Detailed technical details on cable digital set-top boxes

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1. The basic principle of cable digital set-top box

The basic function of the digital TV set-top box is to receive digital TV broadcast programs. The schematic diagram is shown in Figure 1. The tuning module receives the RF signal and downconverts it to an IF signal, then performs A/D conversion to a digital signal, and then sends it to the QAM demodulation module. QAM demodulation is performed to output MPEG transport stream serial or parallel data. The demultiplexing module receives the MPEG transport stream and extracts PES data of a program, including video PES, audio PES, and data PES. The demultiplexing module includes a descrambling engine that descrambles the scrambled data at the transport stream layer and the PES layer, and the output is the descrambled PES. The video PES is sent to the video decoding module, the MPEG video data is taken out, and the MPEG video data is decoded, and then output to the PAL/NTSC encoder, encoded into an analog television signal, and then output through the video output circuit. The audio PES 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.

Figure 1

Figure 1

Key technologies for cable digital set-top boxes

The set-top box consists of the following components: digital TV broadcast receiving front end, MPEG decoding, video and audio and graphics processing, cable modem, CPU and memory, and various interface circuits. The digital television broadcast receiving front end includes a tuner and a QAM demodulator, which can demodulate the MPEG transport stream from the radio frequency signal; the MPEG decoding portion includes a demultiplexing, descrambling engine and MPEG decompression, and the output is MPEG video and audio. Elementary stream and data payload. The video and audio processing and graphics processing sections complete the analog encoding and graphics processing functions of video and audio. The cable modem module consists of a two-way tuner, a downstream QAM demodulator, an upstream QPSK/QAM modulator, and a media access control (MAC) module that implements all of the cable modem functions. The CPU and memory modules are used to store and run software systems and control individual modules. The interface circuit provides a wealth of external interfaces, including universal serial interface USB, high-speed serial interface 1394, Ethernet interface, RS232, AV interface and so on.

The cable TV digital set-top box described in the block diagram is a full-featured set-top box. In fact, when implemented, the manufacturer can cut it as needed.

2. Real-time operating system

Everyone is familiar with the operating system of the PC, such as DOS, Windows98, WindowsNT, Unix, MacOS. Unlike these operating systems, the operating system in the set-top box is not very large, but it requires working in a real-time environment and running in a small memory space. This operating system is called a real-time operating system.

The popular real-time operating systems include VxWorks from WindRiverSystem, pSOS from Integrated Systems Inc., DAVIDOS-9 from Microware, OS20 from ST, WindowsCE from the previous introduction, and PowerTV developed for set-top boxes. These operating systems have their own strengths and are used in set-top boxes. VxWorks, pSOS, OS-9, OS20, etc. are general-purpose real-time operating systems, and are widely used in other embedded applications. In set-top boxes, they should be used in conjunction with the middleware described below. PowerTV is developed for set-top boxes, and the operating system that integrates middleware is widely used in the United States. In addition, with the rise of Linux, embedded Linux has gradually matured. It not only provides a choice for set-top box manufacturers, but also poses a huge threat to existing real-time operating systems due to the openness and advanced structure of Linux.

3. Middleware

Middleware is a software environment that isolates applications from low-level operating systems and hardware details. It is usually composed of various virtual machines, such as HTML virtual machines, JavaScript virtual machines, Java virtual machines, and MHEG-5 virtual machines. Machine and so on.

A complete digital set-top box consists of a hardware platform and a software system, which can be divided into four layers, from bottom to top: hardware, low-level software, middleware, and application software. The hardware provides the hardware platform of the set-top box; the underlying software provides the operating system kernel and various hardware drivers; the application software includes the native storage application and the downloadable application; the middleware separates the application software from the hardware-dependent low-level software. So that the application does not depend on a specific hardware platform.

Mature commercial middleware products include Opentv's EN2, Liberate's TVNavigatorforDTV, Enreach's EnreachTVforDTV, Canel+'s Mediahighway, and Intellibyte's IBEPG, IBSIManager, IBBrowser, and more. These products have a place in the market, but they are not compatible with each other.

Currently, standards organizations have recognized this fact and are beginning to establish open middleware standards. DVB proposed the middleware standard DVB-MHP (Multimedia Home Platform) based on Java virtual machine; ATSC established the T3/S17 technical expert subcommittee to define the software environment for the set-top box, which is called DTV Application Software Environment (DASE). ATVEF (Advanced TV Development Forum) has also created a specification called AdvancedTelevisionEnhancementForumSpecificationforInteractiveTelevision.

