1 files changed, 64 insertions, 0 deletions
diff --git a/src/libsample/sample.c b/src/libsample/sample.c
new file mode 100644
index 0000000..a084b66
--- /dev/null
+++ b/src/libsample/sample.c
@@ -0,0 +1,64 @@
+/* Sample definition
+ *
+ * (C) 2017 by Andreas Eversberg <jolly@eversberg.eu>
+ * All Rights Reserved
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * 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 General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdint.h>
+#include "sample.h"
+
+/*
+ * A regular voice conversation takes place at this factor below the full range
+ * of 16 bits signed value:
+ */
+static double int_16_speech_level = SPEECH_LEVEL * 0.7079; /* 16 dBm below dBm0, which is about 3dBm below full 16 bit range */
+
+/* A sample_t is a value that has virtually infinite precision but will also
+ * support high numbers. 'double' or 'float' types are sufficient.
+ *
+ * When using sample_t inside signal processing of each base station, the
+ * level of +- 1 is relative to the normal speech evenlope.
+ *
+ * When converting sample_t to int16_t, the level of +- 1 is reduced by factor.
+ * This way the speech may be louder before clipping happens.
+ *
+ * When using sample_t to modulate (SDR or sound card), the level is changed,
+ * so it represents the frequency deviation in Hz. The deviation of speech
+ * envelope is network dependent.
+ */
+
+void samples_to_int16(int16_t *spl, sample_t *samples, int length)
+{
+	int32_t value;
+
+	while (length--) {
+		value = *samples++ * int_16_speech_level * 32768.0;
+		if (value > 32767.0)
+			*spl++ = 32767;
+		else if (value < -32767.0)
+			*spl++ = -32767;
+		else
+			*spl++ = (uint16_t)value;
+	}
+}
+
+void int16_to_samples(sample_t *samples, int16_t *spl, int length)
+{
+	while (length--) {
+		*samples++ = (double)(*spl++) / 32767.0 / int_16_speech_level;
+	}
+}
+