SSG

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 noise channel. It is very closely related to the Atari ST's [YM2149].

Programming

Programming the SSG is done through 14 of the YM2610 registers, see YM2610 registers.

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

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 done in 32 steps (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)) = 4000000 / (16 * value)

Example: Note A4 (440Hz) would be 4000000 / (16 * 440) =~ 568 = $238.