantization error value for each frame based on a pixel value difference signal between reconstructed previous frames, and to calculate an average quantization error value for a plurality of preset frames by averaging the calculated quantization error values of the corresponding frame. Way; The calculated average quantization error value of the plurality of frames is referred to as the complexity of the frame to be currently encoded by the protection means, and a preset value for adaptively limiting the frequency passband bandwidth of the current frame input for encoding Control means for generating a bandwidth determination signal corresponding to the calculated average quantization error value among a plurality of bandwidth determination signals; And converting the image signal in the spatial domain with respect to the input current frame signal into two-dimensional DFT transform coefficients in the frequency domain in M × N block units using a two-dimensional discrete Fourier transform, and generating the bandwidth provided from the control means. Determine a high frequency pass band for the transformed 2D DFT transform coefficient blocks based on a determination signal, limit the high pass band of each transformed 2D DFT transform coefficient block to the determined bandwidth, and limit the bandwidth Inverse discrete Fourier transform is performed on each of the quantized DFT blocks to restore the original signal before encoding, and the bandwidth limited frame signal reconstructed as the original signal is transmitted to the encoding means as a current frame signal for the motion estimation and compensation. With a bit generation amount adjustment function, characterized in that it further comprises a frequency selection means for providing Video encoding system. The method of claim 1, wherein each of the predetermined bandwidth determination signals comprises: an average error value for a plurality of frames transmitted per second obtained by averaging the calculated average quantization error value of each frame, the average quantization error value of each frame, and A video encoding system having a bit generation amount adjustment function, which is determined using the standard deviation of the mean error value. 3. The method of claim 1, wherein the predetermined plurality of bandwidth determination signals include four bandwidth determination signals each having a different integer value for selective bandwidth restriction of each of the two-dimensional DFT transform coefficient blocks. An image protection system having a bit generation amount adjustment function. The image having a bit generation amount adjusting function of claim 1, wherein the frequency selecting means converts an image signal in a spatial domain with respect to the current frame signal into two-dimensional DFT transform coefficients in units of 8x8 blocks. Coding system. 5. The method of claim 1 or 4, wherein the frequency selecting means maps high frequency components below the generated bandwidth determination signal to zero values for each of the converted two-dimensional DFT transform coefficient blocks. An image encoding system having a bit generation amount adjusting function.