diff --git a/pwm.c b/pwm.c
index bcc2e1b..23d7e3c 100644
--- a/pwm.c
+++ b/pwm.c
@@ -2,6 +2,7 @@
 #include <pico/types.h>
 
 #include "const.h"
+#include "synth.h"
 
 #include "pwm.h"
 
@@ -13,7 +14,7 @@ void pwm_isr(void) {
 	pwm_clear_irq(slice);
 
 	// TODO:
-	float sample = 0.0f;
+	float sample = get_sample();
 
 	uint16_t level = (uint16_t)((sample + 1.0f) * 0.5f * 3400.0f);
 	pwm_set_chan_level(slice, chan, level);
diff --git a/synth.cc b/synth.cc
new file mode 100644
index 0000000..a1e0e3a
--- /dev/null
+++ b/synth.cc
@@ -0,0 +1,6 @@
+#include "synth.h"
+
+float get_sample(void) {
+	return 0;
+}
+
diff --git a/synth.h b/synth.h
new file mode 100644
index 0000000..1de8ca5
--- /dev/null
+++ b/synth.h
@@ -0,0 +1,4 @@
+#pragma once
+
+float get_sample(void);
+