Latest revision as of 18:47, 11 February 2017

The SSG (Software-controlled Sound Generator as Yamaha calls it) is part of the YM2610 sound chip. It provides 3 square wave channels and 1 LFSR noise channel. It is very closely related to the Atari ST's [YM2149].

Given its limited capacities, the SSG is more often used for simple sound effects (coin sound, cursors in menus...) rather than music.

Envelope generator

File:Ym2610egshapes.png

Selectable EG shapes of the SSG

The Envelope generator (EG) is an unique low-frequency signal generator which can be used to modulate the output of the tone channels.

Envelope shape register ($0D):

Bit	7	6	5	4	3	2	1	0
Def	-				CONT	ATT	ALT	HOLD

CONT: Should always be 1 ?
ATT: Counter direction (0:Down, 1:Up)
ALT: Alternate direction each period
HOLD: Hold value after first period (depends on ALT)

Envelope frequency = 8M / 2 / (256 * value)

Note that each period is 32 analog steps (5-bit internal counter).

Synthesis

If the Mode bit in the volume registers is 1, the EG's output is used instead of the fixed 4 bit value.

Tone frequency = 8M / 2 / (16 * (Coarse * 256 + Fine)) = 250000 / value

Example: Note A4 (440Hz) would be 250000 / 440 =~ 568 = $238.

Each channel is an AND (?) sum of both the channel's tone generator (if enabled) and the unique noise generator (if enabled).

Registers

Note that the YM2149 I/O registers $E and $F were removed in the YM2610.

Address (Z80 port 4)

Data (Z80 port 5)

$00

Bit	7	6	5	4	3	2	1	0
Def	Fine tune

Channel A

$01

Bit	7	6	5	4	3	2	1	0
Def	-				Coarse tune

$02

Bit	7	6	5	4	3	2	1	0
Def	Fine tune

Channel B

$03

Bit	7	6	5	4	3	2	1	0
Def	-				Coarse tune

$04

Bit	7	6	5	4	3	2	1	0
Def	Fine tune

Channel C

$05

Bit	7	6	5	4	3	2	1	0
Def	-				Coarse tune

$06

Bit	7	6	5	4	3	2	1	0
Def	-			Noise tune

Noise channel

$07

Bit	7	6	5	4	3	2	1	0
Def	-		/EN noise C	/EN noise B	/EN noise A	/EN tone C	/EN tone B	/EN tone A

Negative enable flags for mixing

$08

Bit	7	6	5	4	3	2	1	0
Def	-			Mode	Volume