Transmission monitoring device and transmission device

Abstract

This transmission device transmits compressed image data in a step S105. In a step S106, the transmission device stores the transmitted compressed image in a transmission monitoring device at a rate of one frame to the tens of frames of the transmitted compressed image. A reception device transmits a compressed image having the same time stamp as the compressed image stored in the transmission monitoring device among the received compressed images to the transmission device in a step S109. In a step S111, the transmission monitoring device compares the compressed image sent from the reception device with the stored compressed image. By this comparison, the transmission monitoring device can detect the reception state of video data in the reception device.

Description

Transmission monitoring device and transmitting device

[0001] The present invention relates to wireless transmission of video data.

[0002] In recent years, video data has increasingly been transmitted via wireless communication. Unlike wired communication, wireless communication is easily affected by the surrounding environment. For example, if an object is present in the transmission path, the object may block the radio waves, making wireless communication impossible. Wireless communication is also easily affected by external noise.

[0003] When transmitting video data via wireless communication, first, several dozen images are created per second from the video data. Then, one image data piece (this "one image data piece" is called "one frame") is converted into digital data, which is then divided into small units called packets for transmission. If the receiving side is unable to receive a packet sent from the sending side for some reason, it is said that "a packet loss has occurred." "Packet loss" is sometimes abbreviated to "packer loss."

[0004] Protocols for transmitting packets include TCP (Transmission Control Protocol) and RTP (Real-time Transport Protocol). TCP exchanges control information between the sender and receiver, enabling functions such as "acknowledgment" to confirm whether a transmitted packet has arrived at the receiver, "resend control" in which the receiver detects a missing or damaged packet during transmission and requests the sender to resend it, and "sequence control" in which a sequence number is assigned to packets at the time of transmission, so that packets can be reordered at the receiver in their original order even if the order of arrival of the packets is changed. Therefore, transmission using TCP is more reliable than RTP, which will be explained later. However, TCP has the disadvantage that it is difficult to achieve immediacy or high speed due to the complex transmission control.

[0005] RTP allows for high-speed transmission because it omits the above-mentioned functions of "acknowledgement," "retransmission control," and "sequence control." RTP also provides a transmission time (timestamp) field in the header, so the receiving side can arrange multiple packets in chronological order by looking at the timestamp.

[0006] When transmitting images from a surveillance camera system or live video, real-time transmission is required, and RTP is often used. However, as mentioned above, RTP omits the functions of "acknowledgement," "retransmission control," and "sequence control," which can result in packet loss. In the case of video data, packet loss can cause block noise over a wide area on the screen or the entire screen to turn gray.

[0007] In particular, if the transmitting side is unaware of the condition on the receiving side even though such a phenomenon is occurring on the receiving side, the poor transmission condition will continue, causing problems.

[0008] To solve the problem that when transmitting video data by RTP, packet loss occurs, and even if a phenomenon occurs on the receiving side where block noise occurs over a wide area on the screen or the entire screen turns gray, the transmitting side cannot detect the reception status on the receiving side.

[0009] An object of the present invention is to provide a device that can detect the reception state on the transmitting side in the transmission of video data that employs RTP.

[0010] The inventors discovered that the transmitting side can detect the reception status of image data by comparing the image data received by the receiving side with the image data sent from the transmitting side, and thus completed the present invention.

[0011] (1) A first transmission monitoring device according to the present invention is a transmission monitoring device for determining degradation of a received compressed image received by a receiving device in a communication system in which a transmitted compressed image transmitted by a transmitting device is received by a receiving device, and includes a transmitted compressed image storage unit for storing the transmitted compressed image transmitted by the transmitting device, a compressed image comparison unit for comparing the received compressed image received by the receiving device with the transmitted compressed image stored in the transmitted compressed image storage unit, and an instruction creation unit for creating instructions for the transmitting device based on the comparison results of the compressed image comparison unit.

[0012] (2) The transmission compressed image storage unit of the first transmission monitoring device may store the transmission compressed image transmitted by the transmitting device at a rate of once per n frames (n is any natural number).

[0013] (3) When the comparison result of the compressed image comparison unit indicates deterioration in image quality, the instruction creation unit of the first transmission monitoring device may compare the actual data transfer volume with an upper limit calculated from the network bandwidth, and when the actual data transfer volume exceeds the upper limit calculated from the network bandwidth, create an instruction to reduce the data transfer volume.

