hello_ymfm_wasm

Beep For YM2612

This note is about ex03_beep.cpp.

The goal is not to make a rich instrument yet. The goal is to understand one simple YM2612 beep by starting from the final sound and then tracing how that sound is configured.

Start with the browser demo

Before reading the code, open this page and hear the target result:

• YM2612 Beep Demo

That demo applies the same YM2612 settings as ex03_beep.cpp.

So the main question of this tutorial is:

How do we describe that beep in FM terms, and how do we send those settings to ymfm or to the real chip?

First describe the beep in FM terms

Before looking at register numbers, it helps to describe the beep like this:

• use channel 1
• use algorithm 7
• send output to both left and right
• use all 4 operators with simple DT/MULTI settings
• keep operators 1-3 effectively silent
• let operator 4 be the audible part
• set one pitch with BLOCK + F-Number
• send Key ON

This is the real idea behind the beep. The register writes are only the chip-specific way to express that idea.

Channel and operator idea

The YM2612 has 6 FM channels. Each channel has 4 operators.

An operator is a small tone generator. Operators can modulate each other, or they can directly contribute to the final output.

In ex03_beep.cpp, we use channel 1.

In FM synthesis, one channel is basically built from 4 operators. That is why ranges such as 0x30-0x3c exist: the settings are repeated for the 4 operators.

Why Algorithm 7?

In Algorithm 7, all 4 operators work as carriers. This is a simple starting point because every operator directly contributes to the output.

It is not the only way to make a sound, but it is easy to understand.

Beep target configuration

For this beep, the intended setup is roughly:

Channel

• channel: 1
• algorithm: 7
• pan: left + right

Operators

• operator 1: simple frequency setting, almost silent
• operator 2: simple frequency setting, almost silent
• operator 3: simple frequency setting, almost silent
• operator 4: simple frequency setting, audible

More concretely:

• operators 1-4 use DT=0, MULTI=1
• operators 1-3 use TL=0x7f
• operator 4 uses TL=0x00
• operators 1-4 use fast AR
• operators 1-4 use simple sustain/release settings

Pitch

• 0xa4 = 0x22
• 0xa0 = 0x69

Trigger

• 0x28 = 0xf0

Without Key ON, the chip is configured but the note does not actually begin.

Then express that idea as YM2612 writes

Once the FM-side idea is clear, we can look at the chip-side expression.

Each register write follows this pattern:

1. Write the register number to the address port.
2. Write the value to the data port.

For example:

chip.write(0, 0x30);
chip.write(1, 0x01);

This means:

1. Select register 0x30.
2. Write value 0x01 into register 0x30.

So ex03_beep.cpp is really doing two things at once:

• describing a beep in FM terms
• translating that description into YM2612 register writes

Registers used in ex03_beep.cpp

Quick view

• 0x30-0x3c: operator detune and multiple
• 0x40-0x4c: operator total level
• 0x50-0x5c: operator attack rate
• 0x80-0x8c: operator sustain level and release rate
• 0xb0: channel algorithm and feedback
• 0xb4: left/right output enable
• 0xa4 and 0xa0: frequency
• 0x28: key on / key off

1. DT / MULTI

These registers control detune and frequency multiple for each operator.

• 0x30, 0x34, 0x38, 0x3c

In ex03_beep.cpp, they are all set to 0x01.

What happens:

• detune is kept very small
• multiple is set to 1
• all 4 operators start from a simple frequency relationship

More detail:

• Detune (DT) slightly shifts the pitch
• Multiple (MUL) changes the frequency by integer ratios such as 1x, 2x, 3x, and 4x
• MUL=2 is about one octave above the base frequency, and MUL=4 is about two octaves above
• in FM synthesis, these harmonic relationships strongly affect the tone color

2. Total Level

These registers control the output level of each operator.

