The FM (Frequency Modulation) is part of the YM2610 sound chip. It provides 4 channels, each having their own set of 4 operators, panning and amplitude values. It's the most used way of producing music in games.
Common registers
| Address (Z80 port 4)
|
Data (Z80 port 5)
|
|
| $22
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
- |
On | Control |
|
LFO control and frequency (see below).
|
| $28
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
Slot |
- | Channel |
|
Key On/Off for each channel.
|
LFO frequency values are as follows:
- 0 – 3.98Hz
- 1 – 5.56Hz
- 2 – 6.02Hz
- 3 – 6.37Hz
- 4 – 6.88Hz
- 5 – 9.63Hz
- 6 – 48.1Hz
- 7 – 72.2Hz
Channel numbering:
| FM Channel
|
Binary Code
|
| CH1
|
001
|
| CH2
|
010
|
| CH3
|
101
|
| CH4
|
110
|
This strange numbering seems to be due to the fact YM2610 is a YM2610B with 2 removed FM channels (000 and 100).
Channel registers
Depending on which channel you want to write to, the Z80 ports used are different:
- Channels 1 & 2: Ports 4/5
- Channels 3 & 4: Ports 6/7
| Address |
Data |
|
| CH1 |
CH2 |
CH3 |
CH4 |
CH1 |
CH2 |
CH3 |
CH4
|
| Port 4 |
Port 6 |
Port 5 |
Port 7
|
| $A1 |
$A2 |
$A1 |
$A2
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
F-Num 1 |
|
F-Numbers and Block (1/2)
|
| $A5 |
$A6 |
$A5 |
$A6
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
- |
Block | F-Num 2 |
|
F-Numbers and Block (2/2)
(must set this first)
|
| $B1 |
$B2 |
$B1 |
$B2
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
- |
FB | ALGO |
|
Feedback (FB) and Algorithm (ALGO)
|
| $B5 |
$B6 |
$B5 |
$B6
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
L |
R | AMS | - | PMS |
|
Left (L)/Right (R) output, AM Sense (AMS), and PM Sense (PMS)
|
2CH mode additional operator frequencies:
- OP1 frequency is stored in the usual CH2 frequency registers ($A2/$A6)
- write to Z80 ports 4/5
| Address |
Data |
|
| OP2 |
OP3 |
OP4 |
|
| $A8 |
$A9 |
$AA
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
2CH * F-Num 1 |
|
2CH mode F-Num LSB
|
| $AC |
$AD |
$AE
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
- |
2CH * Block | 2CH * F-Num 2 |
|
2CH mode F-Num MSB & Block
(must set this first)
|
Operator registers
The ranges given for the address represent all of the parameter values. Each channel's operators are laid out as follows:
| Operator |
1 |
2 |
3 |
4
|
| Channels 1, 3
|
$x1 |
$x5 |
$x9 |
$xD
|
| Channels 2, 4
|
$x2 |
$x6 |
$xA |
$xE
|
| Address |
Data |
|
| CH1 |
CH2 |
CH3 |
CH4 |
CH1 |
CH2 |
CH3 |
CH4
|
| Port 4 |
Port 6 |
Port 5 |
Port 7
|
| $31-$3E
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
- |
DT | MUL |
|
Detune (DT) and Multiple (MUL)
|
| $41-$4E
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
- |
Total Level |
|
Total Level (Volume)
|
| $51-$5E
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
KS |
- | AR |
|
Key Scale (KS) and Attack Rate (AR)
|
| $61-$6E
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
AM |
- | DR |
|
AM On (AM) and Decay Rate (DR)
|
| $71-$7E
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
- |
SR |
|
Sustain Rate (SR)
|
| $81-$8E
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
SL |
RR |
|
Sustain Level (SL) and Release Rate (RR)
|
| $91-$9E
|
| Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Def |
- |
SSG-EG |
|
Envelope generator (not to be confused with the SSG one)
|