Contents

• Overview
• A little bit about operators
• Register overview
• Envelope specification
• Part I memory map
• The registers
  • Register 22H - LFO
  • Registers 24H & 25H - Timer A
  • Register 26H - Timer B
  • Register 27H - Timers; Ch3/6 mode
  • Register 28H - Key on/off
  • Registers 2AH & 2BH - DAC
  • Register 30H+ - detune; multiple
  • Register 40H+ - total level
  • Register 50H+ - rate scaling; attack rate
  • Register 60H+ - first decay rate; amplitude modulation
  • Register 70H+ - secondary decay rate
  • Register 80H+ - secondary amplitude; release rate
  • Register 90H+ - proprietary
  • Registers A0H-AFH+ - frequency
  • Register B0H+ - feedback; algorithm
• Test program

Original page numbers

Its capabilities include:

• 6 channels of FM sound
• An 8-bit Digitized Audio channel (as replacement for one of the FM channels)
• Stereo output capability
• One LFO (low frequency oscillator) to distort the FM sounds
• 2 timers, for use by software

The standard FM channels each have a single overall frequency and data for how to turn this frequency into the complex final wave form (the voice). This conversion process uses four dedicated channel components called 'operators', each possessing a frequency (a variant of the overall frequency), an envelope, and the capability to modulate its input using the frequency and envelope. The operator frequencies are offsets of integral multiples of the overall frequency.

There are two sets of three FM channels, named channels 1 to 3 and 4 to 6 respectively. Channels 3 and 6, the last in each set, have the capability to use a totally separate frequency for each operator rather than offsets of integral multiples. This works well (l believe) for percussion instruments, which have harmonics at odd multiples such as 1.4 or 1.7 of the fundamental.

The 8-bit Digitized Audio exists as a replacement of FM channel 6, meaning that turning on the DAC turns off FM channel 6. Unfortunately, all timing must be done by software -- meaning that unless the software has been very cleverly constructed, it is impossible to use any of the FM channels at the same time as the DAC. As you probably know, loads of games actually do this.

The LFO, or Low Frequency Oscillator, allows for amplitude and/or frequency distortions of the FM sounds. Each channel elects the degree to which it will be distorted by the LFO, if at all. This could be used, for example, in a guitar solo.

Finally, the system has two software timers, which may be used as an alternative to the Z80 VBLANK interrupt. Unfortunately, these timers do not cause interrupts - they must be read by the software to determine if they have finished counting.

A little bit about operators

• An operator has an input, a frequency and envelope with which to modify the input, and an output.
• The operators have two types, those whose outputs feed into another operator, and those that are summed to form the final wave form. The latter are called "slots".
• The slots may be independently enabled, though Sega's software always enables or disables them all simultaneously.
• Operator 1 may feed back into itself, resulting in a more complex waveform.

Algorithm #	Layout	Suggested uses
0		Distortion guitar, "high hat chopper" (?) bass
1		Harp, PSG (programmable sound generator) sound
2		Bass, electric guitar, brass, piano, woods
3		Strings, folk guitar, chimes
4		Flute, bells, chorus, bass drum, snare drum, tom-tom
5		Brass, organ
6		Xylophone, tom-tom, organ, vibraphone, snare drum, base drum
7		Pipe organ

Slots are indicated by shading.

• timer values and status, software use
• LFO enable and frequency, to distort the FM channels
• DAC enable and amplitude
• output enables for each of the 6 FM channels
• number of frequencies to be used in FM channels 3 and 6
  Usually, an FM channel has only one overall frequency, but if so elected, FM channels 3 and 6 use four separate frequencies, one for each operator.

• frequency number (in the standard case)
• algorithm number
• extent of self-feedback in operator 1
• output type, to L, R, or both speakers. This can only be heard if headphones are used.
• the extent to which the channel is distorted by the LFO.

Registers that refer to each operator make up the remainder. The four operator's connections are determined by the algorithm used, but the envelope is always specified individually for each operator. In the case of FM channels 3 and 6, the frequency may be specified individually for each operator.

The sound starts when the key is depressed, a process called 'key on'. The sound has an attack, a strong primary decay, followed by a slow secondary decay. The sound continues this secondary decay until the key is released, a process called 'key off'. The sound then begins a rapid final decay, representing for example a piano note after the key has been released and the damper has come down on the strings.

The envelope is represented by the above amplitudes and angles, and a few supplementary registers. Used in the above diagram are:

TL	Total level, the highest amplitude of the waveform.
AR	Attack rate, the angle of initial amplitude increase. This can be made very steep if desired. The problem with slow attack rates is that if the notes are short, the release (called 'key off') occurs before the note has reached a reasonable level.
D1R	The angle of initial amplitude decrease.
T1L	The amplitude at which the slower amplitude decrease starts.
D2R	The angle of secondary amplitude decrease. This will continue indefinitely unless 'key off' occurs.
RR	The final angle of amplitude decrease, after 'key off'.

