JPH1118086A - Image communication method and system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image communication method and a system with which a delay in an image frame is reduced and where the communication minimizing deterioration in the image quality is conducted. SOLUTION: A bit number obtained by a bit number coding means 4 for each block is added to image data, that are divided into blocks and subject to variable length coding at an image-coding means 1 at a transmitter side, and the resulting data are transmitted. A receiver side can recognize block borders of the image data. In the case that a channel error takes place in data, subject to variable length coding at the receiver side, since the block is known, error propagation in excess of blocks is not caused and deterioration in the image quality is suppressed. Furthermore, data for the converted image frames are sent to an image decoding means 10 without awaiting the arrival to a reception buffer 8 for all the image frame data without errors. Thus, a large delay in the image frame is not caused, and part of information of the image frame is sent to the receiver side without errors.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image communication method and apparatus for transmitting a digitally encoded image using a two-way communication system, and more particularly to a technique for controlling a reproduced image error.

[0002]

2. Description of the Related Art FIG. 7 shows an ARQ (Automatic Repeat Req) in a conventional image communication method.
west for Repeat Transmission
FIG. 2 is a block diagram specifically illustrating (ion). The transmitting side is composed of an image encoding unit 1, a frame composing unit 2, a transmission buffer 3, and a channel error detecting unit 5, and the receiving side is a frame composing unit 6, a channel error detecting unit 7, a receiving buffer 8, image decoding. Means 10 is constituted.

[0003] A transmitting side and a receiving side are connected by a communication path which enables two-way communication. This communication path may cause transmission errors during signal transmission.

An original image, which is information to be transmitted, is encoded by the image encoding means 1 into compressed image data. The compressed image data is divided by the frame structuring unit 2 into frames each having a fixed length, error check bits are calculated, and these are added to the frames as error detection codes. Then, after the numbers are given, they are stored in the transmission buffer 3. The transmission buffer 3 sequentially transmits frames to the communication path unless a retransmission request is received from the receiving side. The operation when there is a retransmission request from the receiving side will be described later.

[0005] The communication path transmits the frame transmitted from the transmission side to the reception side.

On the receiving side, a channel error detection unit 7 performs channel error detection on a frame given a number received from the channel. If no error is detected, a reception acknowledgment signal is sent to the communication path as a response signal after an acknowledgment signal is added to the communication path as a response signal after the frame acknowledgment code is added by the frame structuring means 6 as a notification for confirming reception of the frame. , To the sender. If an error is detected, a retransmission request signal is transmitted to the communication path as a response signal after adding an error detection code, separately from the frame, and transmitted to the transmission side.

[0007] The transmitting side inputs a response signal from the receiving side to the channel error detecting means 5 and, if there is no error in the response signal, inputs it to the transmission buffer 3 to check whether it is a reception confirmation signal or a retransmission request signal. . If there is a communication path error in the response signal,
Since it cannot be confirmed whether the signal is a reception confirmation signal or a retransmission request signal, it is discarded. If the response signal is a reception acknowledgment signal, the corresponding frame in the transmission buffer 3 is discarded and a free area is reserved for storing a new frame. In the case of a retransmission request signal, a frame in the transmission buffer 3 for which retransmission has been requested is transmitted to the communication path instead of transmitting a new frame to the communication path.

[0008] After detecting the communication path error, the receiving side extracts the compressed image data from the frame, rearranges the compressed image data in the order of the assigned numbers, and stores it in the receiving buffer 8. When compressed image data having a length that can be decoded by the image decoding unit 10 is stored in the reception buffer 8, the compressed image data is output to the image decoding unit 10. Image decoding means 10
Outputs a reproduced image from the received compressed image data.

At the same time as the reproduced image is output from the receiving side, the compressed image data stored in the receiving buffer 8 is discarded, and is reserved as an empty area for storing new data.

[0010] By using the ARQ in the conventional system,
Since the frame in which the channel error has been detected is retransmitted using the error detection code, an image free from deterioration due to the channel error can be obtained on the receiving side.

[0011]

However, in the above-described conventional image communication method using ARQ, since the retransmission control causes a delay in the image frame, the motion of the reproduced image becomes unnatural or the motion of the image temporarily stops. There was a problem that it seemed to stop temporarily.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image communication method and apparatus capable of performing communication with reduced delay of an image frame and minimization of image quality deterioration.

[0013]

The present invention solves the above-mentioned problems by the following inventions.