[0014] (4) If the comparison result of the compressed image comparison unit does not improve even when the data transfer rate is reduced, the instruction creation unit of the first transmission monitoring device may check the surrounding wireless usage status and create an instruction to change the bandwidth used for wireless transmission to an available bandwidth.

[0015] (5) If the comparison result of the compressed image comparison unit does not improve even when the bandwidth used for wireless transmission is changed to an available bandwidth, the instruction creation unit of the first transmission monitoring device may create an instruction to shorten the interval between I frames (eye frames) to be transmitted.

[0016] (6) If the comparison result of the compressed image comparison unit is not improved even when the interval between transmitted I frames (eye frames) is shortened, the instruction creation unit of the first transmission monitoring device may create countermeasures to move the location of the wireless communication device used for wireless transmission or to change the method of wireless transmission.

[0017] (7) A first transmitting device according to the present invention includes a transmitted compressed image storage unit that stores a transmitted compressed image transmitted by the transmitting device, a compressed image comparison unit that compares a received compressed image received by a receiving device with the transmitted compressed image stored in the transmitted compressed image storage unit, and an instruction creation unit that creates instructions to improve degradation of the received compressed image based on the comparison result of the compressed image comparison unit.

[0018] (8) The transmission compressed image storage unit of the first transmission device may store the transmission compressed image transmitted by the transmission device at a rate of once per n frames (n is any natural number).

[0019] (9) When the comparison result of the compressed image comparison unit indicates deterioration in image quality, the instruction creation unit of the first transmitting device may compare the actual data transfer volume with an upper limit calculated from the network bandwidth, and when the actual data transfer volume exceeds the upper limit calculated from the network bandwidth, create an instruction to reduce the data transfer volume.

[0020] (10) If the comparison result of the compressed image comparison unit does not improve even when the data transfer rate is reduced, the instruction creation unit of the first transmission device may check the surrounding wireless usage status and create an instruction to change the bandwidth used for wireless transmission to an available bandwidth.

[0021] (11) When the comparison result of the compressed image comparison unit is not improved even after changing the bandwidth used for wireless transmission to an available bandwidth, the instruction creation unit of the first transmission device may create an instruction to shorten the interval between I frames (eye frames) to be transmitted.

[0022] (12) If the comparison result of the compressed image comparison unit is not improved even when the interval between transmitted I frames (eye frames) is shortened, the instruction creation unit of the first transmission device may create countermeasures to move the location of a wireless communication device used for wireless transmission or to change the method of wireless transmission.

[0023] According to the present invention, the receiving state of image data can be grasped on the transmitting side.

[0024] FIG. 1 is a diagram showing a communication system according to a first embodiment of the present invention. FIG. 2 is a diagram showing a hardware configuration of a transmitting device according to a first embodiment of the present invention. FIG. 3 is a diagram showing a hardware configuration of a receiving device according to a first embodiment of the present invention. FIG. 4 is a diagram showing a hardware configuration of a transmission monitoring device according to a first embodiment of the present invention. FIG. 5 is a functional block diagram of a transmission monitoring device according to a first embodiment of the present invention. FIG. 6 is a diagram showing a communication sequence in a first embodiment of the present invention. FIG. 7 is a diagram showing a hardware configuration of a transmitting device according to a second embodiment of the present invention. FIG. 8 is a functional block diagram of a transmitting device according to a second embodiment of the present invention.

[0025] [First Embodiment] A first embodiment of the present invention will be described below with reference to the drawings. Fig. 1 is a diagram showing a communication system 1000 according to the first embodiment. A transmitting device 1001 includes an imaging unit, and digitizes video data captured by the imaging unit, encodes it, packetizes it, and transmits it. Transmission is performed wirelessly between wireless communication device A and wireless communication device B. The video data received by wireless communication device B is transmitted via the Internet 1005 and received by a receiving device 1006. In the receiving device 1006, the video undergoes decoding and digital-to-analog conversion, and the reproduced video is displayed on the display unit of the receiving device 1006.