RS	Rate scaling, the degree to which envelopes become shorter as frequencies become higher. For example, high notes on a piano fade much more quickly than low notes.
AM	Amplitude Modulation enable, whether or not this operator will allow itself to be modified by the LFO. Changing the amplitude of the slots (those colored gray in the diagram on page 3) changes the loudness of the note; changing the amplitude of the other operators changes its flavor.
SSG-EG	A proprietary register whose usage is unknown. It should be set to 0.

The YM-2612 may be accessed from either the 68000 or the Z80. In both cases, however, the bus is only 8 bits wide.

The YM-2612 is accessed through memory locations 4000H - 4003H in the Z80 case, or A04000H - A04003H in the 68000 case. These will be referred to as 4000 to 4003.

The internal registers of the FM-2612 are divided as follows:

CAUTION: Before writing, read from any address to determine if the YM-2612 I/O is still busy from the last write. Delay until bit 7 returns to 0.

CAUTION: in the case of registers that are "ganged together" to form a longer number, for example the 10-bit Timer A value or the 14-bit frequencies, write the high register first.

READ DATA: Reading from any of the four locations gives:

D7	D6	D5	D4	D3	D2	D1	D0
Busy						Overflow A	Overflow B

Busy	1 if busy, 0 if ready for new data
Overflow	1 if the timer has counted up and overflowed. See register 27H.

Part I memory map

Each of 30H-90H has twelve entries, three channels × four operators:

30H	Ch 1 op 1
31H	Ch 2 op 1
32H	Ch 3 op 1
33H
34H	Ch 1 op 2
35H	Ch 2 op 2
36H	Ch 3 op 2
37H
38H	Ch 1 op 3
39H	Ch 2 op 3
3AH	Ch 3 op 3
3BH
3CH	Ch 1 op 4
3DH	Ch 2 op 4
3EH	Ch 3 op 4
3FH

Channels 1-3 become channels 4-6 in Part II.

A0H	Ch 1
A1H	Ch 2
A2H	Ch 3
A3H

with the exception that A8H and ACH follow the pattern:

A8H	Ch 3 op 2
A9H	Ch 3 op 3
AAH	Ch 3 op 4
ABH

"Part II" is a duplication of 30H - B4H, where channels 1-3 are replaced by 4-6.

LFO enable	1 is enabled, 0 is disabled.
LFO frequency /Hz	0	1	2	3	4	5	6	7
	3.98	5.56	6.02	6.37	6.88	9.63	48.1	72.2

The LFO (Low frequency Oscillator) is used to distort the FM sounds?amplitude and phase. It is triply enabled, as there is:

1. A global enable in register 22H
2. A sensitivity enable on a channel by channel basis, in registers B4H-B6H.
3. An amplitude enable on an operation by operation basis, in registers 60H-6EH.

Registers 24H and 25H are ganged together to form 10-bit Timer A, with register 25H containing the least significant bits. They should be set in the order 24H, 25H. The timer lasts:

8 Bit Timer B lasts:

Register 27H - Timers; Ch3/6 mode

Register 27H controls the software timers and the Channel 3 (and 6) mode, two entirely separate items.

Ch3 mode	D7	D6	Description
Normal	0	0	Channel 3 is the same as the others
Special	0	1	Channel 3 has 4 separate frequencies
Illegal	1	0
	1	1

No one at Sega has used the timer feature, but the Japanese manual says:

Load	1 starts the timer, 0 stops it.
Enable	1 causes timer overflow to set the read register flag. 0 means the timer keeps cycling without setting the flag.
Reset	Writing a 1 clears the read register flag, writing a 0 has no effect.

This register is used for "Key on" and "Key off". "Key on" is the depression of the synthesizer key, "Key off" is its release. The sequence of operations is: set parameters, Key on, wait, key off. When key off occurs, the FM channel stops its slow decline and starts the rapid decline specified by "RR", the release rate.

In a single write to register 28H, one sets the status of all operators for a single channel. Sega always sets them to the same value, on (1) or off (0). Using a special channel 3, I believe it is possible to have each operator be a separate note, so there is possible justification for turning then on and off separately.

Register 2AH contains 8 bit DAC data.

If the DAC enable is 1, the DAC data is output as a replacement for channel 6. The only Channel 6 register that affects the DAC is the stereo output portion of register B4H.

Both DT1 (detune) and MUL (multiple) relate the operator's frequency to the overall frequency.

