diff --git a/src/dsp/pipeline.py b/src/dsp/pipeline.py
index 1fab85c..b7443b4 100644
--- a/src/dsp/pipeline.py
+++ b/src/dsp/pipeline.py
@@ -718,8 +718,14 @@ class AudioPipeline:
             ch0 = Send output (to amp input)
             ch1 = Return output (to amp FX return)
         """
-        ch0 = audio_in[0, :].copy()
-        ch1 = audio_in[1, :].copy()
+        if audio_in.ndim == 1:
+            # Mono input in 4CM mode — treat entire input as guitar
+            # channel, create silent return channel
+            ch0 = audio_in.copy()
+            ch1 = np.zeros_like(audio_in)
+        else:
+            ch0 = audio_in[0, :].copy()
+            ch1 = audio_in[1, :].copy()
 
         # ── 60Hz mains hum notch filter on both channels ────────────
         b0, b1, b2 = self._notch_b0, self._notch_b1, self._notch_b2