[Invention 1] A feature of the present invention 1 for solving the above-mentioned problem is that, when transmitting an encoded image using a two-way communication system in which a communication path error occurs, received image data When an error is present, the error is detected on the receiving side, and in an image communication method or apparatus that requests the transmitting side to retransmit the image data in which the error has been detected, the image is divided into blocks on the transmitting side, and further variable-length coding is performed. By adding data obtained by encoding the number of bits for each block to the obtained image data and transmitting the data together, the receiving side knows the delimiter of the block of the image data.

The difference from the prior art is that data obtained by encoding the number of bits for each block is transmitted to the receiving side.

According to the first aspect of the present invention, even if a communication path error occurs in the variable-length coded data on the receiving side, the number of bits for each block is known on the receiving side. Propagation does not occur, and an effect of suppressing image quality deterioration can be obtained.

[Invention 2] A feature of the present invention 2 for solving the above-mentioned problem is that, in the image communication method or apparatus shown in the present invention 1, the number of bits of a plurality of blocks is encoded and added. That is.

According to the second aspect of the present invention, since the number of bits transmitted simultaneously as additional information is reduced, the effect of preventing the propagation of image quality deterioration across blocks without lowering the compression efficiency can be obtained.

[Invention 3] A feature of the invention 3 for solving the above-mentioned problem is that, when transmitting an encoded image using a bidirectional communication system in which a communication path error occurs, received image data When an error exists, the image is degraded due to a communication path error in a reference image held by the receiving side in an image communication method or apparatus that detects the error on the receiving side and requests the transmitting side to retransmit the image data in which the error was detected. If
When image data correctly received by the retransmission control arrives, the reference image is reconstructed using the data and replaced with a reference image having deteriorated image quality.

The difference from the prior art is that the reference image once stored on the receiving side is rewritten.

According to the third aspect of the present invention, even when the image quality is deteriorated in the reference image on the receiving side, the reference image is replaced with the correct image when correct data is obtained. Even if an image frame is decoded and no error occurs in the data of the next image frame, the error of the previous image frame does not propagate.

[Invention 4] A feature of the invention 4 for solving the above-mentioned problem is that, when transmitting an encoded image using a two-way communication system in which a communication channel error occurs, the received image data When there is an error, in the image communication method or apparatus for detecting the error on the receiving side and requesting the transmitting side to retransmit the image data in which the error has been detected, the image decoding means detects and notifies the error on the receiving side. When decoding the data of the block number including the communication path error, the reproduction is performed by replacing the data with the block of the previous image frame.

The difference from the conventional system is that a block in which an error has occurred is notified to an image decoding unit, and the block is replaced with a previous image frame by the image decoding unit.

According to the fifth aspect of the present invention, the block in which the communication path error has occurred is decoded by the image decoding means as it is, and the previous frame is displayed without deteriorating the image quality, so that the effect of concealing the error is obtained.

[Invention 5] A feature of the invention 5 for solving the above-mentioned problem is that, when transmitting an encoded image using a two-way communication system in which a communication path error occurs, the received image data When an error is present, the operation of the reception buffer is to be shown in the following procedure in an image communication method in which the error is detected on the receiving side and the transmitting side is requested to retransmit the image data in which the error has been detected.

K is the target image frame number, N _k is the youngest ARQ frame number including the image frame k, L
_k is the number of ARQ frames including the image frame k, T _k is the number of ARQ frames to wait until the reception buffer passes data as the image frame k to the image decoding means, l is the number of image frames including the header and additional information. deep.

Procedure 1 The ARQ frame number N _k is set, and it waits for the ARQ frames from the frame numbers N _k to N _k + l to arrive at the reception buffer correctly.

Procedure 2 Set L _k and check whether all ARQ frames up to N _k + L _k -1 have arrived correctly,
If confirmed, proceed to step 6, otherwise, proceed to step 3.
Proceed to.

[0029] Step 3 Calculate the ARQ number of frames T _k possible that the receiving buffer will wait, T _k pieces of A from that point
Wait for the RQ frame to arrive, and if so, go to step 4.

Step 4 AR including previous image frame k-1
Q Wait frame, that is, the ARQ frame up again T _k as T _k = T _k +1 until the ARQ frame of up to N _k-1 can be sure that you are arriving all, proceed to step 5. If it can be confirmed.

Step 5: The ARQ frame of the image frame k arriving at that time is transmitted to the image decoding means, and the ARQ frame numbers _Nk to _Nk + _Lk- 1 of ARQ frame numbers
Wait for the RQ frame to arrive correctly, and if it arrives, go to step 6.

