KR100203623B1 - 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 coded bit generation information generated every frame, and adaptively modulates the high frequency components of the input video signal using one-dimensional low pass filtering according to the calculation result. The present invention relates to an image encoding system having a bit generation amount adjusting function for adaptively adjusting a bit generation amount after encoding. To this end, the present invention is directed to an encoding unit having a motion estimation, a compensation method, a DCT, and a quantization method. Bit amount calculation means for adding each encoded bit stream to calculate a respective bit generation amount in each frame unit; Calculate the activity value of 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 limit the frequency passband of the current frame input based on the calculated activity value. Filter control means for generating a filter control signal for performing; And based on the filter control signal, the input current frame is provided to the encoding means as a current frame signal for motion estimation and compensation, or the one-dimensional low pass filtering is applied to the input current frame to remove the high-frequency components. By including the one-dimensional low-pass filtering means for providing the frame signal to the encoding means as a current frame signal for motion estimation and compensation, it is possible to effectively adjust the amount of bits generated after encoding without excessive step size increase during quantization in the encoding means. It is.

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 an illustration of an 8x8 pixel block as an example in accordance with the present invention.

3 is an exemplary view showing a one-dimensional lowpass filter coefficient of order 7 as an example according to the present invention.

4 is an exemplary view illustrating a horizontal filtering process at (0,4) and a vertical filtering process at (3,0) as an example 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: filter control block

230: low pass filtering 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 and temporal and spatial compression are known to be the most efficient, and these techniques are already established by the World Standards Organization. It is widely disclosed in recommendations such as MPEG-1 and MPEG-2.

Most hybrid coding techniques use motion compensated DCPM (Differential Pulse Code Modulation), two-dimensional DCT (Discrete Cosine Transform), 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, 8x8 blocks, into DCT conversion coefficients by using or removing spatial redundancy between image data. This technique is described in Chen and Pratt's Scene Adaptive Coder, IEEE Transactions on Communication, COM-32, NO.3 (March 1984). The DCT transform coefficient is processed through quantizer zigzag scan, VLC, etc., thereby effectively reducing (or compressing) 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, a movement perpendicular to the image plane), while motion vectors are determined for each pixel. Because of this, it is impossible to send substantially all 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 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. When the next transmission point is stored in the buffer, it 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 no malfunction (data overflow or data underflow) occurs in the transmission buffer.

More specifically, various factors (e.g., image complexity) may cause a different amount of bits to be generated for each frame at the time of encoding. In view of this, in an 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 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 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, the one-dimensional low pass filter. 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 high frequency components 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 adjusting function of compressing and encoding the encoded bit stream by encoding means, wherein the quantization is adjusted based on the full state information of the bit stream stored in an output buffer. A bit amount calculating means for adding each of the encoded bit streams generated from the encoding means to calculate a respective bit generation amount in every frame unit; Calculates an activity value for the calculated bit generation amount of each frame, refers to the calculated activity value as the complexity of the frame to be currently encoded, and based on the calculated activity value, the frequency passband of the input current frame Filter control means for generating a filter control signal for limiting And based on the filter control signal, provide the input current frame to the encoding means as the current frame signal for motion estimation and compensation, or limit the pass band by applying one-dimensional low pass filtering to the input current frame. And a one-dimensional low-pass filtering means for providing a signal from the high frequency component to the encoding means as a current frame signal for motion estimation and compensation. to provide.

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, bit rate calculation block 210, filter control block 220, and low pass filtering block 230. do.

Referring to FIG. 1, the input current frame signal is stored in the first frame memory 100 and then input to the low pass filtering block 230, and the low pass filtering block 230 includes the filter control block 220 described later. The frequency of the input frame signal is selectively filtered according to a control signal (a logic signal of level 0 or 1) calculated based on the complexity of the post-encoding video provided from < RTI ID = 0.0 > 1, < / RTI > The filtering of components (which are insensitive to comparative human vision) will be described later in detail with respect to the specific operation of this low pass filtering block 230. Then, the current frame signal in which the high frequency component is removed or not removed from the input image is provided to the subtractor 110 and the current frame prediction block 180 through the line L11, respectively.

