From 16475fe4081044783125f61aaf1e66a173c0c744 Mon Sep 17 00:00:00 2001
From: Wayne Warthen <wwarthen@gmail.com>
Date: Mon, 9 Jan 2023 16:08:42 -0800
Subject: [PATCH] Audio Driver Cleanup

Resolved some consistency issues between the AY-3-8910 and SN76489 audio drivers.
---
 Source/HBIOS/audio.inc      | 105 +++++++++++++++++++++-------------
 Source/HBIOS/ay38910.asm    | 109 ++++++++++++++++++------------------
 Source/HBIOS/cfg_dyno.asm   |  10 ++--
 Source/HBIOS/cfg_ezz80.asm  |  10 ++--
 Source/HBIOS/cfg_master.asm |  10 ++--
 Source/HBIOS/cfg_mbc.asm    |  10 ++--
 Source/HBIOS/cfg_mk4.asm    |  10 ++--
 Source/HBIOS/cfg_n8.asm     |  10 ++--
 Source/HBIOS/cfg_rcz180.asm |  10 ++--
 Source/HBIOS/cfg_rcz280.asm |  10 ++--
 Source/HBIOS/cfg_rcz80.asm  |  10 ++--
 Source/HBIOS/cfg_rph.asm    |  10 ++--
 Source/HBIOS/cfg_sbc.asm    |  10 ++--
 Source/HBIOS/cfg_scz180.asm |  10 ++--
 Source/HBIOS/hbios.asm      |  37 ++++++++++++
 Source/HBIOS/sn76489.asm    |  94 +++++++++++++++++++------------
 Source/ver.inc              |   2 +-
 Source/ver.lib              |   2 +-
 18 files changed, 276 insertions(+), 193 deletions(-)

diff --git a/Source/HBIOS/audio.inc b/Source/HBIOS/audio.inc
index f821e2cc..fcb686c7 100644
--- a/Source/HBIOS/audio.inc
+++ b/Source/HBIOS/audio.inc
@@ -1,4 +1,5 @@
 #IF AUDIOTRACE
+;
 #DEFINE	AUDTRACE(STR)	PUSH DE \ LD DE, STR \ CALL WRITESTR \ POP DE
 #DEFINE AUDTRACE_A	CALL PRTHEXBYTE
 #DEFINE AUDTRACE_B	PUSH AF \ LD A, B \ CALL PRTHEXBYTE \ POP AF
@@ -9,12 +10,13 @@
 #DEFINE AUDTRACE_BC	PUSH HL \ PUSH BC \ POP HL \ CALL PRTHEXWORDHL \ POP HL
 #DEFINE AUDTRACE_DE	PUSH HL \ PUSH DE \ POP HL \ CALL PRTHEXWORDHL \ POP HL
 #DEFINE AUDTRACE_IY	PUSH HL \ PUSH IY \ POP HL \ CALL PRTHEXWORDHL \ POP HL
-
+;
 #DEFINE AUDDEBUG(S) push hl \ CALL PRTSTRD \ .TEXT S \ .TEXT "$" \ pop hl	; $$$$$$ PRINT STRING S TO CONSOLE - PRTD("HELLO") - NO TRAILING $ REQUIRED
-
+;
 #DEFINE AUDTRACE_CR AUDDEBUG("\r\n$")
-
+;
 #ELSE
+;
 #DEFINE	AUDTRACE(S)
 #DEFINE AUDTRACE_A
 #DEFINE AUDTRACE_B
@@ -25,51 +27,76 @@
 #DEFINE AUDTRACE_HL
 #DEFINE AUDTRACE_DE
 #DEFINE AUDTRACE_IY
-
+;
 #DEFINE AUDDEBUG(STR)
-
+;
 #DEFINE AUDTRACE_CR
+;
 #ENDIF
-
-
-#IFNDEF AUDIOUTILS
-#DEFINE AUDIOUTILS
-
+;
+#IFNDEF AUDIO_INC
+#DEFINE AUDIO_INC
+;
+; NOTE TABLE VALUES ARE SHIFTED A FEW EXTRA BITS TO MAKE THE
+; DIVISION AS ACCURATE AS POSSIBLE.
+;
+AUD_SCALE	.EQU	3
+;
+; ON ENTRY, DE IS ADDRESS OF NOTE TABLE, HL IS NOTE TO PLAY
+; NOTE VALUE 0 MEANS B0b/A0# IN OCTAVE 0 WHICH IS THE FIRST ENTRY
+; OF THE NOTE TABLE.  THE NOTE TABLE REPRESENTS THE FREQUENCIES
+; FOR 1 FULL OCTAVE IN QUARTER NOTES.  SINCE THERE ARE 12 NOTES
+; IN AN OCTAVE, THE TABLE HAS 48 ENTRIES FOR ALL QUARTER NOTES.