Step 6 AR from N _k to N _k + L _k -1
Transmitting the Q frame to the image decoding means,
End the procedure for.

The difference from the conventional method is that the data of the corresponding image frame is transmitted to the image decoding means without waiting for the data to arrive at the receiving buffer without error.

Thus, it is possible to transmit a part of the information of the image frame to the receiving side without error without causing a large delay of the image frame.

[Invention 6] A feature of the present invention 6 for solving the above-mentioned problem is that, in the image communication method shown in the above-mentioned present invention 5, when the value of T _k is set in the receiving buffer, up to L _k. To the remaining number of unreceived ARQ frames, the number of ARQ frames for which a retransmission request has already been transmitted to the transmitting side and the retransmitted ARQ frame can arrive without a large delay, and Value T
It is characterized in that it is set as _k .

The transmission time of the image frame data to the image decoding means differs from that of the conventional method.

According to the sixth aspect of the present invention, since there is no need to wait for an ARQ frame that is expected to be greatly delayed, an effect of preventing an increase in the delay of an image frame can be obtained.

[0038]

Embodiments of the present invention will be described below in detail with reference to the drawings.

[Embodiment] FIG. 1 is a block diagram specifically illustrating an embodiment of an image communication method according to the present invention. The transmitting side is composed of an image encoding unit 1, a frame composing unit 2, a transmission buffer 3, a block bit number encoding unit 4, and a channel error detecting unit 5, and the receiving side is a frame composing unit 6, a channel error detecting unit. 7, a receiving buffer 8, an error block detecting means 9, and an image decoding means 10.

[0040] The transmitting side and the receiving side are connected by a communication path which enables bidirectional communication. This communication path may cause transmission errors during signal transmission.

The original image, which is information to be transmitted, is divided into blocks, encoded into compressed image data by the image encoding unit 1, and transmitted to the frame forming unit 2. The compressed image data is the same as that calculated by the conventional method, and is composed of header and block data. Here, in the present embodiment, the number of bits of each block is transmitted to the bit number encoding means 4 of the block.

The block bit number encoding means 4 encodes the number of bits in each block into a fixed-length binary code and transmits it to the frame construction means 2. As the number of bits of the encoded block increases as the number of blocks increases, the number of bits is encoded as a new block obtained by combining several blocks. As a result, the number of bits of the coded block in the entire data amount of the image frame is reduced, and the coding efficiency is increased. This is an example of the second embodiment of the present invention.

The frame structuring means 2 forms a variable-length image frame as shown in FIG. 3 and further converts this image frame into a fixed-length ARQ frame, which is a unit frame of ARQ control described later. The data is divided and transmitted to the transmission buffer 3.

The configuration of this image frame will be described.
First, the header of the compressed image data calculated by the image coding means 1 is calculated by the bit number coding means 4 of the block,
The number of bits of each encoded block is added. next,
The number of ARQ frames up to the ARQ frame including the first bit of the next image frame after the current image frame is written in a fixed length. This is an example of the first embodiment of the present invention.

The construction method of the ARQ frame will be described below. FIG. 2 shows a configuration example of the ARQ frame. The image frame constructed by the above method is divided into ARQ frames of a fixed length, error check bits are calculated, and an error detection code is added to the ARQ frame. In addition, the header of the ARQ frame writes information necessary for controlling ARQ.

The ARQ frame is stored in the transmission buffer 3. The transmission buffer 3 sequentially transmits the ARQ frames to the communication path unless a retransmission request is received from the receiving side. The operation when there is a retransmission request from the receiving side will be described later.

The communication path transmits the ARQ frame transmitted from the transmitting side to the receiving side.

On the receiving side, the channel error detecting means 7 performs channel error detection on the ARQ frame to which the number received from the channel has been assigned, in ARQ frame units.
If no error is detected, a reception acknowledgment signal is sent to the communication path as a response signal after a reception acknowledgment signal is added to the ARQ frame as a notification for confirming the reception of the ARQ frame after the error detection code is added by the frame construction means 6. And transmitted to the sender. When an error is detected, a retransmission request number is transmitted to a communication path after adding an error detection code, separately from the ARQ frame, and transmitted to the transmission side.

