JPS6341472B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an interframe encoding method for encoding and transmitting a difference signal between two sequential frames of an image signal, which has a level exceeding a threshold level. This is an improved version in which the threshold level is set in relation to image information on a pixel-by-pixel basis.

Generally, when encoding and digitally transmitting image signals such as television signals, in order to improve the transmission efficiency, only the differences between successive frames are encoded instead of encoding and transmitting the image signals as they are. Further, in order to further improve transmission efficiency, only significant differences excluding those due to noise are encoded and transmitted.

However, since such a significant difference increases between two successive frames as the movement of the image represented by the image signal increases, the difference between frames is encoded and transmitted for each pixel of which the image signal is sampled. In this case, as the movement of the image increases, the number of picture elements showing a significant level difference to be transmitted increases. It changes depending on the degree,
The transmission density of the encoded signal changes, resulting in a decrease in transmission efficiency.

In order to prevent such a decrease in transmission efficiency, in the conventional interframe encoding method, a threshold indicating the lower limit of the significant level difference to be encoded and transmitted is established using a circuit device having the configuration shown in FIG. The hold level is changed according to the frequency of occurrence of significant pixel signals, and the transmission density of the encoded signal is controlled to be constant. That is, in the conventional interframe encoding device shown in FIG. 1, an input image signal is supplied to a subtracter 1 to form a difference signal between the input image signal and the image signal of the previous frame period from the frame memory 4,
The inter-frame difference signal is guided to the threshold processor 2 to extract only the significant difference signal having a level exceeding the threshold level from the threshold variable device 7, and the significant difference signal is sent to the buffer memory 5. Once written into the frame memory 4, the signal is read out and transmitted at a constant speed, and the significant inter-frame difference signal is led to the adder 3 and stored in the frame memory 4.
The significant current frame period image signal is added to the image signal of the previous frame period from , and the significant current frame period image signal is written into the frame memory 4 and read out after one frame period, and the significant current frame period image signal is added to the image signal of the previous frame period mentioned above. The image signal is Therefore, the frequency of occurrence of significant inter-frame difference signals written in the buffer memory 5 varies depending on the degree of image movement represented by the input image signal as described above. As the frequency of occurrence of pixel signals having a difference in level between pixels increases, it becomes necessary to increase the memory capacity of the buffer memory 5 and to increase the read speed, and hence the transmission speed. Therefore, in order to suppress the occurrence of such a need, a signal indicating the state of significant pixel signal accumulation in the buffer memory 4, and therefore the amount of transmitted information, is sent to the transmitted information amount calculator 6, and the result of the calculation is sent to the transmitted information amount calculator 6. The threshold variable device 7 is controlled to change the threshold level supplied to the threshold processor 2, usually in a plurality of stages. That is, when the frequency of occurrence of significant pixel signals increases, the threshold level is raised, and control is performed so that the frequency of occurrence of pixel signals having inter-frame level differences exceeding the threshold level decreases. For example, the threshold level is switched at a frame period.

Therefore, when the contour of the image represented by the image signal is smooth and the slope of the change in signal level at the contour is gentle, even if such a smooth contour moves, the difference in level between frames of pixel signals at the contour is small. Therefore, due to a comparison error with the threshold level, such as a comparison error due to noise superimposition, even in continuous contour parts, there may be some areas where it is determined that there is a significant level difference and areas where it is determined that there is no significant level difference. This occurs, and the inter-frame difference signal is transmitted only for the outline of the location where it is determined that there is a significant level difference, and an image in which only the outline of that location has moved is reproduced, so the outline in the reproduced image may be uneven. Therefore, a so-called false contour occurs, and the quality of the reproduced image deteriorates. Moreover, since the level difference between frames is usually small in a smooth contour, if the threshold level is set high, for example, to the same level as the level difference between frames in the contour, the above-mentioned error can be avoided. Contours occur frequently, resulting in significant image quality deterioration. Therefore, if the threshold level is switched at a frame period, as in conventional interframe coding, a constant threshold level is applied to the entire screen. When moving at the same speed, the threshold level is set to a high value due to the frequency of occurrence of significant pixel signals in steep contours, and as a result, the above-mentioned false contours are more likely to occur in smooth contours. . Moreover, as mentioned above, the inter-frame level difference in smooth contour areas is essentially smaller overall than the inter-frame level difference in steep contour areas, so the threshold level is simply Not only is the switching performed finely within one screen, that is, during one frame cycle, depending on the image content, but also the image signal height is adjusted according to whether the image contour is smooth or steep. It is necessary to change the threshold level for the inter-frame level difference depending on the fineness of the picture determined by the magnitude of the spatial frequency component.

An object of the present invention is to solve the above-mentioned conventional problems and eliminate their drawbacks, and to provide an interframe coding method that does not cause false contours in reproduced images depending on the picture pattern represented by the image signal. It is about providing.

That is, the interframe coding method of the present invention is an interframe coding method that encodes and transmits a difference signal having a level exceeding a threshold level among difference signals between two sequential frames of an image signal. The present invention is characterized in that the threshold level can be set depending on the fineness of the picture of the signal.

