What is Digital Satellite TV?

Digital satellite TV system (Digital satellite TV system) for short DVB-S system, also known as DVB-S digital satellite TV system, has developed rapidly in recent years. It uses geostationary satellites to transmit digitally encoded and compressed TV signals to users. A form of radio and television. Many countries in the world are currently developing digital satellite television systems.

Chinese name: Digital satellite television system

Foreign name: Digital satellite TV system
Short name: DVB-S

Digital Satellite TV System Introduction

Digital satellite television system, referred to as DVB-S system, also known as DVB-S digital satellite television system .

^[1] Digital satellite television has developed rapidly in recent years. It uses a geostationary satellite to transmit digitally encoded and compressed television signals to users.

Digital Satellite TV Transmission System Group Radio and television form. There are two main ways. One is to transmit the digital TV signal to the front end of the cable TV, and then the cable TV station converts the analog TV to the user's home. This form has been popularized in countries around the world for many years. Another method is to directly transmit the digital TV signal to the user's home, that is, the Direct to Home (DTH) method. American Direct TV is the first satellite TV operator to apply this technology. Compared with the first method, the DTH satellite has a larger transmitting power and can be received by a smaller antenna, which can be used by ordinary households. At the same time, it can directly provide user authorization and encryption management, and carry out advanced TV services such as digital TV, pay-per-view TV (PPV), and high-definition TV, without being restricted by intermediate links. In addition, the DTH method can also carry out other digital information services other than television services, such as Internet high-speed download, interactive television, and so on.

Digital Satellite TV System System Overview

Many countries in the world are developing digital satellite television systems due to their unique advantages: First, after digital television is transmitted via satellite, the signal quality at the receiving end can be comparable to that at the transmitting end, because it uses digital transmission and bit errors. Protection technology, while ordinary analog TV signals use analog processing and transmission methods, and the reception quality is easily affected by noise and interference. Second, digital satellite TV systems can only transmit 1 channel due to the use of digital compression technology and digital modulation technology. A 36MHz satellite transponder of the analog TV festival can transmit 5-6 digital TV festivals, which greatly saves space frequency resources. Third, the use of large-scale integrated circuits reduces equipment power consumption, reduces volume, and is reliable. Improved performance and easy networking with computers. Digital satellite television has the above-mentioned outstanding advantages, coupled with the development of high-speed signal processing technology and ultra-large-scale integrated circuit technology, it provides a basis for implementing digital satellite television systems. Since 1993, the European Digital Television Broadcasting Group has successively formulated a series of digital television standards, namely DVB (DigitalVideoBroadcast), including the DVB-S standard for satellite links. The European DVB-S standard has received responses in Asia, Australia, and the United States. In 1996, China promulgated a digital broadcasting and television transmission technology system and decided to adopt a digital television satellite broadcasting system that complies with the DVB-S standard.

Core technology of digital satellite TV system

The core technology of DVB system is universal MPEG-2 video and audio coding. The main application for digital satellite and TV broadcasting is MP

DVB system @ML. The first generation of European DVB receivers will provide up to 625 lines of studio quality? ITU-Rec, BT601 images? ITU is the ITU, which can be 4: 3 or 16: 9 aspect ratio. The code rate used can also be determined based on service requirements. Generally speaking, the higher the selected bit rate, the better the image quality, but the wider the occupied frequency band. The choice of bit rate has a lot to do with the content of the image. For more moving images such as sports programs, a higher bit rate can be used, and for cartoons and other programs, a lower bit rate can be used. Therefore, in the case where multiple program bit streams are combined into one bit stream, a statistical multiplexing method is adopted, and the total code rate number can be flexibly allocated among programs of different code rates. In order to meet the requirements of various materials, ITU-Rec, BT601 studio quality requires 9Mbit / s, PAL / SECAM, and broadcast quality requires a digital rate of 5Mbit / s. Data packet structure, so you can easily add appropriate information, integrate various services such as image, sound, and data services, and specify the format of service information in detail. The standard formed is service information Standard ETS300468. The DVB system uses the low-data-rate MPEG-1 layer II audio standard as its universal audio compression coding standard to obtain sounds close to CD quality.