+;
+; ON EXIT, HL CONTAINS THE PERIOD VALUE TO PROGRAM INTO THE PSG
+; DERIVED FROM THE NOTE TABLE SCALED TO THE REQUESTED OCTAVE.
+;
 AUD_NOTE:
 	AUDDEBUG("AUDNOTE ")
 	AUDTRACE_HL
 	AUDTRACE_CR
 	AUDTRACE_DE
 	AUDTRACE_CR
-
-	push	de
-	LD	DE, 48
-	CALL	DIV16
-					; BC IS OCTAVE COUNT
-					; HL is NOTE WITHIN OCTAVE
-	ADD	HL, HL
-	pop	de
-	ADD	HL, DE
-
-	LD	A, (HL)			; RETRIEVE PERIOD COUNT FROM SN7NOTETBL
-	INC	HL
-	LD	H, (HL)
-	LD	L, A
-
-	INC	C
+;
+	; START BY SEPARATING THE NOTE AND OCTAVE PORTION
+	; OF THE INCOMING TONE VALUE IN HL
+	PUSH	DE			; SAVE NOTE TABLE ADR
+	LD	DE,48			; 48 QUARTERNOTES PER OCTAVE
+	CALL	DIV16			; SEPARATE OCTAVE AND NOTE
+;
+	; THE QUOTIENT (BC) IS THE OCTAVE NUMBER REQUESTED
+	; THE REMAINDER (HL) IS THE QUARTER NOTE WITHIN THE OCTAVE.
+;
+	; NOW USE THE QUARTER NOTE VALUE TO LOOKUP THE CORRESPONDING
+	; PSG PERIOD VALUE IN THE NOTE TABLE.
+	ADD	HL,HL			; SCALE FOR 2 BYTE TABLE ENTRY SIZE
+	POP	DE			; RECOVER THE TABLE ADR
+	ADD	HL,DE			; HL := DESIRED TABLE ENTRY ADR
+	LD	A,(HL)			; GET LOW BYTE OF PERIOD TO A
+	INC	HL			; POINT TO HIGH BYTE VALUE
+	LD	H,(HL)			; GET HIGH BYTE OF PERIOD TO H
+	LD	L,A			; PUT LOW BYTE INTO L
+;
+	; NOW WE SCALE THE PERIOD VALUE DOWN BASED ON THE OCTAVE VALUE IN C
+	; AND BY THE AUD_SCALE VALUE THAT WAS USED IN THE NOTE TABLE.
+	; FOR EACH OCTAVE, THE PERIOD IS HALVED WHICH DOUBLES THE FREQUENCY
+	; THAT WILL BE PRODUCED BY THE PSG.  SINCE MOVING UP AN OCTAVE
+	; SHOULD PRODUCE A FREQUENCY THAT IS TWICE THE LOWER OCTIAVE, THIS
+	; WORKS NICELY, ALTHOUGH SOME ERROR MAY BE INTRODUCED.
+;
+	LD	A,AUD_SCALE		; SHIFT BY AUD_SCALE BITS
+	ADD	A,C			; ... AND OCTAVE
+	LD	B,A			; USE AS LOOP COUNTER
 AUD_NOTE1:
-	DEC	C
-	JR	Z, AUD_NOTE2
-	SRL	H
-  	RR	L
-	JR	AUD_NOTE1
-
-AUD_NOTE2:
-	LD	A, L			; IF NOT ZERO
-	OR	H
-	RET	NZ			; RETURN THE CALCULATED PERIOD
-
-	LD	H, $FF			; OTHERWISE RETURN -1 PERIOD (ERROR)
-	LD	L, $FF
+	SRL	H			; RIGHT SHIFT HL
+	RR	L			; ... BY ONE BIT
+	DJNZ	AUD_NOTE1		; LOOP UNTIL DONE
+;
+	; IF THE RESULTANT PERIOD IS ZERO, IT MEANS THAT THE REQUESTED
+	; PERIOD IS TOO LOW (FREQUENCY TOO HIGH) FOR THE PSG TO PRODUCE.
