JPH07264588A - Encoding device for video signal - Google Patents

Abstract

PURPOSE:To evade the deterioration of picture quality due to overload by controlling a quantizing step parameter so as not to generate overload based upon the size of a transformation parameter in a regulated picture area. CONSTITUTION:An overload evading parameter calculating circuit 15 calculates a minimum quantizing step parameter for preventing the generation of overload in a regulated picture area by prescribed operation based upon a coefficient transformed by a DCT circuit 4. An overload evading circuit 16 compares a quantizing step parameter inputted from a quantization control circuit 14 with the minimum quantizating step parameter inputted from the circuit 15, calculates a final quantizing step parameter and sends the parameter value to a quantizing circuit 5. Consequently the generation of overload can be evaded.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal coding apparatus, and more particularly to a video signal coding apparatus for avoiding overload.

[0002]

2. Description of the Related Art FIG. 3 shows, for example, ISO-IEC / JTC1 / SC29 / WG11 M.
FIG. 11 is a block circuit diagram showing a conventional video signal encoding device shown in PEG 93 / N0457 Test Model 5. In the figure,
The digitized video signal input from the input terminal 1 receives the first input of the subtractor 3 and the motion compensation prediction circuit 11
Given to the first input of. The output of the subtractor 3 is the DCT
It is provided to the first input of the quantizer 5 via the circuit 4. The output of the quantization circuit 5 is given to the input of the transmission buffer 13 via the variable length coding circuit 12, and is given to the first input of the adder 8 via the dequantization circuit 6 and the IDCT circuit 7. . The output of the adder 8 is given to the first input of the memory circuit 9, and the output of the memory circuit 9 is
It is provided to the second input of the motion compensation prediction circuit 11 and the first input of the switching circuit 10. The second output of the memory circuit 9 is supplied with the first output of the motion compensation prediction circuit 11.

On the other hand, the zero signal is given to the second input of the switching circuit 10, and the second output of the motion compensation prediction circuit 11 is given to the third input of the switching circuit 10. The output of the switching circuit 10 is given to the second input of the subtractor 3 and the second input of the adder 8. On the other hand, the second output of the transmission buffer 13 is given to the second input of the quantization circuit 5 via the quantization control circuit 14, and the first output of the transmission buffer 13 is output from the output terminal 2.

As one of high-efficiency coding systems for coding a video signal, there is a hybrid coding system which combines inter-picture predictive coding using motion compensation prediction and intra-picture transform coding. The example also employs the above hybrid coding scheme.

Explaining the outline of the operation of this conventional example, the digitized input signal is subjected to a difference between images by using motion compensation prediction in order to reduce redundancy in the time axis direction.
DCT is performed in the spatial axis direction. The transformed coefficients are quantized, variable-length coded, and then transmitted through a transmission buffer.

First, the basic concept of quantization will be described. In general, the coefficient converted by the DCT or the like cannot always be expressed with a finite number of digits. Representing this coefficient with a finite number of digits is called quantization, a discrete value by quantization is called a quantization level, and an interval between quantization levels is called a quantization step. The quantized coefficient value is called a quantization index. In a coding system, this quantized index is the main object of transmission. At this time, if the quantization step is increased, the quantized value becomes zero or a small value close to zero, and as a result, the amount of data to be transmitted is reduced.

The quantization in the conventional example will be described below. The relationship between the transform coefficient and the quantization index in this conventional example is i = d / (2 × mquant) (1) where d is the transformed coefficient and i is the quantization index of d. At this time, the operator / indicates an operation of truncating the fractional part in the zero direction after division.

The mquant in the equation (1) is a variable called a quantization step parameter and has the same meaning as the above quantization step. That is, increasing mquant leads to a decrease in the amount of data to be transmitted.
In addition, mquant can be changed in units of a certain defined pixel block (hereinafter, this is referred to as a “specified pixel area”).

In the conventional example, for example, 1 is displayed on the screen.
A block called a macro block consisting of 6 pixels × 16 lines is used as a specified pixel area. That is, the mquant can be changed in macroblock units. Generally, since the transmission rate is regulated in the video signal encoding system, in the conventional example, the generated code amount is controlled by appropriately changing the size of mquant.

Further, in the conventional example, the conversion coefficient d is 1
It is defined as 2 bits, and its value range is defined as follows. -2048 ≦ d ≦ 2047 (2)

The quantization index i is defined as 9 bits, and the range of its value is defined as follows. (However, not all the quantization indexes) -255≤i≤255 (3)

At this time, mquant = 1 or mq
When uant = 2, not all transform coefficients d can be represented by the quantization index i that satisfies the condition of the above expression (3). For example, when mquant = 1, the equation (3) can be written as follows. −510 ≦ i × 2 × mquant ≦ 510 (4)

From equations (1) and (4), −511 ≦ d ≦ 511 (5) That is, in mquant = 1,
This means that the conversion coefficient d outside the range of the condition of the expression (5) cannot be communicated.

