From Sound Chip to Beep

1. What does the chip do?

The YM2612 makes sound from settings written by a program. It has six FM channels, each with four operators. An operator produces a sine-based signal and controls its loudness over time with an envelope. Operators can become audible output or modulate other operators.

For our first beep, we only need a steady tone: one audible operator, no modulation, and no feedback.

On the original hardware, the CPU sends bytes through the chip's data bus and a clock drives the chip. Here, Playground runs the emulator and sends its generated samples to browser audio. We only need to supply the settings and the timing.

2. A register is a small setting

A register has an address and holds a byte. The address chooses what to change; the byte supplies the setting. JavaScript's 0x prefix means hexadecimal: 0x07 is 7 and 0x7f is 127.

write(0xb0, 0x07); // CH1: algorithm 7, feedback 0

This write(address, value) helper is provided by Playground. It combines the two bus writes used in the C++ tutorial: select a register, then send its value.

// C++ / chip bus interface:
chip.write(0, 0xb0); // Select a register in bank 0.
chip.write(1, 0x07); // Send its value.

// JavaScript / Playground:
write(0xb0, 0x07);

Most channel registers are divided into two banks: bank 0 for CH1–CH3 and bank 1 for CH4–CH6. Playground calls this bank argument port. You can make it explicit with write(0, 0xb0, 0x07). The two-argument form defaults to bank 0.

The C++ bus offsets are different: 0 and 1 select/write bank 0; 2 and 3 select/write bank 1. For this beep, all writes use bank 0.

3. Choose a simple voice

Algorithm 7 connects all four operators directly to the output, in parallel. We make OP1–OP3 very quiet and leave OP4 audible.

The four hardware slot offsets are 0, 4, 8, 12. Adding an offset to a register group's base address selects a slot. The last offset, 12 (0x0c), selects OP4; for example, its total level is at 0x4c.

These are hardware address offsets, not Playground's OP1–OP4 constants. The two middle slots have a different logical order; they receive identical settings here.

We begin with fm.reset(). This leaves the two decay rates, D1R and D2R, at zero, so the tone stays steady until key off. Release then makes it fade quickly.

4. Set the pitch, then press the key

The chip stores pitch as a range called BLOCK and an 11-bit number called FNUM. The C++ beep uses BLOCK 4 and FNUM 0x269. They occupy two registers:

write(0xa4, 0x22); // High: (BLOCK 4 << 3) | upper FNUM bits 2
write(0xa0, 0x69); // Low: remaining eight FNUM bits

Write the high part first, then the low part. The low write commits the combined pitch. These values reproduce the tutorial's pitch; they are not a frequency in Hz.

Finally, register 0x28 controls key on and key off. In 0xf0, the upper four bits turn on all four operators and the lower bits select CH1. Key on starts the envelopes; it does not choose the voice or pitch.

write(0x28, 0xf0); // CH1: key on
await sleep(3);    // Let it sound for three seconds.
write(0x28, 0x00); // CH1: key off

5. Play the beep

Press Run in Playground. This is the complete JavaScript version of the register setup in ex03_beep.cpp, followed by a timed key off. Playground provides fm, write, and sleep.

Open in full Playground →

Change one thing and listen

• Change OP4's TL from 0x00 to 0x20 in the conditional expression: a quieter beep.
• Change pitch high from 0x22 to 0x2a: BLOCK rises from 4 to 5, one octave higher.
• Change sleep(3) to sleep(0.3): a shorter beep.
• Change stereo output from 0xc0 to 0x80 or 0x40: left only or right only.

6. From register bytes to musical controls

Once the register version makes sense, Playground's named methods let you express the same channel settings more directly:

fm.setAlgo(CH1, 7, 0);          // write(0xb0, 0x07)
fm.setPan(CH1, true, true);    // write(0xb4, 0xc0)
fm.setFrequency(CH1, 4, 0x269); // writes 0xa4 and 0xa0
fm.keyOn(CH1);                // write(0x28, 0xf0)
await sleep(3);
fm.keyOff(CH1);               // write(0x28, 0x00)

This fragment replaces the channel and playback lines; it still needs the operator setup above. The sound comes from the same sequence: configure the voice, set the pitch, key on, wait, key off.

Next, explore operators and FM sound design, or use one tone to explore tuning and musical intervals.