177 lines
6.0 KiB
C++
177 lines
6.0 KiB
C++
/* -*- c++ -*- */
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/*
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* NXDN Encoder/Decoder (C) Copyright 2019 Max H. Parke KA1RBI
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*
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* This file is part of OP25
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*
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* This is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 3, or (at your option)
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* any later version.
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*
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* This software is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this software; see the file COPYING. If not, write to
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* the Free Software Foundation, Inc., 51 Franklin Street,
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* Boston, MA 02110-1301, USA.
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*/
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#include <stdio.h>
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#include <stdint.h>
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#include <string.h>
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#include <assert.h>
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#include "bit_utils.h"
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#include "nxdn.h"
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static const uint16_t perm_25_12[300] = { // perumtation schedule for 25x12
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0,12,24,36,48,60,72,84,96,108,120,132,144,156,168,180,192,204,216,228,240,252,264,276,288,
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1,13,25,37,49,61,73,85,97,109,121,133,145,157,169,181,193,205,217,229,241,253,265,277,289,
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2,14,26,38,50,62,74,86,98,110,122,134,146,158,170,182,194,206,218,230,242,254,266,278,290,
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3,15,27,39,51,63,75,87,99,111,123,135,147,159,171,183,195,207,219,231,243,255,267,279,291,
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4,16,28,40,52,64,76,88,100,112,124,136,148,160,172,184,196,208,220,232,244,256,268,280,292,
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5,17,29,41,53,65,77,89,101,113,125,137,149,161,173,185,197,209,221,233,245,257,269,281,293,
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6,18,30,42,54,66,78,90,102,114,126,138,150,162,174,186,198,210,222,234,246,258,270,282,294,
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7,19,31,43,55,67,79,91,103,115,127,139,151,163,175,187,199,211,223,235,247,259,271,283,295,
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8,20,32,44,56,68,80,92,104,116,128,140,152,164,176,188,200,212,224,236,248,260,272,284,296,
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9,21,33,45,57,69,81,93,105,117,129,141,153,165,177,189,201,213,225,237,249,261,273,285,297,
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10,22,34,46,58,70,82,94,106,118,130,142,154,166,178,190,202,214,226,238,250,262,274,286,298,
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11,23,35,47,59,71,83,95,107,119,131,143,155,167,179,191,203,215,227,239,251,263,275,287,299};
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static const uint8_t scramble_t[] = {
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2, 5, 6, 7, 10, 12, 14, 16, 17, 22, 23, 25, 26, 27, 28, 30, 33, 34, 36, 37, 38, 41, 45, 47,
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52, 54, 56, 57, 59, 62, 63, 64, 65, 66, 67, 69, 70, 73, 76, 79, 81, 82, 84, 85, 86, 87, 88,
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89, 92, 95, 96, 98, 100, 103, 104, 107, 108, 116, 117, 121, 122, 125, 127, 131, 132, 134,
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137, 139, 140, 141, 142, 143, 144, 145, 147, 151, 153, 154, 158, 159, 160, 162, 164, 165,
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168, 170, 171, 174, 175, 176, 177, 181};
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static const int PARITY[] = {0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1};
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static inline uint16_t crc16(const uint8_t buf[], int len, uint32_t crc) {
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uint32_t poly = (1<<12) + (1<<5) + (1<<0);
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for(int i=0; i<len; i++) {
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uint8_t bit = buf[i] & 1;
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crc = ((crc << 1) | bit) & 0x1ffff;
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if (crc & 0x10000)
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crc = (crc & 0xffff) ^ poly;
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}
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crc = crc ^ 0xffff;
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return crc & 0xffff;
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}
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// trellis_1_2 encode: source is in bits, result in bits
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static inline void trellis_encode(uint8_t result[], const uint8_t source[], int result_len, int reg)
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{
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for (int i=0; i<result_len; i+=2) {
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reg = (reg << 1) | source[i>>1];
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result[i] = PARITY[reg & 0x19];
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result[i+1] = PARITY[reg & 0x17];
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}
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}
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// simplified trellis 2:1 decode; source and result in bits
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// assumes that encoding was done with NTEST trailing zero bits
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// result_len should be set to the actual number of data bits
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// in the original unencoded message (excl. these trailing bits)
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static inline void trellis_decode(uint8_t result[], const uint8_t source[], int result_len)
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{
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int reg = 0;
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int min_d;
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int min_bt;