As described above, in an operating state in which a certain mquantt is set, a quantization coefficient within a defined range generates a transform coefficient that cannot cover the coefficient value, which is called overload.

[0015]

In the conventional video signal encoding device, no particular reference was made to the processing method relating to the occurrence of overload. In other words, the conversion coefficient can be notified only within the range that can be notified by the quantization index, and when the conversion coefficient outside the range is notified, the limited value is notified, which causes remarkable image quality deterioration. It was

The present invention has been made to solve the above problems, and an object of the present invention is to obtain a video signal coding apparatus capable of avoiding the occurrence of overload.

[0017]

A video signal coding apparatus according to the present invention calculates a minimum quantization step parameter that does not cause an overload in a specified image area based on the size of a transform coefficient in the specified image area. However, the quantization step parameter set in the specified image area without considering the overload avoidance is compared with the minimum quantization step parameter, and it is set in the specified image area without considering the overload avoidance. When the quantization step parameter is smaller than the minimum quantization step parameter, the quantization step parameter of the specified image area is changed to the minimum quantization step parameter.

Further, the minimum quantization step parameter that does not cause overload in the specified image area is calculated based on the power of the conversion target image in the specified image area, and the specified image is considered without avoiding overload avoidance. The quantization step parameter set in the area is compared with the minimum quantization step parameter, and the quantization step parameter set in the specified image area is the minimum quantization step parameter without considering the overload avoidance. If it is smaller than the above, the quantization step parameter of the specified image area is changed to the minimum quantization step parameter.

[0019]

The video signal encoding device according to the present invention calculates the overload avoidance parameter based on the output of the DCT during the encoding process and feeds the result to the quantization circuit via the overload avoidance circuit. Forward, and controls the quantization step parameter of the quantization circuit based on the size of the transform coefficient so as not to cause overload.

Also, during the encoding process, an overload avoidance parameter is calculated based on the image to be converted, and the result is fed forward to the quantizing circuit via the overload avoiding circuit to determine the power of the image to be converted. The quantization step parameter is controlled based on the magnitude so as not to cause overload.

[0021]

【Example】

Example 1. 1 is a block circuit diagram of a first embodiment 1 of the present invention, and the same reference numerals as those in FIG. 3 denote the same or corresponding portions. In the figure, 15 is an overload avoidance parameter calculation circuit, 16 is an overload avoidance circuit, and the DCT circuit 4
Is given to the first input of the quantization circuit 5 and the overload avoidance parameter calculation circuit 15, and the overload avoidance circuit 16
The output of the quantization control circuit 14 is given to the first input of the above, and the output of the overload avoidance parameter calculation circuit 15 is given to the second input of the overload circuit 16.

The operation of the first embodiment will be described below.
The overload avoidance parameter calculation circuit 15 calculates the minimum quantization step parameter that does not cause overload in the specified image area as follows based on the coefficient converted by the DCT circuit 4.

A certain conversion coefficient in the specified image area is dx
, Mq that does not cause overload on dx
The minimum value of ant is uniquely determined in accordance with the quantization characteristic as in the expression (1) in the conventional example. The minimum value of mquantt that does not cause this overload is minx. At this time, if the maximum value of all minx in the specified image area is limit, m is set in the specified image area.
If quant ≧ limit is defined, overload will not occur.

The overload avoidance parameter calculation circuit 15 calculates the limit and sends it to the overload avoidance circuit 16. The overload avoidance circuit 16 compares the mquantt input from the quantization control circuit 14 with the limit, calculates the final mquantt as follows, and sends the value to the quantization circuit 5. IF mquant <limit THEN (final mquant) = limit ELSE (final mquant) = mquant

In the first embodiment, the video signal coding apparatus has been described, but the present invention is not limited to this and can be applied to a video signal coding system.

In the first embodiment, the case where the DCT is used has been described, but the present invention is not limited to this, and can be applied to the case where another conversion is used.

Example 2. 2 is a block circuit diagram of a second embodiment of the present invention, and the same reference numerals as those in FIG. 1 denote the same or corresponding portions. In the figure, 17 is an overload avoidance parameter calculation circuit, and the output of the subtractor 3 is the DCT circuit 4
And given to the overload avoidance parameter calculation circuit 17,
The first input of the overload avoidance circuit 16 is the quantization control circuit 1
4 is given, and the output of the overload avoidance parameter calculation circuit 17 is given to the second input of the overload circuit 16.

The operation of the second embodiment will be described below.
The overload avoidance parameter calculation circuit 17 calculates the minimum quantization step parameter that does not cause overload in the specified image area as follows based on the power of the signal before the DCT conversion.

In general, the power of the signal before and after conversion in the specified image area is preserved, so that in the case of overload, the power of the signal before conversion exceeds a certain fixed value. In this embodiment, this condition is used.