First, subtractor 110 includes current frame prediction block 180 described below via line L19 from a current frame signal from which high frequency components provided from low pass filtering block 230 via line L11 have been selectively removed (or not removed). Subtracts the predicted current frame signal that is motion compensated for the moving object provided from the N-th subfield, and as a result, the data, i.e., the error signal representing the differential pixel value, is transmitted through the image coding block 120 and the discrete cosine transform (DCT) By using any of the quantization methods well known in the art, 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.

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 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 change 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 frame. A 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 on which the high frequency component on the line L11 provided from the low pass filtering block 230 according to the present invention is 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., the search range of the previous frame reconstructed using the block matching algorithm (e.g., 16X16 search range) After predicting the current frame in units of 8 × 8 DCT blocks, 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 a control signal CS from a system controller (not shown). When the switch SW is on, the current encoding mode is an 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.

In addition, the current frame prediction block 180 generates a set of motion vectors for each selected block (8 × 8 block) on the line L17 and provides it to 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 (8 × 8 block) and the preset search area (eg 16 × 16 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 are encoded through a variable length coding technique to generate an encoded bit stream.

Meanwhile, according to the present invention, the encoded bit stream output from the entropy encoding block 130 described above is provided to the transmission buffer 140 on the output side, and at the same time, the bit amount is adjusted through the line L23 to adjust the amount of encoding bit generation according to the present invention. Provided to calculation block 210.

Next, in the bit amount calculation block 210 of the present invention, by adding all of the coded bit streams input through the line L23, that is, bit streams that are finally generated by the epitaxial coding such as DCT, quantization and variable length coding, etc. Calculate the bit generation amount. 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 be increased, and in the opposite case, the amount of bits generated after encoding is small. You will lose.

In general, the average bit amount AF generated in one frame has an average bit amount AF since F frames are transmitted per second when the transmission rate is R (bit / sec) and the frame rate is F. 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 amount of the sum of all the bit streams output from the entropy coding block 130 of FIG. 1 is referred to as bit amount BA. The value represented by comparing this bit amount BA value with the average bit amount AF is relatively expressed. If ACT (Activity) is, 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 the above process is provided to the next filter control block 220.

Meanwhile, the filter control block 220 generates a filter control signal for performing one-dimensional filtering of the input image on the line L25 based on the activity (ACT) value provided from the bit amount calculation block 210, thereby performing low pass filtering. Provided to block 230, i.e., in filter control block 220, if the calculated activity value ACT input from the bit rate calculation block 210 is greater than a preset threshold, a low pass filtering is generated by generating a high level logic signal. If the calculated activity ACT value is less than the predetermined threshold value, the low level logic signal is generated and provided to the low pass filtering block 230. In this case, the preset threshold may be set to an average value of the activity ACT generated during a predetermined time in the course of encoding. As an example, when the frame rate of the video signal is 30, 30 activity (ACT) values calculated for 1 second are averaged, and this value is compared with the activity (ACT) value generated every frame. That is, when the average value of the activity (ACT) generated during the previous 30 frames is C, the filter control block 220, if the activity (ACT) value generated in the current frame is larger than the average value C, filter control on the line L25 A high level logic signal is generated as a signal, and when it is small, a low level logic signal is generated as a filter control signal on the line L25. Of course, the activity (ACT) average value C for the previous 30 frames will be continuously updated by the activity (ACT) value for every frame generated, with the highest temporal of the activity (ACT) values for the previous 30 frames. Old Activity (ACT) values are discarded.

Meanwhile, the low pass filtering block 230 passes the input video signal as it is, based on the filter control signal from the filter control block 220 (that is, the subtractor 110 and the current frame prediction block 180 through the line L11). Or a 1-dimensional low pass filtering is applied to the input video signal to limit the notification band, thereby generating a video signal on the line L11 from which high frequency components are relatively insensitive to human visual characteristics. That is, the low pass filtering block 230 removes high frequency components from the input image through one-dimensional low pass filtering when the filter control signal on the line L25 is a high level logic signal, and the filter control signal on the line L25 is low level logic. In the case of a signal, a high frequency component is removed from the input image without being removed.