Basic composition of digital satellite TV system

The transmitting end is divided into two parts, one is the signal formation, that is, the source coding and multiplexing part (here, only SCPC is introduced); the other is the signal transmission, that is, the channel coding and digital modulation part.

1) Digital satellite television system 1) Source coding and multiplexing

The source coding part mainly completes analog / digital and standard conversion and digital compression functions. The A / D and standard conversion system can process a variety of different formats, different standards, and convert them into a single digital signal that can be compressed; the compression encoding part is to compress the digital signal to reduce its bit rate, so that A conventional analog transmission channel can transmit multiple digital television signals without substantially reducing the signal quality; program multiplexing refers to the mixing of video and audio phase auxiliary data into a data stream through a program multiplexer, which constitutes a program digital stream, while adding some Information for business.

2) Digital satellite television system 2) Channel coding and digital modulation

Due to the large signal attenuation and low signal-to-noise in the satellite channel, certain spectrum utilization must be sacrificed to ensure sufficient power utilization. The DVB-S system has adopted two measures: one is to use cascaded channel coding Solution; the second is to use QPSK modulation.

a) Channel coding The most important thing in the transmission of digital signals is to prevent bit errors. Therefore, in the transmission, some numbers (ie error correction codes) used for error control must be added to the original source coding sequence in order to achieve automatic Error correction, thereby improving the reliability of signal transmission. DVB-S uses forward error correction coding (FEC). Concatenated channel coding consists of RS coding (Reed-Solomon coding) and convolutional codes. Among them, the RS code is also called an outer code, and the code type is RS (204, 188). Its characteristic is to correct the errors related to the group (8 bits as a group), which is very effective for correcting sudden errors. Convolutional coding is inner code. The code pattern can be selected from 1/2, 2/3, 3/4, 5/6, and 7/8. The selection criterion is a trade-off between spectrum utilization and anti-error performance. In addition to correcting errors in this group, convolutional coding also corrects errors in other groups. An interleaver is used between the RS code and the convolutional code, and a convolutional interleaving scheme is adopted. Interleaving is to solve the continuous errors that may occur in convolutional coding. The interleaver can spread the continuous errors, so that the receiving end can effectively correct errors, so that the continuous errors will not exceed the error correction capability.

b) Digital modulation four-phase phase shift keying (QPSK) modulation has high transmission efficiency and excellent error resistance. Its modulation signal is a constant envelope signal. The amplitude attenuation in the transmission channel has no effect on its performance, which is very suitable for satellites. Channel (due to the attenuation of the satellite transmission channel), but its channel utilization is not high, only 0.5Hz / bps. The receiving end is shown in Figure 3. The satellite signal (C-band or Ku-band) received by the antenna is converted into an L-band (0.9-1.4GHz) signal by low-noise amplification and down-conversion, enters the integrated receiving decoder (IRD), and is tuned. The demodulator and QPSK demodulator demodulate the digital signal (digital stream). This data stream is subjected to Viterby decoding, deinterleaving, and RS decoding. Errors introduced in the transmission are corrected, and then the digital stream is processed. The data stream is demultiplexed and sent to the MPEG-2 video and audio decoder. After decompression and digital-to-analog conversion, the analog signal is output. The output analog video signal can be a component signal or a composite signal.

DVB-S Digital Satellite TV System DVB-S Digital Satellite TV Technology

The DVB-S standard provides a complete set of specifications for digital television systems suitable for satellite transmission. The ISO / IECMPEG-2 standard is selected as the audio and video encoding and compression method, and the source encoding is unified; subsequently, MPEG-2 The code streams are packaged to form a transport stream (TS), multiple transport streams are multiplexed, channel coding and digital modulation are performed, and finally transmission is performed via satellite.

1)DVB Digital satellite TV system 1) DVB audio characteristics

The audio coding of DVB-S system uses MPEG-2LayerII two-layer audio coding, also called MUSICAM. Audio's MPEG-2LayerII encoding compression system uses the low sound spectrum masking effect of the sound. This human physiological effect allows us to encode low bit rate for frequencies that are not sensitive to the human ear. The use of this technology can greatly reduce the audio encoding rate. . MPEG-2LayerII audio encoding can be used for encoding of mono, stereo, surround sound and multi-channel multi-language sound.

