This article briefly introduces the new generation of portable mobile TV reception standard DVB-H and mobile TV source compression coding standard, focusing on the composition of DVB-H system, time slicing technology, MPE-FEC, 4K transmission mode and making it adaptable Several main technical features of mobile TV reception.
DVB-H,
Using portable communication tools such as mobile phones, watching TV anytime, anywhere was a dream. With the establishment of source coding technology, channel transmission and the establishment of a new generation of basic communication networks, the portable mobile receiving subsystem also receives from a single text or picture. Turn to more colorful video and audio form reception. In order to adapt to this trend, the television industry has also formulated standards and developed technologies for related technologies. Now do the following discussion on related technologies.
To watch TV on a mobile phone, three links need to be handled technically: signal source, propagation channel and receiving terminal. In terms of signal source, a source compression coding standard with a high compression ratio is required; in terms of propagation channels, there are wireless microwave and network transmission. In order to achieve mobile reception, digital modulation and channel processing techniques with strong anti-interference capability are required. In terms of receiving terminals, it is necessary to develop chips with high integration, small size, light weight, and low power consumption, as well as small and high-capacity rechargeable batteries.
At present, there are three main ways to realize the service:
1. Using mobile network
At present, the mobile TV services launched by mobile operators in the United States and China mainly rely on existing mobile networks. China Mobile's mobile TV business is based on its GPRS network, while China Unicom relies on its CDMA network. Regardless of whether it is a GPRS mobile phone or a CDMA mobile phone, it is necessary to install the corresponding playback software on the mobile phone terminal (generally PDA mobile phone and other high-end products) equipped with an operating system, and the corresponding TV program source is by the mobile communication company or through the corresponding service Provider to organize and provide.
2. Ways to use satellite networks
It is a very new idea to use mobile phones to receive satellite broadcast television program signals. Currently, only South Korea is pushing for mobile TV broadcasting (DMB). This DMB receiver can provide high-quality images, and the use of this receiver module enables users to simultaneously receive terrestrial wireless TV broadcast and satellite TV broadcast signals.
3. The way to install the digital TV receiving module in the mobile phone
At present, the most promising mobile TV technology is through the integration of digital TV and mobile phones. In this way, the microwave digital TV receiving module needs to be installed on the mobile phone terminal, and the digital TV signal can be directly obtained without the link of the mobile communication network. At present, the mobile phone digital TV standard is only the European DVB-H and Japan's single-band broadcast standard.
In China, only CCTV and a few mobile companies have launched mobile TV services. Taking China Central Television as an example, since there is no DVB-H digital broadcasting network in China, they use 2.5G or 2.75G network transmission technology to play "mobile TV" programs, that is, using China Mobile's GPRS / EDGE network or China Unicom. The CDMA network provides users with online streaming or on-demand streaming audio and video programs through the WAP portal.
The following discusses the transmission standards and coding standards for mobile TV:
1. Transmission standard of mobile TV-DVB-H
The DVB-H (early DVB-X) standard is called Digital Video BroadcasTIng Handheld. It is a transmission standard established by the DVB organization to provide multimedia services to portable / handheld terminals through terrestrial digital broadcasting networks. DVB-H is based on DVB-T and is a system that transmits data (mainly digital multimedia data) in IP packets (datagrams). This standard is considered to be an extended application of the DVB-T standard, but compared with DVB-T, DVB-H terminals have lower power consumption, and mobile reception and anti-interference performance are more superior, so this standard is suitable for mobile phones, Small portable devices such as handheld computers receive signals through terrestrial digital TV broadcast networks. In fact, since DVB-H is a standard that supports multimedia services, in addition to TV services, it can also carry out a variety of comprehensive services such as electronic newspapers, electronic auctions, travel guides, games, video on demand, and interaction. In short, the DVB-H standard is based on the current DVB-T transmission system, by adding certain additional functions and improved technology to enable mobile devices such as mobile phones to receive broadcast and television signals stably.
In order to reduce the power consumption of small handheld devices, DVB-H uses a technology called "time slicing" (TIme-slicing), which cuts IP packets into data bursts in a short time period (TIme slots). Burst transmission. The front end circuit of the receiver will only be turned on during the selected data burst period. During this extremely short period of time, the data is received at high speed and can be stored in the buffer provided by the device. This buffer can store downloaded content, and can also directly play live broadcast data files.
