JPH01183980A - System for compressing image signal band - Google Patents

Abstract

PURPOSE:To reduce the deterioration in picture quality caused by the random change of a mode by filtering the distortion quantity of every block in a spatial direction or a time direction, making it as corrected distortion and deciding the mode with the aid of using it. CONSTITUTION:For an image signal 50 inputted from an input terminal, the thinning and the interpolation of the mode 2 is executed by a mode 2 thinning and interpolation circuit 1 and the interpolation signal is inputted to a difference circuit 2. Then, the difference between the interpolation signal and an original signal is calculated and inputted to a distortion quantity calculation circuit 3. A total sum in the block of the absolute value of a difference signal is calculated and made to be the distortion quantity D2 of every block. This distortion signal 52 is inputted to a filter circuit 4 and a corrected distortion signal 53 is outputted. A mode decision circuit 5 decides the mode by using the signal 63. Thus, the random change of the mode in the time direction caused by the influence such as noise is restrained and the visual deterioration of the picture quality is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an image signal band compression method that can be applied to transmitting or recording image signals.

(Prior art) Conventionally, regarding the band compression technology for image signals, there is a technology called TAT method reported in the Institute of Electronics and Communication Engineers Communication Systems Study Group Material C884-7r Time Axis Conversion Band Compression Method for High-Definition Television Signals. Are known.

The TAT method first extracts basic pixels representing the basic structure of an image by coarse subsampling, and then selects additional pixels necessary to represent the fineness of the image from among the remaining pixels.

This corresponds to dividing the image into small blocks and changing the sampling pattern as shown in FIGS. 8(a) and 8(b) depending on the fineness of the image within each block. That is, a plurality of modes are set with one sampling pattern as one mode, the mode is switched and assigned to each block according to the fineness of the image, and pixels are thinned out and band compression is performed according to the assigned mode. By doing so, it is possible to reduce the interpolation error as a whole by thinning out pixels in flat parts of the image without thinning out pixels in fine parts of the image. In determining the mode in the TAT method, the number of modes is as shown in FIG. 8(a).

In the case of the two types shown in (1)), as shown in Figure 9, a histogram of the distortion amount S for each block, which is the intra-block summation of the absolute value of the interpolation error for mode 2 in Figure 8(b), is used. The mode is determined using the value Th of S as a boundary. That is, for blocks exceeding Th, mode 1 is
The following blocks are assigned mode 2.

(Problem to be solved by the invention) In the conventional TAT method, the mode for each block is
As described above, the threshold value is determined as a boundary by looking at the distortion amount value for each block, which is the intra-block summation of the absolute value of the interpolation error. Therefore, in an image region where the amount of distortion for each block is close to the threshold Th, limited types of modes with large differences in image quality are complicated for each block, and this appears as visual image quality deterioration. Furthermore, in this area, even a slight change in the amount of distortion in the time direction due to the influence of noise etc. may cause the mode of blocks at the same position to change randomly from screen to screen, which also causes visual image quality deterioration. It appears.

An object of the present invention is to provide an image signal band in which the above-mentioned problems are alleviated by applying a filter to the distortion amount for each block in the spatial or temporal direction and determining a mode using the filtered distortion amount. The objective is to provide a compression method.

(Means for Solving the Problems) The image signal band compression method of the present invention divides an image into blocks of a predetermined size, and sets a mode indicating a pixel thinning and interpolation method for each block on the transmitting side. The image signal band compression method is determined, and the pixels are thinned out on the transmitting side according to the mode, and the thinned out pixels are interpolated and restored on the receiving side.
When determining the mode, calculate the interpolation error for each mode for each pixel, calculate the distortion amount for each block using at least the interpolation error, and calculate the distortion amount for each block in space or time. The mode is determined by applying a filter to the direction and using this as a modified distortion, and using at least the modified distortion.

(Operation) In the present invention, in image band compression in which pixels are thinned out on the transmitting side and the thinned out pixels are interpolated on the receiving side to restore the image, the amount of distortion for each block used for mode determination is calculated and The key point is how to fix it.

In the present invention, first, the interpolation error for each mode is calculated, and using this, for example, the intra-block sum of the absolute value of the interpolation error is calculated in the distortion amount calculation circuit 100 of FIG. 1, and this is calculated as the distortion amount for each block. do.

Furthermore, as shown in FIG. 1, the filter circuit 20 uses the distortion amount of one or more blocks located around the screen or at the same position but at different times for the distortion amount of each block.
0, a filter is applied to create a modified distortion, and this modified distortion is used to determine the mode in the mode determining circuit 300 in the same manner as in the conventional TAT method. If, for example, a low-pass spatial filter is used as the filter, the change in distortion amount between adjacent blocks will be reduced, and the possibility that limited types of modes with large differences in image quality will be complicatedly interwoven will be reduced.

