SCHG:Sonic the Hedgehog 2 (Master System)/Music Hacking

SCHG: Sonic 2 (Master System)
Main Article
Art Editing
Editing Art Palettes Uncompressed Art Compressed
Level Editing
Editing Levels Level Headers 32x32 MetaTiles Level Layout
Object Editing
Editing Objects Object Layout Object Types
Music Editing
Editing Music Driver Technical Info Module Format

Driver Technical Information

Note: Since this is Z80 code, all words are little-endian unless otherwise stated.

Memory Layout

Address	Size	Description
$DD00	Byte	Unused
$DD01	Byte	Global speed counter
$DD02	Byte	Global speed counter reset
$DD03	Byte	Sound ID Main Trigger
$DD04	Byte	Sound ID Queue, Slot 1
$DD05	Byte	Sound ID Queue, Slot 2
$DD06	Byte	Sound ID Queue, Slot 3
$DD07	Byte	Unused
$DD08	Byte	Pause Mode (00 - off, 01-7F - paused, 80-FF - do unpause)
$DD09	Byte	Fade-out countdown - major value
$DD0A	Byte	Fade-out countdown - minor value
$DD0B	Byte	Fade-out countdown - minor reset value
$DD0C	Byte	Unused
$DD0D	Byte	Current sound effect number
$DD0E	Byte	Unused
$DD0F	Byte	Current Sound priority
$DD10	Byte	Unused (Game Gear Stereo bits in later drivers)
$DD11	Byte	Current PSG noise mode
$DD12-14	3 Bytes	Unused
$DD15	Byte	Unknown (modifies noise mode, never written and should always be 0)
$DD16	Byte	Noise channel attenuation
$DD17	41 Bytes	Unused
$DD40	48 Bytes	Music Channel 1 RAM (PSG 1)
$DD70	48 Bytes	Music Channel 2 RAM (PSG 2)
$DDA0	48 Bytes	Music Channel 3 RAM (PSG 3)
$DDD0	48 Bytes	Music Channel 4 RAM (PSG Noise)
$DE00	48 Bytes	SFX Channel 1 RAM (PSG 2)
$DE30	48 Bytes	SFX Channel 2 RAM (PSG 3)
$DE60	48 Bytes	SFX Channel 3 RAM (PSG Noise)

Channel Structures

Each channel occupies 48 bytes of RAM:

Offset	Type	Description
$00	Byte	Control flags
$01	Byte	PSG channel number
$02	Unsigned Byte	Note length multiplier
$03	Word	Data pointer
$05	Signed Byte	Transposition (added to note value)
$06	Signed Byte	Volume
$07	Byte	Pitch Envelope
$08	Byte	Volume Envelope
$09	Unsigned Byte	Channel branch "stack" pointer
$0A	Unsigned Byte	Note duration counter
$0B	Unsigned Word	PSG tone register value (i.e. the value written to the PSG to generate the note)
$0D	Unsigned Byte	Note duration reset value
$0E	Byte	Note Stop counter (number of frames after the note gets turned off)
$0F	Byte	Note Stop reset value
$10	Word	Data pointer for pitch bend effect.
$12	Signed Word	Pitch bend effect adjustment value. Added to word at $0B before writing to the PSG.
$14	Unsigned Byte	Number of ticks to wait before starting pitch bend.
$15	Unsigned Byte	Number of ticks to wait before increasing the pitch bend. Also used as the pitch envelope index.
$16	Signed Byte	Pitch bend step size.
$17	Unsigned Byte	Number of pitch bend steps before negating step value.
$18	5 Bytes	Unused.
$1D	Byte	Volume effect flags
$1E	Byte	Volume effect mode (00 - normal, 08 - repeat Attack/Decay phase)
$1F	Unsigned Byte	Volume effect value / Volume envelope index
$20	Signed Byte	Volume Effects - Adjustment value
$21	4 Bytes.	Volume effect memory
$25	Signed Byte	Detune value (added to raw PSG frequency)
$26	Byte	Unused (Channel Stereo Bits in later drivers)
$27	3 Bytes	Loop counters
$2A	3 Words	Branch return address stack

Channel Control Flags

Bit	Description
0	Unknown.
1	Maintain effect state across notes
2	Suppress PSG writes (i.e. music channel that is overridden by SFX)
3	Triggers literal read mode
4	Suppress writes to the PSG volume register (channel plays a rest)
5	Unknown
6	Allow PSG volume updates only
7	Channel active flag

Pitch Effect Flags

Bit	Description
0-6	If bit 7 = 0: pitch envelope number otherwise unused.
7	Set = pitch bend; Reset = pitch envelope.