Generally, in the case of hybrid coding in which inter-picture predictive coding using motion-compensated prediction and intra-picture transform coding are combined, two types of signals, that is, an input picture and a difference signal between pictures, are subject to conversion. . Hereinafter, this will be referred to as a “conversion target image”. In general, when the input image is treated as a conversion target image, the converted direct current component (DC component) is different from the alternating current component (AC component) and is quantized without causing overload. Therefore, it is necessary to change the power calculation method and the overload detection method between the case of the input image and the case of the difference signal between the images.

When the image to be converted is the input image, the overload is detected as follows, for example. It is assumed that the specified image area is a block composed of M pixels × N lines on the screen, and the pixel value of the i-th pixel and the j-th column in the specified image area is pix (i, j). At this time, the power of the input image, powerpix,

[0032]

[Equation 1]

Then, the overload is detected as follows. IF powerpix ≧ Kpix THEN (overload) At this time, Kpix is a constant.

When the image to be converted is a difference signal between images, overload is detected as follows, for example. It is assumed that the specified image area is a block composed of M pixels × N lines on the screen, and the difference signal value of the i-th pixel and the j-th column in the specified image area is dif (i, j). At this time,
The power of the input image, powerdif,

[0035]

[Equation 2]

Then, the overload is detected as follows. IF powered ≧ Kdif THEN (overload) At this time, Kdif is a constant.

In the overload avoidance parameter calculation circuit 17,
When the overload is detected as described above, the minimum value limit of mquant that does not cause the overload is sent to the overload avoidance circuit 16 for all the conversion coefficients within the defined range. This minimum value limit is uniquely determined according to the quantization characteristic as in the expression (1) in the conventional example. Also, if no overload is detected, the limi
Assume that t = 1 is sent.

The overload avoidance circuit 16 compares the mquantt input from the quantization control circuit 14 with the above limit, calculates the final mquantt as follows, and sends the value to the quantization circuit 5. IF mquant <limit THEN (final mquant) = limit ELSE (final mquant) = mquant

In the second embodiment, the video signal coding apparatus has been described, but the present invention is not limited to this and can be applied to a video signal coding system.

In the second embodiment, the case where the DCT is used has been described, but the present invention is not limited to this, and the present invention can be applied to the case where another conversion is used.

[0041]

As described above, according to the present invention, the quantization step parameter is controlled during the encoding process so as not to cause an overload based on the size of the transform coefficient in the specified region. There is an effect of obtaining a video signal encoding device capable of avoiding image quality deterioration due to overload.

Further, since the quantization step parameter is controlled so as not to cause an overload on the basis of the power of the image to be converted in the specified area in the course of the encoding process, the deterioration of the image quality due to the overload can be avoided. There is an effect of obtaining a video signal encoding device.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of a first embodiment of the present invention.

FIG. 2 is a block circuit diagram of a second embodiment of the present invention.

FIG. 3 is a block circuit diagram of a conventional video signal encoding device.

[Explanation of symbols]

 3 Subtractor 4 DCT circuit 5 Quantization circuit 6 Inverse quantization circuit 7 IDCT circuit 8 Adder 9 Memory circuit 10 Switching circuit 11 Motion compensation prediction circuit 12 Variable length coding circuit 13 Transmission buffer 14 Quantization control circuit 15 Overload avoidance Parameter calculation circuit 16 Overload avoidance circuit 17 Overload avoidance parameter calculation circuit

Claims (2)

[Claims] 1. A video signal coding apparatus having a quantization circuit capable of changing a quantization step parameter in units of a specified image area, wherein the specified image area is based on the size of a transform coefficient in the specified image area. First calculation of minimum quantization step parameter that does not cause overload
Of the overload avoidance parameter calculation circuit, the quantization step parameter set in the specified image area without considering the overload avoidance, and the minimum quantization step parameter output from the first overload avoidance parameter calculation circuit. If the quantization step parameter set in the specified image area is smaller than the minimum quantization step parameter without considering the above overload avoidance, the quantization step parameter of the specified image area is set to A video signal coding apparatus, comprising: an overload avoiding means for changing to a minimum quantization step parameter. 2. A video signal encoding device having a quantization circuit capable of changing a quantization step parameter for each specified image area, wherein said specified image area is defined based on a power of a conversion target image in the specified image area. A second overload avoidance parameter calculation circuit that calculates a minimum quantization step parameter that does not cause overload in the image region, a quantization step parameter set in the specified image region without considering overload avoidance, and 2 is compared with the minimum quantization step parameter output from the overload avoidance parameter calculation circuit, and the quantization step parameter set in the specified image area is the minimum quantization step parameter without considering the overload avoidance. If it is smaller than the parameter, set the quantization step parameter of the specified image area to the minimum quantization step above. A video signal encoding device, comprising: an overload avoiding means for changing to an up parameter.