diff --git a/papers/doc/7-systems/x1_010.md b/papers/doc/7-systems/x1_010.md
index 83587a2b8..7d5778713 100644
--- a/papers/doc/7-systems/x1_010.md
+++ b/papers/doc/7-systems/x1_010.md
@@ -15,7 +15,7 @@ In furnace, this chip is can be configurable for original arcade mono output or
 - `10xx`: change wave.
 - `11xx`: change envelope shape. (also wavetable)
 - `17xx`: toggle PCM mode.
-- `20xx`: set PCM frequency.*
+- `20xx`: set PCM frequency. (1 to FF)*
 - `22xx`: set envelope mode.
   - bit 0 sets whether envelope will affect this channel.
   - bit 1 sets whether envelope one-shot mode. when it sets, channel is halted after envelope cycle is ended.