Another example of a filter is one that replaces the distortion amount of the block with the same value if the distortion amount of a predetermined number or more blocks within a predetermined range centered on the block is the same value. But you can get the same effect. Furthermore, by applying a filter in the time direction, random changes in the mode in the time direction due to the influence of noise etc. can be suppressed.

By doing this, applying a filter in the spatial direction reduces the difference in the amount of distortion between adjacent blocks, and applying a filter in the time direction makes it possible to suppress changes in the amount of distortion in the time direction. This increases the possibility that the same mode will be assigned in the spatial or temporal direction, so ``limited types of modes with large image quality differences for each block may become spatially complex, or modes may be randomly assigned in the temporal direction due to noise, etc. Image quality deterioration due to changes can be reduced.

(Example) Next, an example of the present invention will be described using FIGS. 2 to 9.

In this embodiment, as an example, a television signal is input, a field is divided into blocks each having a size of 4 pixels x 4 pixels, a mode is determined for each block for each field, and the number of pixels is reduced to 1 of the total. The case of thinning out to /2 is shown. In addition, the modes in this embodiment are - for example, mode 1 and mode 2 shown in FIGS. 8(a) and 8(b).
Type. The compression ratio of each mode is 1 for mode 1 and 1/4 for mode 2, and mode 2 uses intra-field interpolation.

That is, in this embodiment, two types of modes (a) and (b) in Fig. 8 are assigned to each block, and corresponding thinning is performed to perform band compression. Explain the method.

FIG. 2 is a block diagram schematically showing the basic part of a transmitting section of a television signal band compression system according to an embodiment of the present invention.

Mode 2 is applied to the image signal 50 input from the input terminal.
The thinning and interpolation circuit 1 performs mode 2 thinning and interpolation, and the interpolated signal is input to the difference circuit 2, where the difference between the interpolated signal and the original signal is calculated.

The difference signal 51 is input to the distortion amount calculation circuit 3, and for example, the sum of the absolute values of the difference signal within a block is calculated and this is taken as the distortion amount D2 for each block. The amount of distortion for each block is output as a distortion amount signal 52. FIG. 3 shows a field block division method in this embodiment.

The distortion amount signal 52 is input to the filter circuit 4, subjected to, for example, a low-pass spatial filter, and output as a modified distortion signal 53. FIG. 4 is a block diagram of an example of a filter circuit for applying a spatial direction filter to the distortion amount signal.

Three delay circuits each delay the distortion amount signal 52 by one block for the delayed signal 64 which delayed the distortion amount signal 52 by n blocks, and the delayed signal 65 which delayed the distortion amount signal 52 by 2Xn blocks. (Delay circuits 27 to 35) are connected in series, take out distortion amounts for 3×3 blocks spatially centered on the block, and apply fixed coefficient multipliers 36 to 44 to the output signals of delay circuits 27 to 35. are respectively multiplied by fixed filter coefficients, and fixed coefficient multipliers 36 to 4
4 is added by an adder circuit 45 to produce a modified distortion signal 53.
Output.

The mode determining circuit 5 uses the modified distortion signal 53 to
The mode is determined in the same way as the AT method. FIG. 5 shows an example of a mode determining circuit. First, the corrected distortion signal 53 is input to the histogram creation circuit 8, and a histogram of the distortion amount D2 is created in the same manner as shown in FIG.

In the threshold value determination circuit 9, the contents of the histogram created in the histogram creation circuit 8 are read out one after another from the larger D2 and added, and the value of D2 when the value of D2 becomes 1/3 or more of the total number of blocks is set as the threshold value. Th, threshold signal 57
Output as . Furthermore, the modified distortion signal 53 is delayed by the delay circuit 11 until the threshold value Th is determined by the threshold value determination circuit 9. The mode assignment circuit 10 uses the delayed modified distortion signal 58 and the threshold/value signal 57 to determine the mode for all blocks. That is, for each block, the amount of distortion D
For blocks where 2 is greater than or equal to Th, mode 1 is set to Th.
For blocks having smaller values, mode 2 is assigned and a mode signal 54 is output.

The input image signal 50 is delayed by the delay circuit 6 from when the input image signal 50 is input until the mode is determined by the mode determining circuit 5. The pixels of the delayed input image signal 55 are thinned out in accordance with the mode signal 54 in the thinning circuit 7, and the thinned out image signal 56 and the mode signal 54 determined by the mode determining circuit 5 are respectively transmitted from the output terminal to the receiving side. be done.

A mode determination method based on this embodiment will be described below.