+	LD	A, L			; CHECK FOR ZERO
+	OR	H			; ... MEANING PSG CAN'T DO IT
+	RET	NZ			; IF NOT ZERO, RETURN THE CALCULATED PERIOD
+	DEC	HL			; OTHERWISE RETURN -1 PERIOD (ERROR)
 	RET
 #ENDIF
diff --git a/Source/HBIOS/ay38910.asm b/Source/HBIOS/ay38910.asm
index c591e3b8..6a6df229 100644
--- a/Source/HBIOS/ay38910.asm
+++ b/Source/HBIOS/ay38910.asm
@@ -4,8 +4,12 @@
 ;
 ;======================================================================
 ;
-; @3.579545 OCTAVE RANGE IS 2 - 7 (Bb2/A#2 .. A7)
-; @4.000000 OCTAVE RANGE IS 2 - 7 (B2 .. A7)
+; AY-3-8910 & YM2149 PSG CHIPS NEED AN INPUT CLOCK FREQUENCY OF
+; NO MORE THAN 2 MHZ.  THE CLOSEST THING THERE IS TO A STANDARD
+; IS THE MSX FREQ OF 1.7897725 MHZ.
+;
+; @1.7897725 OCTAVE RANGE IS 2 - 7 (Bb2/A#2 .. A7)
+; @2.0000000 OCTAVE RANGE IS 2 - 7 (B2 .. A7)
 ;
 AY_RCSND	.EQU	0		; 0 = EB MODULE, 1=MF MODULE
 ;
@@ -14,7 +18,6 @@ AY_RSEL		.EQU	$9A
 AY_RDAT		.EQU	$9B
 AY_RIN		.EQU	AY_RSEL
 AY_ACR		.EQU	$9C
-AY_CLK		.SET	3579545 	; MSX NTSC COLOUR BURST FREQ = 315/88
 #ENDIF
 ;
 #IF (AYMODE == AYMODE_N8)
@@ -53,7 +56,6 @@ AY_RSEL		.EQU	$A0
 AY_RDAT		.EQU	$A1
 AY_RIN		.EQU	AY_RSEL
 AY_ACR		.EQU	$A2
-AY_CLK		.SET	3579545 	; MSX NTSC COLOUR BURST FREQ = 315/88
 #ENDIF
 ;
 ;======================================================================
@@ -92,14 +94,16 @@ AY_IDAT	.EQU	0			; NO INSTANCE DATA ASSOCIATED WITH THIS DEVICE
 ;
 AY_TONECNT	.EQU	3		; COUNT NUMBER OF TONE CHANNELS
 AY_NOISECNT	.EQU	1		; COUNT NUMBER OF NOISE CHANNELS
+;;
+;#IF (AY_CLK > 3579545)			; DEPENDING ON THE 
+;AY_SCALE	.EQU	2		; INPUT CLOCK FREQUENCY
+;#ELSE					; PRESCALE THE TONE PERIOD
+;AY_SCALE	.EQU	3		; DATA TO MAINTAIN MAXIMUM
+;#ENDIF					; RANGE AND ACCURACY
 ;
-#IF (AY_CLK > 3579545)			; DEPENDING ON THE 
-AY_SCALE	.EQU	2		; INPUT CLOCK FREQUENCY
-#ELSE					; PRESCALE THE TONE PERIOD
-AY_SCALE	.EQU	3		; DATA TO MAINTAIN MAXIMUM
-#ENDIF					; RANGE AND ACCURACY
-;
-AY_RATIO	.EQU	(AY_CLK * 100) / (16 >> AY_SCALE)
+	.ECHO	"SN76489 CLOCK: "
+	.ECHO	SN7CLK
+	.ECHO	"\n"
 ;
 #INCLUDE "audio.inc"
 ;
@@ -306,38 +310,8 @@ AY_VOLUME:
 ;======================================================================
 ;
 AY_NOTE:
-	AUDTRACE(AYT_NOTE)
-	AUDTRACE_HL
-	AUDTRACE_CR
-;
-;	CALL	PRTHEXWORDHL
-;	CALL	PC_COLON
-;
-	LD	DE, AY3NOTETBL		; ON ENTRY HL IS THE NOTE TO PLAY
-	PUSH	DE			; AND DE IS THE START OF NOTE TABLE
-	LD	DE, 48			; LOAD DE WITH NOTE TABLE SIZE
-	CALL	DIV16			; AND CALCULATE OCTAVE COUNT IN BC
-;			
-	ADD	HL, HL			; HL IS THE REMAINDER FROM ABOVE DIVISION (0-47) AND THE NOTE 
-	POP	DE			; TO PLAY IN THE OCTAVE. ADD IT TO THE START OF THE NOTE TABLE
-	ADD	HL, DE			; TO POINT TO THE PERIOD FOR THE NOTE WE WANT TO PLAY.