The transmitting side inputs the response signal from the receiving side to the communication path error detecting means 5, and if there is no error in the response signal, inputs it to the transmission buffer 3 to check whether it is a reception confirmation signal or a retransmission request signal. . When a reception acknowledgment signal is received from the reception side, the corresponding ARQ frame in the transmission buffer 3 is discarded, and a free ARQ frame is reserved for accumulation. Further, when a retransmission request is issued from the receiving side, instead of transmitting a new ARQ frame to the communication path, the ARQ frame in the transmission buffer 3 for which the retransmission is requested is transmitted to the communication path. If there is a communication path error in the response signal, the response signal is discarded because it cannot be confirmed whether it is a reception confirmation signal or a retransmission request signal.

The receiving side accumulates the ARQ frame to which the number received from the communication channel is assigned in the receiving buffer 8 at every fixed length. The receiving buffer 8 outputs the compressed image data to the image decoding means 10 according to the following control method.

FIG. 4 is a flowchart showing a method of controlling the reception buffer 8 according to a fifth embodiment of the present invention.

When the header of the image frame k-1 is correctly transmitted to the receiving side, the ARQ frame number _Nk including the first bit of the header of the image frame k is known from the header.

The receiving side waits until all of the maximum ARQ frame numbers N _k +1 that may include the last bit of the header from the first ARQ frame N _k are confirmed. Here, l is a fixed value. Then, when all the ARQ frames including the header can be confirmed on the receiving side, the ARQ frame is transmitted to the error block detecting means 9.

In the error block detecting means 9, the ARQ
The number of bits of each block from the frame and the number of remaining ARQ frames L _k from the next image frame are decoded, and each is notified to the reception buffer 8. On the receiving side, the number of ARQ frames required to receive all the ARQ frames including the information of the image frame _k from L _k is set as T _k . Here, when _Tk is set too large, the effect of reducing delay does not appear. Also, T _k
Is set to be small, there is a possibility that there is an ARQ frame that is too late even if there is no delay due to retransmission.
Therefore, when setting T _k , the remaining block including the image frame k is calculated, and an ARQ that has issued a retransmission request before that block may arrive before all the remaining blocks arrive without retransmission. Set by adding the number of frames. This is an embodiment of the method shown in the present invention 6.

At this point, if all the ARQ frames up to N _k + L _k -1 have arrived, all the data of the image frame k has arrived, so that the data is transmitted to the image decoding means 10. Otherwise, when T _k ARQ frames arrive at the receiving side, it is confirmed that all ARQ frames including image frames up to k−1 have been received without error, and the ARQ of the number N _k has been confirmed. The data of the image frame k is extracted from the last ARQ frame confirmed on the receiving side from the frame, and transmitted to the image decoding means 10. k-
If there is an ARQ frame that has not been received among ARQ frames including up to 1 image frame,
Image decoding means 10 until the ARQ frame arrives
, The data of the image frame k is not sent, and _Tk is incremented by 1 to wait for the next ARQ frame. And all k-1
The data of the image frame k is sent to the image decoding means 10 when the image frames up to are completed.

By this processing, if a bit error has been detected on the receiving side up to that point and all the information of some image frames k has not been collected on the receiving side, the bit number of the block written in the header is calculated as follows. A that didn't arrive
The block number included in the RQ frame will be known on the receiving side. Therefore, the reception buffer 8 arranges the ARQ frames from the number N _k in numerical order, replaces all unreceived ARQ frames with data of 0, and transmits the data to the image decoding unit 10, and simultaneously transmits the missing block number.

The image decoding means 10 decodes the ARQ frame received correctly and the bit number of the block written in the header for each block, and the missing blocks are correctly decoded in the frame memory by the processing described later. Is replaced with the image frame k-1, and a reproduced image is output. Then, the output reproduced image is stored in the frame memory. This is the fourth embodiment of the present invention.

In addition, since an unreceived ARQ frame is requested to be retransmitted by the same processing as that of normal ARQ,
When the information of the image frame k arrives completely error-free, the data of the image frame is decoded by the image decoding means 1.
0, and replaces the image frame k, which is present in the frame memory of the image decoding means 10 and is partially replaced by the previous image frame k-1, with a correct reproduced image. This is the third embodiment of the present invention.

The compressed image data stored in the receiving buffer 8 is all discarded after being retransmitted to the complete image decoding means 10, and is reserved as an empty area for storing new data.

According to this method, the next image frame k + 1
At the time of decoding, the correct information is sent on the receiving side for all image frames before that, so that no error exists in the reference image frame k. Therefore, inter-frame error propagation does not occur. Further, by setting T _k sufficiently fast, a delay corresponding to the response delay time that occurs when normal ARQ is performed does not occur. When header retransmission occurs, T
_Although there is a possibility that _k may be set erroneously, data is sent to the image decoding means 10 when all ARQ frames including the image frame k have been delivered to the receiving side, similarly to normal ARQ. Does not increase the delay more than ARQ.