4. Add descrambling technique

The descrambling technique is used to encrypt and decrypt digital programs. Currently, there are two international standards: the POD defined by OpenCable and the SimulCrypt and MultiCrypt standards defined by DVB. The POD defined by OpenCable is a module connected to the set-top box through the PCMCIA interface. In addition to the interference function, the module also completes the interaction function with the front-end. DVB's MultiCrypt is also connected to the set-top box using the PCMCIA interface, but it only has the descrambling function. SimulCrypt for DVB only requires a set-top box with an ISO7816 SmartCard interface, but requires a set-top box with a hardware descrambling engine. The basic principles of conditional access to DVB are briefly described below.

The basic principle of conditional access is shown in Figure 4. Before the program is broadcasted, it is subjected to scrambling. The scrambling process is to modulate the multiplexed transport stream (TransportStream) with a pseudo-random scrambling sequence, and the pseudo-random sequence is generated by the control word generator. The provided control word (ControlWord, referred to as CW) is determined. The core of conditional access is actually the control of control word transmission. In the MPEG transport stream, there are two data streams associated with control word transmission: Entitlement Control Information (ECMs) and Entitlement Management Information (EMMs). The control words encrypted by the service key (SK) are transmitted in the ECMs, and also include program information such as program source, time, content classification, and program price. The service key encrypted for the control word is transmitted in the authorization management information, and the service key is subjected to encryption processing by the user's personal distribution key (PDK) before being transmitted. The EMMs also include an address, user authorization information, and 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 (SmartCard).

At the user end, the set-top box must acquire relevant conditional access control information in order to reproduce the descrambled random sequence. First, the set-top box obtains the PID values ​​of the EMM and ECM based on the CA_descriptor in the PMT and CAT tables. Then, the ECMs and EMMs are filtered out from the TS stream and sent to the SmartCard through the SmardCard interface. SmardCard 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 SmartCard interface. The descrambling engine uses the CW. It is possible to descramble the stream that has been scrambled

5. Cable modem

So far, various research institutes have conducted extensive research on cable modems, and these research results have finally formed several internationally defined standards: IEEE-defined IEEE802.14 protocol, DAVIC-defined DAVIC protocol, and MCNS-defined DOCSIS. protocol.

Among these protocols, the purpose of the MCNS standard is the simplest and clearest, that is, transparently transmitting IP data packets on a wired network, so the protocol supports IP best; the purpose of DAVIC is mainly to provide users with interactive digital video and audio. The service, while also providing data transmission, is the best support for digital video; the purpose of IEEE802.14 is to build an HFC-based metropolitan area network and enable the network to support a variety of services, including fixed bit rates. CBR, variable bit rate VBR and effective bit rate ABR service, this purpose makes it very good support for ATM.

Different purposes make great differences between standards. These differences are mainly reflected in the physical layer defined by each standard, the frame format of the MAC layer, and the MAC protocol.

IEEE 802.14 has good support for ATM cells and can support OoS well with ATM cells. In terms of transmitting ATM cells, this protocol has less delay and delay jitter than MCNS. However, in terms of supporting IP, since it must be supported by AAL5, it is lower in the throughput of transmitting IP packets than the MCNS standard, which is also applicable to the DAVIC standard.

At the MAC layer, the MCNS standard uplink channel access method and collision resolution algorithm are simpler than IEEE802.14 and DAVIC, making the user equipment simpler, plus the best support for IP, making the MCNS standard in the current market. Most optimistic. But it is also because of its simplicity, there are some shortcomings when using MCNS CableModem for voice and video services. Specifically, in the use of MCNS standard CableModem to support IP telephony and video telephony, it is also necessary to increase the mechanism to support QoS. . But with the release of DOCSIS 1.1, the standard has gradually improved.

In support of digital video services, DAVIC has an important influence on the standards of digital set-top boxes due to its compatibility with digital television broadcasting and the advantages of having an out-of-band channel. In fact, North America's OpenCable standard is the product of organically combining MCNS and DAVIC, while Europe's EuroBox and EuroModem actually use the DAVIC standard. The IEEE 802.14 protocol, which was first developed, has gradually been abandoned by the market.

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