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asyncmsk.c
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#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <math.h>
#include <unistd.h>
#include "rtl-ais.h"
#define INC 16
float fast_atanf(float x);
static float rcos_filter_coeffs[84] = {
-0.0195706589, -0.0019463516, +0.0174879305, +0.0376500445,
+0.0573162726, +0.0751855693, +0.0899533523, +0.1003910752,
+0.1054272579, +0.1042253298, +0.0962535644, +0.0813425924,
+0.0597264537, +0.0320638792, -0.0005625513, -0.0366705982,
-0.0744258758, -0.1117113668, -0.1462163400, -0.1755408960,
-0.1973113466, -0.2093008549, -0.2095492535, -0.1964757729,
-0.1689785575, -0.1265153337, -0.0691603972, +0.0023658260,
+0.0866970749, +0.1818523559, +0.2852972395, +0.3940331739,
+0.5047111565, +0.6137647860, +0.7175566441, +0.8125311731,
+0.8953667896, +0.9631199126, +1.0133539072, +1.0442466315,
+1.0546712948, +1.0442466315, +1.0133539072, +0.9631199126,
+0.8953667896, +0.8125311731, +0.7175566441, +0.6137647860,
+0.5047111565, +0.3940331739, +0.2852972395, +0.1818523559,
+0.0866970749, +0.0023658260, -0.0691603972, -0.1265153337,
-0.1689785575, -0.1964757729, -0.2095492535, -0.2093008549,
-0.1973113466, -0.1755408960, -0.1462163400, -0.1117113668,
-0.0744258758, -0.0366705982, -0.0005625513, +0.0320638792,
+0.0597264537, +0.0813425924, +0.0962535644, +0.1042253298,
+0.1054272579, +0.1003910752, +0.0899533523, +0.0751855693,
+0.0573162726, +0.0376500445, +0.0174879305, -0.0019463516,
-0.0195706589, 0.0, 0.0, 0.0
};
static float rrc_filter_run_buf(asynchronous_msk_demod_t *f, float *in, float *out, unsigned int len)
{
float maxval = 0.0f;
unsigned int id = 0, filtidx = f->rrc_filter_bufidx;
while (id < len) {
f->rrc_filter_buffer[f->rrc_filter_bufidx] = in[id];
// look for peak volume
if (in[id] > maxval)
maxval = in[id];
out[id++] = INVGAIN*scalarproduct_float_sse(&f->rrc_filter_buffer[filtidx - RRC_COEFFS_L], rcos_filter_coeffs, 84);
/* the buffer is much smaller than the incoming chunks */
if (filtidx == RRC_BUFLEN-1) {
memcpy(f->rrc_filter_buffer,
f->rrc_filter_buffer + RRC_BUFLEN - RRC_COEFFS_L,
RRC_COEFFS_L * sizeof(float));
filtidx = RRC_COEFFS_L - 1;
}
filtidx++;
}
f->rrc_filter_bufidx = filtidx;
return maxval;
}
void putbit(asynchronous_msk_demod_t *ch, unsigned char v)
{
ch->outbits <<= 1;
if(v>0) {
ch->outbits |= 1;
}
if (ch->nbits >= 8) {
/* feed to the decoder */
ais_bytearray_append(ch->outbits);
ch->nbits = 0;
ch->outbits = 0;
}
/* feed to the decoder */
//protodec_decode(&b, 1, &fm->decoder);
}
void init_async_msk(asynchronous_msk_demod_t *async_msk)
{
async_msk->rrc_filter_bufidx = RRC_COEFFS_L;
async_msk->lastbit = 0;
async_msk->pll = 0;
async_msk->pllinc = 0x10000 / 5;
async_msk->prev = 0;
async_msk->last_levellog = 0;
}
float polar_disc_fast(FFTComplex a, FFTComplex b)
{
float x = a.re*b.re + a.im*b.im;
float y = a.im*b.re - a.re*b.im;
float z;
if ((y != y) || (x != x))
return 0.0f;
y += 1e-12f;
if (x < 1e-12f) {
z = (float)M_PI * 0.5f;
} else {
/* compute y/x */
z=fast_atanf(fabsf(y/x));
if (x < 0.0f) {
z = (float)M_PI - z;
}
}
if (z != z) {
z = 0.0f;
}
if (y < 0.0f) {
z = -z;
}
return (z * 0.31831f);
}
void async_msk_decode(asynchronous_msk_demod_t *async_msk, FFTComplex *inbuf, unsigned int inbuf_len)
{
float out, maxval = 0.0f;
unsigned int i;
int curr, bit;
unsigned char b;
float filtered[4096];
async_msk->fm_demod[0] = polar_disc_fast(inbuf[0], async_msk->prev_fm);
for (i = 1; i < inbuf_len; i++) {
async_msk->fm_demod[i] = polar_disc_fast(inbuf[i], inbuf[i-1]);
}
async_msk->prev_fm = inbuf[inbuf_len - 1];
maxval = rrc_filter_run_buf(async_msk, async_msk->fm_demod, filtered, inbuf_len);
for (i = 0; i < inbuf_len; i++) {
out = filtered[i];
curr = (out > 0);
if ((curr ^ async_msk->prev) == 1) {
if (async_msk->pll < (0x10000 / 2)) {
async_msk->pll += async_msk->pllinc / INC;
} else {
async_msk->pll -= async_msk->pllinc / INC;
}
}
async_msk->prev = curr;
async_msk->pll += async_msk->pllinc;
if (async_msk->pll > 0xffff) {
/* slice */
bit = (out > 0);
/* nrzi decode */
b = !(bit ^ async_msk->lastbit);
putbit(async_msk, b);
async_msk->lastbit = bit;
async_msk->pll &= 0xffff;
}
}
}