KR100203625B1 - An improved image coding system having functions for controlling generated amount of coded bit stream - Google Patents

Abstract

The present invention calculates the complexity of an image to be encoded based on the encoded bit generation amount information generated every frame, and selectively removes high frequency components of an input video signal using discrete Fourier transform according to the calculation result. A video encoding system having a bit generation amount adjustment function for adaptively adjusting a bit generation amount after encoding, comprising adding a hatched bit stream generated from an encoding means having a motion estimation, a compensation method, a DCT, and a quantization method. Bit amount calculation means for calculating each bit generation amount in every frame unit; Calculate an activity value for the calculated bit generation amount of each frame, refer to the calculated activity value as the complexity of the frame to be currently encoded, and adaptively limit the frequency passband of the current frame input for encoding. Control means for generating a bandwidth determination signal corresponding to the calculated activity value among a plurality of preset bandwidth determination signals for the mobile terminal; 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. And performing frequency inverse discrete Fourier transform on each to restore the original signal before encoding, and providing the encoding means as a current frame signal for motion estimation and compensation to the encoding means. Effect of bit amount generated after encoding without excessive step size increase in quantization by encoding means Can be adjusted by

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: bit amount 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 video encoding system for encoding an image signal. The present invention relates to a video encoding system having a bit generation amount adjustment function suitable for adaptively adjusting the amount of generated bits after encoding with reference to complexity.

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 DCPM (Differential Pulse Code Modulation), 2-D Discrete Cosine Transform (DCT), Quantization of DCT Coefficients, VLC (Variable Coding) rather than motion. 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 (1982, January).

In general, two-dimensional DCT converts digital image data blocks, for example, 8x8 blocks, into DCT conversion coefficients by using or removing spatial redundancy between image data. This technique is described in Chen and Pratt's Scenc 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.

Among the motion estimation methods for estimating the displacement of an object as described above, when the pixel-based motion estimation method is used, the displacement is calculated for each pixel. 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, 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 coding scheme as described above, that is, coding techniques such as motion compensation DCPM, two-dimensional DCT, DCT coefficient quantization, and VLC (or entropy encoding) is provided on the output side of the video encoding system. The next transmission point stored in the transmission buffer is sent to the transmitter for transmission to the remote receiver. 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 no malfunction (data overflow or data underflow) occurs 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 adjusts the amount of bits to be allocated in the current frame, based on the data fullness state information of the output side transmission buffer, to investigate the amount of bits generated before the frame currently encoded. 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 increasing the bit generation amount by adjusting the quantization step size small.

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, resulting in a large quantization step size, 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 calculates the complexity of an image to be encoded based on the coded bit generation amount information generated every frame, and according to the calculation result, two-dimensional discrete Fourier transform It is an object of the present invention to provide a video encoding system having a bit generation amount adjusting function capable of adaptively adjusting a bit generation amount after encoding by selectively removing a high frequency component of an input video signal by using a?

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. To a video encoding system having a bit generation amount adjustment function of which a step size is adjusted based on the full state information of the bit stream stored in an output buffer. A bit amount calculating means for calculating each bit generation amount in every frame unit by adding the encoded bit stream generated from the encoding means: calculating an activity value for the calculated bit generation amount of each frame, and calculating The current activity value Refers to the complexity of the frame to be generated, and generates a bandwidth determination signal corresponding to the calculated activity value among a plurality of preset bandwidth determination signals for adaptively limiting the frequency passband bandwidth of the current frame input for encoding. And control means for converting an image signal in a spatial domain with respect to the input current frame signal into two-dimensional DFT transform coefficients in a frequency domain in units of M × N blocks using a two-dimensional discrete Fourier transform, and providing from the control means. Determine a high frequency pass band for the converted 2D DFT transform coefficient blocks based on the generated bandwidth determination signal, and limit the high pass band of each of the converted 2D DFT transform coefficient blocks to the converted bandwidth; And an inverse discrete Fourier transform for each of the bandwidth-limited quantized DET blocks. And a frequency selecting means for restoring the original signal before encoding and providing the bandwidth limited frame signal restored to the original signal as the current frame signal for motion estimation and compensation. 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. A block 150, an adder 160, a second frame memory 170, a current frame prediction block 180, a bit amount calculation block 210, a control block 220, and a frequency selection block 230 are included.

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 after encoding, that is, using a two-dimensional discrete Fourier transform (DFT). By removing the high frequency components (which are insensitive to the comparative human vision) of the input image, the NCP operation process for this frequency selection block 230 will be described in detail later. Then, the current frame signal from which the high frequency component is adaptively removed is provided to the subtractor 110 and the current frame prediction block 180 through 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. In this case, the quantization of the error signal in the image encoding block 120 is 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.

Next, the quantized DCT transform coefficients on line L13 are sent to entropy encoding 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. By generating the frame signal, the reconstructed previous frame signal 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 above-described second frame is removed. Based on the reconstructed previous frame signal on the line L15 provided from the frame memory 170, a predetermined block (eg, a predetermined search range (eg, 16 × 16 search range) of the previous frame reconstructed using the block matching algorithm For example, the current frame is predicted in units of 8 × 8 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, 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 (8 × 8 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, the encoded bit stream output from the entropy encoding block 130 according to the present invention is provided to the transmission buffer 140 on the output side, and at the same time, the bit amount is calculated through the line L23 to adjust the amount of encoding bit generation according to the present invention. Provided to block 210.

Next, in the bit amount calculation block 210 of the present invention, all of the encoded bit streams input through the line L23, that is, bit streams that are entropy coded and finally generated such as DCT, quantization, variable length coding, and the like, are added together. Calculate the amount of generation. In this case, the amount of bits generated may be related to the amount of information of a video signal. If the current input image is complex, the amount of bits generated after encoding will increase, and vice versa. You will lose.