-;
-	LD	A, (HL)			; HL POINT TO CURRENT PERIOD COUNT WE WANT TO PLAY
-	INC	HL			; SO LOAD PERIOD COUNT FROM NOTE TABLE INTO HL
-	LD	H, (HL)			; SO WE CAN UPDATE IT FOR THE REQUIRED OCTAVE
-	LD	L, A
-;
-	;LD	A,AY_SCALE - 1		; THE NOTE TABLE PERIOD DATA HAS BEEN
-	LD	A,AY_SCALE		; THE NOTE TABLE PERIOD DATA HAS BEEN
-	ADD	A,C			; PRESCALED TO MAINTAIN RANGE SO ALLOW
-	LD	B,A			; FOR THIS WHEN CHANGING OCTAVE
-AY_NOTE1:
-	SRL	H			; ADJUST THE PERIOD DATA
-  	RR	L			; FOR THE DESIRED OCTAVE
-	DJNZ	AY_NOTE1		; FALL THROUGH TO SET PERIOD AND RANGE CHECK
-;
-;	CALL	PRTHEXWORDHL
-;	CALL	NEWLINE
+	LD	DE, AY3NOTETBL
+	CALL	AUD_NOTE		; RETURNS PERIOD IN HL, FALL THRU
 ;
 ;======================================================================
 ;	SOUND DRIVER FUNCTION - PERIOD
@@ -347,21 +321,22 @@ AY_PERIOD:
 	AUDTRACE(AYT_PERIOD)
 	AUDTRACE_HL
 	AUDTRACE_CR
-
-	LD	A, H			; IF ZERO - ERROR
+;
+	LD	A,H			; IF ZERO - ERROR
 	OR	L
-	JR	Z, AY_PERIOD1
+	JR	Z,AY_PERIOD1
 ;
-	LD	A, H			; MAXIMUM TONE PERIOD IS 12-BITS
+	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
-	LD	(AY_PENDING_PERIOD), HL	; SAVE AND RETURN SUCCESSFUL
+	JR	NZ,AY_PERIOD1		; RETURN NZ IF NUMBER TOO LARGE
+	LD	(AY_PENDING_PERIOD),HL	; SAVE AND RETURN SUCCESSFUL
+	XOR	A			; SET SUCCESS
 	RET
 ;
 AY_PERIOD1:
-	LD	A, $FF			; REQUESTED PERIOD IS LARGER
-	LD	(AY_PENDING_PERIOD), A	; THAN THE DEVICE CAN SUPPORT
-	LD	(AY_PENDING_PERIOD+1), A; SO SET PERIOD TO FFFF
+	LD	HL,$FFFF		; REQUESTED PERIOD IS LARGER
+	LD	(AY_PENDING_PERIOD),HL	; THAN PSG CAN SUPPORT, SO
+	OR	$FF			; SET PERIOD TO $FFFF
 	RET				; AND RETURN FAILURE
 ;
 ;======================================================================
@@ -612,10 +587,32 @@ AYT_REGWR		.DB	"\r\nOUT AY-3-8910 $"
 ;	QUARTER TONE FREQUENCY TABLE
 ;======================================================================
 ;
-; THE FREQUENCY BY QUARTER TONE STARTING AT A0# OCTAVE 0
-; USED TO MAP EACH OCTAVE (DIV BY 2 TO JUMP AN OCTAVE UP)
-; FIRST PLAYABLE NOTE WILL BE 0
-; ASSUMING A CLOCK OF 1843200 THIS MAPS TO A0#
+; THE FOLLOWING TABLE MAPS A FULL OCTAVE OF QUARTER-NOTES
+; STARTING AT A# IN OCTAVE 0 TO THE CORRESPONDING PERIOD
+; VALUE TO USE ON THE PSG TO ACHIEVE THE DESIRED NOTE FREQUENCY.
+;
+; THE FREQUENCY PRODUCED BY THE AY-3-8910 IS:
+; FREQ = CLOCK / 16 / PERIOD
+;
+; SO, TO MAP A DESIRED FREQUENCY TO A PERIOD, WE USE:
+; PERIOD = CLOCK / 16 / FREQ
+;
+; IN ORDER TO IMPROVE THE RESOLUTION OF THE FREQUENCY
+; VALUE USED, WE ALSO MULTPLY BOTH SIDES OF THE EQUATION
+; BY 100:
+; PERIOD * 100 = (CLOCK / 16 / FREQ) * 100
+;
+; THE RESULTING PERIOD VALUE CAN BE REPEATEDLY HALVED
+; TO TO JUMP UP AS MANY OCTAVES AS DESIRED.
+;
+; THE FINAL VALUE IS SHIFTED BY AUD_SCALE BITS
+; IN ORDER TO IMPROVE THE RESOLUTION.  THIS FINAL SHIFT
+; IS REMOVED WHEN IN THE AY_NOTE ROUTINE.