The overall basic configuration of the interframe encoding device according to the present invention having the above-mentioned characteristics is exactly the same as the configuration of the interframe encoding device according to the conventional method shown in FIG. Not only does it have almost the same function, but the effect of setting the encoding threshold level in the threshold variable device 7 in the configuration shown in FIG. In addition to this under the control of the transmission information amount calculator 6 based on the information of This is also done based on the result of detecting fineness. Therefore, in the interframe encoding device according to the present invention, the encoding threshold level for determining a pixel signal having a significant interframe level difference to be encoded and transmitted is set to such a significant pixel level. Settings are made by switching the fineness of each pixel depending on both the frequency of signal occurrence and the fineness of the picture.

Therefore, in order to detect the fineness of the picture elements represented by the image signal, for the analog image signal, for example, a high-pass filter having an appropriate cutoff frequency is used to extract a high frequency component corresponding to the steepness of the contour. The fineness of the picture can be detected based on the extracted output signal level, and the encoding threshold level can be changed according to the high frequency component signal level, which is supplied to the interframe encoding device. Since the image signal is sampled for encoding, it is preferable to judge the picture definition suitable for the image signal sampled on a picture element basis using an appropriate sampling frequency.

FIG. 2 shows an example of the configuration of a threshold variable device adapted to judge the picture definition of such a sampled image signal. In the threshold variable device 7 having the configuration shown in FIG. 2, the sampled image signal from the input terminal is supplied to the one-pixel delay device 8 having an appropriate delay amount corresponding to the sampling frequency. 1
The sampled image signal delayed by the picture element intervals and the non-delayed sampled image signal from the input terminal are supplied to a subtracter 9 to form a difference signal between picture elements, and the difference output signal is guided to the absolute value unit 10. The absolute value of the inter-picture element signal is extracted, and the absolute value is guided to the low-pass filter 11, and the inter-picture element difference signal of an appropriate low frequency component corresponding to the required encoding threshold level switching period is extracted and read-only. is applied to the access input terminal of memory 12. In this read-only memory 12,
A threshold level control signal related to the frequency of occurrence of significant pixel signals calculated at an appropriate period by the transmission information calculator 6 in the configuration shown in FIG. 1 is also applied to the access input terminal. Therefore, if the above-mentioned inter-pixel difference signal and threshold level control signal applied to the read-only memory 12 are quantized in response to digital transmission of the image signal, from a practical point of view, both are 10 steps. A degree of quantization is sufficient. Therefore, in consideration of the frequency of occurrence of significant picture element signals that may actually occur in the image signal and the level range of the inter-picture element difference signal, each stage of the quantization control signal and the quantization between the quantization picture elements that are actually required are determined. Appropriate multi-stage encoding threshold level values whose addresses are combinations with each stage of the difference signal are a finite number within a practical range, so such a finite number of assumed threshold level values can be determined in advance. The threshold value is stored in the read-only memory 12, and accessed by both the encoding threshold control signal and the inter-pixel difference signal applied to the access input terminal to read the appropriate encoding threshold level value and set the threshold value. It can be supplied to the threshold processor 2 as a variable output signal of the hold variable device 7. Therefore, for example, for an inter-frame difference signal in a smooth contour, the encoding thread should be set so that the generation interval of significant pixel signals is narrow enough that even if a false contour occurs, it will not be noticeable. Encoding threshold levels can be switched and set to suit desired conditions, such as by setting a threshold level.

As is clear from the above description, according to the present invention, the occurrence of false contours in smooth contours that become noticeable due to a high encoding threshold level can be suppressed, and the generation of false contours in steep contours can be suppressed. The encoding threshold level can be set sufficiently high to suppress the frequency of occurrence of pixel signals with significant inter-frame level differences. Digital transmission of image signals using highly efficient interframe coding is achieved by appropriately switching and controlling the transmission bit rate of encoded signals without causing deterioration in image quality of reproduced images, and with high transmission efficiency. can be done.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic configuration of an interframe encoding device, and FIG. 2 is a block diagram showing an example of the configuration of a variable encoding threshold device according to the method of the present invention. 1, 9...Differentiator, 2...Threshold processor, 3...Adder, 4...Frame memory, 5...Buffer memory, 6...Transmission information amount calculator, 7...Threshold variable device, 8...1 Picture element delay device, 10... Absolute value device, 11... Low pass filter, 12... Read only memory.

Claims (1)

[Claims] 1. In an interframe coding method in which a difference signal between two sequential frames of an image signal having a level exceeding a threshold level is encoded and transmitted, An interframe coding method characterized in that the threshold level is set according to the magnitude of a spatial frequency component. 2. In the encoding method according to claim 1, the magnitude of the high spatial frequency component is detected based on the level difference between two sequential pixel signals in the image signal, and the magnitude of the high spatial frequency component is detected between the two sequential frames. 1. An interframe coding method, characterized in that the threshold level is set for each pixel according to both the transmission amount of the difference signal and the magnitude of the high spatial frequency component. 3. In the encoding system according to claim 2, a read-only memory in which the threshold levels of a plurality of predetermined stages are stored in advance is read-only according to the amount of transmission of the inter-frame difference signal and the level difference of the pixel signal. An interframe encoding system characterized in that the required threshold level is read by accessing the threshold level.