Radio and TV launches interactive TV services, which can fully rely on the advantages of HFC networks to carry out broadcast, on-demand and other switched services. Some cities and towns have built a small number of LAN cells, and in these places, the same technical model of telecom IPTV can be used to carry out services using multicast switches. Luo Feng of Wuxi Digital TV Co., Ltd. discussed several major technical modes including IPTV and interactive TV, and conducted preliminary comparative analysis.
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1. Broadcasting with DVB, on-demand and interactive DVB+CM mode with CableModem
DVB transmits a large number of TV broadcast programs, which has the advantages of maturity, reliability and low price, and can be translated into thousands of households with digital TV. On-demand and interactive services are performed using CableModem, and the on-demand program stream is transmitted through the CableModem downlink channel. The DVB+CM mode enables a complete interactive TV solution. The CableModem module is best integrated into the DVB set-top box to reduce cost and increase reliability, and to increase the full service to the user with just one device. At present, chip manufacturers have introduced a single chip solution with DVB+CableModem function.
Since DVB uses MPEG-2 encoding, and the CableModem system generally uses MPEG-4, H.264 or AVS encoding format due to bandwidth limitation, it requires DVB set-top box to add at least one encoding to MPEG-4, H.264 or AVS. Format support. Otherwise, there is only an interactive function, and the VOD service cannot be realized. When the CableModem system adopts the 64QAM modulation mode, the downlink rate per head end is 38Mbps, and the 256QAM modulation mode uses a downlink rate of 5lMbps per head end. In a very good quality HFC network, 256QAM modulation can be used. However, most HFC networks currently use the 664QAM modulation mode, so that each video stream has an average of 1.5 Mbps, and each head end supports up to 25 simultaneous on-demand broadcasts. According to the 10% simultaneous on-demand rate calculation, up to 250 interactive TV users can be developed per head.
If the head end purchases single-type small equipment, each about 130,000 yuan, plus the cost of CableModem terminal, the cost per line reaches 900 yuan, which does not include DVB set-top box, authentication billing gateway, two-way HFC network. The cost of waiting. If you purchase a centralized head, the cost per line should be around 1,200 yuan. According to the ARPU value of 40 yuan/month for each on-demand user, it takes 2-2.5 years to recover the line cost. It can be seen that the price of the DVB+CM model is too high, and it is not suitable for large-scale commercial applications. It is only suitable for small-scale applications in the initial stage of business development and the urgent need for rapid business development.
2. Broadcasting with DVB, on-demand and interactive LAN DVB+LAN mode
This mode is basically the same as the DVB+CM mode, except that the CableModem bidirectional channel is changed to a LAN channel. The on-demand program stream is changed to the LAN channel formed by the switch, while the broadcast program still uses the traditional DVB system. LAN access has the advantage of sufficient bandwidth. The bandwidth of each corridor switch can reach 100M, and the rate of uplinking of the cell aggregation switch can reach 1000M. Therefore, it can fully meet the video on demand requirements of each user, and has sufficient bandwidth to ensure the needs of the Internet service. A 500-cell community, if the on-demand user is 20%, and the on-demand rate is 60%, the average bandwidth of each video stream is 1.5 Mbps.
Due to the rich bandwidth, the encoding format can be MPEG-2 in addition to MPEG-4, H.264 or AVS. Similarly, in the above cell, when the average MPEG-2 video stream is 4 Mbps, the total cell bandwidth is 240 M, and a Gigabit link is required. By adopting the same MPEG-2 encoding format as DVB broadcast, the set-top box decompression algorithm can be simplified and the cost of the set-top box can be reduced. At the same time, however, the bandwidth requirements of the backbone network are higher, and a larger chip storage system is required. Since the broadcast has been completed by DVB, this mode is different from the telecom LAN access mode. The switch does not need to support multicast, and a simpler switch can be used. A network network with network management and operation-oriented operation. When the network access rate is about 20%, the cost per line is about 700-1000 yuan, which is slightly lower than the DVB+CM mode. Therefore, it is suitable for large-scale applications when the network access rate is high, or when there are many LAN cells.
3, broadcast and on-demand broadcast the same DVB IPQAM mode
Digital TV can transmit the code stream of hundreds of programs. If it is divided into regions and uses dozens or even 200 program streams in each sub-region for VOD services, it can provide complete broadcast and on-demand services. This is the basic idea of ​​the IPQAM mode. The sub-area can be composed of several or even dozens of optical nodes, covering about 20,000 to 50,000 users, and each sub-area is placed with one or more IPQAM devices. Each sub-area is actually the sub-center of the HFC network. The IPQAM device is a device that multiplexes and upconverts a DVB/IP video stream and outputs it as an RF signal. The different RF signals output by IPQAM will be RF-mixed with the broadcast signals sent from the front-end, respectively, and the mixed signals will be sent to different optical transmitting modules (optical contacts) and then directly transmitted to the last access network. The channel resource space reuse technology will be adopted in the network frequency resource allocation mode. Therefore, the entire VOD system will require only a small number of analog channels to achieve large-scale VOD applications.
Each IPQAM modulator typically has 16 or 24 RF ports, and each RF port outputs a 64QAM modulated signal with a total code rate of 608M (16 ports). Each video stream uses MPEG-2 format, which requires about 4M, so a 16-port IPQAM can support 150 concurrent on-demand requests. According to the 10% simultaneous on-demand rate, 1,500 interactive TV users can be developed. Such a sub-area of ​​30,000 households can only be supported by two IPQAM devices when the network access rate of interactive TV users is 10%.
Since the on-demand also uses the same MPEG-2 encoding format as DVB broadcast, the set-top box decompression algorithm is simplified and the cost of the set-top box is reduced. Since the cost of the domestic IPOAM modulator is about 4,000 yuan per frequency point, this does not include the backbone network and the set-top box. When the cable modem is used for the backhaul, the cost per line of the access network is about 400 yuan. Users who do not need to access the Internet, and only need to order the service, can also use the method of SMS backhaul. At this time, the cost per line of the access network is about 200 yuan, which is the lowest among the above technical modes.
Through the above analysis of the mainstream technology modes of IPTV and interactive TV, it can be seen that both IPTV and interactive TV will have different emphasis on the business model. IPTV focuses on the development of VOD on-demand services, and uses multicast technology to implement broadcast services. It also vigorously develops other Internet-based interactive services. Radio and TV's interactive TV is equally focused on the broadcasting service and the on-demand service, and will promote the popularity of the broadcasting service through the development of on-demand services. With the different technology modes of interactive TV, Internet-based interactive services can also enter thousands of households.
Several technical modes are mainly different in the following points:
(1) Broadcasting business--on the broadcasting service that meets the needs of the masses. IPTV adopts controllable multicast technology and needs to solve a series of new problems. It is still in the trial operation stage; and interactive TV adopts mature DVB solution, and many cities have successfully operated for many years. Therefore, IPTV is difficult to catch up with in the broadcasting service.
(2) On-demand service--On the on-demand service that meets the individual needs, IPTV reflects its own characteristics and advantages. ADSL point-to-point connection mode and high bandwidth of LAN access can carry out business very well. The method of on-demand broadcasting by CableModem in interactive TV is limited by the bandwidth of the headend and the price of the equipment, and it is difficult to meet the requirements for large-scale on-demand services. The IPQAM mode enables SVA to give full play to the advantages of HFC networks. In particular, digital TVs have vacated a large amount of frequency resources after full translation, which is more conducive to the development of on-demand services.
(3) Video coding standard - In the video coding standard, IPTV is plagued by what standards are adopted. There are multiple standard branches in MPEG-4, and it is necessary to consider compatibility issues; the H.264 industry chain is not mature enough, and there are problems with patent fees. The IPQAM mode of the radio and television interactive TV is only based on the MPEG-2 standard, the technology is mature, the chip price is low, and the cost of the set-top box is also low, which is conducive to large-scale promotion.
(4) On HDTV - Due to the larger video stream required for HDTV, IPTV is difficult to support. Radio and TV's interactive TV is easy to implement, and this will be the strength of broadcasting and television for a long time.
(5) Access cost--ADSL access in IPTV and IPQAM mode in interactive TV all have lower construction costs, so this is close to competitiveness. The DVB+CM mode and the DVB+LAN mode in LAN access and interactive TV in IPTV are relatively high in cost.
In addition, there are some minor problems with each branch mode. For example, ADSL access has a selection rate problem, overall QoS guarantee, etc.; LAN access has security issues, multicast capability issues, and the like. Interactive TV also has some problems, such as the reverse noise problem when CableModem is connected. But these will be resolved in the operation, without having a big impact on the overall solution.
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