+;
+; ASSUMING A CLOCK OF 1.7897725 MHZ, THE FIRST PLAYABLE
+; NOTE WILL BE A0#/B0b (HBIOS NOTE CODE 0).
+;
+AY_RATIO	.EQU	(AY_CLK * 100) / (16 >> AUD_SCALE)
 ;
 AY3NOTETBL:
 	.DW	AY_RATIO / 2913		; A0#/B0b	178977250 / 2913 = 61440; PROOF: 61440 >> 3 = 7680, 3579545 / 7680 / 16 = 29.13
diff --git a/Source/HBIOS/cfg_dyno.asm b/Source/HBIOS/cfg_dyno.asm
index 2bffd67a..fb5cb3c0 100644
--- a/Source/HBIOS/cfg_dyno.asm
+++ b/Source/HBIOS/cfg_dyno.asm
@@ -196,13 +196,13 @@ PIO_SBC		.EQU	FALSE		; PIO: ENABLE PARALLEL PORT DRIVER FOR 8255 CHIP
 ;
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_NONE	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_NONE	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_ezz80.asm b/Source/HBIOS/cfg_ezz80.asm
index fde81ae0..28fea35d 100644
--- a/Source/HBIOS/cfg_ezz80.asm
+++ b/Source/HBIOS/cfg_ezz80.asm
@@ -240,13 +240,13 @@ PIO_SBC		.EQU	FALSE		; PIO: ENABLE PARALLEL PORT DRIVER FOR 8255 CHIP
 ;
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_RC	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_RCZ80	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_master.asm b/Source/HBIOS/cfg_master.asm
index 1f09a9f2..968a488f 100644
--- a/Source/HBIOS/cfg_master.asm
+++ b/Source/HBIOS/cfg_master.asm
@@ -320,13 +320,13 @@ PIOSBASE	.EQU	$60		; PIO: PIO REGISTERS BASE ADR FOR SBC PPI
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 UFBASE		.EQU	$0C		; UF: REGISTERS BASE ADR
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_NONE	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_NONE	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_mbc.asm b/Source/HBIOS/cfg_mbc.asm
index d9ce9007..2820b8fb 100644
--- a/Source/HBIOS/cfg_mbc.asm
+++ b/Source/HBIOS/cfg_mbc.asm
@@ -244,13 +244,13 @@ PIOSBASE	.EQU	$60		; PIO: PIO REGISTERS BASE ADR FOR SBC PPI
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 UFBASE		.EQU	$0C		; UF: REGISTERS BASE ADR
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_NONE	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	TRUE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	3579545 / 2	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_MBC	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	TRUE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_mk4.asm b/Source/HBIOS/cfg_mk4.asm
index 3f9e7024..555b40b7 100644
--- a/Source/HBIOS/cfg_mk4.asm
+++ b/Source/HBIOS/cfg_mk4.asm
@@ -246,13 +246,13 @@ PIOSBASE	.EQU	$60		; PIO: PIO REGISTERS BASE ADR FOR SBC PPI
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 UFBASE		.EQU	$0C		; UF: REGISTERS BASE ADR
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_VGM	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_SCG	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_n8.asm b/Source/HBIOS/cfg_n8.asm
index 1a8ef6a3..2653c052 100644
--- a/Source/HBIOS/cfg_n8.asm
+++ b/Source/HBIOS/cfg_n8.asm
@@ -241,13 +241,13 @@ PIOSBASE	.EQU	N8_PPI0		; PIO: PIO REGISTERS BASE ADR FOR SBC PPI
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 FIFO_BASE	.EQU	$0C		; UF: REGISTERS BASE ADR
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_VGM	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_N8	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_rcz180.asm b/Source/HBIOS/cfg_rcz180.asm
index 30792515..8e87167c 100644
--- a/Source/HBIOS/cfg_rcz180.asm
+++ b/Source/HBIOS/cfg_rcz180.asm
@@ -253,13 +253,13 @@ PIO_SBC		.EQU	FALSE		; PIO: ENABLE PARALLEL PORT DRIVER FOR 8255 CHIP
 ;
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_RC	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_RCZ180	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_rcz280.asm b/Source/HBIOS/cfg_rcz280.asm
index 66926fb6..9501676b 100644
--- a/Source/HBIOS/cfg_rcz280.asm
+++ b/Source/HBIOS/cfg_rcz280.asm
@@ -268,13 +268,13 @@ PIO_SBC		.EQU	FALSE		; PIO: ENABLE PARALLEL PORT DRIVER FOR 8255 CHIP
 ;
