AGSC (File Format)

AGSC is the sound effect format for Metroid Prime and Metroid Prime 2: Echoes. Each AGSC file contains a group of sound effects. The first two Metroid Prime games utilize the MusyX audio engine created by Factor5 and as a result, AGSC files are essentially just embedded MusyX files.

The audio codec used in AGSC is the standard GameCube DSP ADPCM codec.

To do: Better descriptions for how SoundMacros work and a description for what each command does.

This file format needs more research The proj chunk and the sdir chunk still need some reverse-engineering.

Format

The AGSC format is essentially four data chunks combined into one resource, each of which is a standard MusyX file. Of the four data chunks (pool, proj, samp, and sdir), there's one for sound engine scripts, one for sound properties, one for actual ADPCM sound data, and one for sound metadata. The main difference between Prime 1 and 2 is the header, and some slight changes in the way the four chunks are organized. In Metroid Prime, each chunk begins with its own size value; in Metroid Prime 2, every chunk instead has its size listed at the beginning of the file, at the end of the header. In addition, in Metroid Prime, the third chunk is samp, and the fourth is sdir; in Metroid Prime 2, it's the other way around.

Header

Metroid Prime

Offset	Size	Description
0x0	D	Audio Directory. Always "Audio/." Zero-terminated.
0x0 + D	N	Audio Group Name. Zero-terminated.
0x0 + D + N	End of header

Metroid Prime 2

Offset	Size	Description
0x0	4	Unknown; always 1
0x4	D	Audio Group Name. Zero-terminated.
0x4 + D	4	Pool size
0x8 + D	4	Project size
0xC + D	4	Sample size
0x10 + D	4	Sample directory size
0x14 + D	End of header

Pool

The Pool chunk denotes MusyX's SoundMacros, small scripts that apply various effects on the sounds in the game. Each macro is composed of a number of pre-defined commands; each command specifies its type through a single-byte command ID, then specifies the parameters of that particular command, which vary.

Each 4 bytes were originally little endian, but have been swapped to big endian in the AGSC files (despite not being longs). To read the data as originally formatted, every four bytes needs to be byte-swapped.

Each command is structured as follows:

Offset	Size	Description
0x0	1	Command ID (varies; there are 79 known commands in the MusyX audio engine.)
0x1	7	Command arguments (varies between commands)
0x8	End of entry

The SoundMacro terminates when the END command is executed. The command ID for END is 0 and has null command arguments; the next SoundMacro begins after reading it. The entire Pool chunk is terminated by a value of 0xFFFFFFFF.

These are the possible commands:

ID	Name	Arguments
0x1	STOP
0x2	SPLITKEY	Keynumber	SoundMacro ID		SoundMacro step
0x3	SPLITVEL	Velocity	SoundMacro ID		SoundMacro step
0x4	WAIT_TICKS	Keyoff	Random	Sampleend	Absolute	ms switch	Ticks/Millisec.
0x5	LOOP	Keyoff	Random	Sampleend	SoundMacro step		Times
0x6	GOTO		SoundMacro ID		SoundMacro step
0x7	WAIT_MS	Keyoff	Random	Sample end	Absolute		Millisec.
0x8	PLAYMACRO	Addnote	SoundMacro ID		SoundMacro step		Priority	MaxVoices
0x9	SENDKEYOFF	Variable	Last started
0xA	SPLITMOD	Mod value	SoundMacro ID		SoundMacro step
0xB	PIANOPAN	Scale	Centerkey	Centerpan
0xC	SETADSR	Table-ID (ADSR)		DLS mode
0xD	SCALEVOLUME	Scale	Add	Table-ID (Curve)		Org vol
0xE	PANNING	Pan position	Time ms		Width
0xF	ENVELOPE	Scale	Add	Table-ID (Curve)		Org vol
0x10	STARTSAMPLE	Sample-ID		Mode	Offset
0x11	STOPSAMPLE
0x12	KEYOFF
0x13	SPLITRND	RND	SoundMacro ID		SoundMacro step
0x14	FADE-IN	Scale	Add	Table-ID (Curve)		ms switch	Ticks/Millisec.
0x15	SPANNING	Pan position	Time ms		Width
0x16	SETADSRCTRL	Attack	Decay	Sustain	Relase
0x17	RNDNOTE	Note Lo	Detune	Note Hi	Fixed/Free	Abs/Rel
0x18	ADDNOTE	Add	Detune	org key		ms switch	Ticks/Millisec.
0x19	SETNOTE	Key	Detune			ms switch	Ticks/Millisec.
0x1A	LASTNOTE	Add	detune			ms switch	Ticks/Millisec.
0x1B	PORTAMENTO	Port. State	Port. Type			ms switch	Ticks/Millisec.
0x1C	VIBRATO	Level note	Level fine	Modwheel flag		ms switch	Ticks/Millisec.
0x1D	PITCHSWEEP1	Times	Add			ms switch	Ticks/Millisec.
0x1E	PITCHSWEEP2	Times	Add			ms switch	Ticks/Millisec.
0x1F	SETPITCH	Frequency (Hz)			Fine
0x20	SETPITCHADSR	Table-ID (ADSR)			Note range	Detune range
0x21	SCALEVOLUME DLS	Scale		Org vol
0x22	MOD2VIBRANGE	Key range	Cent range
0x23	SETUP TREMOLO	Tremolo scale			Modw. add scale
0x24	RETURN
0x25	GOSUB		SoundMacro ID		SoundMacro step
0x28	TRAP_EVENT	Event	SoundMacro ID		SoundMacro step
0x29	SEND_MESSAGE	IsVar	Macro		VID	Varibale
0x2A	GET_MESSAGE	Varibale
0x2B	GET_VID	Varibale	PLAY_MACRO
0x30	ADDAGECOUNT		Add
0x31	SETAGECOUNT		Counter
0x32	SENDFLAG	Flag-ID	Value
0x33	PITCHWHEELR	Range up	Range down
0x34	SETPRIORITY	Prio
0x35	ADDPRIORITY		Add
0x36	AGECNTSPEED				Time
0x37	AGECNTVEL		AGE Base		AGE Scale
0x40	VOL_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x41	PAN_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x42	PitchW_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x43	ModW_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x44	PEDAL_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x45	PORTA_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x46	REVERB_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x47	SPAN_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x48	DOPPLER_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x49	TREMOLO_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x4A	PREA_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x4B	PREB_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x4C	POSTB_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x4D	AUXAFX_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x4E	AUXBFX_SELECT	MIDI Contr.	Scaling percentage		Combine	is var.	Fine scaling
0x50	SETUP_LFO	LFO Nr.	Period in ms
0x58	MODE_SELECT	DLS vol	ITD
0x59	SET_KEYGROUP	group	kill
0x5A	SRCMODE_SELECT	SRC type	Type 0 SRC filter
0x60	ADD_VARS	Var/Ctrl	A =	Var/Ctrl	B +	Var/Ctrl	C
0x61	SUB_VARS	Var/Ctrl	A =	Var/Ctrl	B -	Var/Ctrl	C
0x62	MUL_VARS	Var/Ctrl	A =	Var/Ctrl	B *	Var/Ctrl	C
0x63	DIV_VARS	Var/Ctrl	A =	Var/Ctrl	B /	Var/Ctrl	C
0x64	ADDI_VARS	Var/Ctrl	A =	Var/Ctrl	B +	Immediate
0x70	IF_EQUAL	Ctrl	A ==	Ctrl	B	Not	SoundMacro Step
0x71	IF_LESS	Ctrl	A <	Ctrl	B	Not	SoundMacro Step
0x0	END

