getbits.c - acarsdec - an ACARS decoder
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getbits.c (4418B)
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1 /*
2 * Copyright (c) 2007 by Thierry Leconte (F4DWV)
3 *
4 * $Id: getbits.c,v 1.4 2007/04/15 15:06:54 f4dwv Exp $
5 *
6 * This code is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU Library General Public License version 2
8 * published by the Free Software Foundation.
9 *
10 * This program is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 * GNU Library General Public License for more details.
14 *
15 * You should have received a copy of the GNU Library General Public
16 * License along with this library; if not, write to the Free Software
17 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
18 *
19 */
20
21 #include <stdlib.h>
22 #include <stdio.h>
23 #include <math.h>
24
25 #include "acarsdec.h"
26
27
28 #define Fe 48000.0
29 #define Freqh 4800.0/Fe*2.0*M_PI
30 #define Freql 2400.0/Fe*2.0*M_PI
31 #define BITLEN ((int)Fe/1200)
32
33 static float h[BITLEN];
34
35 static struct bstat_s {
36 float hsample[BITLEN];
37 float lsample[BITLEN];
38 float isample[BITLEN];
39 float qsample[BITLEN];
40 float csample[BITLEN];
41 int is;
42 int clock;
43 float lin;
44 float phih,phil;
45 float dfh,dfl;
46 float pC,ppC;
47 int sgI, sgQ;
48 float ea;
49 } bstat[2];
50
51
52 void init_bits(void)
53 {
54 int i;
55 for (i = 0; i < BITLEN; i++)
56 h[i] = sin(2.0 * M_PI * (float) i / (float) BITLEN);
57
58 for (i = 0; i < BITLEN; i++) {
59 bstat[0].hsample[i] = bstat[0].lsample[i] =
60 bstat[0].isample[i] = bstat[0].qsample[i] =
61 bstat[0].csample[i] = 0.0;
62 bstat[1].hsample[i] = bstat[1].lsample[i] =
63 bstat[1].isample[i] = bstat[1].qsample[i] =
64 bstat[1].csample[i] = 0.0;
65 }
66 bstat[0].is = bstat[0].clock = bstat[0].sgI = bstat[0].sgQ = 0;
67 bstat[1].is = bstat[1].clock = bstat[1].sgI = bstat[1].sgQ = 0;
68 bstat[0].phih = bstat[0].phil = bstat[0].dfh = bstat[0].dfl =
69 bstat[0].pC = bstat[0].ppC = bstat[0].ea = 0.0;
70 bstat[1].phih = bstat[1].phil = bstat[1].dfh = bstat[1].dfl =
71 bstat[1].pC = bstat[1].ppC = bstat[1].ea = 0.0;
72 bstat[0].lin=bstat[1].lin=1.0;
73
74 }
75
76 void resetbits(int ch)
77 {
78 bstat[ch].sgI = bstat[ch].sgQ = 0;
79 }
80
81 #define VFOPLL 0.7e-3
82 #define BITPLL 0.2
83
84 int getbit(short sample, unsigned char *outbits, int ch)
85 {
86 int i, bt;
87 float in, in2;
88 float C;
89 float I, Q;
90 float oscl, osch;
91 struct bstat_s *st;
92
93 bt = 0;
94 st = &bstat[ch];
95
96 in = (float) sample;
97 st->lin = 0.003 * fabs(in) + 0.997 * st->lin;
98 in /= st->lin;
99 in2 = in * in;
100
101 st->is--;
102 if (st->is < 0)
103 st->is = BITLEN - 1;
104
105 /* VFOs */
106 st->phih += Freqh - VFOPLL * st->dfh;
107 if (st->phih >= 4.0 * M_PI)
108 st->phih -= 4.0 * M_PI;
109 st->hsample[st->is] = in2 * sin(st->phih);
110 for (i = 0, st->dfh = 0.0; i < BITLEN / 2; i++) {
111 st->dfh += st->hsample[(st->is + i) % BITLEN];
112 }
113 osch = cos(st->phih / 2.0);
114
115 st->phil += Freql - VFOPLL * st->dfl;
116 if (st->phil >= 4.0 * M_PI)
117 st->phil -= 4.0 * M_PI;
118 st->lsample[st->is] = in2 * sin(st->phil);
119 for (i = 0, st->dfl = 0.0; i < BITLEN / 2; i++) {
120 st->dfl += st->lsample[(st->is + i) % BITLEN];
121 }
122 oscl = cos(st->phil / 2.0);
123
124 /* mix */
125 st->isample[st->is] = in * (oscl + osch);
126 st->qsample[st->is] = in * (oscl - osch);
127 st->csample[st->is] = oscl * osch;
128
129
130 /* bit clock */
131 st->clock++;
132 if (st->clock >= BITLEN/4 + st->ea) {
133 st->clock = 0;
134
135 /* clock filter */
136 for (i = 0, C = 0.0; i < BITLEN; i++) {
137 C += h[i] * st->csample[(st->is + i) % BITLEN];
138 }
139
140 if (st->pC < C && st->pC < st->ppC) {
141
142 /* integrator */
143 for (i = 0, Q = 0.0; i < BITLEN; i++) {
144 Q += st->qsample[(st->is + i) % BITLEN];
145 }
146
147 if (st->sgQ == 0) {
148 if (Q < 0)
149 st->sgQ = -1;
150 else
151 st->sgQ = 1;
152 }
153
154 *outbits =
155 ((*outbits) >> 1) | (unsigned
156 char) ((Q * st->sgQ >
157 0) ? 0x80 : 0);
158 bt = 1;
159
160 st->ea = -BITPLL * (C - st->ppC);
161 if(st->ea > 2.0) st->ea=2.0;
162 if(st->ea < -2.0) st->ea=-2.0;
163 }
164 if (st->pC > C && st->pC > st->ppC) {
165
166 /* integrator */
167 for (i = 0, I = 0.0; i < BITLEN; i++) {
168 I += st->isample[(st->is + i) % BITLEN];
169 }
170
171 if (st->sgI == 0) {
172 if (I < 0)
173 st->sgI = -1;
174 else
175 st->sgI = 1;
176 }
177
178 *outbits =
179 ((*outbits) >> 1) | (unsigned
180 char) ((I * st->sgI >
181 0) ? 0x80 : 0);
182 bt = 1;
183
184 st->ea = BITPLL * (C - st->ppC);
185 if(st->ea > 2.0) st->ea=2.0;
186 if(st->ea < -2.0) st->ea=-2.0;
187 }
188 st->ppC = st->pC;
189 st->pC = C;
190 }
191 return bt;
192 }