1.DVB-C front-end system induction
The hardware platform of 1. 1 includes: ① satellite digital TV receiver array, ②MPEG-2 encoder array, ③ stream multiplexer/distributor array, ④ scrambler array, ⑤ video server array, ⑤ Si server, ⑦64QAM modulator array, ⑧ digital matrix switch, ⑨ gateway, switch, router and ⑩.
1.2 central software platform includes: ① SMS system, ②CA system, ③EPG system, ④ middleware system, ⑤ network management system, ⑤ billing system and ⑤ broadcast control system.
2. Each part can be better explained.
2. 1 signal input part:
Through certain format conversion, different formats of video and audio signals and data signals are converted into ts stream signals conforming to DVB-C standard.
2.2 signal processing part:
According to the corresponding relationship of code rates, the multi-channel TS stream signals output by the signal input part are multiplexed and scrambled to form a multi-channel digital TV program transmission code stream conforming to DVB/MPEG-2 standard.
2.3 signal output part:
Multiplexed and scrambled multi-channel digital TV program transmission streams are encoded and modulated by channels to form radio frequency signals suitable for cable network transmission. The main functions include baseband interface, synchronous inversion, data randomization and scrambling, RS coding, convolutional interleaving, byte-to-m symbol mapping, differential coding, baseband shaping and QAM modulation.
2.3. 1? Baseband interface:
Data adaptation signal source format, intra-cell signal frame structure and MPEG-? 2? Transport layer consistency.
2.3.2? Data randomization scrambling code:
According to MPEG? -? 2? The frame structure inverts the synchronization bytes, and in order to realize spectrum shaping, the data stream is randomized to make the energy of the data sequence evenly distributed.
TS packet scrambling structure diagram and RS packet coding structure diagram.
TS packet length: 1 sync byte+187 bytes = 188 bytes.
RS packet length: 1 synchronization byte+187 bytes+16 supervision byte =204 bytes.
2.3.3? RS coding:
Use truncated Reed-Solomon code? Rs (204, 188, t = 8) performs forward error correction and processes each randomized transmission packet to generate an error protection packet.
2.3.4? Convolutional interleaving:
Byte convolution interleaving of error protection packets with interleaving depth of I = 12 is adopted to generate interleaved error protection frames.
2.3.5? Byte to m-bit symbol mapping:
Convert the bytes generated by the interleaver into QAM? Symbol.
2.3.6? Differential coding:
The upper two bits of the mapping symbol starting from most significant bit (MSB) are differentially encoded to form an in-phase component Ik and a quadrature component Qk.
2.3.7 baseband shaping:
In QAM? Before modulation, I signal and Q signal are shaped by raised cosine square root roll-off filtering, and the roll-off coefficient α = 0. 15.
2.3.8QAM modulation:
QAM signal and q signal.
QAM modulation
The modulated signal passes through the intermediate frequency IF.
And radio frequency RF
Send to RF channel. What is the threshold of CNR in 64QAM modulation mode? =25.5? [dB], transmission rate =38.4? [Mb/s], spectrum utilization ratio =4.8? [Bit/sec/Hz]. Related calculation formula
Modulation level? m？ =? 2n？ (? N is the number of bits per symbol)? ;
Bit rate? r？ =M×BW？ (? BW？ For channel bandwidth)? ;
Symbol rate? BS？ =? BW/？ (? 1+α? )
Bit rate? r？ =? BS？ (? 1+α? ) ×n(α is the filter roll-off coefficient of MPEG-2 =0. 15? );
Spectrum utilization? =R (transmission bit rate)/? BW (channel bandwidth)/? (? 1+α? )=n？ /? (? 1+α? )? [(? bps/Hz？ )]
2.4 System management part:
Including (1) conditional access system (CAS); (2) User Management System (SMS); (3) Network Management System (NMS).