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1 IntroductionSet Top Box (STB) is the best solution for the transition from analog broadcast to digital broadcast TV. By combining a digital set-top box with a normal analog TV, digital TV functions can be realized. Digital set-top boxes can convert digital TV signals from satellite, terrestrial or cable to PAL/NTSC signals that analog TVs can receive. They can also support video on demand, Internet browsing and Other functions. The set-top boxes with practical development value can be divided into digital satellite receiving (DVB-S) set-top boxes and digital cable television (DVB-C) set-top boxes.
The cable television (CATV) network has the advantages of wide coverage, rich frequency resources and low construction cost, and can support traditional analog services, new digital on-demand (VOD) and data information services. China has not yet developed a CATV digital TV broadcasting standard, but adopted the European DVB-C standard. This paper introduces the design principle and solution of cable TV set-top box system using DVB-C standard.
2 Design principles and key technologies?
The main functions that cable TV set-top boxes should have are: receiving digital broadcast signals (including audio, video and data signals on CATV) and video on demand.
The digital data stream is demodulated, demultiplexed, and decoded through each chip of the set top box. Its principle block diagram is shown in Figure 1.
2.1 Data modulation and demodulation techniques
At present, the modulation technologies used in digital high-definition television transmission systems in the world mainly include: four-phase phase shift keying (QPSK), multi-level quadrature amplitude modulation (MQAM), and multi-level vestigial sideband modulation (MVSB). And Orthogonal Frequency Division Multiplexing (OFDM). In the cable TV, the MQAM modulation method is mainly adopted. QAM modulation is a frequency-saving digital amplitude modulation method (16QAM spectrum utilization is 4 (bit/s)/Hz, 64QAM spectrum utilization is 6 (bit/s)/Hz), and has a high signal-to-noise ratio. . The QAM modulation method is often used in medium and high speed modulation above 2400 bit/s, and is widely used in downlink transmission of cable television and terrestrial broadcast transmission of HDTV.
2.2 plus descrambling technology
The plus descrambling technique is used to encrypt and decrypt digital programs. The basic principle is to use the scrambling control word encryption transmission method, and the user end uses the IC card to decrypt. In the MPEG transport stream, there are two data streams associated with control word transmission: Entitlement Control Information (ECMs) and Authorization Management Information (EMMs), and control words encrypted by the Service Key (SK) are transmitted in the ECMs. It also includes program information such as program source, time, content classification, and program price. The service key encrypted by the control word is transmitted in the authorization management, and the service key is subjected to encryption processing by the user's personal distribution key (PDK) before being transmitted. The EMMs also include the address and the user authorization information, such as the user can view. Program or time period, user paid viewing fees, etc. The User Personal Distribution Key (PDK) is stored in the user's Smart Card. On the user side, the set-top box obtains the PID values ​​of the EMM and ECM according to the CA?descriptor in the PMT and CAT tables, and then filters out the ECMs and EMMs from the TS stream and sends them to the Smart Card through the Smart Card interface. The Smart Card first reads the user's personal distribution key (PDK), decrypts the EMM with the PDK, extracts the SK, then decrypts the ECM with the SK, extracts the CW, and sends the CW to the descrambling engine through the Smart Card interface. The descrambling engine The disturbed transport stream can be descrambled using CW. The plus descrambling technique is divided into the same density and multi-density technology.
The same-density technology is to apply two or more conditional access (CA) systems to the same network platform, which is a technology choice and a competitive environment for cable TV stations.
The multi-density technology requires the set-top box to adopt CI technology, so that the same set-top box can receive encrypted programs of different CA systems. From the user's point of view, there is no limit to which CA's set-top box to purchase, and users have the possibility to choose their CA service.
2.3 Multiplexing and Demultiplexing Technology
The entire multiplexing process can be described as two different levels of multiplexing. ?
Program layer multiplexing: The video stream, audio stream, and data stream are multiplexed into a transmitted bit stream of a program by a multiplexer. ?
System layer multiplexing: The transport bit streams of multiple programs are multiplexed into a systematic bit stream by a multiplexer. ?
