correct misconception about txbuffer, signal generation working

fl2k-psk.cpp

@@ -9,9 +9,19 @@
 #include <iostream>
 #include <signal.h>
 #include <math.h>
+#include <map>
+#include <vector>
 
 #include "osmo-fl2k.h"
 
+#define SIGNAL_MAX 99			// 99 * 0.70710678 = 70.00.... keeps rounding error in signal to minimum
+								// 17 * 0.70710678 = 12.02.... keeps rounding error low and low signal keeps rest of the spectrum relatively clean
+#define SIGNAL_MIN -SIGNAL_MAX
+#define SIGNAL_FREQ 28000000
+#define SF_RATIO 4				// {2,4,8} samplerate / signal frequency ratio (max sample rate is about 140 MS/s on USB3)
+
+#define SIN_1_4_PI 0.70710678		// sin(1/4*pi)
+
 int8_t *tx_buffer = nullptr;
 int phase_curr = 0;
 
@@ -36,20 +46,21 @@ void fl2k_callback(fl2k_data_info_t *data_info) {
 		cout << "WARNING: device error" << endl;
 	}
 
-	// sample rate is twice the frequency, so half a wavelength is in de buffer
-	phase_curr = (phase_curr + 180) % 360;
-	int idx_txbuf = ((phase_curr/180) * FL2K_BUF_LEN);
-
 	data_info->sampletype_signed = 1;
-	data_info->r_buf = (char *)tx_buffer + idx_txbuf;
+	data_info->r_buf = (char *)tx_buffer;
 }
 
 void init_txbuffer() {
-	tx_buffer = (int8_t*)malloc(FL2K_BUF_LEN * 3); // put 1.5 wavelengths in buffer
+	int8_t sin14pi = (int8_t)(SIGNAL_MAX * SIN_1_4_PI);
+	map<int,vector<int8_t>> sines;
+	sines[2] = {SIGNAL_MIN, SIGNAL_MAX};
+	sines[4] = {0, SIGNAL_MIN, 0, SIGNAL_MAX};
+	sines[8] = {0, (int8_t)-sin14pi, SIGNAL_MIN, (int8_t)-sin14pi, 0, sin14pi, SIGNAL_MAX, sin14pi};
+
+	tx_buffer = (int8_t*)malloc(FL2K_BUF_LEN);
+	auto sine = sines[SF_RATIO];
 	for(int i = 0; i < FL2K_BUF_LEN; i++) {
-		tx_buffer[i] = (sin(((double)i/FL2K_BUF_LEN) * M_PI) * 127) +.5; // add .5 because integers round down
-		tx_buffer[i + FL2K_BUF_LEN] = -tx_buffer[i];
-		tx_buffer[i + (FL2K_BUF_LEN*2)] = tx_buffer[i];
+		tx_buffer[i] = sine[i % SF_RATIO];
 	}
 }
 
@@ -67,7 +78,7 @@ void attach_sighandlers() {
 }
 
 void set_freq_carrier(uint32_t freq) {
-	uint32_t samplerate = freq * 2; // sample rate twice the frequency gives the smoothest output (TODO: experiment)
+	uint32_t samplerate = freq * SF_RATIO;
 
 	// Set the sample rate
 	int r = fl2k_set_sample_rate(fl2k_dev, samplerate);
@@ -77,7 +88,7 @@ void set_freq_carrier(uint32_t freq) {
 
 	/* read back actual frequency */
 	samplerate = fl2k_get_sample_rate(fl2k_dev);
-	cout << "Actual {sample rate,frequency} = {" << samplerate << "," << samplerate/2 << "}" << endl;
+	cout << "Actual {sample rate,frequency} = {" << samplerate << "," << samplerate/SF_RATIO << "}" << endl;
 }
 
 
@@ -93,28 +104,28 @@ int main(int argc, char **argv) {
 		cout << "Opened device" << endl;
 
 		int r = fl2k_start_tx(fl2k_dev, fl2k_callback, nullptr, 0);
-		set_freq_carrier(28000000);
-	}
-
-	cout << "Press u,d,q to raise or lower frequency, or quit: " << flush;
-	char c;
-	while(cin.get(c)) {
-		switch(c) {
-		case 'u':
-			set_freq_carrier((fl2k_get_sample_rate(fl2k_dev) /2) +1000000);
-			break;
-		case 'd':
-			set_freq_carrier((fl2k_get_sample_rate(fl2k_dev) /2) -1000000);
-			break;
-		case 'q':
-			exit(0);
-			break;
-		default:
-			break;
+		set_freq_carrier(SIGNAL_FREQ);
+
+		cout << "Press u,d,q to raise or lower frequency, or quit: " << flush;
+		char c;
+		while(cin.get(c)) {
+			switch(c) {
+			case 'u':
+				set_freq_carrier((fl2k_get_sample_rate(fl2k_dev) /SF_RATIO) +1000000);
+				break;
+			case 'd':
+				set_freq_carrier((fl2k_get_sample_rate(fl2k_dev) /SF_RATIO) -1000000);
+				break;
+			case 'q':
+				exit(0);
+				break;
+			default:
+				break;
+			}
+
+			cin.ignore();
+			cout << "> " << flush;
 		}
-
-		cin.ignore();
-		cout << "> " << flush;
 	}