According to this method, error propagation in a frame can be performed by the block synchronization information without increasing the delay time.
It is considered that ARQ can prevent inter-frame error propagation.

[Computer simulation results]
The following shows the specifications of the computer simulation.

[0063]

[Table 1]

In the following discussion, the “conventional method” is a method of retransmitting all ARQ frames for which errors have been detected on the receiving side using ARQ. The transmitting side adjusts the quantization step and the frame skip according to the channel quality to control the generated code amount.

First, the effect of the delay reduction according to the present invention will be described. FIG. 5 shows an example of a delay time of each image frame with respect to a time change when the CNR is 25 dB. From this, it can be seen that in the conventional method, image frames may be greatly delayed due to retransmission, but this can be avoided by the proposed method according to the present invention.

Next, the effect of error resilience according to the present invention will be described. FIG. 6 shows an SNR characteristic of an image with respect to a time change measured simultaneously with the above. The number of blocks to be transmitted and transmitted as additional information is one. Since the additional information is transmitted, the SNR of the present invention is lower than that of the conventional system when there is no communication path error. As can be seen from the comparison with FIG. 5, when the delay is avoided, there is a block that does not arrive at the image decoding unit, so that the SNR is degraded. This is distortion caused by replacement with the previous image frame, and the visual deterioration is not so large. It can be seen that distortion can be recovered without error propagation in the next frame.

[0067]

As described above, according to the present invention, even if an ARQ frame that does not arrive at the receiving side exists,
This is known because the number of blocks is transmitted to the receiving side. Therefore, no error propagates between the blocks. This is the effect of the present invention 1.

Further, since the blocks included in the ARQ frame that has not arrived are replaced with the previous image frame, distortion occurring in the image is minimized. This is Invention 4
The effect is.

Further, when there is no communication path error, the number of bits of the coded block becomes redundant, but this information can be reduced by collecting the number of blocks.
This is the effect of the present invention 2.

Further, when decoding the image frame k + 1 next to a certain image frame k, since the correct information has been sent on the receiving side for all the image frames before k, there is no error in the reference image frame k. Therefore, inter-frame error propagation does not occur. This is the effect of the third invention.

When the header is retransmitted, the number of ARQ frames T _{k in} which the reception buffer can wait may be erroneously set.
Since data is sent to the image decoding means when all ARQ frames including the image frame k have been delivered to the receiving side, the delay does not increase more than in the normal ARQ.
This is the effect of the fifth invention.

Since the above T _k can be set appropriately,
There is no delay corresponding to the response delay time that occurs when normal ARQ is performed. This is the effect of the present invention 6.

According to the present invention, the intra-frame error propagation can be controlled by the block synchronization information without increasing the delay time.
It is considered that RQ can prevent inter-frame error propagation.

[Brief description of the drawings]

FIG. 1 is a block diagram for describing an embodiment of a configuration and a method of an image communication device according to the present invention.

FIG. 2 is a diagram showing a specific example of an ARQ frame according to the present invention.

FIG. 3 is a diagram showing a specific example of an image frame according to the present invention.

FIG. 4 is a flowchart illustrating an embodiment of a control method in a reception buffer according to the present invention.

FIG. 5 is a diagram showing an example of the effect of the delay reduction of the present invention.

FIG. 6 is a diagram showing an example of the effect of error resilience of the present invention.

FIG. 7 is a block diagram for explaining a problem of a conventional image communication method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Image coding means 2 ... Frame construction means 3 ... Transmission buffer 4 ... Block bit number coding means 5 ... Communication path error detection means 6 ... Frame construction means 7 ... Communication path error detection means 8 ... Receiving buffer 9 ... Error Block detecting means 10 ... Image decoding means 100 ... Sending side 200 ... Receiving side

Claims (10)

