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/*
tdec.c
David Rowe
Jan 2017
Trivial non filtered decimator for high ration sample rate conversion.
build: gcc tdec.c -o tdec -Wall -O2
*/
#include <assert.h>
#include <getopt.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#define SIGNED_16BIT 0
#define SIGNED_8BIT 1
#define UNSIGNED_8BIT 2
void freq_shift_complex_buf(short buf[], int n, int lo_i[], int lo_q[]);
void display_help(void) {
fprintf(stderr, "\nusage: tdec inputRawFile OutputRawFile DecimationRatio [-c]\n");
fprintf(stderr, "\nUse - for stdin/stdout\n\n");
fprintf(stderr, "-c complex signed 16 bit input and output\n");
fprintf(stderr, "-d complex signed 8 bit input (e.g. HackRF), complex signed 16 bit output\n");
fprintf(stderr, "-e complex unsigned 8 bit input (e.g. RTL-SDR), complex signed 16 bit output\n");
fprintf(stderr, "-f -Fs/4 freq shift\n\n");
}
int main(int argc, char *argv[]) {
FILE *fin, *fout;
short dec;
int lo_i[3], lo_q[3];
if (argc < 3) {
display_help();
exit(1);
}
if (strcmp(argv[1], "-") == 0)
fin = stdin;
else
fin = fopen(argv[1], "rb");
assert(fin != NULL);
if (strcmp(argv[2], "-") == 0)
fout = stdout;
else
fout = fopen(argv[2], "wb");
assert(fout != NULL);
dec = atoi(argv[3]);
int channels = 1;
int freq_shift = 0;
lo_i[0] = -1; lo_i[1] = 0;
lo_q[0] = 0; lo_q[1] = -1;
int opt;
int format = SIGNED_16BIT;
while ((opt = getopt(argc, argv, "cdef")) != -1) {
switch (opt) {
case 'c': channels = 2; break;
case 'd': channels = 2; format = SIGNED_8BIT; break;
case 'e': channels = 2; format = UNSIGNED_8BIT; break;
case 'f': freq_shift = 1; break;
default:
display_help();
exit(1);
}
}
if (format == SIGNED_16BIT) {
short buf[dec*channels];
while(fread(buf, sizeof(short)*channels, dec, fin) == dec) {
if (freq_shift)
freq_shift_complex_buf(buf, dec*channels, lo_i, lo_q);
fwrite(buf, sizeof(short), channels, fout);
}
}
else {
uint8_t inbuf[dec*channels];
short outbuf[dec*channels];
short sam, i;
while(fread(inbuf, sizeof(uint8_t)*channels, dec, fin) == dec) {
for (i=0; i<dec*channels; i++) {
assert((format == SIGNED_8BIT) || (format == UNSIGNED_8BIT));
if (format == SIGNED_8BIT) {
sam = (short)inbuf[i];
sam <<= 8;
} else {
sam = (short)inbuf[i] - 127;
sam <<= 8;
}
outbuf[i] = sam;
}
if (freq_shift)
freq_shift_complex_buf(outbuf, dec*channels, lo_i, lo_q);
fwrite(outbuf, sizeof(short), channels, fout);
}
}
fclose(fout);
fclose(fin);
return 0;
}
void freq_shift_complex_buf(short buf[], int n, int lo_i[], int lo_q[]) {
int i;
for (i=0; i<n; i+=2) {
/* update local osc recursion */
lo_i[2] = -lo_i[0]; lo_q[2] = -lo_q[0];
/* freq shift down input samples */
int a = buf[i];
int b = buf[i+1];
int c = lo_i[2];
int d = -lo_q[2]; /* conj LO as down shift */
buf[i] = a*c - b*d;
buf[i+1] = b*c + a*d;
//fprintf(stderr, "% d % d % 5d % 5d\n", lo_i[2], lo_q[2], buf[i], buf[i+1]);
/* update LO memory */
lo_i[0] = lo_i[1]; lo_i[1] = lo_i[2];
lo_q[0] = lo_q[1]; lo_q[1] = lo_q[2];
}
}
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