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_RC	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_RCZ80	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_rcz80.asm b/Source/HBIOS/cfg_rcz80.asm
index 34908eb8..b85d6df7 100644
--- a/Source/HBIOS/cfg_rcz80.asm
+++ b/Source/HBIOS/cfg_rcz80.asm
@@ -260,13 +260,13 @@ PIO_SBC		.EQU	FALSE		; PIO: ENABLE PARALLEL PORT DRIVER FOR 8255 CHIP
 ;
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_RC	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_RC	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_RCZ80	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_rph.asm b/Source/HBIOS/cfg_rph.asm
index 33bb97f1..5010d8a5 100644
--- a/Source/HBIOS/cfg_rph.asm
+++ b/Source/HBIOS/cfg_rph.asm
@@ -241,13 +241,13 @@ PIOSBASE	.EQU	RPH_PPI0		; PIO: PIO REGISTERS BASE ADR FOR SBC PPI
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 FIFO_BASE	.EQU	$0C		; UF: REGISTERS BASE ADR
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_NONE	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_N8	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_sbc.asm b/Source/HBIOS/cfg_sbc.asm
index 068d884b..48c1de7a 100644
--- a/Source/HBIOS/cfg_sbc.asm
+++ b/Source/HBIOS/cfg_sbc.asm
@@ -244,13 +244,13 @@ PIOSBASE	.EQU	$60		; PIO: PIO REGISTERS BASE ADR FOR SBC PPI
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 UFBASE		.EQU	$0C		; UF: REGISTERS BASE ADR
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_VGM	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_VGM	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_SCG	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/cfg_scz180.asm b/Source/HBIOS/cfg_scz180.asm
index 1bd33caa..154cb8e4 100644
--- a/Source/HBIOS/cfg_scz180.asm
+++ b/Source/HBIOS/cfg_scz180.asm
@@ -248,13 +248,13 @@ PIO_SBC		.EQU	FALSE		; PIO: ENABLE PARALLEL PORT DRIVER FOR 8255 CHIP
 ;
 UFENABLE	.EQU	FALSE		; UF: ENABLE ECB USB FIFO DRIVER (UF.ASM)
 ;
-SN76489ENABLE	.EQU	FALSE		; SN76489 SOUND DRIVER
+SN76489ENABLE	.EQU	FALSE		; SN: ENABLE SN76489 SOUND DRIVER
 AUDIOTRACE	.EQU	FALSE		; ENABLE TRACING TO CONSOLE OF SOUND DRIVER
-SN7CLK		.EQU	3579545		; DEFAULT TO NTSC
-SNMODE		.EQU	SNMODE_NONE	; DRIVER MODE: SNMODE_[NONE|RC|VGM]
+SN7CLK		.EQU	3579545		; SN: PSG CLOCK FREQ, ASSUME MSX STD
+SNMODE		.EQU	SNMODE_RC	; SN: DRIVER MODE: SNMODE_[NONE|RC|VGM]
 ;
-AY38910ENABLE	.EQU	FALSE		; AY: AY-3-8910 / YM2149 SOUND DRIVER
-AY_CLK		.EQU	CPUOSC / 4	; DEFAULT TO CPUOSC / 4
+AY38910ENABLE	.EQU	FALSE		; AY: ENABLE AY-3-8910 / YM2149 SOUND DRIVER
+AY_CLK		.EQU	1789772		; AY: PSG CLOCK FREQ, ASSUME MSX STD
 AYMODE		.EQU	AYMODE_RCZ180	; AY: DRIVER MODE: AYMODE_[SCG|N8|RCZ80|RCZ180|MSX|LINC|MBC]
 ;
 SPKENABLE	.EQU	FALSE		; SPK: ENABLE RTC LATCH IOBIT SOUND DRIVER (SPK.ASM)
diff --git a/Source/HBIOS/hbios.asm b/Source/HBIOS/hbios.asm
index 636d869c..1969d3a2 100644
--- a/Source/HBIOS/hbios.asm
+++ b/Source/HBIOS/hbios.asm
@@ -2812,6 +2812,43 @@ INITSYS3:
 ;
 #ENDIF
 ;
+#IF 0
+;
+	LD	HL,0
+	CALL	DBG_NOTE
+	LD	HL,48
+	CALL	DBG_NOTE
+	LD	HL,204
+	CALL	DBG_NOTE
+	LD	HL,268
+	CALL	DBG_NOTE
+	LD	HL,436
+	CALL	DBG_NOTE
+;
+	JP	INITSYS4
+;
+DBG_NOTE:
+	PUSH	HL
+	CALL	NEWLINE
+	PRTS("AY: $")
+	CALL	PRTDEC16
+	PRTS("=$")
+	CALL	AY_NOTE
+	LD	HL,(AY_PENDING_PERIOD)
+	CALL	PRTDEC16
+	POP	HL
+;
+	PRTS(" SN: $")
+	CALL	PRTDEC16
+	PRTS("=$")
+	CALL	SN7_NOTE
+	LD	HL,(SN7_PENDING_PERIOD)
+	CALL	PRTDEC16
+;
+	RET
+;
+#ENDIF
+;
 INITSYS4:
 ;
 #IF (MEMMGR == MM_Z280)
diff --git a/Source/HBIOS/sn76489.asm b/Source/HBIOS/sn76489.asm
index acfcad1e..59570c07 100644
--- a/Source/HBIOS/sn76489.asm
+++ b/Source/HBIOS/sn76489.asm
@@ -1,9 +1,13 @@
 ;======================================================================
-;	SN76489 sound driver
+;	SN76489 SOUND DRIVER
 ;
 ;	WRITTEN BY: DEAN NETHERTON
 ;======================================================================
 ;
+; SN74489 PSG CHIP NEEDS AN INPUT CLOCK FREQUENCY OF
+; NO MORE THAN 4 MHZ.  THE CLOSEST THING THERE IS TO A STANDARD
+; IS THE MSX FREQ OF 3.579545 MHZ.