1. DVB-H system structure
DVB-H supports small mobile terminal devices such as mobile phones and is the standard for mobile phone digital TV transmission. DVB-H is a standard built on DVB data broadcasting and DVB-T transmission, with more emphasis on protocol implementation. The front end of the system is composed of a DVB-H packaging machine and a DVB-H modulator. The DVB-H packaging machine is responsible for encapsulating IP data into an MPEG-2 system transport stream, and the DVB-H modulator is responsible for channel coding and modulation; the system terminal is DVB-H The H demodulator and the DVB-H terminal are composed. The DVB-H demodulator is responsible for channel demodulation and decoding, and the DVB-H terminal is responsible for displaying and processing related services.
The DVB-H transmission system also has the following special requirements:
(1) In order to extend battery life, the terminal periodically turns off part of the receiving circuit to save power consumption;
(2) Can roam, and can still receive DVB-H business very smoothly while roaming;
(3) The transmission system can ensure the smooth reception of DVB-H services at various mobile rates;
(4) The system has strong anti-interference ability;
(5) The system has considerable flexibility to adapt to different transmission bandwidth and channel bandwidth applications.
2. Protocol level division
The DVB-H standard will implement the data link layer and the physical layer.
(1) Data link layer-adopts time slicing technology to reduce average power consumption and facilitate smooth and seamless service exchange; adopts MPE (multi-protocol encapsulation) forward error correction technology to improve mobile use C / N threshold and Doppler performance enhance the ability to resist pulse interference.
(2) Physical layer-Compared with DVB-T, 4k transmission mode and deep symbol interleaving are added.
Other technical features include: adding DVB-H signaling to Transmission Parameter Signaling (TPS) bits to increase the speed of service development; cellular identification (in TPS) to support fast signal scanning and mobile reception Frequency exchange; add 4k mode to adapt to mobile reception and single frequency cellular network, improve the flexibility of network design and planning; 2k and 4k mode for deep symbol interleaving, further improve the robustness in mobile environment and impact noise environment.
3. Key new technologies
(1) Power consumption: DVB-H requires that the power consumption of the radio frequency receiving and channel demodulation and decoding sections is less than 100mW.
(2) Network design
Because the receiving antenna is small and single when the DVB-H terminal moves in the network, the single frequency network must be optimized. To this end, DVB-H added new technical modules, mainly including:
â‘ Time slicing-based on time division multiplexing technology, saving power consumption of receiving terminal and facilitating network exchange;
②MPE-FEC——Based on RS error correction coding technology, additional forward error correction coding is added to improve the system's movement and anti-pulse interference capability;
â‘¢ 4k mode-used to improve the flexibility of network design;
④DVB-H TPS——DVB-H dedicated transmission parameter signaling, used to improve system synchronization and service access speed.
The following is a detailed introduction to time slicing, MPE-FEC, 4k mode and DVB-H TPS:
â‘ Time slicing
Time slicing technology is the most important new technology module in DVB-H. It uses burst mode to transmit data. Each burst time slice transmits a service. In the service transmission time slice, the service will occupy all the data bandwidth separately, and it is pointed out The moment when the next same service time slice is generated, so that the handheld terminal can receive the selected service at a specified time and do energy-saving processing during the idle time of the service, thereby reducing the total average power consumption. During this period, the front-end radiograph has been working all the time, and other business data will be transmitted between two time slices of the same service. The DVB-H signal is composed of many such time slices. From the receiver's point of view, the received service data is not the same as the traditional constant rate continuous input method. The data arrives at discrete intervals, which is called burst transmission. If the decoding terminal requires a lower data rate but it must be constant Bit rate, the receiver can buffer the received burst data first, and then generate a data stream with a constant rate. Not only can it effectively reduce the average power consumption of handheld terminals, but it is also the basis for achieving a smooth and seamless service exchange between different networks.
