SSG

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The SSG (Software-controlled Sound Generator as Yamaha calls it) is part of the Chipicon.png 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

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 - CONTATTALTHOLD
  • 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 - ModeVolume
Channel A
$09
Bit 7 6 5 4 3 2 1 0
Def - ModeVolume
Channel B
$0A
Bit 7 6 5 4 3 2 1 0
Def - ModeVolume
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.