Literal Read Mode

If bit 3 of the channel's control byte is set the driver enters literal read mode. This changes the way that data is read from the channel's stream: instead of reading bytes, the driver starts reading big-endian words (command bytes are still processed). These words are interpreted as PSG tone register values and are copied into offset $0B. The pitch adjustment at $05 is sign-extended to 16-bits and treated as a detune value which is added to the word before copying to the offset. Each word must be followed by a duration byte.

Module Editing

Module Header

Offset	Size	Description
$00	Word	Unused. Was probably intended as a pointer to volume envelope data but the driver ignores it.
$02	Byte	Number of channels.
$03	Byte	Unused. Skipped by the driver.
$04	Byte	Note length multiplier. Copied into offset $02 of each channel structure.
$05	Byte	Global speed. Copied into $DD01 and $DD02

Following the module header are the channel headers. The number of channel headers must match the value at offset $02 in the module header.

Offset	Size	Description
$00	Word	Channel data pointer (absolute).
$02	Byte	Pitch adjustment. (transposition)
$03	Byte	Volume adjustment.

Commands

Command	Description
$00 - $7F	Duration values
$80 - $DF	Notes ($80 = rest)
$E0vvwwxxyyzz	Volume effect / ADSR envelope (Attack, Decay, Sustain, Release) vv - Attack Rate (volume starts at silence and increases with this speed until it reaches the maximum) ww - Decay Rate (how fast the volume decreases after the Attack Phase) xx - Decay Level (volume level where the Decay Phase ends) yy - Sustain Rate (how fast the volume decreases after the Decay Phase) zz - Release Rate (how fast the volume decreases after being turned off)
$E1xx	Detune - xx = amount (signed)
$E2xx	Adjust volume - same as $E6
$E3xx	Nothing (empty slot)
$E4xx	Adjust noise volume - xx = amount (signed)
$E5xx	Change ADSR Mode. 00 - normal, 01-FF - repeat Attack/Decay phases
$E6xx	Adjust volume - xx = amount (signed)
$E7	Maintain effect state (sets bit 1 of channel control)
$E8xx	set Note Stop timeout (number of frames after that the note is turned off forcibly, 0 - off)
$E9xx	Nothing (empty slot)
$EAxx	Nothing (empty slot)
$EBxx	Nothing (empty slot)
$ECxx	Nothing (empty slot)
$EDxx	Set global speed - xx = new speed
$EExx	Nothing (empty slot)
$EF	Invalid command (points into data)
$F0wwxxyyzz	Pitch Bend ww - Number of ticks to wait. xx - Number of ticks before increasing pitch bend. yy - Pitch bend step size. zz - Number of ticks before negating step size.
$F1	Invalid command (points into data)
$F2	Stop channel playback
$F3??	Unknown
$F4xx	Pitch envelope - xx = envelope number
$F5xx	Volume envelope - xx = envelope number
$F6xxxx	Jump - xxxx = absolute address
$F7xxyyzzzz	Loop xx - Slot number yy - Loop count zzzz - Absolute address
$F8xxxx	Subroutine branch - xxxx = absolute address
$F9	Return from subroutine
$FAxx	Set channel speed - xx = new speed
$FBxx	Transpose channel by xx (added to current transpose value, which is added to all notes before looking up the PSG frequency)
$FCxx	Nothing (empty slot)
$FDxx	Set read literal mode. Turned on if xx == $01, otherwise off.
$FExx	Nothing (empty slot)
$FFxx	Nothing (empty slot)