The working principle of digital TV set-top box
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The digital TV set-top box is composed of high-frequency head, QAM demodulator, TS stream demultiplexer, MPEG-2 decoder, PAUNTSC video encoder, embedded CPU system and peripheral interface, CA module and upstream data modulator. The working principle is as shown in the figure. 
The working process of the digital TV set-top box is as follows: the high-frequency head receives the high-frequency signal from the cable network, completes the channel decoding through the QAM demodulator, and separates the transmission stream containing audio, video and other data information from the carrier. The transport stream generally includes multiple audio and video streams and some data information. The demultiplexer is used to distinguish different programs, extract the corresponding audio, video and data streams, and send them to the MPEG-2 decoder and corresponding parsing software to complete the restoration of digital information. For pay TV, the conditional access module descrambles audio and video streams, and uses a smart card that includes user identification and billing functions to ensure that legitimate users watch normally. The MPEG-2 decoder completes the decompression of audio and video signals. After video encoder and audio D / A conversion, analog audio and video signals are restored, high-quality images are displayed on conventional color TV sets, and multi-channel stereo programs are provided. .
Digital TV set-top boxes are functionally a fusion product of computers and TV sets, but the structure is different from the two. From the perspective of signal processing and application operations, set-top boxes include the following layers:
(1) Physical layer and connection layer: including high-frequency tuner, QPSK, QAM, OFDM, VSB demodulation, convolutional decoding, deinterleaving, Reed-Solomon decoding, and energy dispersal solution.
(2) Transport layer: includes demultiplexing, which divides the transport stream into video, audio, and data packets.
(3) Program layer: including MPEG-2 video decoding. MPEG / AC-3 audio decoding.
(4) User layer: including service information, electronic program table, graphical user interface (GUI), browser, remote control, conditional access, and data decoding.
(5) Output interface: including analog video and audio interface, digital video and audio interface, data interface, keyboard, mouse, etc.
2. Key technology of digital TV set-top box
The digital set-top box reflects the development level of multimedia, computer, data compression coding, encryption and descrambling algorithm, encryption and decryption algorithm, communication technology and network technology, so the technical content is very high.
(1) Multiplexing and decompression technology: after the analog signal is digitized. The amount of information has increased dramatically, and data compression is essential. The MPEG-2 video compression standard is widely adopted in digital TV, and is suitable for various definition image quality:
MPEG-4 uses an object-based compression coding method, which divides the image and video into different objects and processes them separately, which not only improves the data compression ratio, but also realizes many content-based interactive functions, which provides more For a broad platform.
With the wide application of large-scale integrated circuit technology and embedded system technology, the digital set-top box hardware mostly uses dedicated chips or digital signal processing (DSP, Digital Signal Processing) chips to demultiplex the CPU core from the MPEG-2 and MPEG4 transmission streams. , DVB general descrambler, MPEG audio video decoder and NTSC / PAL encoder integration, forming the core chip of STB. To achieve real-time demultiplexing and real-time data information processing.
(2) Downlink data demodulation and channel decoding technology: the compressed data cannot be directly transmitted on the channel, and channel coding and modulation are also required. The digital TV and value-added services usually transmitted in the cable TV network mostly use the QAM (QuadratureAmplitudeModulaTIon) modulation method and RS (ReeDSOlomon) error correction coding.
(3) Modulation and coding of upstream data: In interactive applications, three methods are generally adopted, that is, using telephone lines to transmit upstream data. Use Ethernet to transmit uplink data and transmit uplink data via wired network. Because the upstream data is much less than the downstream data, the OP-SK (QuadraturePhaseShiftKeying) or 160AM mode is used for modulation.
(4) Web browsing technology: In order to achieve real web browsing, the connection between the cable TV network and the Internet must consider running IP (InternetProtocol) on it in addition to the upstream and downstream data.
(5) Real-time operating system: The operating system in the digital set-top box uses a real-time operating system (RTOS.Real-TImeOperaTIngSvstem). RTOS can work in a real-time environment and takes up less memory space. It is responsible for managing local resources and network resources. Provide basic operation functions and device access control.
(6) Middleware technology: Middleware (Middleware) technology, that is, open business platform, has become the core technology of interactive TV. Middleware is usually composed of Java virtual machine, web browser, image and multimedia modules, etc. By defining a complete set of standard application program interfaces, the application program is independent of the operating system and hardware platform. Makes the development of application programs simpler, and the openness and portability of products are stronger to ensure the expandability of set-top boxes and the effective recovery of investment.
Conclusion
Interactive TV is the future development direction of cable TV, and plays an important role in the construction of multimedia broadband networks and national information infrastructure. We are currently in a critical period of development of digital TV and high-definition TV, the interactive TV industry is booming, so it is very necessary to understand digital TV technology and digital set-top boxes.