The SSG (S oftware-controlled S ound G enerator as Yamaha calls it) is part of the YM2610 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
Channel A
$09
Bit 7
6
5
4
3
2
1
0
Def -
Mode Volume
Channel B
$0A
Bit 7
6
5
4
3
2
1
0
Def -
Mode Volume
Channel C
$0B
Bit 7
6
5
4
3
2
1
0
Def Volume envelope period fine tune
$0C
Bit 7
6
5
4
3
2
1
0
Def Volume envelope period coarse tune
$0D
Bit 7
6
5
4
3
2
1
0
Def -
Volume envelope shape
See diagram.