2)DVB Digital Satellite TV System 2) DVB Video Features

The video of the DVB-S system uses standard MPEG-2 compression coding. The MPEG-2 video coding system is composed of a large family. Each subsystem has compatibility and commonality. According to the difference of image definition, it is divided into Four source formats, or "levels," range from low-resolution video on video tapes (VCR) to high-definition television. In addition to the "level" defined according to image sharpness, the DVB-S video standard also defines the concept of "profile". Each different "profile" can provide the compression tools and compression algorithms that constitute the encoding system. .

a) "Profile"

There are 5 "profiles" in the MPEG-2 system, and each "profile" will be more complex and complete than its previous "profile", providing more tools, and its corresponding The price of the equipment is also higher.

The first level of "profile" is called SimpleProfile, followed by MainProfile, which adds the function of encoding bidirectional prediction than SimpleProfile (B-FRAMES), when the same code stream is used. The quality will be better, but the algorithm is more complicated and more chips are used. The main profile decoding chip is compatible with decoding of simple profile encoding. This downward compatibility runs through the "grade" of the entire series.

After the main profile, there are SNRScalable profile and SpatialityScalableProfile. These two "profiles" can adjust the relationship between the signal-to-noise ratio and the code stream rate and the image sharpness. The relationship between the bit rate and the complexity of the encoding and the expensive receiver equipment do not support these two "grades" in the DVB-S standard. The highest-level "grade" is HighProfile, which is not only compatible with the previous low-grade "grade", and has all the functions, and can be coded on multiple lines at the same time, while the previous "grade" is line-by-line. There are two image sampling methods in the grade, 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). In the 4: 2: 2 format, 4 samples are taken for the luminance signal and 2 are used for the chrominance signal (RY, BY). See Figure 1 for chroma signals (RY, BY) in the 4: 2: 0 format. Only interlaced sampling is used. If 8-bit sampling is used, we can calculate 4: 2 after sampling a standard PAL TV signal: The bitstream rate of the 2 format image is as follows: The bitstream rate of the luminance signal: 720 × 576 × 25 frames / second × 8bit = 82.944Mb / s The bitstream rate of the chrominance signal: 2 × 1/2 × 720 × 576 × 25 frames /Sec×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 bit rate is very high, the occupied bandwidth is too wide, and it is not applicable For satellite transmission, even with 4: 2: 0 format, the image bit rate is as high as 124.416Mb / s. The compression algorithm of MPEG-2 uses an algorithm that removes the temporal and spatial redundancy of TV video signals, which reduces the bit rate to 3 to 8 Mb / s and still obtains a clear image, making digital television transmission possible. The core of the compression algorithm mainly includes some key technical links such as discrete cosine transform, shape scan and run length encoding, entropy encoding, channel buffering, motion estimation, and motion compensation.

b) "Level"

According to the definition of the image program source from low to high, the MPEG-2 standard is divided into many "levels". The lowest LowLevel definition is a quarter of the IU-R-BT.601 recommendation, namely: 352 × 288 × 25 frames / second; MainLevel is in full compliance with the standards recommended by IU-R-BT.601, namely: 720 × 576 × 25 frames / second; High-1440Level uses the method of 1440 samples per line; HighLevel uses a higher 1920 sampling method per line. The most commonly used MPEG-2 standard in the world is MP @ ML, that is: MainProfile @ MainLevel, which is the basis of the first generation of digital satellite television. Program providers can provide 625-line quality programs, and the length and width of images The ratio can be 4: 3 or 16: 9; its bitstream rate is selected by the program provider according to the program quality. The higher the image quality, the higher the required bitstream rate, and vice versa.

3) MPEG-2 stream multiplexing and service information

The audio, video and digital signals are first compressed by the MPEG-2 encoder, and the elementary code stream (ES) is formed by the program multiplexer. After the elementary code stream is packed, the elementary code stream (PES) with the header is formed. 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. It is necessary to add service information (SI) to the system multiplexer so that the receiving end can identify different programs.

DTH Digital Satellite Television System DTH

There are two international standards for DTH, the European standard DVB-S and the American standard DigiCipher. However, the DVB standard is gradually widely used around the world, and the Dish Network of the later American DTH company has also adopted the DVB standard.