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static const int NTEST = 4;
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static const int NTESTC = 1 << NTEST;
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uint8_t bt[NTEST];
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uint8_t tt[NTEST*2];
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int dstats[4];
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int sum;
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for (int p=0; p < 4; p++)
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dstats[p] = 0;
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for (int p=0; p < result_len; p++) {
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for (int i=0; i<NTESTC; i++) {
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bt[0] = (i&8)>>3;
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bt[1] = (i&4)>>2;
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bt[2] = (i&2)>>1;
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bt[3] = (i&1);
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trellis_encode(tt, bt, NTEST*2, reg);
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sum=0;
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for (int j=0; j<NTEST*2; j++) {
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sum += tt[j] ^ source[p*2+j];
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}
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if (i == 0 || sum < min_d) {
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min_d = sum;
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min_bt = bt[0];
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}
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}
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result[p] = min_bt;
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reg = (reg << 1) | min_bt;
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dstats[(min_d > 3) ? 3 : min_d] += 1;
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}
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// fprintf (stderr, "stats\t%d %d %d %d\n", dstats[0], dstats[1], dstats[2], dstats[3]);
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}
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void nxdn_descramble(uint8_t dibits[], int len) {
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for (int i=0; i<sizeof(scramble_t); i++) {
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if (scramble_t[i] >= len)
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break;
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dibits[scramble_t[i]] ^= 0x2; // invert sign of scrambled dibits
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}
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}
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static inline void decode_cac(const uint8_t dibits[], int len) {
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uint8_t cacbits[300];
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uint8_t deperm[300];
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uint8_t depunc[350];
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uint8_t decode[171];
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int id=0;
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uint16_t crc;
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dibits_to_bits(cacbits, dibits, 150);
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for (int i=0; i<300; i++) {
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deperm[perm_25_12[i]] = cacbits[i];
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}
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for (int i=0; i<25; i++) {
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depunc[id++] = deperm[i*12];
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depunc[id++] = deperm[i*12+1];
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depunc[id++] = deperm[i*12+2];
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depunc[id++] = 0;
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depunc[id++] = deperm[i*12+3];
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depunc[id++] = deperm[i*12+4];
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depunc[id++] = deperm[i*12+5];
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depunc[id++] = deperm[i*12+6];
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depunc[id++] = deperm[i*12+7];
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depunc[id++] = deperm[i*12+8];
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depunc[id++] = deperm[i*12+9];
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depunc[id++] = 0;
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depunc[id++] = deperm[i*12+10];
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depunc[id++] = deperm[i*12+11];
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}
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trellis_decode(decode, depunc, 171);
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crc = crc16(decode, 171, 0xc3ee);
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if (crc != 0)
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return; // ignore msg if crc failed
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uint8_t msg_type = load_i(decode+8, 8) & 0x3f;
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// todo: forward CAC message
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}
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void nxdn_frame(const uint8_t dibits[], int ndibits) {
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uint8_t descrambled[182];
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uint8_t lich;
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uint8_t lich_test;
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uint8_t bit72[72];
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assert (ndibits >= 170);
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memcpy(descrambled, dibits, ndibits);
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nxdn_descramble(descrambled, ndibits);
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lich = 0;
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for (int i=0; i<8; i++)
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lich |= (descrambled[i] >> 1) << (7-i);
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/* todo: parity check & process LICH */
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if (lich >> 1 == 0x01)
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decode_cac(descrambled+8, 150);
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/* todo: process E: 12 dibits at descrambed+158; */
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}
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