In general, if the average bit rate AF generated in one frame is R (bit / sec) and the frame rate is F, since F frames are transmitted per second, the average bit amount AF is It becomes R / F. Therefore, by comparing the bit amount generated in the current frame with the R / F, it is possible to relatively obtain the complexity of the image that is currently input and encoded. That is, if the bit amount generated in the current frame is larger than the average bit amount AF, the image is relatively complicated. This complexity can be used as the complexity of the next input image by comparing the bit amount generated in the current frame with the average bit amount AF since the video signal does not change rapidly every frame.

That is, the sum of all the bit streams output from the entropy coding block 130 of FIG. 1 is referred to as bit amount BA, and the value represented by comparing this bit amount BA value with the average bit amount AF is relatively expressed. Is ACT (Activity), the ACT value is calculated by the following equation (1).

ACT = (BA / AF) --------------------- (1)

Therefore, the above operation is performed every frame. For the new frame, the generated bit amount is calculated from the beginning, the activity ACT of each frame is calculated, and the calculated activity ACT is imaged for this frame. It is used as the complexity of the signal. In this case, when the calculated activity ACT value is small, it corresponds to a simple image, and when the calculated activity ACT value is large, it corresponds to a complex image. Then, the activity ACT value calculated through this process is provided to the next control block 220.

Meanwhile, the control block 220 generates a frequency bandwidth determination signal B on the line L25 for limiting the frequency of the input image based on the activity ACT value provided from the bit amount calculation block 210. Provided at 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 second. It will be described later in detail with reference to the drawings.

In more detail, the process of outputting the bandwidth determination signal B for frequency domain classification using the activity ACT value output from the bit amount calculation block 210 in the control block 220 is as follows.

The process of obtaining MACT and SACT in the above formula (2) is as follows. That is, when the frame rate of the video signal is 30, the average of 30 activity (ACT) values calculated for one second is MACT, and the standard deviation of the values is SACT. Accordingly, by comparing the MACT and SACT values thus obtained and the activity (ACT) values generated in each frame, the bandwidth determination signal B for area division can be obtained. That is, the control block 220 generates the bandwidth determination signal B by using the average and the standard deviation of the activity ACT values generated during the previous 30 frames. As a result, the currently generated activity (ACT) values obtained through this process are again used to calculate the 30 mean values and standard deviations. Therefore, the control block 220 discards the activity value ACT (the oldest activity value in time) first obtained from the 30 activity ACT values previously generated, and newly inputs the activity value from the bit amount calculation block 210. (ACT) is to find the mean and standard deviation.

Of course, the currently generated activity value ACT is also not used to calculate the mean and standard deviation after the 31st frame.

As is apparent from the above equation (2), the bandwidth determination signal B for frequency domain division has an integer value between 1 and 4, and subtracts the current frame signal output from the frame memory 100 of FIG. This is to adaptively adjust the bandwidth according to the activity value (ACT) of the previous frame before providing it with.

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 limits the high frequency components relatively insensitive to the time in the input image, the process is substantially It can be divided into a two-dimensional frequency conversion process and a frequency selection process, where a discrete Fourier transform (DFT) is used in the two-dimensional frequency conversion process, and in the frequency selection process to the bandwidth determination signal B provided from the control block 220 described above. Based on this, the passband of the DFT-converted 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. 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 3 below. For example, two-dimensional DFT transform coefficients of a frequency domain of 8 × 8 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 a converted signal, and k and 1 mean 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 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 and 1 are integer values 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. That is, when the value of B is 4 in FIG. 2, frequencies below the dotted line such as Z (1,7), Z (2,6), etc., are all mapped to 0. FIG.

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 (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 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 even though it is a 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 the image to be currently encoded is calculated by using the bit amount generated after the encoding of the immediately preceding frame, and according to the calculation result, if the current input image is a complex image, By giving weights, the high frequency components of the image insensitive to human vision are first removed, and then coding such as MC-DCT and quantization is performed to effectively control the amount of bits generated after encoding without increasing an excessive step size in the quantization step. Can be. 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)

Bits encoded by compression encoding by means of encoding means including discrete cosine transform quantization and entropy encoding for a difference signal between an input current frame and a prediction frame obtained through motion estimation and compensation using the current frame and the reconstructed previous frame. A video encoding system having a bit generation amount adjustment function of generating a stream, wherein the quantization is adjusted based on the full state information of the bit stream stored in an output buffer, wherein the coded generated from the encoding means is encoded. Bit amount calculation means for calculating the respective bit generation amount in every frame unit by adding the bit stream: calculating the activity value for the calculated bit generation amount of each frame, and complexity of the frame to currently encode the calculated activity value As a reference Control means for generating a bandwidth determination signal corresponding to the calculated activity value among a plurality of preset bandwidth determination signals for adaptively limiting the frequency passband bandwidth of the current frame input for encoding; 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 An 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 restored to the original signal is transmitted to the encoding means as a current frame signal for 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 image having a bit generation amount adjusting function of claim 1, wherein the complexity of each corresponding frame is calculated based on an average bit amount calculated by using a transmission rate and a frame rate and a generation bit amount of the corresponding frame. Coding system. 3. The method of claim 1, wherein the plurality of predetermined 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 encoding system having a bit generation amount adjusting function. The image having the bit generation amount adjusting function of claim 1, wherein the frequency selecting means converts the image signal in the spatial domain with respect to the current frame signal into two-dimensional DET conversion 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.