+;
 ; TODO:
 ; 1. PROVIDE SUPPORT FOR NOISE CHANNEL
 ; 2. DOES THIS WORK FOR FASTER CPUS? ONLY BEEN TESTED ON A Z80 7MHZ UNIT
@@ -15,15 +19,13 @@
 #IF (SNMODE == SNMODE_VGM)
 SN76489_PORT_LEFT	.EQU	$C6	; PORTS FOR ACCESSING THE SN76489 CHIP (LEFT)
 SN76489_PORT_RIGHT	.EQU	$C7	; PORTS FOR ACCESSING THE SN76489 CHIP (RIGHT)
-;SN7CLK			.EQU	3575950	; CLOCK SPEED DRIVING THE SN76489 CHIPS
 #ENDIF
-
+;
 #IF (SNMODE == SNMODE_RC)
 SN76489_PORT_LEFT	.EQU	$FF	; PORTS FOR ACCESSING THE SN76489 CHIP (LEFT)
 SN76489_PORT_RIGHT	.EQU	$FB	; PORTS FOR ACCESSING THE SN76489 CHIP (RIGHT)
-;SN7CLK			.EQU	3575950	; CLOCK SPEED DRIVING THE SN76489 CHIPS
 #ENDIF
-
+;
 SN7_IDAT		.EQU	0
 SN7_TONECNT		.EQU	3	; COUNT NUMBER OF TONE CHANNELS
 SN7_NOISECNT		.EQU	1	; COUNT NUMBER OF NOISE CHANNELS
@@ -32,15 +34,13 @@ CHANNEL_0_SILENT	.EQU	$9F
 CHANNEL_1_SILENT	.EQU	$BF
 CHANNEL_2_SILENT	.EQU	$DF
 CHANNEL_3_SILENT	.EQU	$FF
-
-SN7RATIO	.EQU	SN7CLK * 100 / 32
-
+;
 	.ECHO	"SN76489 CLOCK: "
 	.ECHO	SN7CLK
 	.ECHO	"\n"
-
+;
 #INCLUDE "audio.inc"
-
+;
 SN76489_INIT:
 	LD	IY, SN7_IDAT		; POINTER TO INSTANCE DATA
 	LD	BC, SN7_FNTBL		; BC := FUNCTION TABLE ADDRESS
@@ -125,33 +125,29 @@ SN7_VOLUME:
 SN7_NOTE:
 	LD	DE, SN7NOTETBL
 	CALL	AUD_NOTE		; RETURNS PERIOD IN HL, FALL THRU
-					; TO SET THIS PERIOD
+;
 SN7_PERIOD:
 	AUDTRACE(SNT_PERIOD)
 	AUDTRACE_HL
 	AUDTRACE_CR
-
-	LD	A, H			; IF ZERO - ERROR
+;
+	LD	A,H			; IF ZERO - ERROR
 	OR	L
-	JR	Z, SN7_QUERY_PERIOD1
-
-	LD	(SN7_PENDING_PERIOD), HL ;ASSUME SUCCESS
-
-	OR	A			; IF >= 401 ERROR
-	LD	DE, $401
-	SBC	HL, DE
-	JR	NC, SN7_QUERY_PERIOD1
-
-	XOR	A			; SIGNAL SUCCESS
-	RET
-
-SN7_QUERY_PERIOD1:			; REQUESTED PERIOD IS LARGER THAN THE SN76489 CAN SUPPORT
-	LD	A, $FF
-	LD	L, A
-	LD	H, A
-	LD	(SN7_PENDING_PERIOD), HL
+	JR	Z,SN7_PERIOD1
+;
+	LD	A,H			; MAXIMUM TONE PERIOD IS 10-BITS
+	AND	11111100B		; ALLOWED RANGE IS 0001-03FF (1023)
+	JR	NZ,SN7_PERIOD1		; RETURN NZ IF NUMBER TOO LARGE
+	LD	(SN7_PENDING_PERIOD),HL	; SAVE AND RETURN SUCCESSFUL
+	XOR	A			; SET SUCCESS
 	RET
-
+;
+SN7_PERIOD1:
+	LD	HL,$FFFF		; REQUESTED PERIOD IS LARGER