[0026] In the communication system 1000 according to the first embodiment, compressed image data received by the receiving device 1006 is sent back to the transmitting device 1001 at a rate of approximately one frame per several tens of frames. However, "one frame per several tens of frames" is merely an example, and the rate may be "one frame per any natural number of frames." Here, the compressed image sent back by the receiving device 1006 is referred to as a "received compressed image." The transmission monitoring device 1002 compares the returned received compressed image data with the transmitted compressed image data corresponding to the returned received compressed image data. Here, "transmitted compressed image data corresponding to the returned received compressed image data" means "transmitted compressed image data having the same timestamp as the timestamp of the returned received compressed image data." Furthermore, the transmitted compressed image corresponding to the returned received compressed image data is referred to as a "transmitted compressed image."

[0027] The comparison result can determine the degree of degradation of the compressed image received by the receiving device 1006. In order to perform the above comparison, the transmitting device 1001 stores compressed images to be transmitted in advance in the transmission monitoring device 1002 at a rate of about one image per several tens of frames.

[0028] Based on the results of the comparison, the transmission monitoring device 1002 determines measures to reduce the degradation of the received compressed image, and notifies the transmitting device 1001 of the determined measures. Alternatively, the transmission monitoring device 1002 outputs the measures to reduce the degradation of the received compressed image to an output unit of the transmission monitoring device 1002.

[0029] 2 is a diagram showing the hardware configuration of the transmitting device 1001 according to the first embodiment of the present invention. The transmitting device 1001 includes an imaging unit 201, a CPU 202, a RAM 203, a ROM 204, an encoding unit 205, a communication unit 206, and a bus 208. The imaging unit 201 converts incident light into an electrical signal to acquire frame data (image data). The imaging unit 201 includes a lens, a photoelectric conversion element that converts light that has passed through the lens into an electrical signal, and the like. The CPU 202 controls the overall operation of the transmitting device 1001 and performs calculations. The CPU 202 executes application programs, an operating system (OS), control programs, and the like, and temporarily stores information, files, and the like required for program execution in the RAM 203.

[0030] The RAM (Random Access Memory) 203 is a random access memory used as a work area for calculations and a buffer. The ROM (Read Only Memory) 204 is a read-only memory that stores basic programs, basic data, etc. The encoding unit 205 performs a predetermined encoding process on image data converted into an electrical signal by the imaging unit 201. During the encoding process, the image is also compressed. The encoding unit 205 may be implemented by a GPU (Graphics Processing Unit). Data transmission between the transmitting device 1001 and the receiving device 1006 involves communication of encoded image data. The communication unit 206 is used to send and receive frame data to and from external devices. The bus 208 is used to send and receive data between each of the blocks 201 to 206.

[0031] 3 is a diagram showing the hardware configuration of the receiving device 1006 according to the first embodiment of the present invention. The receiving device 1006 includes a CPU 301, a RAM 302, an OM 303, a decoding unit 304, a communication unit 305, a display unit 306, and a bus 307. The CPU 301 controls the overall operation of the receiving device 1006 and performs calculations. The CPU 301 executes application programs, an operating system (OS), control programs, etc., and temporarily stores information, files, etc. required for executing the programs in the RAM 302.

[0032] The RAM 302 is a random access memory used as a work area for calculations and as a buffer. The ROM 303 is a read-only memory that stores basic programs, basic data, etc. The decoding unit 304 performs processing to decode received coded data. The communication unit 305 communicates with the outside. Image data is received via the communication unit 305. The display unit 306 displays received videos, etc. The bus 307 is a bus for transmitting and receiving data between each of the blocks 301 to 306.

[0033] Fig. 4 is a diagram showing the hardware configuration of a transmission monitoring device 1002 according to the first embodiment of the present invention. In the transmission monitoring device 1002 shown in Fig. 4, a CPU 401 executes processes for implementing various functions of the transmission monitoring device 1002 in accordance with a program stored in a ROM 402 or a program loaded into a RAM 403. The RAM 403 also stores data necessary for the CPU 401 to execute various processes as appropriate. The CPU 401, ROM 402, and RAM 403 are interconnected via a bus 408. An input / output interface 409 is also connected to this bus 408.

[0034] The input / output interface 409 is connected to an input unit 404, an output unit 405, a storage unit 406, and a communication unit 407. The input unit 404 is composed of keys, buttons, a touch panel, etc. The output unit 405 is composed of a display, a speaker, etc. The storage unit 406 is composed of a hard disk, etc. The communication unit 407 is composed of a communication module that performs wired communication, etc.

