KR100203684B1 - Improved image coding system having functions for controlling generated amount of coded bit stream - Google Patents

Abstract

The present invention calculates a complexity of an image to be currently encoded based on quantization parameter information determined in units of macroblocks based on data fullness state information of an output transmission buffer, and according to the calculation result, two-dimensional discrete Fourier transform 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. An average value calculating means for calculating an average quantization parameter value of a previous frame by averaging each quantization parameter value respectively generated for each macroblock of a previous frame previously encoded by encoding means having entropy encoding; The calculated average quantization parameter value is referred to as the complexity of the frame to be currently encoded, and the average value of the calculated quantization parameter among a plurality of preset bandwidth determination signals for adaptively limiting the frequency passband of the current frame input for encoding Control means for generating a bandwidth determination signal corresponding to the signal; Converts 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 applies the generated bandwidth determination signal provided from the control means. Determine a high 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 each of the quantized DFT blocks with limited bandwidth A frequency selection means for performing an inverse discrete Fourier transform on the signal to restore the original signal before encoding, and providing the bandwidth limited frame signal restored to the original signal to the encoding means as a current frame signal for motion estimation and compensation. In the quantization in the means, the amount of bits generated after encoding is effectively avoided without excessive step size increase. It can be adjusted with.

Description

Image Coding System with Bit Rate Control

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: QP average value 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, when encoding a video in a video encoder that compresses and encodes a video signal, each macroblock unit is based on data fullness state information of an output transmission buffer. 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 quantization parameter information determined as.

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 quality 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 techniques combining probabilistic coding techniques with temporal and spatial compression techniques are known to be the most efficient, and these techniques have already been enacted by the standards standard, for example, by the World Organization for Standardization. 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), 2-D Discrete Cosine Transform (DCT), quantization of DCT coefficients, VLC (variable length coding), and the like. The motion compensation DPCM determines the movement of the object between the current frame and the previous frame, and predicts the current frame according to the movement of the object to produce a differential signal representing the difference between the current frame and the 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 Interfram 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, such as 8x8 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-by-block motion estimation method using a block matching algorithm and the other is a pixel-by-pixel 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-by-block 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 the 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, for 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, the encoding scheme 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 transmission time point here is related to the size (ie, capacity) and 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 varies due to various factors (for example, the complexity of the image). In view of this, in the image encoding system, the average bit rate may be kept constant. Control of the 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 has been performed in the case of encoding and transmitting video data corresponding to each frame at the same data rate. However, when the image to be encoded is complex (a large amount of high frequency components are generated), a large amount of bits are generated. As a result, the quantization step size becomes large, resulting in serious image quality degradation in the reproduced image. The high frequency component generated here is a component that is substantially insensitive to human vision (a component that hardly affects the image quality of a reproduced video).

Accordingly, the present invention solves the problems of the prior art described above, and based on the quantization parameter information determined in units of macroblocks based on the data fullness state information of the output transmission buffer, the complexity of the image to be currently encoded is determined. A video encoding system having a bit generation amount adjustment function capable of adaptively adjusting a bit generation amount after encoding by calculating and selectively removing high frequency components of an input video signal using a two-dimensional discrete Fourier transform according to the calculation result. Its purpose is to.

In order to achieve the above object, the present invention includes a discrete cosine transform, quantization and entropy encoding on the difference signal between the input current frame and the prediction frame obtained through motion estimation and compensation using the current frame and the reconstructed previous frame. A bit stream generated by compression encoding through an encoding means to generate an encoded bit stream, wherein the quantization is a bit generation amount adjusting function whose step size is adjusted based on a quantization parameter determined according to the full state information of the bit stream stored in an output buffer. An image encoding system comprising: an average value calculating means for calculating an average quantization parameter value of the previous frame by averaging each quantization parameter value generated for each macroblock of a previously encoded previous frame; The calculated average quantization parameter value is referred to as a complexity of a frame to be currently encoded, and the calculated one of a plurality of predetermined bandwidth determination signals for adaptively limiting a frequency passband bandwidth of the current frame input for encoding is calculated. Control means for generating a bandwidth determination signal corresponding to a mean value of the quantization parameter; 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, QP average value 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 restricts the frequency of the input frame signal according to a control signal (bandwidth determination signal for frequency domain classification) calculated based on the complexity of the image generated based on the quantization parameter determined in the quantization process during encoding, that is, two-dimensional The Discrete Fourier Transform (DFT) is used to remove the high frequency components (which are insensitive to comparative human vision) of the input image, which will be described later in detail. Will be described. Then, the current frame signal in which the high frequency component is adaptively removed is provided to the subtractor 110 and the current frame prediction block 180 through the line L11, respectively.