The standard MPEG-2 TS code stream is generated by the two-stage multiplexing.
The demultiplexing unit is composed of a dedicated processor, which is responsible for parsing the MPEG-2 TS stream and the packetized elementary stream (PES), recovering the code stream required by the audio and video decoder, and is responsible for identifying the transmission error. The transport system uses a fixed-length 188-byte transport stream packet scheme defined by MPEG-2, which identifies video, audio or data information through a packet identifier (PID) in the packet header. The transport stream contains one or more programs, each program consisting of one or more elementary streams multiplexed together. In the single channel program transmission multiplexing, the PID information of the transmission code stream constituting the program, the application (such as audio, video, etc.) identifiers transmitted in the code stream, and the relationship between these code streams, etc., by the program map table (PMT) )To represent. The PID number of the code stream containing the PMT is given by the Program Association Table (PAT) carried by the system level control code stream of PID=0 defined at the time of multiplexing of the program system. Both PMT and PAT are part of Program Specific Information (PSI) that contains program and system information. The demultiplexer identifies the PID of the bit stream carrying the desired program map table (PMT) by using the program association table (PAT) of PID=0, and then obtains the PID of the elementary stream constituting the program from the program map table (PMT). No., by setting a filter in the demultiplexer, to receive the transport stream of the desired program. The audio and video elementary stream is transmitted in the transport layer in the form of a packetized elementary stream (PES), and the compressed audio and video stream data after the PES unpacking of the selected program is delivered to the audio and video decoding unit.
2.4 MPEG-2 video coding
The MPEG-2 video coding system consists of a large family. Each system has compatibility and commonality. According to the image clarity, it is divided into four source formats or “levelsâ€. (VCR) low image clarity to high definition TV. In addition to the "level" defined by image sharpness, the DVB video standard also defines the concept of "profile", each of which provides the compression tools and compression algorithms that make up the coding system.
2.4.1 "Levels"
At present, there are five "grades" in the MPEG-2 system. Each "grade" is more complicated and more complete than its previous "grade", providing more tools, and the price of its corresponding equipment is also more high.
The initial level of "grade" is called Simple Profile, followed by the Main Profile, which adds the ability to encode bi-directional predictions, ie B-FRAMES. In the case of using the same code stream, its quality will be better, but the algorithm is more complicated and uses more chips. The decoding chip of the main grade can be compatible with the decoding of the simple grade code. This downward compatibility runs through the entire series of "grades".
After the main grade, it is the SNR Scalable and the Space Scalable Profile. These two “grades†can adjust the relationship between signal-to-noise ratio and code rate, and image clarity. The relationship between code stream rates, the DVB standard does not currently support these two "grades" due to the complexity of its encoding and the high cost of receiving devices. The most advanced "grade" is HIGH PROFILE, which is not only compatible with the previous low-level "grade", but also has all the functions, and can be multi-line simultaneous encoding.
2.4.2 "Level"
Depending on the clarity of the image source, the DVB MPEG 2 standard is divided into many “levelsâ€. The lowest LOWLEVEL resolution is one quarter of the IU-R-BT and 601 recommendations, namely: 352 × 288 × 25 frames / sec. MAIN LEVEL is fully compliant with the IU-R-BT, 601 recommended standard, namely: 720 × 576 × 25 frames / sec. The HIGH-1440 LEVEL uses a method of 1440 samples per line. HIGH LEVEL uses a higher sampling method of 1920 lines per line.
At present, the most commonly used MPEG-2 standard in the world is MP@ ML, namely: MAIN PROFLE@ MAIN LEVEL, which is the basis of the first generation of digital cable TV and digital satellite TV, and the program provider can provide 625 line quality programs. The aspect ratio of the image can be 4:3 or 16:9. As for the code stream rate, it is selected by the program provider according to the program quality. The higher the image quality, the higher the required code stream rate, and vice versa.