[0035] Fig. 5 is a functional block diagram of a transmission monitoring device 1002 according to the first embodiment of the present invention. The CPU 401 of the transmission monitoring device 1002 executes an operating program stored in the ROM 402 to realize the functions of a transmitted compressed image storage unit 501, a compressed image comparison unit 502, a bandwidth usage status detection unit 503, and an instruction creation unit 504 shown in Fig. 5. The transmitted compressed image storage unit 501 stores transmitted compressed images transmitted by the transmitting device 1001 at a rate of one frame per n frames (n is any natural number).

[0036] The compressed image comparison unit 502 compares the transmitted compressed image stored in the transmitted compressed image storage unit 501 with the received compressed image corresponding to the transmitted compressed image and returned from the receiving device 1006. To compare the images, OpenCV is used to calculate histograms for each of the transmitted compressed image and the received compressed image, and the ratio of the two histograms is calculated. If the ratio is "1," the image quality of the transmitted compressed image and the received compressed image is equivalent. If the ratio is close to "1," the image quality is determined to be not significantly different. If the ratio is below a predetermined threshold (e.g., 0.8), the image quality is determined to be significantly degraded. The bandwidth usage status detection unit 503 detects the usage status of multiple bandwidths. One example of the bandwidth usage status detection unit 503 can be configured by providing a receiver with a variable reception frequency, sequentially changing the reception frequency, and checking for the presence of radio waves in each bandwidth.

[0037] The instruction creation unit 504 creates instructions for the transmitting device 1001 based on the comparison result from the compressed image comparison unit 502. If the comparison result from the compressed image comparison unit 502 indicates a significant deterioration in image quality, the instruction creation unit 504 calculates an upper limit for the amount of data transfer from the bandwidth of the network over which the video is transmitted. Then, the instruction creation unit 504 compares the actual amount of data transfer with the calculated upper limit, and if the actual amount of data transfer exceeds the calculated upper limit, creates an instruction to reduce the amount of data transfer and transmits it to the transmitting device 1001.

[0038] Furthermore, if the comparison results in the compressed image comparison unit 502 do not improve even when the data transfer volume is reduced, the instruction creation unit 504 checks the surrounding wireless usage status based on the detection results of the bandwidth usage status detection unit 503, and creates and transmits an instruction to the transmitting device 1001 to change to an available bandwidth.

[0039] Furthermore, if the comparison result in the compressed image comparison unit 502 does not improve even after changing to an available band, it is possible that the wireless communication device A 1003 has moved and is being affected by an obstruction, so the instruction creation unit 504 creates and transmits an instruction to the transmitting device 1001 to shorten the interval between I frames (eye frames) to be transmitted. Here, an I frame (eye frame) is a frame that holds all of the information of a frame when compressing a moving image using inter-frame prediction.

[0040] Furthermore, if the comparison results in the compressed image comparison unit 502 do not improve even when the interval between transmitted I frames is shortened, countermeasures such as moving the location of the wireless communication device A1003 or changing the wireless method are created and displayed on the display of the output unit 405.

[0041] 6 is a diagram showing a communication sequence in the first embodiment of the present invention. In step S101, the receiving device 1006 sets various parameters for compression and transmission. Specific examples of the various parameters include the resolution of the video to be displayed, the interval between I-frame insertions, and the transmission bit rate.

[0042] In step S102, the receiving device 1006 transmits the various set parameters to the transmitting device 1001. In step S103, the transmitting device 1001 sets the received various parameters in its own device. In step S104, the transmitting device 1001 creates transmission data based on the video captured by the imaging unit 201.

[0043] In step S105, the transmitting device 1001 transmits the created transmission data. In step S106, the transmitting device 1001 transmits the transmission compressed image to the transmission monitoring device 1002. In step S107, the transmission monitoring device 1002 stores the transmission compressed image in the transmission compressed image storage unit 501.

[0044] In step S108, the receiving device 1006 displays the transmitted video on the display unit 306 and stores the received compressed image. In step S109, the receiving device 1006 transmits the stored received compressed image to the transmitting device 1001.

[0045] In step S110, the transmitting device 1001 transfers the received compressed image to the transmission monitoring device 1002. In step S111, the compressed image comparison unit 502 of the transmission monitoring device 1002 compares the received compressed image with the transmitted compressed image. The instruction creation unit 504 of the transmission monitoring device 1002 creates an instruction based on the comparison result. In step S112, the transmission monitoring device 1002 transmits the created instruction to the transmitting device 1001.