Project

The Project properties chunk contains values for the sounds, including priority, polyphony, volume, etc.

Offset	Size	Description
0x0	4	Chunk size
0x2	2	Unknown
0x4	2	Unknown; Always 0x1
0x8	4	Unknown; Always 0x28
0xC	4	Unknown
0x10	4	Unknown; possible offset?
0x14	4	Unknown; always +2 than previous long
0x18	4	Unknown; always +2 than previous long
0x1C	4	Unknown; always +4 than previous long
0x20	WIP

The first table terminates with 0xFFFFFFFF.

Offset	Size	Description
0x0	2	Number of soundmacros
0x2	2	Padding
0x4	2	ID; referenced in EVNT
0x6	2	Unknown; looks like another ID
0x8	1	Max number of voices; The maximum of voices allowed for this soundeffect.
0x9	1	Priority; voices are limited, so priority is used to play more important sounds over others
0xA	1	Definite Velocity; volume (usually 7F)
0xB	1	Panning
0xC	2	Definite Key; The default pitch (usually 0x3C00...the second byte may possibly be the MIDI channel)

Sample

The Sample chunk is all the sound data encoded using the standard Gamecube DSP ADPCM codec. It can be decoded the same way as a DSP file. Each sound's size is padded to 32 bytes before the next sound's data begins.

Sample Directory

The Sample Directory chunk (chunk 4 in Metroid Prime, chunk 3 in Metroid Prime 2) is made up of two sets of tables. The structure of both these tables is identical between both games.

Table A

The first metadata table has one entry per sound, and is terminated with 0xFFFFFFFF; since there's no known sound count anywhere in the file, the only way to read this correctly is to read until you reach the terminator value. Each entry is 0x20 bytes long.

Offset	Size	Description
0x0	2	Sound ID
0x2	2	Padding; always 0
0x4	4	Sound start offset, relative to the start of the ADPCM chunk
0x8	4	Unknown
0xC	2	Pitch; stored as a half-float, always 0x3C00 (1.0)
0xE	2	Sample rate
0x10	4	Number of samples
0x14	4	Loop start sample
0x18	4	Loop length, in samples. To get the loop end sample, add this to the start sample and subtract 1.
0x1C	4	Table B entry offset, relative to the start of the sound metadata chunk
0x20	End of entry

Table B

These are accessed through the offsets in table A's entries; note that it might not match the sound count, because the same entry in this table can be used with multiple sounds. Each entry is 0x28 bytes long.

Offset	Size	Description
0x0	2	Unknown; always 8
0x2	1	Initial predictor/scale (matches first frame header)
0x3	1	Loop predictor/scale (matches loop start frame header)
0x4	2	Loop context sample history 1
0x6	2	Loop context sample history 2
0x8	2 × 16	Decode coefficients
0x28	End of entry

Tools

• Prime Audio Decoder by Parax will dump all sound effects contained in a given AGSC file.