3 hardware implementation
Most of the set-top boxes currently on the market only have a single function of converting digital signals into analog signals that can be received by ordinary TV sets. Services such as video on demand (VOD) have been launched in individual cells in individual cities. Since the network used by most cable TV users in China is still a one-way network, the investment in two-way transformation is relatively large and cannot be completed in the near future. Therefore, Modem can be considered to transmit back uplink data through the telephone line. This article describes a solution for implementing a CATV set-top box with ST's series of chips, as shown in Figure 2.
3.1 Front end part
The main function of the front-end part is to reduce the high-frequency signal transmitted by the cable TV network to the intermediate frequency signal by the tuner. After filtering and amplifying, the signal is sent to the QAM demodulation chip to complete the demodulation work. The solution uses MicroTune's MT2040 Silicon Tuner tuner and ST's STV0297QAM demodulation chip.
The front end structure is shown in Figure 3. The MT2040 Silicon Tuner can receive high frequency signals from 48MHz to 860MHz and convert them to the IF band required by the user. After a surface acoustic wave filter (SAW Filter) and an MT1230 intermediate frequency amplifier, the down-converted intermediate frequency signal is transmitted to the STV0297 for QAM demodulation.
The STV0297 integrates a high-performance A/D converter on-chip to directly sample the input IF signal into A/D. The STV0297 also provides two PWM outputs for the AGC, one for the Tuner (AGC1) and the other for the IF Amplifier (AGC2) for AGC adjustment of the input signal. The digital signal obtained after A/D conversion is processed by the Nyquist filter to achieve a transmission roll-off factor of 0.13-0.15. The energy loss of the signal through the Nyquist filter is compensated by the on-chip digital AGC module. In addition, STV0297 also needs to complete carrier recovery, equalization, deinterleaving and FEC decoding. Carrier recovery eliminates residual carrier frequency and phase offset. Channel equalization properly eliminates various echo and linear channel distortions. In order to reduce the impulse noise, the error correction capability of the RS FEC coding is increased, and the data is interleaved after the RS coding at the transmitting end. Therefore, the deinterleaving operation is performed before the RS decoding at the receiving end. The Forney-type deinterleaver is used in STV0297 with a default deinterleaving depth of 12 and a cell depth of 17. Since MPEG data stream header information is very important for correct signal reception, STV0297 uses Reed-Solomon error correction code to ensure the correctness of the received digital bit stream.
3.2 Backend part?
The back end is shown in Figure 4. The QAM demodulator performs channel decoding, separating the transport stream (TS) containing the audio, video and other data information from the carrier. A transport stream typically contains multiple audio, video streams, and data information. The demultiplexer is used to distinguish different programs and extract corresponding audio streams, video streams, and data streams. The demultiplexing module includes a descrambling engine that descrambles the scrambled data at the transport stream layer and the PES layer. Its output is the descrambled PES. The video PES stream is sent to the video decoding module, the MPEG video data is taken out, and the MPEG video data is decoded and output to the PAL/NTSC encoder, and the encoded analog signal is output through the video output circuit. The audio PES stream is sent to the audio decoding module, the MPEG audio data is taken out, and the MPEG audio data is decoded, and the PCM audio data is output to the PCM decoder, and the PCM decoder outputs a stereo analog audio signal, which is output through the audio output circuit. For pay TV, the conditional access (CA) module descrambles the audio and video streams, and uses a smart card containing the identification user and the billing function to ensure that the legitimate user can watch normally.
The STI5518 uses the OS20 Real-Time Embedded Operating System (RTOS). Its characteristic is that it is not very large, and can be tailored according to user requirements (scalable, through conditional compilation in C language), but requires that it can work in real-time environment, and can be in a small memory space. run. OS20 is an operating system developed by ST that supports real-time processing. Its CPU is ST20C2+ 32-bit processor, which supports real-time features such as multitasking, kernel management, process scheduling, and task priority.
4 Conclusion
TV digitalization is the trend of development in the future. Although it is mature in theory, it is limited by the problem that the existing cable TV network is a one-way network and cannot transmit back data in the specific implementation process. The design method needs to be changed according to the specific situation. The method of using the Modem for uplink data backhaul mentioned in this paper can utilize the existing cable television transmission network without large-scale circuit transformation, and is a relatively economical and practical solution.
references
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