[0046] Information regarding the instruction executed by the transmitting device 1001 may be transmitted from the transmitting device 1001 to the transmission monitoring device 1002 .

[0047] Second Embodiment In the first embodiment of the present invention, the transmission monitoring device 1002 has been described as a device separate from the transmitting device 1001, but the functions of the transmission monitoring device 1002 may be incorporated into the transmitting device 1001. A transmitting device incorporating the functions of this transmission monitoring device will be described as a second embodiment of the present invention.

[0048] 7 is a diagram showing the hardware configuration of a transmitting device 1001A according to the second embodiment of the present invention. The imaging unit 701, CPU 702, RAM 703, ROM 704, encoding unit 705, communication unit 706, and bus 708 in Fig. 7 are the same as the imaging unit 201, CPU 202, RAM 203, ROM 204, encoding unit 205, communication unit 206, and bus 207 in Fig. 2, respectively, and therefore description thereof will be omitted. The output unit 707 is composed of a display, a speaker, etc.

[0049] Fig. 8 is a functional block diagram of a transmitting device 1001A according to the second embodiment of the present invention. The CPU 702 of the transmitting device 1001A shown in Fig. 7 executes an operating program stored in the ROM 704 to realize the functions of a transmitted compressed image storage unit 801, a compressed image comparison unit 802, a bandwidth usage status detection unit 803, an instruction creation unit 804, and an instruction transmission unit 805 shown in Fig. 8.

[0050] The transmission compressed image storage unit 801, compressed image comparison unit 802, and bandwidth usage status detection unit 803 in Figure 8 are similar to the transmission compressed image storage unit 501, compressed image comparison unit 502, and bandwidth usage status detection unit 503 in Figure 5, so their description will be omitted.

[0051] The instruction creation unit 804 creates instructions for improving the degradation of the received compressed image based on the comparison result in the compressed image comparison unit 802. The instruction sending unit 805 sends the instructions created by the instruction creation unit 804 to appropriate units such as the encoding unit 705 and wireless communication device A 1003.

[0052] If the comparison result from the compressed image comparison unit 802 indicates a significant deterioration in image quality, the instruction creation unit 804 calculates an upper limit for the data transfer amount from the bandwidth of the network over which the video is transmitted.The instruction creation unit 804 then compares the actual data transfer amount with the calculated upper limit, and if the actual data transfer amount exceeds the calculated upper limit, creates an instruction to reduce the data transfer amount.The instruction sending unit 805 sends the created instruction to the encoding unit 705.

[0053] Furthermore, if the comparison result in the compressed image comparison unit 802 does not improve even when the data transfer rate is reduced, the instruction creation unit 804 checks the surrounding wireless usage status based on the detection result of the bandwidth usage status detection unit 803, and creates an instruction to change to an available bandwidth. The instruction sending unit 805 sends the created instruction to the wireless communication device A 1003.

[0054] Furthermore, if the comparison result in the compressed image comparison unit 802 does not improve even after changing to an available band, it is possible that the wireless communication device A 1003 has moved and is being affected by an obstruction, so the instruction creation unit 804 creates an instruction to shorten the interval between I frames to be transmitted.The instruction sending unit 805 then sends the created instruction to the encoding unit 705.

[0055] Furthermore, if the comparison result in the compressed image comparison unit 802 does not improve even when the interval between transmitted I frames is shortened, the instruction creation unit 804 creates countermeasures such as moving the location of the wireless communication device A 1003 or changing the wireless system. The instruction sending unit 805 displays the created instructions on the display of the output unit 707.

[0056] According to the first and second embodiments of the present invention, the transmission monitoring device or the transmitting device on the transmitting side can grasp the reception status of the receiving device. Furthermore, if it is determined that the reception status of the receiving device is poor, various measures can be taken on the transmitting side.