In the embodiment shown in FIG. 2, the mode is determined for each field, but the mode may be determined for a plurality of fields at once.

Furthermore, before thinning out in the mode 2 thinning interpolation circuit 1 and thinning circuit 7 in FIG. 2, the signal band may be limited in order to prevent aliasing distortion in correspondence with mode 2.

Furthermore, as a method for determining the threshold value Th in the threshold value determining circuit 9 in FIG.
You can start with the smaller one. However, in that case,
Let Th be the value of D2 when the Kagure result exceeds 2/3 of the total number of blocks.

In addition, as a spatial filter in the filter circuit 4 of FIG. 2, a filter circuit as shown in the block diagram of FIG. If the quantities have the same value,
It is also possible to replace the distortion amount of the block with the same value. That is, the counting circuit 22 uses the distortion amount signal 52 to create a histogram of the amount of distortion within a range of, for example, a 3×3 block centered on the block in question, and the value of the amount of distortion having the maximum frequency in this histogram is set as Ds. The power with respect to Ds is set as H(Ds) and output as a maximum power distortion amount signal 60 and a maximum power signal 61, respectively. Comparison circuit 2
3, the maximum frequency signal 61 and the predetermined value signal 62 are input, a comparison is made as to whether H(Ds) is larger or smaller than the predetermined value, and a comparison signal 63 is output. In the selection circuit 21,
If H(Ds) is larger than the predetermined value, the maximum frequency distortion signal 60 may be selected, and if H(Ds) is less than or equal to the predetermined value, the distortion signal 52 may be selected as the modified distortion signal 53.

Moreover, as a spatial filter in the filter circuit 4 of FIG. 2, a filter circuit as shown in the block diagram of FIG. 7 may be used to apply a low-pass filter to the distortion amount signal 52 in the time direction. That is, with respect to the distortion amount signal 52 and the delayed signal 66 obtained by delaying the distortion amount signal 52 by one field of nXmXm blocks, and the delayed signal 67 which delayed the distortion amount signal 52 by 2 hand,
One delay circuit each delays one block (
Delay circuits 14 to 16) Connection ・Take out the distortion amounts for three blocks centered on the block in the time direction, and apply the fixed coefficient multiplier 1 to the output signals of the delay circuits 14 to 16.
Multiply each fixed filter coefficient by 7 to 19,
The outputs of the fixed coefficient multipliers 17 to 19 may be added by the adding circuit 20 and output as the corrected distortion signal 53.

Further, the type of mode used may be different from those shown in FIGS. 8(a) and 8(b).

According to this embodiment, the signal band of the television signal is halved.
The bandwidth of the transmission path required for transmission is 1/2.
This enables an image signal band compression method with even better image quality. Note that the compression rate of the image signal can be varied by controlling the compression rate of each mode or the value of Th.

(Effects of the Invention) As described above, according to the present invention, it is possible to reduce the difference in the amount of distortion between adjacent blocks and suppress the change in the amount of distortion in the time direction, and therefore the same mode can be assigned. Therefore, it is possible to reduce image quality deterioration caused by limited types of modes with large image quality differences for each block becoming complicated or by modes changing randomly in the time direction due to the influence of noise or the like. Therefore, conventional TA
It is possible to perform band compression of visually higher-quality images than the T method.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a method for calculating and correcting the amount of distortion in the present invention, FIG. 2 is a block diagram showing an outline of the basic part of a transmitter in an embodiment of the present invention, and FIG. 3 is an embodiment of the present invention. FIG. 4 is a diagram showing an example of a filter circuit according to an embodiment of the present invention. FIG. 5 is a diagram showing an example of a mode determining circuit according to an embodiment of the present invention. , FIG. 6 is a diagram showing an example of a filter circuit in one embodiment of the present invention, FIG. 7 is a diagram showing an example of a filter circuit in one embodiment of the present invention, and FIG.
) and (b) are explanatory diagrams showing sampling patterns of the TAT method, and FIG. 9 is an explanatory diagram showing a mode determination method in the TAT method. In the figure, 100...distortion amount calculation circuit, 200...
Filter circuit, 300...mode determining circuit.

Claims (1)

[Claims] An image is divided into blocks of a predetermined size, a mode indicating a pixel thinning and interpolation method is determined for each block on the transmitting side, and pixels are thinned out on the transmitting side and thinned out on the receiving side according to the mode. In an image signal band compression method that restores pixels by interpolation, when determining the mode, calculate an interpolation error for each mode for each pixel, and calculate a distortion amount for each block using at least the interpolation error, An image signal band compression method, characterized in that the amount of distortion for each block is filtered in the spatial or temporal direction to obtain a modified distortion, and at least the modified distortion is used to determine the mode.