+	LD	(SN7_PENDING_PERIOD),HL	; THAN PSG CAN SUPPORT, SO
+	OR	$FF			; SET PERIOD TO $FFFF
+	RET				; AND RETURN FAILURE
+;
 SN7_PLAY:
 	AUDTRACE(SNT_PLAY)
 	AUDTRACE_D
@@ -408,12 +404,38 @@ SNT_PERIOD		.DB	"\r\nSN7_PERIOD: $"
 SNT_PLAY		.DB	"\r\nSN7_PLAY CH: $"
 SNT_REGWR		.DB	"\r\nOUT SN76489, $"
 #ENDIF
-
-; THE FREQUENCY BY QUARTER TONE STARTING AT A0#
-; USED TO MAP EACH OCTAVE (DIV BY 2 TO JUMP AN OCTAVE UP)
-; ASSUMING A CLOCK OF 3575950 HZ, THE FIRST PLAYABLE
+;
+;======================================================================
+;	QUARTER TONE FREQUENCY TABLE
+;======================================================================
+;
+; THE FOLLOWING TABLE MAPS A FULL OCTAVE OF QUARTER-NOTES
+; STARTING AT A# IN OCTAVE 0 TO THE CORRESPONDING PERIOD
+; VALUE TO USE ON THE PSG TO ACHIEVE THE DESIRED NOTE FREQUENCY.
+;
+; THE FREQUENCY PRODUCED BY THE SN76489 IS:
+; FREQ = CLOCK / 32 / PERIOD
+;
+; SO, TO MAP A DESIRED FREQUENCY TO A PERIOD, WE USE:
+; PERIOD = CLOCK / 32 / FREQ
+;
+; IN ORDER TO IMPROVE THE RESOLUTION OF THE FREQUENCY
+; VALUE USED, WE ALSO MULTPLY BOTH SIDES OF THE EQUATION
+; BY 100:
+; PERIOD * 100 = (CLOCK / 32 / FREQ) * 100
+;
+; THE RESULTING PERIOD VALUE CAN BE REPEATEDLY HALVED
+; TO TO JUMP UP AS MANY OCTAVES AS DESIRED.
+;
+; THE FINAL VALUE IS SHIFTED BY AUD_SCALE BITS
+; IN ORDER TO IMPROVE THE RESOLUTION.  THIS FINAL SHIFT
+; IS REMOVED WHEN IN THE AY_NOTE ROUTINE.
+;
+; ASSUMING A CLOCK OF 3.579545 MHZ, THE FIRST PLAYABLE
 ; NOTE WILL BE A2 (HBIOS NOTE CODE 92).
-
+;
+SN7RATIO	.EQU	(SN7CLK * 100) / (32 >> AUD_SCALE)
+;
 SN7NOTETBL:
 	.DW	SN7RATIO / 2913		; A0#/B0b
 	.DW	SN7RATIO / 2956         ;
diff --git a/Source/ver.inc b/Source/ver.inc
index a28e2b6c..04683e23 100644
--- a/Source/ver.inc
+++ b/Source/ver.inc
@@ -2,4 +2,4 @@
 #DEFINE	RMN	1
 #DEFINE	RUP	1
 #DEFINE	RTP	0
-#DEFINE BIOSVER	"3.1.1-pre.190"
+#DEFINE BIOSVER	"3.1.1-pre.191"
diff --git a/Source/ver.lib b/Source/ver.lib
index 21ab6b99..1974dabc 100644
--- a/Source/ver.lib
+++ b/Source/ver.lib
@@ -3,5 +3,5 @@ rmn	equ	1
 rup	equ	1
 rtp	equ	0
 biosver	macro
-	db	"3.1.1-pre.190"
+	db	"3.1.1-pre.191"
 	endm