diff --git a/papers/doc/7-systems/sms.md b/papers/doc/7-systems/sms.md
index 53560a976..882085f16 100644
--- a/papers/doc/7-systems/sms.md
+++ b/papers/doc/7-systems/sms.md
@@ -6,10 +6,13 @@ surely had better graphics than NES, but its sound (fairly weak, 4ch PSG with A-
 
 this console is powered by a derivative of the Texas Instruments SN76489.
 
+the original iteration of the SN76489 used in the TI-99/4A computers was clocked much lower at 447 kHz, being able to play as low as 13.670 Hz (A -1). consequentially, pitch accuracy for higher notes is compromised.
+
+
 # effects
 
 - `20xy`: set noise mode.
-  - `x` controls whether to inherit frequency from hannel 3.
+  - `x` controls whether to inherit frequency from channel 3.
     - 0: use one of 3 preset frequencies (C: A-2; C#: A-3; D: A-4).
     - 1: use frequency of channel 3.
   - `y` controls whether to select noise or thin pulse.