First, in the subtractor 110, a moving object provided from the current frame prediction block 180 described later through the line L19 from the current frame signal from which the high frequency component provided from the frequency selection block 230 is selectively removed through the line L11. Subtract the motion-compensated predicted current frame signal, and as a result, the data, i.e., the error signal representing the differential pixel value, is subjected to discrete cosine transform (DCT) and quantization methods well known in the art through the image coding block 120. By using either one, it is encoded into 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.

On the other hand, as described above in the quantization step of the image coding block 120, the process of quantizing the video signal with a step size according to the quantization parameter (QP) determined according to the data full state information of the output transmission buffer 140, discrete The DCT transform coefficient of the cosine transformed M × N block is divided by a value of 2 * QP. The quantization parameter (QP) is different for each macroblock (16 × 16 block) according to the data filling state of the transmission buffer 140. It can have a value, and also has an integer value between 1 and 31. Accordingly, in the image encoding block 120, when the data-filled state of the transmission buffer 140 rises above a predetermined value, the QP value is increased to reduce the amount of bits generated after entropy encoding, and conversely, the data-filled state of the transmission buffer 140 is filled. When the state falls below a predetermined value, the QP value is lowered to reduce the amount of bits generated after entropy encoding.

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.

On the other hand, 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 the lie L19. 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.

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. Based on the reconstructed previous frame signal on the line L15 provided from the memory 170, a predetermined block (e.g., a predetermined search range (e.g., 16x16 search range) of the previous frame reconstructed using the block matching algorithm) For example, the current frame is predicted in units of 8x8 DCT blocks, and the predicted current frame signal is generated on the line L19 and provided to the subtractor 110 and the adder 160, respectively. 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 on, the switch SW on the line L19 indicates that the current encoding mode is inter mode. On the contrary, when off, this means that the current encoding mode is an 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.

In addition, the current frame prediction block 180 generates a set of motion vectors for each of the selected blocks (8x8 blocks) on the line L17 and provides the entropy coding block 130 described above. Here, the sets of motion vectors detected are the displacements between the most similar block predicted in the block of the current frame (8x8 block) and the preset search area (e.g., 16x16 search range) in the previous frame. Accordingly, in the entropy coding block 130 described above, the quantized DCT transform coefficients on the line L13 together with the sets of the motion vectors on the line L17 generate a coded bit stream by, for example, a variable length coding technique. .

Meanwhile, according to the present invention, the quantization parameter value QP determined in units of macroblocks in the quantizer (not shown) in the image encoding block 120 according to the data fullness state of the output side transmission buffer 140 is quantized through the line L23. The parameter QP is input to the average value calculation block 210.

Accordingly, the QP average value calculation block 210 calculates a QP average value, that is, an AQP value, for every frame. That is, in one QP average value calculation block 210, after encoding (quantization, DCT, and entropy encoding) image data of a frame, an AQP value is calculated by averaging QP values for all macroblocks of the encoded frame. This means that the AQP value of the currently encoded frame is used to calculate the complexity in the encoding process for the next frame using the characteristics of the video signal that the frames adjacent to each other in time are not significantly changed. As a result, when the current encoded image is a complex image, the AQP value calculated by the QP average value calculation block 210 will have a large value, and when the simple image is relatively easy to encode, the AQP value may be a small value. Will be.

More specifically, if the video signal of one frame to be encoded currently has the size of M × N and each macroblock has the size of L × L, the number of macroblocks of this video signal is (ML) × (NL ) For example, assuming 16 × 16 macroblocks for a 352 × 288 input video signal, the number of macroblocks is (352/16) × (288/16), which corresponds to 22 × 18, or 396. . Therefore, in the present invention, the AQP value calculated by averaging the QPs of all such macroblocks is used as the complexity of the next video signal input for encoding. In this case, the calculated AQP value may be regarded as related to the information amount (bit generation amount) of the image. If the current encoded image is complex, the calculated AQP value will be relatively large, and vice versa. Will be smaller. Then, the AQP value calculated in the QP average value calculation block 210 is provided to the next control block 220.

Meanwhile, the control block 220 generates a frequency bandwidth determination signal B for limiting the frequency of the input image on the line L25 based on the AQP value provided from the QP average value calculation block 210 and thus selects the frequency selection block 230. To provide. Here, the bandwidth determination signal B for region division generated and provided to the frequency selection block 230 limits the frequency band of the frame signal input for encoding. 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.

In more detail, the process of outputting the bandwidth determination signal B for frequency domain division using the AQP value output from the QP average value calculation block 210 in the control block 220 is as follows.

In the above formula (1), ROUND is an operation that takes an integer value by rounding. Therefore, as a result of Equation (1), in the conventional encoder having an integer value between 1 and 31, the B value for frequency domain division has an integer value between 1 and 4. That is, when the calculated AQP value is 31, since the ROUND {(31-16) / 5} is 3, the frequency domain division value B becomes 4. Therefore, according to the value B calculated in this way, as shown in FIG. 2 as an example, a frequency range to be limited is determined. This operation is to automatically adjust the bandwidth limiting in the two-dimensional low pass filtering before the quantization step in the image coding block 120 according to the AQP value.

