see the block diagram
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The working phase of the digital television set-box is roughly as follows:
1. The R.F.head receives the high frequency signal from cable network, finish the signal channel to decode through QAM demodulator, isolate the sound, video and other data information to flow from the carrier. Generally sound, video flowing and some data information while conveying and flowing.The digital television set-box is the integration products of computer and television in view of function, the set-box includes the following levels:
( 1) Physical layer and connecting layer: Including high-frequency tuner, QPSK, QAM, OFDM, VSB demodulation, the convolution is decoded, de-interleaving, a Saloman of Reed decodes, solve energy dispersal.
( 2) Transport layer: Including demultiplexing, it divides transport stream into video, audio frequency and data packet.
( 3) Program layer: Including MPEG- 2 video decoding. MPEG/AC- 3 audio decoding.
( 4) User level: Including serving information, the electronic listing, graphical user interface (GUI) ,The browser, remote control, conditional acceptance, the data are decoded.
( 5) Output interface: Look at the audio interface including dividing simulation, the figure looks at the audio interface, data interface, the keyboard, the mouse,etc..
2.Set-box critical technology of the digital television
The top box of digital machine embodies a concentrated reflection of the multimedia, computer, data compressed encoding, adds the descrambling algorithm, adds decoding algorithm, communication skill and network state-of-the-art, so the technological content is very high, the critical technology that it involves is mainly as follows,
( 1) Multiplex and decompression technique: After the digitization of analog signal. The information volume is increased severely, data compaction is essential. MPEG- 2 video compression standard adopts in the digital television extensively, suitable for the image quality of many kinds of sharpness:
MPEG- 4 adopt on the basis of compressed coded method of target, it cut different target apart into, deal with respectively picture and video, not only raise the compression ratio of the data, still can realize a lot of mutual functions based on content, have offered the broader platform for compressed encoding of multimedia data.
It is employed extensively that technical with large-scale integrated-circuit technique and embedded system, the hardware implementation of the top box of digital machine adopt the special purpose chip or digital signal processing (DSP more, DigitalSignalProcessing) The chip, integrate CPU kernel and MPEG-2, MPEG4 transport stream NULL, DVB common descrambler, MPEG audio frequency video decoder and NTSC/PAL Coder, form the key chip of STB. In order to realizing demultiplexing in real time and real time data information processing.
( 2) Demodulation of data of descenting and signal channel decode technology: The data after compressing can't be transmitted on the signal channel directly, need to carry on channel coding and modulate. Usually transmissive digital television and value-added service in cable television network, adopt more QAM (QuadratureAmplitudeModulation)s Modulation mode and RS (ReeDSOlomon) Error correction coding.
( 3) The machine format modulation code of up run: In the interactive application, generally adopt 3 ways, namely adopt the telephone line to convey the upstream data. Adopt Ethernet to convey the upstream data and convey the upstream data through the wired network. Because upstream data should be few much to data of descenting, so use more OP-SK (QuadraturePhaseShiftKeying)s Or 160AM way is modulated.
( 4) The network browses through technology: In order to realize the real network browses through, the Cable News Network connects with Internet except considering the up run and data of descenting, must also consider that operate IP (InternetProtocol) on it .