a. Time slicing and power consumption
Time slicing technology uses burst transmission of data, which has a higher instantaneous rate than traditional data streaming services. In order to meet the requirements of saving power consumption, the burst bandwidth is generally about 10 times the fixed bandwidth. For example, a service stream with a constant rate of 350 kbit / s means that it requires a burst bandwidth of about 4 Mbit / s. Burst bandwidth can save 50% of power consumption when the fixed bandwidth is twice, so if the bandwidth is 10 times, it can save 90%.
b. Time slicing and PSI / SI
The DVB-H standard stipulates that PSI / SI (Program Specific InformaTIon, PSI / Service Information, SI) information will not be time-sliced, they will be allocated a fixed bandwidth for transmission, mainly because of the current use PSI / SI information does not support time-sliced ​​transmission. If it is changed, it will be difficult to be compatible with the current data table. PSI information uses 4 tables to define the structure of the code stream: Program Association Table (PAT), Program Map Table (PMT), Network Information Table (NIT), and Conditional Access Table ( Conditional Access Table, CAT).
In the DVB-H system, the handheld terminal needs to access the NIT (Network Information Table, NIT Network Information Table) and intermediate code INT table in the SI. The purpose of the NIT table is to provide information about the physical network. Its content is fixed. When a handheld terminal joins a new network, it must first receive the table to determine the network parameters. When switching between different transport streams, the handheld terminal needs to read the INT table. Unless the INT table is changed in the future, the terminal will no longer receive the INT table. The change information of the INT table is in the PMT (Program Map Table, PMT) of PSI Program mapping table) table. The PMT table indicates the PID number of each code stream that constitutes the program service (Service), and describes each code stream
As the DVB standard stipulates that PSI information must be retransmitted every 100ms, if the service transmission time of the burst is longer than 100ms, the handheld terminal can access all PSI information while receiving the service; if the service transmission time is less than 100ms, the handheld terminal Need to continue to maintain a period of work after the completion of business reception to ensure the completion of the reception of the requested PSI form.
c. Time slicing and business exchange
Using time slicing technology enables handheld terminals to monitor adjacent cells during idle periods of service transmission, scan other frequency signals and test signal strength, but does not interrupt the reception of this service. When users enter a new network, they switch to different transport streams with the same service in the idle period according to the monitoring results, so as to achieve a better seamless service exchange. If the services are synchronously and accurately arranged at the front end, the same services can be made to appear on different time slots of adjacent peaks and nests in time without the user being aware of such changes.
d. Time slice and conditional access
DVB-H can use two methods to achieve conditional access, one is IP-based conditional access system (IP-CAS, IP data broadcast encryption). All CAS (Conditional Access System) related information is in IP data and can support time slicing technology to ensure power saving. But the DVB-H standard does not need to support two-way transmission between CAS and receivers, IP-CAS only needs to support the broadcast environment.
Another way is to use the conditional acceptance system (DVB-CAS, TV encryption system) of the DVB general scrambling algorithm. At this time, there will be some problems in transmitting CAS information in the DVB-H system. Because DVB-CAS uses Electronic Counter Measure (Electronic Counter Measure) to transmit descrambling keys, ECM cannot perform time slicing. In addition, DVB-CAS also uses management information EMM (EMM-Entitlement Management Message) to transmit authorization management information Because the time interval of EMM is random, the terminal must always work to ensure that EMM will not be lost, and directly using DVB-CAS will affect the network roaming service.
To ensure the descrambling work, the receiver must complete ECM reception, and the system identifies the ECM minimum repetition period through the ECM repetition rate descriptor. If the handheld terminal completes at least one ECM minimum period of reception before starting to receive service data, it can receive at least one ECM to obtain the descrambling key. The effective time of the descrambling key is usually 10s. For this, the receiver must ensure that the descrambling key is received 10s before the service data arrives.
The EMM will be transmitted in time slices. First, the EMM is encapsulated in the form of an IP datagram. After encapsulation, the time slice method of the EMM-IP data is the same as other IP data, and MPE-FEC is used to reduce data loss. From the perspective of the receiving terminal, the IP data carrying EMM is an additional service, which must be received, and the recovered EMM-IP data will be transmitted to the DVB-CAS specific module to process the EMM information.
After processing in the above manner, DVB-CAS will not cause any impact on user roaming.
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