More specifically, the low pass filtering method of an input video signal according to the present invention is performed by multiplying a low pass filter by an input video signal. That is, when the value of each pixel for an image of one block of NXM (for example, 8 × 8 block) is f (x, y), one block is 8 × as shown in FIG. 2 as an example. In the case of a block of 8, the position values x and y of the pixel in the horizontal and vertical directions have integer values between 0 and 7, and each value has a level value between 0 and 255. That is, as can be seen from FIG. 2, the horizontal and vertical position values of each pixel of the 8x8 block have a value of f (0,0) to f (7,7).

On the other hand, as the low pass filter in the present invention, as shown in FIG. 3 as an example, it is assumed that the one-dimensional low pass filter coefficient has seven orders. The low pass filter is an example of a low pass filter that band-limits the signal to have a frequency bandwidth of fs / 4 because the frequency bandwidth is fs / 2 when the sampling frequency of the input video signal is fs.

Accordingly, in the low pass filtering block 230, the low pass filtering of the video signal according to the present invention is performed by performing one-dimensional low pass filtering on each direction since the input video signal is a two-dimensional signal in the horizontal and vertical directions. Can be implemented. This process is illustrated in detail in FIG. 4, which shows the horizontal filtering at position (0,4) and the vertical filtering at position (3,0).

In other words, if the low-pass filtered signal in the vertical direction at the position of (x, y) is z (x, y), it is calculated by the following expression (2).

In the above formula (2), T means the order of the filter, so T = 7.

Thus, the u, v values have integer values between -3 and 3. In the above equation (2), the k (u) value is a filter coefficient value, and the f (x, y) value is a pixel value. In the above equation (2), if the value of f (x, yu) is less than 0, the value is 0. If the value is larger than the M-1 value corresponding to the image size of one frame, the value of M-1 is M-1. Do it. This is a method of setting the end value when the pixel value exists only in the area, that is, 0 to M-1, and this filtering method is a technique known in the art.

Therefore, if filtering is performed at all pixel positions as in Equation (2), as shown in FIG. 4, for example, one-dimensional filtering in the horizontal and vertical directions can be obtained. In this case, according to the present invention, in the process of performing such filtering at all pixel positions, the horizontal filtering may be performed first, and the vertical filtering may be performed again or the order of the horizontal filtering may be performed again.

As a result, the low pass filtering block 230 applies one-dimensional low pass filtering to the input video signal when the filter control signal on the line L25 is high level, that is, when it is determined that the complexity of the image is large. By restricting, a video signal from which a high frequency component relatively insensitive to human visual characteristics is removed is generated on a line L11 and provided to the subtractor 110 and the current frame prediction block 180 of FIG.

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 that is relatively insensitive to human vision is removed through one-dimensional low pass filtering as described above. Therefore, the low frequency signal, which is a visually important component, can be coded while generating less quantization error. If the passband of the frequency is not limited (high frequency component removal) 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. In the low frequency band), a lot 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 a bit amount generated after the encoding of a previous frame, and when the current input image is a complex image according to the calculation result, the one-dimensional low pass Pass filtering removes the high-frequency components of the image insensitive to human vision first and then performs coding such as MC-DCT and quantization to effectively control the amount of bits generated after encoding without excessive step size increase 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 displayed in a reproduced image when the encoded image is restored and displayed.

Claims (5)

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 the full state information of the bit stream stored in an output side buffer, wherein the quantization is generated from the encoding means. Bit amount calculation means for adding each encoded bit stream to calculate a respective bit generation amount in each frame unit; Calculates an activity value for the calculated bit generation amount of each frame, refers to the calculated activity value as the complexity of the frame to be currently encoded, and based on the calculated activity value, the frequency passband of the input current frame Filter control means for generating a filter control signal for limiting And based on the filter control signal, provide the input current frame to the encoding means as the current frame signal for motion estimation and compensation, or limit the pass band by applying one-dimensional low pass filtering to the input current frame. And one-dimensional low-pass filtering means for providing a frame signal from which high frequency components are removed to the encoding means as a current frame signal for motion estimation and compensation. 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. The video encoding system according to claim 1, wherein the filter control signal is a logic signal of a high or low level. The video encoding system of claim 1, wherein the one-dimensional low pass filtering of the current frame is performed in units of 8 × 8 blocks. 5. The method of claim 1 or 4, wherein the one-dimensional low pass filtering for the 8x8 block is performed sequentially in the horizontal-vertical or vertical-horizontal direction of each pixel position in the 8x8 block. An image encoding system having a bit generation amount adjustment function.