Interframe coding system and method

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an interframe coding system, wherein an image signal is coded on a frame-to-frame difference basis.

2. Discussion of the Related Art

It is a common practice to remove redundant components in an image signal for a highly efficient coding. Especially with a dynamic image signal, an interframe coding is one of the preferable and typical arts. The interframe coding is a coding technique which codes a difference between a newly input uncoded image signal and a previously coded image signal.

Fig. 9 is a block diagram of a conventional interframe coding apparatus disclosed in Japanese Unexamined Patent Publication No.208382/1988. The interframe coding apparatus according to the figure has a frame memory 1, a motion-vector detector 2, a subtractor 3, an encoder 4, a local decoder 5, an adder 6, a filter 7 and a filter controller 8. The frame memory 1 stores an image signal of the previous frame.

Operation of the conventional interframe coding system with a local decoding loop is now described with reference to the Fig. 9.

The input image signal 12 is compared with the image signal of the previous frame 11 stored in the frame memory 1 by a block-matching technique in the motion vector detector 2. The motion vector detector 2 detects the quantity and direction of motion of the input image signal 12 and outputs a motion vector signal 13. The frame memory 1 outputs a motion compensation predictive signal 14 based upon the motion vector signal 13. The subtractor 3 subtracts the motion compensation predictive signal 14 from the input image signal 12 to output a predictive error signal 15 or a difference signal.

The predictive error signal 15 is coded by quantization at the encoder 4, and a coded error signal 16 is output. The coded error signal 16 is decoded in the local decoder 5, and a local decoded error signal 17 is output. The local decoded error signal 17 is added to the motion compensation predictive signal 14 in the adder 6, and a local decoded signal 18 is output. The local decoded signal 18 is filtered by the filter 7 to eliminate higher frequency components in the signal. The filter smooths the local decoded signal and outputs a smoothed local decoded signal 19. Filtering is controlled by a control signal 20 output by the filter controller 8. The filter control signal 20 controls the filter based upon the motion vector signal 13.

The coded error signal 16 and the motion vector signal 13 are transmitted via a transmission line to an external decoding system.

Image coding is generally processed by the unit or block of 16x16 or 8x8 pixels of an image signal.

With reference to other conventional coding techniques, the filter 7 can be placed after the frame memory 1 in the local decoding loop instead of after the adder 6 as shown in Fig. 9. Further, filtering can be accomplished by an intra-block filter which processes pixels within a block, and by an inter-block filter which processes pixels involving the pixels in the neighboring blocks. Furthermore, a motion detection can be achieved with a smaller unit of a pixel than a full pixel. This is designed to detect the optimal block of pixels in the previous frame which match a block of pixels in the input image signal 12. These conventional techniques can contribute to eliminate higher frequency components or redundancy in the input image signal based upon the quantity of motion. Thus, they are effective for removing noise in the signal. Consequently, coding efficiency can be improved greatly with these conventional arts.

An intraframe coding with a local decoding loop is processed in the following manner according to the conventional coding with reference to Fig 9. The input image signal 12 is directly coded by quantization in the encoder 4, where the coded error signal 16 is output. The coded error signal 16 is decoded in the local decoder 5, where the local decoded error signal 17 is output. The local decoded error signal 17 is stored in the frame memory 1. The coded error signal 16 is transmitted via a transmission line to an external decoding system.

The conventional interframe coding generally controls filtering based upon the motion vector, whereby an image signal can be filtered based upon the quantity of motion. In other words, a block of pixels representing motion of an image signal, or a motion block, is filtered with a low-pass filter (LPF) thereby eliminating the original definition to eliminate noise in the signal. On the other hand, a block of pixels representing no motion or a very small amount of motion, is not filtered.

The conventional interframe coding system carries a problem of quantization. Coding performance, according to the conventional system, is based upon a limited quantization which is designed optimally for a certain pattern of predictive error signal 15. In other words, the limited quantization can not deal effectively with coding signals of various patterns. When the encoder quantizes the signal with the limited quantization, the predictive error signal 15 is characterized with a poor coding efficiency and results in producing a poor coded error signal.

