atsci_reed_solomon.cc

这是用python语言写的一个数字广播的信号处理工具包。利用它

/* -*- c++ -*- */
/*
 * Copyright 2002 Free Software Foundation, Inc.
 * 
 * This file is part of GNU Radio
 * 
 * GNU Radio is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3, or (at your option)
 * any later version.
 * 
 * GNU Radio is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with GNU Radio; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#include <atsci_reed_solomon.h>
#include <assert.h>
#include <string.h>

extern "C" {
#include "rs.h"
}

static const int rs_init_symsize =     8;
static const int rs_init_gfpoly  = 0x11d;
static const int rs_init_fcr     =     0;	// first consecutive root
static const int rs_init_prim    =     1;	// primitive is 1 (alpha)
static const int rs_init_nroots  =    20;

static const int N = (1 << rs_init_symsize) - 1;	// 255
static const int K = N - rs_init_nroots;		// 235

static const int amount_of_pad	 = N - ATSC_MPEG_RS_ENCODED_LENGTH;	  // 48

atsci_reed_solomon::atsci_reed_solomon ()
{
  d_rs = init_rs_char (rs_init_symsize, rs_init_gfpoly,
		       rs_init_fcr, rs_init_prim, rs_init_nroots);

  assert (d_rs != 0);
}

atsci_reed_solomon::~atsci_reed_solomon ()
{
  if (d_rs)
    free_rs_char (d_rs);
  d_rs = 0;
}

void
atsci_reed_solomon::encode (atsc_mpeg_packet_rs_encoded &out, const atsc_mpeg_packet_no_sync &in)
{
  unsigned char tmp[K];

  assert ((int)(amount_of_pad + sizeof (in.data)) == K);
  
  // add missing prefix zero padding to message
  memset (tmp, 0, amount_of_pad);
  memcpy (&tmp[amount_of_pad], in.data, sizeof (in.data));

  // copy message portion to output packet
  memcpy (out.data, in.data, sizeof (in.data));

  // now compute parity bytes and add them to tail end of output packet
  encode_rs_char (d_rs, tmp, &out.data[sizeof (in.data)]);
}

int
atsci_reed_solomon::decode (atsc_mpeg_packet_no_sync &out, const atsc_mpeg_packet_rs_encoded &in)
{
  unsigned char tmp[N];
  int		ncorrections;

  assert ((int)(amount_of_pad + sizeof (in.data)) == N);
  
  // add missing prefix zero padding to message
  memset (tmp, 0, amount_of_pad);
  memcpy (&tmp[amount_of_pad], in.data, sizeof (in.data));

  // correct message... 
  ncorrections = decode_rs_char (d_rs, tmp, 0, 0);
  
  // copy corrected message to output, skipping prefix zero padding
  memcpy (out.data, &tmp[amount_of_pad], sizeof (out.data));

  return ncorrections;
}