On the other hand, the frequency selection block 230, based on the bandwidth determination signal B for frequency domain classification provided from the above-described control block 220 to limit the high frequency components in which the relatively low visual characteristics in the input image, the process May be substantially divided into a two-dimensional frequency conversion process and a frequency selection process, in which the two-dimensional frequency conversion process uses a two-dimensional discrete Fourier transform (DFT), and in the frequency selection process, the bandwidth provided from the control block 220 described above. The passband of the 2D DFT-converted video signal is determined based on the decision signal B.

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 image signal is selected based on the bandwidth determination signal B for frequency domain classification. It demonstrates in detail.

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 (2) below. For example, two-dimensional DFT transform in a frequency domain of 8x8 units.

In formula (2), 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, in the case of 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 the frequency domain division provided from the control block 220 of FIG. 1 through the line L25 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, as shown in FIG. 2 as an example, the pass frequency band will be determined.

As shown in FIG. 2, when the bandwidth determination signal B value for frequency domain division provided from the control block 220 of FIG. 1 to the frequency selection block 2330 through the line L25 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. 2, frequencies below the dotted line corresponding to each are not selected as the mode 0. That is, when the B value is 4 in FIG. 2, Z (1,7), Z (2,6), and KX are all mapped to 0 with frequencies below the dotted line.

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 in accordance with the bandwidth determination signal B value determined based on the complexity of the image are shown below (3). Is converted back to the original spatial signal.

In Equation (3), 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 subtractor 110 and the current frame prediction block 180 of FIG. 1 through the line L11 are applied to the image signal from which the frequency of the specific region is removed according to the complexity of the image, that is, the complexity of the image. According to the bandwidth determination signal B calculated based on this, a video signal (video signal in which a high frequency component of a specific region is replaced with a zero value) from which a high frequency component of an image is selectively (or adaptively) removed is provided.

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 complexity of an image to be currently encoded is calculated by using an AQP value obtained by averaging QP values generated for each macroblock at the time of quantization in a frame immediately encoded, and calculating the calculated value. According to the result, if the current input image is a complex image, the corresponding weight is first applied to remove the high frequency components of the image insensitive to human visual characteristics, and then the MC-DCT, quantization, and the like are encoded to perform the encoding. The amount of bits generated after encoding can be effectively adjusted without increasing the excessive step size. 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 (4)

The differential signal between the input current frame and the prediction frame obtained through motion estimation and compensation using the current frame and the reconstructed previous frame is encoded by compression encoding by encoding means including discrete cosine transform, quantization, and entropy encoding. A video encoding system having a bit generation amount adjusting function for generating a bit stream, wherein the quantization is adjusted based on a quantization parameter determined according to the full state information of the bit stream stored in an output buffer. An average value calculating means for calculating an average quantization parameter value of the previous frame by averaging each quantization parameter value generated for each of the macroblocks of the encoded previous frame; The calculated average quantization parameter value is referred to as a complexity of a frame to be currently encoded, and the calculated one of a plurality of predetermined bandwidth determination signals for adaptively limiting a frequency passband bandwidth of the current frame input for encoding is calculated. Control means for generating a bandwidth determination signal corresponding to a mean value of the quantization parameter; Converts 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 determines the generated bandwidth provided from the control means. Determine a high frequency pass band for the converted 2D DFT transform coefficient blocks based on the signal, limit the high pass band of each transformed 2D DFT transform coefficient block to the determined bandwidth, and limit the bandwidth to quantization Limiting the high pass band of each of the DFT blocks to the determined bandwidth, performing inverse discrete Fourier transform on each of the bandwidth-limited quantized DFT blocks to restore the original signal before encoding, and recovering the original signal. The coded number is a bandwidth-limited frame signal as the current frame signal for motion estimation and compensation. And a frequency selecting means provided to the stage. The bit bandwidth of claim 1, wherein the plurality of predetermined bandwidth determination signals include four bandwidth determination signals having different integer values for selective bandwidth limitation of each of the two-dimensional DFT transform coefficient blocks. Image coding system with generation amount control function. The bit generation amount control according to claim 1 or 2, wherein the frequency selecting means converts the image signal in the spatial domain with respect to the current frame signal into two-dimensional DFT transform coefficients in units of 8x8 blocks. Image coding system with a function. The method of claim 1, wherein the frequency selecting means maps a high frequency component below the generated bandwidth determination signal to a zero value for each of the converted two-dimensional DFT transform coefficient blocks. An image encoding system having a bit generation amount adjusting function.