Compressor: state the input level it stays clean at

lib/audioeffects/compressor.py

@@ -42,6 +42,9 @@
 tail: the gain multiplies the audio, so silence in is silence out on the same
 sample.
 
+**Levels.** Keep the input under -6 dBFS and the defaults and patches stay
+off the rails.
+
 **Two traits this palette cannot carry, recorded rather than hidden**
 (dossier App. J.3, J.4):
 
@@ -204,6 +207,11 @@ class Compressor(_component.Component):
     #: "Detector and coupling state" in docs/reference/component-api.md.
     TAIL_SAMPLES = 0
 
+    #: The defaults and every patch play the ceiling kit's materials
+    #: clean at this level; at -3 dBFS, patch 3 rails on
+    #: a 100 Hz square.
+    INPUT_CEILING_DBFS = -6
+
     MACRO_LABELS = ("Character", "Threshold", "Ratio", "Attack", "Release",
                     "Release Slow", "Memory", "Knee", "Detector",
                     "RMS Window", "Emphasis", "Emphasis Freq", "Makeup",

tests/test_cpython_effects_compressor.py

@@ -385,3 +385,15 @@ def test_patch_14_and_back_mid_stream_settles(self):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+import kit_ceiling                                               # noqa: E402
+
+
+class TheCeiling(unittest.TestCase):
+    def test_clean_at_the_stated_input_level(self):
+        # square is where it rails first: p3 at -3 dBFS.
+        self.assertEqual(
+            kit_ceiling.stated_level_reds(compressor.Compressor, "square"), [])
+        self.assertTrue(kit_ceiling.stated_level_reds(
+            compressor.Compressor, "square", db=-3, keys=("p3",)))