[0057] 201 Imaging unit 202 CPU 203 RAM 204 ROM 205 Encoding unit 206 Communication unit 207 Bus 301 CPU 302 RAM 303 ROM 304 Decoding unit 305 Communication unit 306 Display unit 307 Bus 401 CPU 402 ROM 403 RAM 404 Input unit 405 Output unit 406 Storage unit 407 Communication unit 408 Bus 409 Input / output interface 501 Transmission compressed image storage unit 502 Compressed image comparison unit 503 Bandwidth usage status detection unit 504 Instruction creation unit 701 Imaging unit 702 CPU 703 RAM 704 ROM 705 Encoding unit 706 Communication unit 707 Output unit 708 Bus 801: Transmission compressed image storage unit 802: Compressed image comparison unit 803: Bandwidth usage status detection unit 804: Instruction creation unit 805: Instruction transmission unit 1001: Transmission device of first embodiment 1001A: Transmission device of second embodiment 1002: Transmission monitoring device 1003: Wireless communication device A 1004: Wireless communication device B 1005: Internet 1006: Receiving device

Claims

1. A transmission monitoring device for determining degradation of a received compressed image received by a receiving device in a communication system in which a transmitted compressed image transmitted by a transmitting device is received by a receiving device, the transmission monitoring device comprising: a transmitted compressed image storage unit for storing the transmitted compressed image transmitted by the transmitting device; a compressed image comparison unit for comparing the received compressed image received by the receiving device with the transmitted compressed image stored in the transmitted compressed image storage unit; and an instruction creation unit for creating instructions to the transmitting device based on the comparison results of the compressed image comparison unit.

2. A transmission monitoring device as described in claim 1, wherein said transmission compressed image storage unit stores a transmission compressed image transmitted by said transmitting device once every n frames (n is any natural number).

3. The transmission monitoring device of claim 1, wherein the instruction creation unit, when the comparison result of the compressed image comparison unit indicates degradation of image quality, compares the actual data transfer volume with an upper limit calculated from the network bandwidth, and when the actual data transfer volume exceeds the upper limit calculated from the network bandwidth, creates an instruction to reduce the data transfer volume.

4. The transmission monitoring device of claim 3, wherein the instruction creation unit, if the comparison result of the compressed image comparison unit does not improve even when the data transfer volume is reduced, checks the surrounding wireless usage status and creates an instruction to change the bandwidth used for wireless transmission to an available bandwidth.

5. The transmission monitoring device of claim 4, wherein said instruction creation unit creates an instruction to shorten the interval between transmitted I frames (eye frames) if the comparison result of said compressed image comparison unit does not improve even when the bandwidth used for wireless transmission is changed to an available bandwidth.

6. The transmission monitoring device of claim 5, wherein the instruction creation unit creates countermeasures to move the location of the wireless communication device used for wireless transmission or to change the method of wireless transmission if the comparison result of the compressed image comparison unit does not improve even when the interval between transmitted I frames (eye frames) is shortened.

7. A transmitting device comprising: a transmitted compressed image storage unit that stores a transmitted compressed image transmitted by the transmitting device; a compressed image comparison unit that compares a received compressed image received by a receiving device with the transmitted compressed image stored in the transmitted compressed image storage unit; and an instruction creation unit that creates instructions to improve degradation of the received compressed image based on the comparison result of the compressed image comparison unit.

8. The transmitting device according to claim 7, wherein said transmission compressed image storage unit stores a transmission compressed image once every n frames (n is any natural number) for the transmission compressed image transmitted by said transmitting device.

9. The transmitting device according to claim 7, wherein the instruction creation unit, when the comparison result of the compressed image comparison unit indicates degradation in image quality, compares the actual data transfer volume with an upper limit calculated from the network bandwidth, and, when the actual data transfer volume exceeds the upper limit calculated from the network bandwidth, creates an instruction to reduce the data transfer volume.

10. A transmitting device as described in claim 9, wherein the instruction creation unit, if the comparison result of the compressed image comparison unit does not improve even when the data transfer volume is reduced, checks the surrounding wireless usage status and creates an instruction to change the bandwidth used for wireless transmission to an available bandwidth.

11. The transmitting device according to claim 10, wherein the instruction creating unit creates an instruction to shorten the interval between transmitted I frames (I frames) if the comparison result of the compressed image comparison unit does not improve even when the band used for wireless transmission is changed to an available band.

12. The transmitting device described in claim 11, wherein the instruction creation unit creates countermeasures to move the location of the wireless communication device used for wireless transmission or change the method of wireless transmission if the comparison result of the compressed image comparison unit does not improve even when the interval between transmitted I frames (eye frames) is shortened.

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