Sound drivers: support/rejection of zero value for period

SN76489:
The SN76489 chip will produce a note for value 0 - but the driver will reject
this value - but it does allow value $400 - (logically the same) to produce the lowest note

AY38910:
This chip does not produce a sound for period of 0 - so its also rejected
The max value for this driver is still $0FFF

Source/HBIOS/ay38910.asm

@@ -217,7 +217,11 @@ AY_PERIOD:
 	AUDTRACE(AYT_PERIOD)
 	AUDTRACE_HL
 	AUDTRACE_CR
-;
+
+	LD	A, H			; IF ZERO - ERROR
+	OR	L
+	JR	Z, AY_PERIOD1
+
 	LD	A, H			; MAXIMUM TONE PERIOD IS 12-BITS
 	AND	11110000B		; ALLOWED RANGE IS 0001-0FFF (4095)
 	JR	NZ, AY_PERIOD1		; RETURN NZ IF NUMBER TOO LARGE

Source/HBIOS/sn76489.asm

@@ -113,11 +113,9 @@ SN7_PERIOD:
 	AUDTRACE_HL
 	AUDTRACE_CR
 
-	; TODO: MAKE ZERO BE AN INVALID VALUE (USE 400) TO INDICATE LOWEST NOTE
-	; NEED TO UPDATE TUNE.ASM TO SUPPORT THIS?
-	; LD	A, H			; IF ZERO - ERROR
-	; OR	L
-	; JR	Z, SN7_QUERY_PERIOD1
+	LD	A, H			; IF ZERO - ERROR
+	OR	L
+	JR	Z, SN7_QUERY_PERIOD1
 
 	LD	(SN7_PENDING_PERIOD), HL ;ASSUME SUCCESS
 
@@ -130,11 +128,10 @@ SN7_PERIOD:
 	RET
 
 SN7_QUERY_PERIOD1:			; REQUESTED PERIOD IS LARGER THAN THE SN76489 CAN SUPPORT
-	LD	L, $FF
-	LD	H, $FF
+	LD	A, $FF
+	LD	L, A
+	LD	H, A
 	LD	(SN7_PENDING_PERIOD), HL
-
-	OR	$FF			; SIGNAL FAILURE
 	RET
 
 SN7_PLAY: