diff --git a/src/midi_vst.rs b/src/midi_vst.rs
index 1afe888..bb0d4c6 100644
--- a/src/midi_vst.rs
+++ b/src/midi_vst.rs
@@ -132,6 +132,7 @@ impl OutputSource {
     }
 }
 
+// Produces audio output PCM samples from VST.
 impl Iterator for OutputSource {
     type Item = f32;