The present invention is devoted to solve this problem. An objective of this invention is to provide a coding controller which allows the encoder to code efficiently with coding signals of various patterns. This can lead to the overall coding efficiency of the interframe coding system.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, an interframe coding system for frame-to-frame coding of an input signal, having a local decoding loop which includes an encoder for interframe coding the input signal, decoding means for producing a decoded signal, and a frame memory which receives the decoded signal and outputs a predictive signal, may include a controller for receiving the input signal and the predictive signal from the frame memory, for calculating a difference between the input signal and the predictive signal, for calculating a characteristic of the input signal and for generating a coding control signal based on the difference and the characteristic for controlling the encoder.

The controller may include a difference calculator for receiving the input signal and the predictive signal for calculating a difference between the input signal and the predictive signal and for outputting a difference signal, a pixel difference calculator for receiving the difference signal, for calculating a value of the difference per each pixel, and for outputting the difference value per each pixel to the encoder, and wherein the encoder may include means for quantizing the input signal with a step size and for changing the step size of quantization so that coding error becomes smaller than the difference value per each pixel.

The decoding loop may include a filter, and the controller may include means for generating a filter control signal to control the filter based on the characteristic.

In accordance with another aspect of the invention, an interframe coding system for frame-to-frame coding of an input signal, having a local decoding loop which includes an encoder for interframe coding the input signal, decoding means for producing a decoded signal, and a frame memory which receives the decoded signal and outputs a predictive signal, may include a controller for receiving at least one of the input signal and the predictive signal, calculating an activity value from the received signals, and outputting an activity signal to the encoder to control the coding therein.

In accordance with yet another aspect of the invention, an interframe coding method for frame-to-frame coding of an input signal, wherein the coding system includes a local decoding lop including an encoder for interframe coding an input signal, decoding means for producing a decoded signal, and a frame memory for receiving the decoded signal and outputting a predictive signal, may include the steps of: <ul id="ul0001" list-style="none" compact="compact"><li>(a) calculating the difference between the input signal and the predictive signal;</li><li>(b) changing a step size of quantization in the encoder responsive to the difference; and</li><li>(c) coding the input signal based on the step size of quantization.</li></ul>

In accordance with yet another aspect of the invention, an interframe coding method for frame-to-frame coding of an input signal, wherein the coding system includes a local decoding loop including an encoder for interframe coding the input signal, decoding means for producing a decoded signal, and a frame memory for receiving the decoded signal and outputting a predictive signal, may include the steps of: <ul id="ul0002" list-style="none" compact="compact"><li>(a) providing a characteristic signal indicating relative characteristic of one of the input signal and the predictive signal;</li><li>(b) changing a step size of quantization in the encoder based on the characteristic signal; and,</li><li>(c) coding the input signal based on the step size of quantization.</li></ul>

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0003" list-style="none" compact="compact"><li>Fig. 1 shows a block diagram illustrating a configuration of an interframe coding system according to an embodiment of the present invention;</li><li>Fig. 2 shows a block diagram of a configuration of a filter controller according to the embodiment with reference to Fig. 1;</li><li>Fig. 3 is an explanation diagram of an operation of an encoder according to the embodiment with reference to Fig. 1;</li><li>Fig. 4 shows a block diagram of a configuration of an interframe coding system according to another embodiment of the present invention;</li><li>Fig. 5 shows a block diagram of a configuration of a controller according to the embodiment with reference to Fig. 4;</li><li>Fig. 6 is an explanation diagram of the "Activity" of an image signal according to the embodiment with reference to Fig. 4;</li><li>Fig. 7 is another explanation diagram of the "Activity" of an image signal according to the embodiment with reference to Fig. 4;</li><li>Fig. 8 is an explanation diagram of frame and field according to another embodiment of the present invention; and</li><li>Fig. 9 shows a block diagram of a conventional configuration of an interframe coding system.</li></ul>

Embodiment 1.

