DVB
| DVB standard composition | DVB standard analysis | DVB standard transmission system |
Based on the painful lessons of the past, the industry decided to use the commercial needs of the market as a guide for standard setting. In Europe, since 1991, TV stations, home appliance manufacturers and standard setters have come together to discuss the formation of a working group to jointly develop the development plan for digital TV. The members of the working group have developed rapidly. The sponsored organization soon attracted many members of the United States and Japan and became a worldwide organization. In September 1993, the working group drafted a memorandum to rename the working group as DVB (Digital Video BroadcasTIng) organization, the International Digital Video Broadcasting Organization.
From the beginning, everyone chose the ISO / IEC MPEG-2 standard as the audio and video encoding compression method, unified the source encoding, and then packaged the MPEG-2 code stream to form a transport stream (TS) for multiple The transmission streams are multiplexed and finally transmitted through different media transmission methods such as satellite, cable TV and open circuit TV. In 1995, the DVB organization established the standard DVB-S for digital satellite TV. In 1996, the digital cable TV DVB-C digital shared antenna TV, digital microwave TV and other standards were established accordingly.
(1) Composition of DVB standard1. DVB broadcast transmission system
The DVB digital broadcast transmission system utilizes all general TV broadcast transmission media including satellite, cable, terrestrial, SMATV, MNDSD. They correspond to the DVB standards: DVB-S, DVB-C, DVB-T, DVB-SMATV, DVB-MS and DVB-MC.
(1) DVB-S (ETS 300 421)-digital satellite direct broadcast system standard
The standard uses satellites as the transmission medium. The compressed digital signal forwarded by the satellite is processed by the satellite set-top box after passing through the satellite receiver, and outputs a signal that can be received by the existing analog television. This kind of transmission has a wide coverage and a large program volume. The modulation of the data stream adopts four-phase phase shift keying modulation (QPSK) mode, and the operating frequency is 11 / 12GHz. When using the MPEG-2 MP @ ML (main class @ main level) format, the code rate of the client to achieve CCIR 601 studio quality is 9Mb / s, and the code rate to achieve PAL quality is 5Mb / s. The transmission rate of a 54MHz transponder can reach 68Mb / s, and can be multiplexed by multiple programs. After the DVB-S standard was announced, almost all satellite live digital TVs adopted this standard, including the US Echostar. China has also adopted the DVB-S standard.
(2) DVB-C (ETS 300 429)-digital cable broadcasting system standard
The standard uses the cable TV network as the transmission medium and has a wide range of applications. It has three methods of 16, 32, 64QAM, and the operating frequency is below 10GHz. When 64QAM quadrature amplitude modulation is used, the transmission rate of a PAL channel is 41.34Mb / s, which can also be used for multiple program multiplexing. The front end of the system can obtain signals from satellite and ground transmission, and a cable set-top box is required at the terminal.
(3) DVB-T (ETS 300 744)-Digital terrestrial broadcasting system standard
This is the most complex DVB transmission system. The transmission capacity of terrestrial digital transmission is roughly equivalent to that of cable TV system in theory, and the coverage in this area is good. Now adopts coded orthogonal frequency division multiplexing (COFDM) modulation method, which can transmit 4 sets of TV programs within 8MHz bandwidth, and the transmission quality is high. But this system has a high reception cost and few channels.
(4) DVB-SMATV (ETS 300 473)-Digital SMATV (satellite shared antenna TV) broadcasting system standard
The standard is based on DVB-S and DVB-C.
(5) DVB-MS (ETS 300 748)-Digital broadcasting MMDS distribution system standard higher than 10GHz
MMDS is a system that uses amplitude modulation microwave to transmit to multiple points and distribute multi-channel TV programs. The standard is based on DVB-S, which allows microwave signals carrying a large number of programs to enter the home directly. A DVB-S receiver can be used with a MMDS frequency converter to receive DVB-MS signals.
