At present, IPTV is a hot technology that has received much attention. At home and abroad, a large number of IPTV trial networks and commercial trial operation networks have been built. There are many users everywhere, and it can be said that IPTV technology is in full swing. The various services provided by IPTV are rich and colorful, especially the video service meets various applications of people, and will also become the revenue growth point of operators.
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At present, the technical standards for IPTV in China have not yet been determined, and the products of various manufacturers still lack rigorous testing and wide-ranging applications in all aspects. The network supporting IPTV services is mainly IPv4, and the access methods are mainly ADSL, Ethernet, CableModem, etc. Tests by various research institutes and operators' experimental networks show that with the increase in the number of IPTV users, the contradiction between the support of IPv4 networks, especially the quality of service (QoS) at the access layer, and the requirements of users for service quality is becoming increasingly prominent. These contradictions directly indicate that the image quality is degraded due to network performance. Therefore, the transmission quality of video streams in the IP environment has become a key issue affecting the development of IPTV services.
For IPTV operators, when users are suspicious of video quality, how to quickly find fault points and eliminate faults, and then build a monitoring system that can facilitate IPTV service management becomes a key technical issue for large-scale operation of IPTV in the future.
Video image quality test
Video quality testing for video communication systems has always been a difficult point for people to study. Video quality testing for digital video codecs is usually measured by PSNR values ​​and is commonly used for static image quality analysis. However, the human visual system is a complex system. The subjective perception of video quality cannot be expressed by PSNR alone, because observers are also affected by other factors such as environment, position, scene, and likes and dislikes when observing video images. At the same time, these factors are difficult to accurately measure and quantify.
ITU-RBT.500-11 has developed a method for testing video quality, namely the DSCQS (Double Stimulus Continuous Quality Scale) method, as shown in Figure 1.
DSCQS test schematic
The DSCQS method is to alternately display the original image and the measured image, let enough observers observe the contrast, score the measured image, and then calculate the average value. This is a subjective test method. Although it can accurately reflect the image quality, it is very difficult to implement.
ITU-T has also issued J.144 recommendations for video quality testing, and the following reference models are recommended in the recommendations.
1.BT full-referencemodel(British Telecommunications
2.CPqD-IESalgorithm(TelecommunicationsResearchand Development Center)
3.Edge PSNR
(Yonsei University/SK Telecom/Radio Research Laboratory)
J.144 also gives the similarity of these models and DSCQS test methods, PSNR, and points out the specific test method, as shown in Figure 2.
J.144 recommended video quality test method
In FIG. 2, a source video stream that is not video-compressed (as a reference video source) and a coded video stream (a video stream under test) are input to a test device, and are compared in a test device, and given The reference value of the video quality is used to quantitatively evaluate the image quality of the video system. It should be noted that this test method has nothing to do with the encoding method of the video stream under test.
VQEG (VideoQualityExpertGroup) is an organization that specializes in video quality technology. For DSCQS and ITU-T J.144 recommendations, VQEG recommends that there are currently dedicated test equipment, such as Tektronix's PQA300 video quality tester, which uses its own test sequence as a test source, sends it to the device under test, and then The decoded video is input to the test equipment, and the result of the video stream quality is obtained by calculation and comparison. On the one hand, this test method requires a dedicated test sequence. On the other hand, its test requires a lot of calculations, and the real-time performance is poor. In addition, VQEG recommends another video quality test equipment, K-will's video quality tester, which can input uncoded original video (without dedicated video sequence) and processed video as measured video into the device. The reference value of the video quality is calculated in real time, and the value is expressed as a percentage. The lower the percentage value, the closer the measured video is to the original. The device can get the measurement results in real time.
This test method requires two channels of video for comparison to be input into the tester, commonly referred to as a double-ended test. In the IPTV test application, since the video source is an already encapsulated IP data stream, it cannot be input to the dedicated test device described above. And because the test equipment supporting J.144 is expensive, it is impossible to apply it to actual operation and maintenance.
After a lot of experiments, the factors affecting the video quality of the IPTV system are also the performance of the codec itself and the adaptability to the quality of the video stream transmission. On the other hand, the transmission problem of the IP network, that is, the performance of the IP network has a video streaming function. Important impact, the performance parameters of the IP network, Latency, Jitter, and PacketLoss are important parameters that affect video transmission. ITU-TG.1050 recommends classifying the performance of IP networks, namely WellManaged IP Network, Managed IP Network, Unmanaged IP Network, and defines these three types of networks in detail, and gives three categories defined by them. Performance parameters of the network. G.1050 also gives the quality parameters of the voice service (as assessed by PESQ, MOS, etc.) under different IP network performance parameters, that is, the relationship between voice quality and IP network performance parameters. However, for the relationship between video streams and IP network performance parameters, G.1050 is not given. In this case, how to establish an operationally manageable IPTV system becomes an important issue in the development of IPTV.
MDI (Media Delivery Index)
The video streaming performance indicator MDI (MediaDeliveryIndex) was proposed by Cisco and IneoQuest Technologies and adopted by the IETF, and the standard number is RFC4445. The proposed MDI provides an effective method for the transmission performance of video streams in an IPTV system. MDI defines the performance indicators of two video streaming: DF (Delay Factor) and MLR (Media Loss Rate), and evaluates the video streaming performance according to DF and MLR. The DF value expresses the interval value at which each packet is displayed at the test point or the time of arrival of the record. The MLR gives the number of packet losses or the number of out-of-sequence packets in the selected time interval.
By calculating the DF value and the MLR value, the size of the video buffer required at a certain video stream rate can be obtained, which is used to smooth the degradation of the output video image quality of the decoder due to the deterioration of the network performance. Therefore, the MDI parameters are currently closely related to the quality of video image transmission in the IP network environment.
In practical applications, the MDI parameters can be conveniently measured and calculated. The calculation method of MDI parameters is described in detail in RCF4445. This makes it convenient and feasible to test the transmission quality of video streams in an IPTV system. The test of the video image is converted into the test of the quality of the video stream transmission, and the complex double-ended test (such as the DSCQS method) becomes a single-ended test, which has strong practicability. MDI, like the DSCQS test, has nothing to do with the coding standard of the video stream under test.
IPTV video transmission quality centralized monitoring system
At present, MDI has been adopted by many IPTV equipment vendors and is planned to be used in products. This provides convenience for the maintenance of IPTV video streaming.
In the IPTV video quality centralized monitoring system we developed, the MDI indicator is also used as an indicator for video streaming. The monitoring system is shown in Figure 3.
In Figure 3, we use the Network Probe Probe as a monitoring of MDI values, which can be connected to any Ethernet port in the IPTV network and measure the DF and MLR values ​​of the video stream. At the same time, Probe supports the SNMP protocol, which can be connected to the network management network through the control port. Through the network management system, the operation and maintenance personnel can remotely monitor the operation status of the IPTV system without leaving the home. For example, when a user questions the quality of the video program he watches, the network management center can query the probe connected to the user step by step to check the MDI value submitted by the network to determine which part of the network has a problem in the network. In order to troubleshoot.
IPTV video transmission quality centralized monitoring system
The main functions of the system are:
1. It is possible to evaluate the quality of a video communication system for installation and commissioning tests;
2. By measuring and calculating the DF parameter value, it can confirm and locate the network jitter (Jitter) and delay, and then track and check the network fault point (congested node);
3. By measuring the MLR, it can determine the network packet loss situation and locate the fault point;
4. When monitoring the network operation status in real time, the DF and MLR thresholds can be preset. If the threshold is lower or higher, the alarm can be issued in time.
Testing for video image quality is a complex and difficult problem. The use of instruments to achieve objective testing of video quality, there is no mature technical means. The proposed MDI provides an effective method for the transmission of video streams. The IPTV centralized monitoring system we developed is based on MDI parameters and provides a convenient and efficient means for effectively maintaining IPTV video transmission. Further research is needed on the relationship between network performance parameters, MDI parameter values ​​and video image quality.
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