MUL ranges from 0 to 15 (decimal), and multiplies the overall frequency, with the exception that 0 results in multiplication by 1/2. That is, MUL=0 to 15 gives ×1/2, ×1, ×2, ... ×15.

DT1 gives small variations from the overall frequency × MUL. The MSB of DT1 is a primitive sign bit, and the two LSB's are magnitude bits. See the next page for a diagram.

Register 40H+ - total level

TL (total level) represents the envelope's highest amplitude, with 0 being the largest and 127 (decimal) the smallest. A change of one unit is about 0.75 dB.

To make a note softer, only change the TL of the slots (the output operators). Changing the other operators will affect the flavor of the note.

Register 50H contains RS (rate scaling) and AR (attack rate). AR is the steepness of the initial amplitude rise, shown on page 5.

RS affects AR, D1R, D2R and RR in the same way. RS is the degree to which the envelope becomes narrower as the frequency becomes higher.

The frequency's top five bits (3 octave bits and 2 note bits) are called KC (Key code) in the following rate formulae:

RS	Final rate
0	2×Rate+(KC/8)
1	2×Rate+(KC/4)
2	2×Rate+(KC/2)
3	2×Rate+(KC/1)

(KC/n) is always rounded down

As Rate ranges from 0-31, this means that the RS influence ranges from small (at 0-3) to very large (at 0-31).

D1R (first decay rate) is the initial steep amplitude decay rate (see page 4). It is, like all rates, 0-31 in value and affected by RS.

AM is the amplitude modulation enable, whether of not this operator will be subject to amplitude modulation by the LFO. This bit is not relevant unless both the LFO is enabled and register B4's AMS (amplitude modulation sensitivity) is non-zero.

Register 70H+ - secondary decay rate

D2R (secondary decay rate) is the long tail off of the sound that continues as long as the key is depressed.

D1L is the secondary amplitude reached after the first period of rapid decay. It should be multiplied by 8 if one wishes to compare it to TL. Again as TL, the higher the number, the more attenuated the sound.

RR is the release rate, the final sharp decrease in volume after the key is released. All rates are 5 bit numbers, but there are only four bits available in the register. Thus, for comparison and calculation purposes, these four bits are the MSBs and the LSB is always 1. In other words, double it and add one.

Register 90H+ - proprietary

This register is proprietary and should be set to zero.

Channel 1's frequency is in A0 and A4H.
Channel 2's frequency is in A1 and A5H.
Channel 3's frequency, if it is in normal mode (please see page 12), is in A2 and A6H.

If Channel 3 is in special mode:
Operator 1's frequency is in A2 and A6H
Operator 2's frequency is in A8 and ACH
Operator 3's frequency is in A9 and ADH
Operator 4's frequency is in AA and AEH

Low                                                              High
617   653   692   733   777   823   872   924   979  1037  1099  1164
   635   372   392   755   800   847   898   951   1008  1131  1199
   (36)  (39)  (41)  (44)  (46)  (49)  (52)  (55)  (58)  (62)  (70)

This sequence should be used inside each octave.

Feedback is the degree to which operator 1 feeds back into itself. In the voice library, self feedback is represented as this:

The algorithm is the type of inter-operator connection used. Please see the list of the eight operators on page 3.

Register B4H+ - stereo; LFO sensitivity

Register B4H contains stereo output control and LFO sensitivity control.

L	Left Output, 1 is on, 0 is off.
R	Right Output, 1 is on, 0 is off.

NOTE: The stereo may only be heard by headphones.

AMS	0	1	2	3
Sensitivity /dB	0	1.4	5.9	11.8

FMS	0	1	2	3	4	5	6	7
% of a halftone	0	?.4	?.7	?0	?4	?0	?0	?0

Register	Value	Comments
22H	0	LFO off
27H	0	Channel 3 mode normal
28H	0	All channels off
	1
	2
	4
	5
	6
2BH	0	DAC off
30H	71H	DT1/MUL
34H	0DH
38H	33H
3CH	01H
40H	23H	Total Level
44H	2DH
48H	26H
4CH	00H
50H	5FH	RS/AR
54H	99H
58H	5FH
5CH	94H
60H	5	AM/D1R
64H	5
68H	5
6CH	7
70H	2	D2R
74H	2
78H	2
7CH	2
80H	11H	D1L/RR
84H	11H
88H	11H
8CH	A6H
90H	0	Proprietary
94H	0
98H	0
9CH	0
B0H	32H	Feedback/algorithm
B4H	C0H	Both speakers on
28H	00H	Key off
A4H	22H	Set frequency
A0H	69H
28H	F0H	Key on
<Wait>
28H	00H	Key off

Notes:

1. Write address then data.
2. Loop until read register D7 becomes 0
3. Follow MSB/LSB sequence.