(6) DVB-MC (ETS 300 749)-Digital broadcasting MMDS distribution system standard below 10GHz
The standard is based on DVB-C, which allows microwave signals carrying a large number of programs to enter the home directly. A DVB-C receiver can be used with a MMDS frequency converter to receive DVB-MC signals.
2. DVB baseband additional information system
In addition to transmitting ordinary video and audio signals, the DVB digital broadcasting system also needs to transmit and receive information such as IRD tuning, program guide, and graphics, subtitles, and icons. The DVB standards applicable to such baseband additional information systems include: DVB-SI, DVB-TXT and DVB-SUB.
(1) DVB-SI (ETS 300 468)-Digital broadcasting service information system standard
This standard is used for IRD self-setting and provides users with broadcast service guidelines. DVB-SI consists of a series of tables and descriptors, which give all the parameters related to DVB services or programs.
(2) DVB-TXT (ETS 300 472)-digital teletext broadcasting system standard
This standard is used for fixed format teletext transmission.
(3) DVB-SUB (ETS 300 743)-Digital broadcasting subtitle system standard
This standard is used for the transmission of subtitles and icons (station logos, etc.).
3. DVB interactive business system
Many services in the DVB digital broadcasting system can provide some form of interactive services as needed. On the basis of the general DVB digital broadcasting system, the elements that further constitute the interactive business system include independent protocols of the interactive business network compatible with other relevant international standards, and return channels for transmitting interactive service process commands and control signals. The corresponding DVB standards of the interactive service system are: DVB-NIP, DVB-RCC and DVB-RCT.
(1) DVB-NIP (ETS 300 802)-Independent protocol standard of DVB interactive service network
(2) DVB-RCC (ETS 300 800)-CATV system DVB backhaul channel standard
(3) DVB-RCT (ETS 300 801)-PSTN / ISDN DVB backhaul channel standard
4. DVB conditional access and interface standards
Some services in the DVB digital broadcasting system transmit scrambled conditional acceptance information. The universal interface of conditional access enables IRD (Integrated Receiver Decoder) to descramble the scrambling information using the general scrambling algorithm. The connection of the DVB digital broadcasting system with other telecommunication networks (such as PDH, SDH, ATM, etc.) expands the application scope of the DVB technology, and its interface with these telecommunication networks enables the transition from DVB to telecommunication networks. There are also interfaces for connecting professional equipment and IRDs. The DVB standards for these interfaces include: DVB-CI, DVB-PDH, DVB-SDH, DVB-ATM, DVB-PI and DVB-IRDI.
(1) DVB-CI (EN 50221)-General interface standard for conditional access and other applications
Conditional access determines which digital receivers can receive specific broadcast and television programs. This is an essential part of pay TV broadcasting and is therefore crucial to the successful development of digital TV operations.
(2) DVB-PDH (ETS 300 813)-PDH (quasi-synchronous digital series) network DVB interface standard
(3) DVB-SDH (ETS 300 814)-SDH (synchronous digital series) network DVB interface standard
(4) DVB-ATM (ETS 300 815)-ATM network DVB interface standard
(5) DVB-PI (EN 50083-9)-CATV / SMATV front-end and similar professional equipment interface standards
(6) DVB-IPDI (EN 50201)-DVB-IRD interface standard
(2) DVB standard analysis1. The core of the DVB standard
â— The system uses MPEG compressed audio, video and data formats as data sources;
â— The system adopts public MPEG-2 transport stream (TS) multiplexing mode;
â— The system uses public system service information (SI) for describing broadcast programs;
â— The first channel coding of the system adopts RS forward error correction coding protection;
â— Modulation and other auxiliary channel coding methods are determined by different transmission media;
â— Use general scrambling method and conditional access interface.
2. DVB audio features
The audio coding of the DVB system uses MPEG-1 Layer II second layer audio coding, also known as MUSICAM. The MPEG-1 Layer II encoding and compression system for audio utilizes the low-sound spectrum masking effect of sound. This physiological effect of the human body allows us to perform low-bit-rate encoding on frequencies that are less sensitive to the human ear. Audio encoding rate. MPEG-1 Layer II audio coding can be used to encode mono, stereo, surround sound and multi-channel multi-language sound.
