diff --git a/doc/7-systems/ym2203.md b/doc/7-systems/ym2203.md
index 03950444..f79f95d2 100644
--- a/doc/7-systems/ym2203.md
+++ b/doc/7-systems/ym2203.md
@@ -99,3 +99,9 @@ several variants of this chip were released as well, with more features.
 - `5Dxx`: **set D2R/SR of operator 2.**
 - `5Exx`: **set D2R/SR of operator 3.**
 - `5Fxx`: **set D2R/SR of operator 4.**
+
+# system modes
+## extended channel 3
+in ExtCh mode, channel 3 is split into one column for each of its four operators and feedback are shared. the frequency of each operator may be controlled independently with notes and effects. this can be used for more polyphony or more complex sounds.
+
+all four operators are still combined according to the algorithm in use. for example, algorithm 7 acts as four independent sine waves. algorithm 4 acts as two independent 2op sounds. even with algorithm 0, placing a note in any operator triggers that operator alone.