The DTH system consists of six parts:

1Headend) Digital Satellite TV System 1) Headend System

The front-end system is mainly composed of video and audio compression encoder, multiplexer and so on. The main task of the front-end system is to digitally encode and compress the television signal, and use statistical multiplexing technology to transmit more programs on the limited satellite transponder band. DTH compresses video and audio signals according to the MPEG-2 standard. Using dynamic statistical multiplexing technology, it can transmit up to 10 sets of TV programs on a 27MHz repeater.

2Uplink) Digital satellite TV system 2) Transmission and uplink system (Uplink)

The transmission and uplink system includes communication equipment and uplink equipment from the front end to the uplink station. There are two main transmission methods: intermediate frequency transmission and digital baseband transmission.

3Satellite) Digital Satellite TV System 3) Satellite

DTH system uses high-power direct broadcast satellite or communication satellite. Due to technical and cost reasons, some DTH systems use high-power communication satellites, and DTH companies in the United States and Canada have adopted more suitable dedicated high-power direct broadcast satellites (DBS).

4SMS Digital Satellite TV System 4) User Management System (SMS)

The user management system is the heart of the DTH system and mainly completes the following functions:

A. Register and manage user data.

B. Buying and packaging programs.

C. Establish program charging standards and user charges.

D. Market forecasting and marketing.

The user management system is mainly composed of a database management system of user information and program information, and a Call Center that provides user services and provides a variety of customer services.

5CA Digital Satellite Television System 5) Conditional Access System (CA)

The conditional access system has two main functions:

A. Encrypt program data.

B. Authorize programs and users.

The conditional access systems used by DTH systems in the world are: US NDS, Israel Irdeto, France Via Access, Switzerland Nagra Vision, etc.

Direct TV in the US and Star Choice in Canada using Direct TV technology use the NDS conditional access system; Dish Network (Echostar) in the United States and Bell ExpressVu in Canada using the Echostar technology use the Nagra Vission conditional access system.

6IRD Digital satellite television system 6) User receiving system (IRD)

The DTH user receiving system consists of a small dish-shaped satellite receiving antenna (Dish), an integrated receiving decoder (IRD), and a smart card.

IRD is responsible for four main functions:

A. Decode the program data stream and output it to the TV.

B. Use the key in the smart card for decryption.

C. Receive and process various user commands.

D. Download and run various applications.

The IRD in the DTH system is no longer a pure hardware device, it also includes an operating system and a large number of application software. The more successful IRD operating system is Open TV. The American Dish Network company has begun to gradually upgrade the user's IRD to the Open TV system.

DVB Digital Satellite TV System DVB Introduction

Digital Satellite TV System Introduction

The DVB organization is an international organization from 33 countries and 230 organizations. The State Administration of Radio, Film, and Television of the Chinese Academy of Sciences joined the DVB organization as a member of the association on April 13, 1999. At the same time, DVB (DigitalVideoBroadcasting) is also a European digital video broadcasting standard. The main standards include satellite TV DVB-S, cable TV DVB-C, terrestrial broadcast ordinary TV DVB-T and high-definition television (HDTV) broadcast and transmission. The numbers of the above three transmission standards are ETS300421, ETS300429 and ETS300744? ETS are European telecommunications standards. DVB-S is a set of digital broadcasting technical specifications formulated to change the situation of analog television technology due to different standards. It is responsible for making it an international standard for digital broadcasting technology, and promoting international exchanges, cooperation and cooperation between radio and television technologies. Development tasks.

Digital satellite television system requirements

The main goal of the DVB project is to find a digital television technology and system that is suitable for all transmission media. The requirements for it are:

1. Flexible transmission of MPEG-2 video, audio and other data signals.

2. Use unified MPEG-2 to transmit bit stream multiplexing.

3. Use a unified service information system to provide details such as broadcast programs.

4. Use a unified first-level Reed-Solomon forward error correction system.

5. Use a unified scrambling system, but different encryption methods are available.

6. Select the modulation method and channel coding method suitable for different transmission media and any necessary additional error correction methods.

7. Encourage regions outside Europe to use the DVB standard and promote the establishment of worldwide digital video broadcasting standards.

8. Support teletext system in digital system.