3. DVB video features
For video, the international standard MPEG-2 compression coding is used. The MPEG-2 video coding system is composed of a large family, and each system has compatibility and commonality. According to the difference in image clarity, it is divided into four source formats or "levels" (LEVEL), from video tape (VCR) low image clarity, to high-definition television. In addition to the levels defined according to image clarity, the DVB video standard also defines the concept of "profile" (PROFILE). Each different profile can provide a compression tool and a compression algorithm that constitute the encoding system (Figure 07-05-3).
—— (a) Outline—— At present, there are 5 profiles (PROFILE) in the MPEG-2 system, each profile will be more complex and more complete than its previous profile, provide more tools, and its corresponding The price of the equipment is also higher.
The most basic "profile" is called SIMPLE PROFILE, followed by MAIN PROFILE. It adds the function of encoding bidirectional prediction to SIMPLE PROPILE, that is: B-FRAMES, using the same code In the case of streaming, its quality will be better, but the algorithm is more complicated and more chips are used. The main profile (MAIN PROFILE) decoder chip can be compatible with decoding simple profile (SIMPLE PROFILE) encoding. This downward compatibility runs through the entire series of profiles.
After the main profile (MAIN PROFILE), it is the signal-to-noise ratio scalable profile (SNR SCALABLE) and the spatial spectrum scalable profile (SPACILLY SCALABLE PROFILE). These two "profiles" can adjust the relationship between the signal-to-noise ratio and the code rate, and The relationship between image clarity and bit rate is due to the complexity of encoding and the problem of expensive receiving equipment. The DVB standard does not currently support these two profiles. The highest-level profile is HIGH PROFILE, which is not only compatible with the previous low-level profile, but also has all the functions. It can also encode multiple lines at the same time, while the previous profile is encoded line by line.
There are two image sampling methods in the outline, namely 4: 2: 2 and 4: 2: 0 formats. We know that the TV composite signal can be divided into a luminance signal component (Y) and a chrominance signal component (RY, BY). The 4: 2: 2 format is 4 samples of the luminance signal and 2 chrominance signals (RY, BY). Sampling, see figure 07-04-4; the chrominance signal (RY, BY) in 4: 2: 0 format is only interlaced, see figure 07-04-2. If 8-bit sampling is used, we can calculate the 4: 2: 2 format image stream rate after sampling the standard PAL TV signal as follows:
The bit rate of the luminance signal is: 720 × 576 × 25 frames / second × 8bit = 82.944Mb / s
Chroma signal bit rate is: 2 × 1/2 × 720 × 576 × 25 frames / second × 8bit = 82.944Mb / s
The total bit rate is: 82.944Mb / s + 82.944Mb / s = 165.888Mb / s
——We see that the uncompressed TV image stream rate is very high, and the occupied bandwidth is too wide, which is not suitable for transmission. Even if the 4: 2: 0 format is used, the image stream rate is as high as 124.416Mb / s. The compression algorithm of MPEG-2 adopts the algorithm of removing the temporal and spatial redundancy of the TV video signal, so that the code stream rate is reduced to 3 ~ 8Mb / s and still obtains a clear quality image, making digital TV transmission possible.
(b) Level (LEVEL)
According to the difference of the definition of the image program source from low to high, the DVB MPEG-2 standard is divided into many levels, the lowest LOW LEVEL resolution is one-fourth recommended by IU-R-BT 601, namely: 352 × 288 × 25 frames per second. MAIN LEVEL is fully compliant with the standard recommended by IU-R-BT 601, namely: 720 × 576 × 25 frames / second. HIGH-1440 LEVEL uses the method of 1440 samples per line. HIGH LEVEL uses a higher sampling method of 1920 per line.
