diff --git a/Source/Doc/SystemGuide.md b/Source/Doc/SystemGuide.md
index 6cd9c50e..bf1a2d99 100644
--- a/Source/Doc/SystemGuide.md
+++ b/Source/Doc/SystemGuide.md
@@ -1241,7 +1241,7 @@ The Status (A) is a standard HBIOS result code.
 Assign the specified character Attribute (E) code to be used for all 
 subsequent character writes/fills on the specified Video Unit (C). This 
 attribute is used to fill new lines generated by scroll operations. The 
-character attributes values are listed abovev. Note that a given video 
+character attributes values are listed above. Note that a given video 
 display may or may not support any/all attributes.  The Status (A) is a 
 standard HBIOS result code.
 
diff --git a/Source/HBIOS/ay38910.asm b/Source/HBIOS/ay38910.asm
index 6a6df229..45b97f00 100644
--- a/Source/HBIOS/ay38910.asm
+++ b/Source/HBIOS/ay38910.asm
@@ -11,6 +11,13 @@
 ; @1.7897725 OCTAVE RANGE IS 2 - 7 (Bb2/A#2 .. A7)
 ; @2.0000000 OCTAVE RANGE IS 2 - 7 (B2 .. A7)
 ;
+; DIFFENCES BETWEEN AY-3-8910 AND YM2149
+;  THE AY-3-8910 HAS 16 ENVELOPE LEVELS, YM2149 HAS 32.
+;  THIS AFFECTS AUDIO OUTPUT ONLY. THERE IS NO PROGRAMMING IMPACT.
+;  UNUSED BITS IN REGISTERS ARE READ AS ZERO ON AY-3-8910. 
+;  UNUSED BITS CAN BE READ BACK AND WRITTEN ON YM.
+;  VOLTAGE LEVEL OUTPUT ON A AY-3-8910 IS LOW AND AROUND 2V ON YM2149.
+;
 AY_RCSND	.EQU	0		; 0 = EB MODULE, 1=MF MODULE
 ;
 #IF (AYMODE == AYMODE_SCG)
@@ -101,8 +108,8 @@ AY_NOISECNT	.EQU	1		; COUNT NUMBER OF NOISE CHANNELS
 ;AY_SCALE	.EQU	3		; DATA TO MAINTAIN MAXIMUM
 ;#ENDIF					; RANGE AND ACCURACY
 ;
-	.ECHO	"SN76489 CLOCK: "
-	.ECHO	SN7CLK
+	.ECHO	"AY38910 CLOCK: "
+	.ECHO	AY_CLK
 	.ECHO	"\n"
 ;
 #INCLUDE "audio.inc"