diff options
| author | Julius Durst <julius.durst@student.kit.edu> | 2017-05-29 01:14:10 +0200 |
|---|---|---|
| committer | Martin Braun <martin.braun@ettus.com> | 2018-12-13 20:19:03 -0800 |
| commit | a50812d2b2f8aeb0bf9bd8806b49a32e98866f72 (patch) | |
| tree | 946b060145516e67ab3e210e91d087a4be5d70ca /gr-dtv | |
| parent | 76538055531207b36528484d86e9cb1a632c9d7b (diff) | |
gr-dtv: Implement the dvb bch encoder using an improved algorithm
This commit uses a bitset to represent the shift register
and precomputes all dividents for the bch encoder
to efficiently compute the checksum.
written by Team "Barkhausens Army" @ Rohde & Schwarz Engineering
Competition 2017
Diffstat (limited to 'gr-dtv')
| -rw-r--r-- | gr-dtv/lib/dvb/dvb_bch_bb_impl.cc | 69 | ||||
| -rw-r--r-- | gr-dtv/lib/dvb/dvb_bch_bb_impl.h | 4 |
2 files changed, 55 insertions, 18 deletions
diff --git a/gr-dtv/lib/dvb/dvb_bch_bb_impl.cc b/gr-dtv/lib/dvb/dvb_bch_bb_impl.cc index 8d89f9b04a..fef580acdc 100644 --- a/gr-dtv/lib/dvb/dvb_bch_bb_impl.cc +++ b/gr-dtv/lib/dvb/dvb_bch_bb_impl.cc @@ -490,6 +490,31 @@ namespace gr { sr[0] = (sr[0] >> 1); } + //precalculate the crc from: http://www.sunshine2k.de/articles/coding/crc/understanding_crc.html - cf. CRC-32 Lookup + void dvb_bch_bb_impl::CalculateCrcTable() + { + for (int divident = 0; divident < 256; divident++) /* iterate over all possible input byte values 0 - 255 */ + { + std::bitset<192> curByte(divident); + curByte <<= 184; + + for (unsigned char bit = 0; bit < 8; bit++) + { + if ((curByte[191]) != 0) + { + curByte <<= 1; + curByte ^= polynome; + } + else + { + curByte <<= 1; + } + } + + crcTable[divident] = curByte; + } + } + void dvb_bch_bb_impl::bch_poly_build_tables(void) { @@ -555,6 +580,13 @@ namespace gr { len = poly_mult(polyn12, 17, polyout[1], len, polyout[0]); poly_pack(polyout[0], m_poly_n_12, 192); + //pack the polynome in a bitset + for (int i = 0; i < 192; i += 1) { + polynome[i] = polyout[0][i]; + } + CalculateCrcTable(); + + len = poly_mult(polys01, 15, polys02, 15, polyout[0]); len = poly_mult(polys03, 15, polyout[0], len, polyout[1]); len = poly_mult(polys04, 15, polyout[1], len, polyout[0]); @@ -592,33 +624,34 @@ namespace gr { unsigned char *out = (unsigned char *) output_items[0]; unsigned char b, temp; unsigned int shift[6]; + std::bitset<192> parity_bits; int consumed = 0; switch (bch_code) { case BCH_CODE_N12: for (int i = 0; i < noutput_items; i += nbch) { - //Zero the shift register - memset(shift, 0, sizeof(unsigned int) * 6); - // MSB of the codeword first - for (int j = 0; j < (int)kbch; j++) { - temp = *in++; - *out++ = temp; - consumed++; - b = (temp ^ (shift[5] & 1)); - reg_6_shift(shift); - if (b) { - shift[0] ^= m_poly_n_12[0]; - shift[1] ^= m_poly_n_12[1]; - shift[2] ^= m_poly_n_12[2]; - shift[3] ^= m_poly_n_12[3]; - shift[4] ^= m_poly_n_12[4]; - shift[5] ^= m_poly_n_12[5]; + for (int j = 0; j < (int)kbch/8; j++) { + b = 0; + + // calculate the crc using the lookup table, cf. http://www.sunshine2k.de/articles/coding/crc/understanding_crc.html + for (int e = 0; e < 8; e++) { + temp = *in++; + *out++ = temp; + consumed++; + + b |= temp << (7 - e); } + + unsigned long msB_CRC = (parity_bits >> 184).to_ulong(); + /* XOR-in next input byte into MSB of crc and get this MSB, that's our new intermediate divident */ + unsigned char pos = (unsigned char)(msB_CRC ^ b); + /* Shift out the MSB used for division per lookuptable and XOR with the remainder */ + parity_bits = (parity_bits << 8) ^ crcTable[pos]; } // Now add the parity bits to the output for (int n = 0; n < 192; n++) { - *out++ = (shift[5] & 1); - reg_6_shift(shift); + *out++ = (char) parity_bits[191]; + parity_bits <<= 1; } } break; diff --git a/gr-dtv/lib/dvb/dvb_bch_bb_impl.h b/gr-dtv/lib/dvb/dvb_bch_bb_impl.h index 56b4772b26..0a73ee5078 100644 --- a/gr-dtv/lib/dvb/dvb_bch_bb_impl.h +++ b/gr-dtv/lib/dvb/dvb_bch_bb_impl.h @@ -23,6 +23,7 @@ #include <gnuradio/dtv/dvb_bch_bb.h> #include "dvb_defines.h" +#include <bitset> namespace gr { namespace dtv { @@ -38,6 +39,9 @@ namespace gr { unsigned int m_poly_n_12[6]; unsigned int m_poly_s_12[6]; unsigned int m_poly_m_12[6]; + std::bitset<192> polynome; + std::bitset<192> crcTable[256]; + void CalculateCrcTable(); int poly_mult(const int*, int, const int*, int, int*); void poly_pack(const int*, unsigned int*, int); void poly_reverse(int*, int*, int); |