1 files changed, 143 insertions, 0 deletions
diff --git a/src/fsk_mod_ext_vco.c b/src/fsk_mod_ext_vco.c
new file mode 100644
index 0000000..b4cfd6a
--- /dev/null
+++ b/src/fsk_mod_ext_vco.c
@@ -0,0 +1,143 @@
+/*---------------------------------------------------------------------------*\
+
+  FILE........: fsk_mod_ext_vco.c
+  AUTHOR......: David Rowe
+  DATE CREATED: Feb 2018
+
+  Converts a stream of bits to mFSK raw file of "levels" suitable for
+  driving an external VCO, e.g. legacy FM transmitter in data mode.
+   
+\*---------------------------------------------------------------------------*/
+
+/*
+  Copyright (C) 2018 David Rowe
+
+  All rights reserved.
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU Lesser General Public License version 2.1, as
+  published by the Free Software Foundation.  This program 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 Lesser General Public License
+  along with this program; if not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <assert.h>
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+
+#define OVERSAMPLE 100
+
+int main(int argc,char *argv[]){
+    int   os, m, log2m, i, bit_i, sym, legacy_mode, rpitx_mode;
+    float d;
+    double shiftHz, symbolRateHz;
+    uint32_t time_sample;
+    FILE *fin,*fout;
+    
+    if(argc<5){
+        fprintf(stderr, "usage: %s InputOneBitPerCharFile OutputVcoRawFile MbitsPerFSKsymbol\n",argv[0]);
+        fprintf(stderr, "[--legacy OutputSamplesPerSymbol deviationPerlevel]\n");
+        fprintf(stderr, "[--rpitx ShiftHz SymbolRateHz\n]");
+        exit(1);
+    }
+    
+    /* Extract parameters */
+    
+    if (strcmp(argv[1],"-")==0){
+        fin = stdin;
+    } else {
+        fin = fopen(argv[1],"r");
+    }
+	
+    if (strcmp(argv[2],"-")==0){
+        fout = stdout;
+    } else {
+        fout = fopen(argv[2],"w");
+    }
+    
+    m = atoi(argv[3]); log2m = log2(m);
+    printf("log2m: %d\n", log2m);
+
+    legacy_mode = rpitx_mode = os = 0;
+    if (!strcmp(argv[4],"--legacy")) {
+        os = atoi(argv[5]);
+        d = atof(argv[6]);
+        legacy_mode = 1;
+    }
+    if (!strcmp(argv[4],"--rpitx")) {
+        shiftHz = atof(argv[5]);
+        symbolRateHz = atof(argv[6]);
+        rpitx_mode = 1;
+        time_sample = 1E9/symbolRateHz;
+        fprintf(stderr, "time_sample: %d\n", time_sample);
+    }
+
+    assert(legacy_mode || rpitx_mode);
+    fprintf(stderr, "legacy_mode: %d rpitx_mode: %d\n", legacy_mode, rpitx_mode);
+    
+    uint8_t tx_bits[log2m];
+    int16_t rawbuf[os];
+
+    /* Modulate m bits to levels to drive external VCO */
+
+    while( fread(tx_bits, sizeof(uint8_t), log2m, fin) == log2m ){
+
+        /* generate the symbol number from the bit stream, 
+           e.g. 0,1 for 2FSK, 0,1,2,3 for 4FSK */
+
+        sym = bit_i = 0;
+        for( i=m; i>>=1; ){
+            //fprintf(stderr, "tx_bits[%d] = %d\n", i, tx_bits[bit_i]);
+            uint8_t bit = tx_bits[bit_i];
+            bit = (bit==1)?1:0;
+            sym = (sym<<1)|bit;
+            bit_i++;
+        }
+        //fprintf(stderr, "sym = %d\n", sym);
+
+        if (legacy_mode) {
+            /* map 'sym' to VCO drive signal symmetrically about 0,
+               separate tones by constant "d"  */
+            /* 2 FSK -d/2, +d/2                */
+            /* 4 FSK -3*d/2, -d/2, +d/2, 3*d/2 */
+
+            /* note: drive is inverted, a higher tone drives VCO voltage lower */
+
+            float symf = sym;
+            float level = d*(((float)m-1)*0.5 - symf);
+            assert(level <= 32767.0);
+            assert(level >= -32768.0);
+            short level_short = (short)level;
+            //fprintf(stderr, "level = %f level_short = %d\n\n", level, level_short);
+            for(i=0; i<os; i++) {
+                rawbuf[i] = level_short;
+            }
+            fwrite(rawbuf, sizeof(int16_t), os, fout);
+        }
+
+        if (rpitx_mode) {
+	    short frequencyHz;
+            frequencyHz = shiftHz*(sym+1);
+            fwrite(&frequencyHz, sizeof(short), 1, fout);
+        }
+
+        if(fout == stdout){
+            fflush(fout);
+        }
+    }
+    
+    fclose(fin);
+    fclose(fout);
+
+    exit(0);
+}
+
+