Fig. 1 shows a configuration of the interframe coding system in accordance with the invention. The filter controller 21 outputs a pixel difference signal 25 or a control signal to the encoder 4 as well as the filter control signal 23 to the adaptive filter 22. The pixel difference signal 25 is the difference per pixel which is calculated by dividing the value of the difference signal 32 by the number of pixels in a block of pixels (e.g. 8x8=64) in the filter controller 21.

Fig. 2 shows a configuration of the filter controller 21 according to this embodiment which includes a difference calculator 30, a decision unit 31 and a pixel difference calculator 33. The difference calculator 30 basically calculates a differnce between the input image singal 12 and the motion compensation predictive signal 14 to output the difference signal 32. Specifically, the difference signal 32 is output in the following process: the difference calculator 30 calculates an absolute difference or squared difference between the signals for each pixel. Then the absolute values or squared values of pixel differnces are calculated for the processing unit or a block of 8x8 (64) pixels and output as the difference signal 32.

The value of the difference signal 32 is divided in the pixel difference calculator 33 by the number of pixels in a block of pixels (8x8=64) to produce the average value of the difference signal 32 in order to output a pixel difference signal 25. The pixel difference signal 25 is the threshold value of quantization which is output to the encoder 4. The encoder 4 decides the optimal quantization step size for coding the image signal under the condition that qantization error does not exceed the value of the pixel difference signal 25.

Fig. 3 shows the pixel difference of an image signal illustrating the operation of deciding the quantization step size in the encoder 4 in accordance with this embodiment.

In Fig. 3(a), D1 designates the pixel difference of a block containing eight pixels, D2 designates the pixel difference of another block of eight pixels, and D3 designates the pixel difference of another block of eight pixels. Those pixel differences are supposed to be given the following condition in the filter controller 21 as shown in Fig. 3(b): D1&gt;D2&gt;D3.

When the pixel difference signal 25 is input into the encoder 4, the encoder 4 selects the optimal guantization step size for an input image signal based upon the pixel difference signal 25, so that the value of quantization error should be less than the value of the pixel difference signal 25. With D1 as the pixel difference signal 25, for instance, the encoder 4 selects a quantization step size SS1 (SS1&lt;D1), with D2 as the signal a step size SS2 (SS2&lt;D2) is selected; and a step size SS3 (SS3&lt;D3) is selected for D3.

Under that condition, the encoder 4 quantizes an input image signal by selecting the optimal quantization step sizes based upon the value on the pixel difference signal 25 in the following relations as shown in Fig. 3(d): SS1&gt;SS2&gt;SS3.

Thus, the quantization step size becomes larger when the difference between the input image signal 12 and the motion compensation predictive signal 14 is large. On the other hand, the quantization step size becomes smaller when the difference between the two signals is smaller. Consequently this leads to a highly efficient coding performance minimizing the quantization error to have a high quality decoded image.

The pixel difference signal 25 is not the only signal used to decided the quantization step size in the encoder 4. The quantization step size is also controlled or limited by the size of an output buffer which temporarily stores coded signals to be output from the encoder 4. The size of the buffer can limit the quantity of coding signals. In this case, the quantization step size is limited by the size of the output buffer.

Embodiment 2.

With further reference to Fig. 1, the interframe coding system in accordance with the invention can provide a controller 21a to output a control signal 25 to the encoder 4 for controlling the quantization step size. In this embodiment the adaptive filter 22 and the filter control signal 23 are not necessarily provided as shown in Fig. 4.

Fig. 5(a) shows a configuration of the controller 21a according to this embodiment. Fig. 5(a) comprises Fig. 2, as modified by removing the decision unit 31. The operation of the difference calculator 30 and the pixel difference calculator 33 corresponds to that described in reference to Fig. 2.

Fig. 5(b) shows another configuration of the controller 21a according to this embodiment providing a decision unit 31a which outputs the control signal 25 of the "Activity" of an image signal. The "Activity" is calculated with the input image signal 12 as shown in Figs. 6 and 7.