At present, the most commonly used MPEG-2 standard in the world is MP @ ML, that is: MAIN PROFLE @ MAIN LEVEL, which is the basis of the first generation of digital cable TV and digital satellite TV. Program providers can provide 625-line quality programs, The aspect ratio of the image can be 4: 3 or 1 6: 9; as for the bit rate, it is selected by the program provider according to the quality of the program. Lower.
Figure 07-05-3 Schematic diagram of outline and level list
4. MPEG-2 stream multiplexing and service information
Referring to the digital TV transmission block diagram 07-04-10, audio, video and digital signals are first compressed by the MPEG-2 encoder, and a basic code stream (ES) is formed by the program multiplexer, and the basic code stream is packaged to form a packet header Elementary stream (PES). PES streams representing different audio and video signals are sent to a transmission multiplexer for system multiplexing. The multiplexed code stream is called a transport stream (TS). The transport stream includes different signals from multiple program sources. In order to distinguish these signals , Service information (SI) needs to be added to the system multiplexer, so that the receiving end can identify different programs. In order to facilitate the understanding of the service information of the DVB transmission system, we give a rough introduction to the structure of the transmission code stream. The length of the transmission code stream is defined as 188 bytes long, see Figure 07-05-4.
Figure 07-05-4 Structure of the transmission stream
The first 4 bytes of each transport stream are headers, and behind the headers are the useful information that needs to be transmitted, including audio, video or data information, usually 184 bytes in length, sometimes in useful data (Useful Data ) Inserts an adaptation field (AdaptaTIon Field) to supplement the transport stream with incomplete length and place the decode clock (PCR). The prefix of the transport stream is the key to identify the transport stream. Its structure is shown in Figure 07-05-5, which consists of 32 bits, and its meaning is shown in the following figure 07-05-5.
Figure 0 7-05-5 transmission header and the meaning of each bit
Among the 32-bit prefix, the 13-bit PID code is particularly important. It is the key to discerning the nature of the code stream information. It is the "identity card" of program information. Different TV programs and service information (SI) have different PID codes. . For a decoding receiver, in order to find the TV program it wants to receive, it first finds the different tables (Table) corresponding to the service information (SI) through the PID code. The DVB standard defines the following service information table:
PAT: Program AllocaTIon Table (program allocation table)
CAT: CondiTIonal Access Table (Conditional Access Table)
PMT: Program Map Table (program map table)
NIT: Network Information Table
SDT: Service Description Table
EIT: Event Information Table
TDT: Time and Date Table
Through these service information tables, you can find the PID code of the program you want to receive and the corresponding clock PCR, and the program can be restored.
(3) DVB standard transmission system The transmission system of the DVB standard is divided into two parts: Source Coding and Channel Coding. Source coding uses MPEG-2 code stream. First, multiplex audio and video programs, and then multiplex multiple digital TV program streams.
—— Channel coding includes: forward error correction coding, decoding, modulation, demodulation, and up-down variable amount. The forward error correction code uses different combinations according to different transmission media. Satellite transmission uses QPSK modulation, cable transmission uses QAM modulation, and terrestrial broadcasting uses COFDM modulation or I6VSB modulation. See figure 07-05-6.
Figure 07-05-6 DVB transmission system
1. DVB Standard Huai Satellite Transmission System (DVB-S
)
The transmission of digital satellite TV is not designed to meet the bandwidth of satellite transponders and the transmission characteristics of satellite signals. The satellite system is a single carrier system. If we call the useful information to be transmitted as a "core", then it is surrounded by many protective layers to make the signal more resistant to interference during transmission, video, audio and The data is put into a fixed-length packed MPEG-2 transport stream, and then processed by the channel. In a satellite system, the channel processing process includes:
—— (a) Invert the sync byte first. The length of the inverted byte is every 8 bytes.
—— (b) Then perform energy diffusion of the data, randomize the data, and avoid long strings of 0s or 1s.
—— (c) Add RS code for forward error correction to each data packet, also called outer code. The addition of RS encoding will increase the original data length from the original 188 bytes to 204 bytes.
—— (d) Perform data interleaving.
—— (e) Error correction by adding convolutional code, also called inner code, the number of inner code can be adjusted according to the transmission environment of the signal.