• 0x40, 0x44, 0x48, 0x4c

In ex03_beep.cpp, operator 4 is louder than the others:

• 0x40 = 0x7f
• 0x44 = 0x7f
• 0x48 = 0x7f
• 0x4c = 0x00

What happens:

• operators 1-3 are nearly silent
• operator 4 is loud
• this makes the result easier to understand as a simple beep

More detail:

• Total Level (TL) is the final output level of an operator
• lowering the TL of a carrier makes the audible volume smaller
• lowering the TL of a modulator reduces brightness and harshness, making the tone rounder

3. Attack Rate

These registers control how quickly the sound rises.

• 0x50, 0x54, 0x58, 0x5c

In ex03_beep.cpp, they are all set to 0x1f.

What happens:

• the note starts quickly
• this helps make the sound feel like a short, clear beep

More detail:

• Attack Rate (AR) is how fast the sound reaches its peak after Key On
• larger values create a fast, sharp start
• smaller values create a slower, softer rise

4. Sustain / Release area

These registers are part of the envelope settings.

• 0x80, 0x84, 0x88, 0x8c

In ex03_beep.cpp, they are all set to 0x0f.

What happens:

• the envelope stays simple
• the example does not try to shape a rich instrument yet

More detail:

• Sustain Level (SL) is the level held after the decay stage
• Release Rate (RR) is how fast the sound disappears after Key Off
• a larger RR makes the sound stop quickly
• a smaller RR leaves a longer tail

5. Algorithm

• 0xb0 = 0x07

This selects Algorithm 7 for channel 1.

What happens:

• all 4 operators act as carriers
• they go directly to the output instead of forming a deeper modulation chain

More detail:

• Algorithm (ALG) defines how the 4 operators are connected
• different connections produce different musical characters, such as organ-like, bell-like, or bass-like tones
• 0xb0 also contains Feedback (FB)
• Feedback routes an operator output back into itself, which can add sharpness, noise, or distorted character

6. Stereo output

• 0xb4 = 0xc0

This enables both left and right output.

What happens:

• the sound is sent to both MOL and MOR
• the beep is heard in both left and right channels

More detail:

• this register also controls PAN
• you can send the sound to the left channel, the right channel, or both
• enabling both sides makes the sound appear centered

7. Frequency

• 0xa4 = 0x22
• 0xa0 = 0x69

These two registers set the pitch.

What happens:

• the channel gets one specific note value
• if you change these values, the beep pitch changes

More detail:

• this is the main pitch control area of the YM2612
• 0xa4 contains upper information such as BLOCK
• 0xa0 contains the lower F-Number bits
• together they define the final note pitch

8. Key ON

• 0x28 = 0xf0

This turns on operators 1-4 for channel 1.

What happens:

• the note begins
• without this write, the configured sound will not actually start

More detail:

• 0x28 is the actual note trigger
• it contains the channel number and the operator enable bits
• Key On starts the note, and Key Off moves the sound into its release phase

Read the code as a register dictionary

If you want to understand the YM2612 more deeply and write your own setup by yourself, it is a good idea to read ymfm source code directly.

In this repository, src/ymfm_opn.h:209 is a very useful register dictionary.

These lines are especially helpful:

• src/ymfm_opn.h:221 0x30 range -> op_detune, op_multiple
• src/ymfm_opn.h:223 0x40 range -> op_total_level
• src/ymfm_opn.h:224 0x50 range -> op_ksr, op_attack_rate
• src/ymfm_opn.h:229 0x80 range -> op_sustain_level, op_release_rate
• src/ymfm_opn.h:210 0xb0 -> ch_feedback, ch_algorithm
• src/ymfm_opn.h:212 0xb4 -> channel output / pan related fields
• src/ymfm_opn.h:209 0xa4 + 0xa0 -> ch_block_freq

For 0x28 key on/key off handling, src/ymfm_opn.cpp:176 is the place to read.

Once you can connect ex03_beep.cpp with these definitions, you are already very close to being able to build your own YM2612 beep setup.

Important idea

The beep is not made by one special “beep command”.

The sound appears because:

• the operators are configured
• the channel is configured
• the pitch is set
• Key ON starts the note

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