Fig. 6 is an explanation diagram of the "Activity" illustrating that the "Activity" is based upon the difference between the maximum and minimum values of luminance intensity of pixels of the input image signal 12. The figure offers a brief explanation of the "Activity" by using only eight pixels instead of 64 pixels. The "Activity" of an image signal according to the figure is the difference value between the values of luminance intensity of the fourth pixel (maximum intensity) and of the fifth pixel (minimum intensity) in the eight pixels of the image signal.

Fig. 7 shows another example of calculating the "Activity" of the image signal which can be either the sum of the absolute or squared difference values between the mean value of luminance intensity of the pixels and the individaul values.

Fig. 7(a) illustrates the "Activity" calculation of the mean value of luminance intensity of the pixels of the image signal. X1 through X8 in the diagram show the difference between the luminance intensity of the pixel and the mean value of luminance intensity of the eight pixels. Fig. 7(b) shows one of the calculation formulas of "Activity" X which is the sum of the absolute difference values of X1 through X8. Fig. 7(c) shows another calculation formula of "Activity" X which is the sum of the squared difference values of X1 through X8.

The "Activity" denotes, as shown in Figs. 6 and 7, nothing like an absolute intensity but a relative value of an image signal.

Specifically, "Activity" X becomes smaller with an image signal containing a large amount of lower frequency components, "Activity" x becomes larger with an image signal containing a large amount of higher frequency components.

Accordingly, the control signal of the "Activity" enables the encoder 4 to perform the optimal coding by selecting flexibly the optimal quantization step size for an image signal.

With further reference to Fig. 5(b), the decision unit 31b in accordance with the invention can have the motion compensation prediction signal 14 as input information for calculating its "Activity" as shown in Fig. 5(c). The overall operation of the decision unit 31b is equivalent with that described in Fig. 5(b).

Embodiment 3.

With further reference to the embodiments hereinbefore, the frame memory 1 in accordance with the invention can store an image signal not by the frame but by the field as shown in Fig. 8. Fig. 8 shows the relation between the frame and field of an image signal. Fig. 8(a) illustrates that the frame of an image signal is consisted of two fields of an image signal, the first and second fields. Fig. 8(b) illustrates a frame composed of the first and second fields of an image signal by using an interlace mode.

With further reference to this embodiment, the filtering of an image signal can also be processed by the field.

With further reference to the embodiments hereinbefore, the frame memory 1 in accordance with the invention can store a multiple number of time-varying frames or fields including the past, present, and forthcoming frames or fields.

Embodiment 4.

With further reference to the embodiments hereinbefore with Fig. 2, the decision unit 31 of the invention can use the motion compensation predictive signal 14, instead of the input image signal 12, to calculate its "Activity" for normalizing the difference signal 32.

A distinctive feature of the interframe coding system of the present invention is that the coding controller controls the quantization step size based upon one of the three signals which are a difference signal between the input image signal and the signal from the frame memory, the input image signal, and the signal from the frame memory. This can contribute to reducing the amount of coding error in a coded image signal.

Another distinctive feature of the present invention is that the coding controller controls coding in the encoder by the mode signal based upon the signal with the optimal "Activity" or characteristic of either the input image signal or the predictive error signal. This can contribute to a highly efficient coding.

As stated hereinbefore, the interframe coding system and method of the present invention have the following advantageous effects.

An advantageous feature is that filtering can produce a highly coding efficient image signal by eliminating higher frequency components in the signal in the event that the predictive signal still contains a large amount of quantization error.

Another advantageous feature is that a pixel difference is used to control coding error, so that a decoded image can carry less coded error. This can contribute to a highly efficient prediction for the forthcoming frames of an image signal.

As stated hereinbefore, the interframe coding system and method of the present invention also have the following advantageous effects.

One of the advantageous features is that the coding controller outputs the coding control signal based upon the "Activities" of the input image signal and the predictive error signal. This can improve coding efficiency.