-(F) Finally, QPSK modulation is performed on the data stream. See figure 07-05-7.
Figure 07-05-7 DVB-S transmission system
In short, the transmission system first discretizes the burst error codes, and then adds RS external error correction code protection. The internal error correction code can be adjusted and changed according to the transmission power, antenna size and code rate. For example, a satellite transponder with a bandwidth of 36MHz using a 3/4 convolutional code can achieve a code rate of 39Mb / s, which can transmit 5-6 high-quality TV signals. See figure 07-05-8.
Figure 07-05-8 DVB-C transmission system
2. DVB standard cable transmission system (DVB-C)
Digital cable TV uses the same "core" as the satellite, that is, MPEG-2 compression-encoded transport stream. Because the transmission medium uses a coaxial line, compared with satellite transmission, the external interference is small, and the signal strength is relatively high, so the inner code is canceled in the forward error correction code protection. Modulation mode is changed to 64-QAM mode, sometimes 16-QAM, 32-QAM or higher 128-QAM, 256-QAM can also be used. For QAM modulation, the higher the amount of transmitted information, the lower the anti-interference ability. In an 8MHz standard TV channel, if 64-QAM is used, the transmitted data rate is 38.5Mb / s.
3. DVB standard terrestrial broadcasting system (DVB-T
)
The standard of the terrestrial broadcasting system was approved in February 1998. The first formal terrestrial broadcasting system began operations at the beginning of I998. MPEG-2 digital audio and video compression coding is still the core of terrestrial broadcasting. The other characteristic is that it adopts the COFDM modulation method. In this modulation method, it can be divided into a 2k carrier method suitable for single transmitter operation in a small range and an 8k carrier method suitable for a large range of multiple transmitters. The COFDM modulation method distributes information to many carriers. This technology has been successfully applied to digital audio and video broadcasting DAB to avoid the multipath reflection effect caused by the transmission environment at the cost of introducing a transmission "guard interval". . These "guard intervals" will occupy a part of the bandwidth. Generally, the more COFDM carriers, the smaller the transmission capacity loss for a given maximum reflection delay time. But there is always a plateau. Increasing the number of carriers will increase the complexity of the receiver and destroy the phase noise sensitivity.
Due to the anti-multipath reflection function of the COFDM modulation method, it can potentially allow the electromagnetic coverage of adjacent networks in a single frequency network to overlap. In the overlapping area, the electromagnetic waves from two transmission towers can be regarded as one transmission tower The superposition of the electromagnetic wave and its own reflected wave. However, if the two transmission towers are far apart and the time delay of the electromagnetic waves sent from the two towers is relatively long, the system requires a larger guard interval.
—— From the perspective of forward error correction codes, due to the complexity of the transmission environment, the DVB-T system includes not only inner and outer codes (Outer Code, Inner Code). And joined the internal and external organizations (Outer Interleave, Inner Interleave).
(4) Conditional access-Conditional access to digital TV is a relatively complex topic. Various countries and companies want to keep their secrets. It is difficult for everyone to reach an agreement. Finally, the DVB standard reaches the following consensus:
——A) Two kinds of scrambling and descrambling methods coexist in the market, the first one is "Simulcrypt ', each receiver can only use a single descrambling method, excluding other descrambling methods. The second one is" Multcrypt', Each receiver allows multiple descrambling methods through a defined common interface (Common Interface).
——B) Define a common scrambling algorithm to enable consumers to use a single decoder.
——C) Require the supplier of conditional access to provide an interface method to enter the digital decoder.
——D) publish the technical specifications of the conditional access common interface (Common Interface).
——E) Draft anti-piracy recommendations.
——F) The products provided by the conditional access system suppliers to other digital TV manufacturers must be reasonable products, and products that prohibit the exclusion of the common interface (Common Interface).
——G) The conditional access system must allow conditional control transfer between program operators. For example, after a satellite conditional access program enters the cable network, the original conditional access system can be replaced by a new conditional access system .
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