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• #11503 wassermann Topikgazda 4ron #11502

  Új Válasz 2012-09-30 10:29:56 #11503

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  wassermann

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  válasz 4ron #11502 üzenetére

  Angolul értőknek a DVB-T - DVB-T2 közti különbségek (korszerűbb, komplexebb hibajavítás, kódolás, stb...)

  Some of the main differences are:

  Transport streams - that each can carry more multiplexed bit-streams - can be allocated to Physical Layer Pipes or PLP's. Each PLP can be encoded with its own QAM modulation and FEC protection - some can be highly protected, but have low useful bit-rate while another PLP can have less protection and a high bit-rate - all on the same DVB-T2 multiplex.

  DVB-T2 uses a very much stronger Forward Error Correcting code - the LDPC/BCH code. DVB-T uses the rather weak Viterbi algorithm followed by a ReedSolomon code. The RS code itself uses 8% of the bit-rate, while the BCH code used with DVB-T2 just uses 0.2-0.3%.

  DVB-T2 allows many more FFT modes 1k, 2k, 4k, 8k, 16k, 32k (DVB-T is only using the 8k mode except for the 2k mode at pre-DSO UK transmitters and maybe 4k with DVB-H). Larger FFT modes lowers the overhead with SFN's very significantly. But Doppler effects will make the larger FFT modes of little use in a true mobile environment.
  Note all PLP's within a DVB-T2 multiplex must all use the same FFT mode.

  DVB-T2 enables 256-QAM modulation. This will allow 8 bits to be encoded at each constellation/frequency slot.

  DVB-T has a fixed and large overhead (8%) from 'pilot-tones' that are used for synchronisation and channel estimation. DVB-T2 has 8 different 'pilot-patterns' - some with very low overhead.

  DVB-T2 uses something called 'Rotated Constellations' that divides each constellation into two parts, which both contains all the information (2,4,6 or 8 bits). One half is then swapped with the same half of a another constellation/frequency slot before transmission. Assume aA is the two parts of one frequency slot and bB is another. Then these two are transmitted as aB and bA at different frequencies. If say aB is lost due to noise or fading then all bits can be recovered from the bA frequency slot. This increases very significantly the chance of error free reception in difficult situations. This will also enable the code-rates 4/5 and 5/6 to be used in real system (the DVB-T code-rates 5/6 and 7/8 are rather fragile and useless)
  (In practice 'a' is moved to 'B', 'b' to 'C', 'c' to 'D', 'd' to 'E' .... and 'z' to A' before the frequency interleave)

  forrás: (klikk)

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