FM

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The FM (Frequency Modulation) is part of the Chipicon.png 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)
$21
Bit 7 6 5 4 3 2 1 0
Def ?
Test register. Ignore or set to $00 for normal operation.
$22
Bit 7 6 5 4 3 2 1 0
Def - OnControl
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 Chipicon.png 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 - BlockF-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 - FBALGO
Feedback (FB) and Algorithm (ALGO)
$B5 $B6 $B5 $B6
Bit 7 6 5 4 3 2 1 0
Def L RAMS-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 * Block2CH * 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 - DTMUL
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)