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Important module of Digital set-top boxes

time2011/03/18

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1,A modem module

Digital cable set-top box work in the state, cable TV with analog transmission, it is essential to modulate and demodulate digital signals can be transmitted in the analog channel, the modem is a key component of the system, is technically similar to the principles of the present telephone modem , but with a higher modulation, 64QAM, or down to use more 256QAM, in the DVB-C (Digital Video Broadcast by Cable) and DAVIC used as a standard 64QAM modulation, demodulation chips to Motorola's MC92305QAM example, 7M analog bandwidth using 64QAM modulation on the digital signal rate of up to 42Mbit / S, up now in two ways, one is using phone line as the uplink channel, another is a two-way HFC network upstream channel using the HFC network QPSK as the modulation scheme used. QAM or QPSK modulator to MPEG format, the data flow modulation in a channel within the PAL standard, along with other video modulation signal sent Combiner.

2, codec module

As the analog channels in order to ensure reliability of data transmission and low bit error rate, forward error correction coding is essential, DVB with Reed Solomon codes, RS codes are a class of error correcting binary BCH very capable of multi- code. MPEG II decoding module is the core of digital set-top boxes fordigital video technology, MPEGII decoding module can be called outside the CPU core module, MPEG II digital transmission using interleaved coded, first of all need to stream to de-interlacing, video, audio, and data stream separation, as well as video codes decoding. After the above steps MPEGII stream into video (CCIR656 format) and audio digital signals. MPEG program stream multiplexer to separate ways, combined with the data stream to 188 bytes in a frame of MPEG2 data format sent to the RF modulator and provides electronic program guide (EPG).

3 ,digital video encoder and audio DAC

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Digital set top box "peripheral" was TV and sound system, digital audio video signal must be converted to analog audio and video signals to the company's MC44724 Motorola example, MC44724 can ITU601, 656 standard 4:2:2 video data in parallel PAL or NTSC format is converted to video, S-Video, Y / Cb / Cr or R / G / B, expansion of VBI (Vertical Blanking Interval) of information input to display graphic information. Audio DAC is generally not introduced here. Modern sound system support Dolby Pro Logic and Lucasfilm Home THX home theater systems, required before the audio DAC with digital audio signal processing chips for digital audio signal processing, there are special processing chips and digital signal processing chip DSP to handle two way.

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4 graphical control module

Digital set-top boxes and video graphics mixer similar to the other hand the need for complex computer graphics and even three-dimensional graphics, requires the system with strong two-dimensional and three-dimensional graphics acceleration chip, but also the graphics and video signals can be mixed stack shown that such functions similar to the existing CG. MAC media access control module in the network traffic on the digital set-top box can be seen as Cable Modem, IP router receives the client through the PSTN (public telephone network) came the demand signal and transmitted to the Internet server / router. At present the main reference DAVIC and IEEE 802.14 standard, IEEE802.14 advocating the use of ATM access in the 802.14 MAC layer defines the ATM layer, DAVIC also defines Ethernet and ATM standards.

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5 ,rewritable DVD

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Existing digital set-top box Completely work in the network environment, only use of smart cards similar personal account information such as storage of decoding numbers do not have prominent local data storage capabilities, all of the information Xuyao from online access, regardless of network status now or in the future, Xiang CD-ROM medium type large-capacity information transmission is still a major way, CD-ROM The disadvantage is that information can not be updated. Use rewritable DVD disc can be used not only great information, more effective network of data through the Internet update, so that the data on DVD disk to date, DVD digital set-top box should be an important backup module.

6 ,add descrambling module and Receiving Antenna

In the cable operators, the pay-TV is a major business, digital set-top boxes required to have television signal descrambling functions increase, due to digital signal, plus descrambling easily than analog signals and descrambling plus high secrecy, DVB add descrambling functions on the relevant definitions, on the other hand, using the copyright protection on digital signal increased difficulty of the image by Macrovision generator protection activities.