[Claims] When transmitting coded image data using a two-way communication system in which a communication path error occurs, when an error exists in received image data, the error is detected on a receiving side, In an image communication method for requesting the transmitting side to retransmit image data in which an error has been detected, the transmitting side divides data obtained by coding the number of bits per block into image data that has been divided into blocks and further subjected to variable length coding. At the receiving end, based on the delimiter of the block of the received image data, which is known from the number of bits for each block,
Decoding the received image data. 2. The image communication according to claim 1, wherein the transmission side adds, to the image data, data obtained by encoding the number of bits obtained by combining a plurality of blocks with respect to the number of bits for each block. Method. 3. When transmitting coded image data using a two-way communication system in which a communication path error occurs, when an error exists in the received image data, the error is detected on the receiving side. In an image communication method for requesting the transmitting side to retransmit image data in which an error has been detected, when the image quality has deteriorated due to a communication path error in a reference image held by the receiving side, an image of a block received without error by retransmission control. When the data arrives, the image data is replaced with a reference image in which the image quality has deteriorated, and the reference image is reconstructed. 4. When transmitting coded image data using a two-way communication system in which a communication path error occurs, if an error exists in the received image data, the error is detected on the receiving side. In the image communication method for requesting the transmitting side to retransmit the image data in which an error is detected, when decoding the image data of the block including the detected channel error, the receiving side replaces the image data with the block of the previous image frame. An image communication method characterized by reproducing. 5. When transmitting coded image data using a two-way communication system in which a communication path error occurs, when an error exists in the received image data, the error is detected on the receiving side. In an image communication method for requesting a transmission side to retransmit image data in which an error has been detected, the reception buffer sets k as a target image frame number,
N _k is the youngest ARQ frame number including image frame k, L _k is the number of ARQ frames including image frame k,
Receive T _k buffers the number of ARQ frames to wait before passing to the picture decoding unit data as an image frame k, l
Where A is the number of image frames including the header and the additional information, ARQ frame number N _k is set, and procedure 1 for waiting for ARQ frames from frame numbers N _k to N _k + l to correctly arrive at the reception buffer; _k , setting procedure 2 to check whether all ARQ frames up to N _k + L _k -1 have arrived correctly, and the number of ARQ frames T _k that the reception buffer can wait for
And waiting for T _k ARQ frames to arrive from that point in time, and ARQ frame including the previous image frame k−1, that is, N
ARQ things up again T _k as T _k = T _k +1 until confirm the ARQ frame of up to _k-1 has arrived all
Step 4 of waiting for a frame, transmitting the ARQ frame of the image frame k arriving at that time to the image decoding means, and further, ARQ frames from ARQ frame numbers N _k to N _k + L _k -1 arrive correctly. And a procedure 6 for transmitting ARQ frames from frame numbers N _k to N _k + L _k -1 to the image decoding means and ending the procedure for the image frame k. ARQ from frame numbers N _k to N _k +1
If the frame arrives at the receive buffer correctly,
In step 2, if it has been confirmed that all ARQ frames up to N _k + L _k -1 have arrived correctly, the procedure proceeds to step 6,
If not, proceed to step 3. In step 3, if T _k ARQ frames arrive after setting T _k , proceed to step 4. In step 4, all ARQ frames up to N _k-1 have arrived. If it is confirmed that it is, go to Step 5, in step 5, if the ARQ frames from N _k to N _k + L _k -1 was confirmed that have arrived all, the process proceeds to step 6, An image communication method characterized by performing the following operation. 6. In the reception buffer, when setting the value of T _k in step 3, the remaining unreceived ARs up to L _k
To the number of Q frames, add the number of ARQ frames for which a retransmission request has already been transmitted to the transmitting side and the retransmitted ARQ frame can arrive without a large delay, and the value is _defined as T _k The image communication method according to claim 5, wherein the setting is performed. 7. When transmitting coded image data using a two-way communication system in which a communication path error occurs, when an error exists in the received image data, the error is detected on the receiving side. In an image communication apparatus that requests the transmitting side to retransmit image data in which an error is detected, the transmitting side encodes the number of bits per block of the image data divided into blocks and further subjected to variable length coding. Means, and the encoded data of the number of bits per block,
Means for adding to the image data, the receiving side, based on the delimiter of the block of the received image data that is known by the number of bits for each block,
Means for detecting a block in which a channel error exists, and means for decoding the received image data based on a block delimiter of the received image data that is known from the number of bits for each block. An image communication device, comprising: 8. The method according to claim 7, wherein said means for encoding the number of bits for each block encodes the number of bits obtained by combining a plurality of blocks with respect to the number of bits for each block. An image communication device according to claim 1. 9. A method for decoding received image data, comprising: when image quality deterioration due to a communication path error occurs in a reference image, when image data of a block received without error by retransmission control arrives, The image communication device according to claim 7, wherein the image communication is replaced with a reference image in which the image quality has deteriorated, and the reference image is reconfigured. 10. The decoding means for decoding the received image data, when decoding the image data of the block containing the detected channel error, replaces the block with the block of the previous image frame and reproduces it. The image communication device according to claim 7 or 8, wherein: