JPS63191490A - Image encoding and transmitting method - Google Patents

Abstract

PURPOSE:To obtain an image encoding and transmitting means which is invariably good in encoding efficiency by selecting the best variable length characteristics among plural variable length characteristics prepared corresponding to threshold value adjustments. CONSTITUTION:A variable length encoding part 4 is provided with variable length encoding circuits 4a-4c which have different kinds of variable length encoding characteristics, and the respective encoding characteristics are selected according to a threshold value Th outputted by a threshold value generating circuit 6 and controlled by a variable length encoding characteristics switch 4d. When a variation quantity is small with a small threshold value Th, codes are concentrically short almost at 0 in variable length encoding characteristics of a differential data value and when the threshold value Th is large, on the other hand, the variable length code of a small difference data value is made slightly long to obtain short codes even for the large value. Thus the characteristics are prepared and selected according to the threshold value. Consequently, the encoding efficiency is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an image encoding and transmitting method, particularly to an improvement of an image encoding and transmitting method for video conferences, video telephones, and the like.

[Prior Art] In general, image transmission used for video conferences or video calls involves a huge amount of image information, but due to line costs and transmission speeds during transmission, the amount of image information for transmission is limited. A compression encoding method that reduces (compresses) the amount of data has been put into practical use.

An image encoding and transmission method using interframe differential pulse modulation is widely known, in which an input image signal is taken in for each frame, and a difference between the image signal and the image signal between adjacent frames is determined and quantized and encoded.

Regarding the encoding method, a variable length encoding method has been put into practical use as a method with high encoding efficiency.

Hereinafter, a conventional image encoding and transmission method will be explained based on the drawings.

FIG. 3 shows an image encoding and transmitting apparatus to which an interframe differential pulse modulation method including a variable length encoding method, which is a conventional image encoding and transmitting method, is applied.

In the figure, the image encoding and transmitting device has an image input signal (
100) and the predicted signal (107) and outputs difference data; and a valid/invalid discriminator circuit (2) that smoothes the amount of transmitted information using a threshold value to be described later. A quantization circuit (3) that quantizes the difference data and outputs a quantized signal, an encoding circuit (4) that encodes the quantized signal with variable length, and temporarily stores the encoded signal for each frame. and the transmission buffer (5) to be sent and the data temporarily stored in the transmission buffer (5). ], and a threshold generation circuit (6) for controlling the threshold according to the northernmost signal interval.

First, an image input signal (100) is subtracted from a predicted signal (107) by a subtracter (1), and difference data (101) is sent to a valid/invalid determining circuit (2).

Then, the validity/invalidity determination circuit (2) performs determination as shown in the following equation.

If the above judgment is invalid, it is considered that there is no change, and if it is valid, it is necessary to transmit the difference data value, and the quantization circuit (3
) is sent.

Here, it can be seen that the larger the threshold value is, the more the amount of information is reduced.

Then, in the quantization circuit (3), the quantized signal (103) is encoded in the variable length encoding circuit (3), and the transmission buffer (
5).

The encoding method in the variable length encoding circuit (3) is performed as follows.

Generally, in image signals used in video conferences, etc., the probability of occurrence of the differential data value (101) is centered around 0, and as the absolute value of the difference increases, the probability of occurrence decreases, as shown in Figure 4 (a). .

Therefore, the variable length encoding circuit (3) has one variable length encoding characteristic, and the variable length encoding characteristic is shown in FIG. 4(b) based on the information occurrence probability. Thus, a short code length is assigned to a small absolute difference value with a high probability of information occurrence, and a long code length is assigned to a large absolute value of a difference with a low probability of occurrence.

By performing variable length encoding as described above, the amount of encoded information can be reduced.

On the other hand, the threshold value is controlled by the amount of encoded information per frame temporarily stored in the transmission buffer (5), and when the amount of information is large, the threshold value is set to a large value, and when the amount of information is small, the threshold value is set to a large value. The threshold value is adjusted by the threshold generation circuit (6) to a small value.

[Problems to be Solved by the Invention] As mentioned above, the conventional image encoding transmission method transmits a moving image, so the probability of occurrence of a difference data value between the current frame image and the previous frame image is small for each frame. However, the variable-length encoding characteristics were constant, so when transmitting image signals that change rapidly, threshold control generates only large differential data with poor variable-length encoding efficiency. There was a problem that encoding efficiency was poor.

The present invention has been made in order to solve the problem described in 1.It is an object of the present invention to obtain an image encoding and transmission method that always has high encoding efficiency. .

Means for Solving Problem E] In order to achieve the two objectives, the present invention provides that the threshold value for smoothing the information to be quantized is adjusted for each frame according to the amount of information generated. Focusing on the fact that the probability of occurrence of differential data values can be predicted using a threshold value, this method selects encoding characteristics from multiple types of variable-length encoding characteristics according to the threshold value. .

[Operation] As described above, according to the present invention, an image encoding and transmission method with high encoding efficiency can be obtained by selecting variable length encoding characteristics according to a threshold value.

[Example] Hereinafter, an image encoding and transmission method according to the present invention will be explained based on the drawings.

FIG. 1 is a block diagram of an image transmission apparatus to which the method of the present invention is applied.

In the drawings, in FIG. 3, the same or equivalent parts as in the conventional example are denoted by the same reference numerals, and the explanation thereof will be omitted.

The variable length encoding unit (4) includes a variable length encoding circuit (4a) having different types of variable length encoding characteristics.

(4b) and (4c) are provided, and each encoding characteristic is selected based on the threshold value (Th) output from the threshold generation circuit (6). (4d) controls.

As shown in FIG. 2(a), when the threshold value (Th) changes, the occurrence probability distribution of differential data values changes.

That is, a small threshold value (Tb) means that the amount of information generated is small, and the difference from the previous frame is small, and the difference data values are concentrated near θ.

Furthermore, a large threshold value means a large amount of information, and the image changes rapidly, so differential data values occur over a wide range.

Therefore, the threshold value (Th
) is small (when the amount of change is small), the variable-length encoding characteristics of the differential data value are concentrated around θ with short codes, and
When the threshold value (TI+) is large, the variable-length code with a small difference data value is made a slightly long code, so that a characteristic that allows a short code to be given even for a large value is prepared, and the threshold value is By selecting accordingly, encoding efficiency is improved.

Therefore, according to this embodiment, since multiple types of variable length No. 11 northernmost characteristics are provided and the characteristics are selected based on the threshold value, the information on the effective block actually encoded is Therefore, efficient variable length encoding can be performed.

In addition, although Embodiment 1 shows an example of an image encoding method using inter-frame differential pulse modulation including a variable length encoding method, it is also possible to use a method using a vector quantization method and a vector quantization method including motion compensation. A similar effect can be achieved in the image transmission encoding method.

[Effects of the Invention] As described above, according to the present invention, the variable length coding characteristics are selected according to the threshold value, so the variable length coding characteristics are selected according to the probability distribution of differential data values for each frame. It is possible to provide an image encoding and transmission method with high encoding efficiency.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a program for an image encoding and transmitting apparatus to which an embodiment of the image encoding method according to the present invention is applied, and FIG.
Fig. 2(b) is a diagram showing the relationship between the threshold and variable length coding characteristics in the embodiment of Fig. 1; 4(a) is a diagram of the generation probability distribution of differential data values, and FIG. 4(b) is a diagram of the conventional differential data value. FIG. 3 is a diagram showing the relationship between code lengths. In the figure, (1) is a subtracter, (2) is a valid/invalid discrimination circuit, and (4) is a variable length? No. 1 North Circuit, (4a), (
4b), (4c) are variable length encoding characteristics, (4d) are encoding characteristic switchers, (5) are transmission buffers, (6)
is a threshold generation circuit. Note that the same reference numerals in the figures indicate the same or equivalent parts. Figure 2 (b) Figure 3 Figure 4 (b) Code length

Claims (1)

[Claims] An input image signal is captured for each frame, and a difference in image signals is determined between adjacent frames for each pixel or block,
An image code that encodes inter-frame difference data that exceeds a predetermined threshold value into a variable length code based on predetermined variable length characteristics and transmits the data, and smoothes the amount of data to be transmitted by adjusting the threshold value according to the amount of data to be transmitted. 1. An image encoding transmission method, characterized in that an optimal variable length characteristic is selected from a plurality of variable length characteristics prepared in advance in correspondence with threshold adjustment.