# What is FM Tone Generation? Before going deeper into YM2612 details, these Yamaha pages are a good starting point for the general idea of FM tone generation. - [`FM Tone Generators and the Dawn of Home Music Production`](https://web.archive.org/web/20260724133543/https://usa.yamaha.com/products/contents/music_production/synth_50th/history/chapter002.html) - [`How FM Tone Generation Works`](https://web.archive.org/web/20260821160530/https://usa.yamaha.com/products/contents/music_production/synth_50th/anecdotes/004.html) # About YM2612 The YM2612 is a sound chip developed by Yamaha. It was used in Sega's Mega Drive / Genesis video game console. This chip has: - 6 channels of 4-operator FM tone generation - two interval timers - a sine-wave low-frequency oscillator (LFO) - stereo output - a simple DAC playback path for PCM-style sample playback on channel 6 # YM2612 Chip Information ``` http://www.chipdir.nl/pinusr/ym2612.txt +--()--+ GND | 1 24| 0M(CLK) D0 | 2 23| Vcc D1 | 3 22| A.Vcc D2 | 4 21| MOL D3 | 5 20| MOR D4 | 6 19| A.GND D5 | 7 18| A1 D6 | 8 17| A0 D7 | 9 16| /RD ? |10 15| /WR /IC |11 14| /CS GND |12 13| /IRQ +------+ ``` ## Pin summary - `GND`: digital ground - `A.GND`: analog ground for the audio output section - `Vcc`: digital power supply - `A.Vcc`: analog power supply for the audio output section - `0M (CLK)`: master clock input - `MOL`: left audio output pin - `MOR`: right audio output pin - `D0-D7`: 8-bit bidirectional data bus - `A0`: address select pin - `A1`: address select pin - `/RD`: read signal (active low) - `/WR`: write signal (active low) - `/CS`: chip select signal (active low) - `/IRQ`: interrupt request signal (active low) - `/IC`: reset signal (active low) - `Pin 10`: usually unused in normal applications Some references describe it as `NC`, while others describe it as a `TEST` pin. ## FM sound design Before going deeper into PCM / DAC, it is helpful to first look at how YM2612 makes FM tones. The main idea is: - parameters such as `DT`, `MULTI`, `TL`, `AR`, `D1R`, `D2R`, `SL`, and `RR` shape the sound of each operator - multiple operators can then be combined with different algorithms to make more complex sounds You can try these interactive pages first: - [`operator1.html`](../demos/operator1.html) Learn how a single operator changes when you adjust `DT`, `MULTI`, `TL`, `AR`, `D1R`, `D2R`, `SL`, and `RR`. - [`operator2.html`](../demos/operator2.html) Try two operators together and see how modulation and algorithm changes affect the tone. - [`YM2612 synth demo`](../synth/index.html) Combine multiple operators and channels to design more complex sounds in a playable demo. ## FM sound chip control `A0` and `A1` choose which control port is being accessed. They switch between address/data access and port 0/port 1 access. - `A1=0, A0=0`: address port 0 - `A1=0, A0=1`: data port 0 - `A1=1, A0=0`: address port 1 - `A1=1, A0=1`: data port 1 ## How `chip.write()` maps to YM2612 pins `void ym2612::write(uint32_t offset, uint8_t data)` is a simplified interface. - `offset` corresponds to the access selected by `A1` and `A0` - `data` corresponds to the 8-bit value on `D0-D7` The mapping is: - `offset = 0`: `A1=0, A0=0` -> address port 0 - `offset = 1`: `A1=0, A0=1` -> data port 0 - `offset = 2`: `A1=1, A0=0` -> address port 1 - `offset = 3`: `A1=1, A0=1` -> data port 1 Example: ```cpp chip.write(0, 0x30); chip.write(1, 0x01); ``` This means: 1. Select register `0x30` on address port 0. 2. Write `0x01` to that register on data port 0. In other words, software is not changing the sound by magic. It is sending YM2612-specific control values through these chip pins and writing them into YM2612 registers. ## PCM / DAC on YM2612 The YM2612 is mainly known as a 6-channel FM chip, but it also has a simple DAC playback path. - `0x2A`: DAC data - `0x2B`: DAC enable When DAC playback is enabled, channel 6 can be used for PCM-style sample playback instead of normal FM output. This is not a separate large PCM engine like later chips. It is a simpler DAC path where software keeps writing sample bytes to the chip. In practice, this is important for: - voice samples - drum hits - VGM playback that uses YM2612 DAC streaming In this repository, the current PCM-related browser support is mainly handled through the VGM playback path. That path reads VGM DAC/stream commands and forwards the sample bytes to YM2612 DAC register writes. ## Questions Q: Why are there two types of ground, `GND` and `A.GND`? A: The chip has both digital and analog sections. The analog section is used for audio output, so `A.Vcc` and `A.GND` are used for the audio output circuitry behind `MOL` and `MOR`. Q: Why do we need `/CS`? A: `/CS` tells the YM2612 that the current bus access is for this chip. Without it, multiple chips on the same bus could respond at the same time. Q: Why do we need `/IRQ`? A: `/IRQ` is mainly used to notify the CPU when an internal timer event occurs. Q: Why do we need `/RD` and `/WR`? A: `/RD` and `/WR` are control signals, not data lines. The actual data is carried on `D0-D7`. Q: What are `D0-D7` for? A: `D0-D7` are 8-bit data pins. They carry digital data between the CPU and the YM2612. Q: What is `/IC` for? A: `/IC` is the reset signal. It initializes the chip and clears its registers. ## References - http://www.chipdir.nl/pinusr/ym2612.txt - https://www.vgmpf.com/Wiki/index.php?title=YM2612 - https://gendev.spritesmind.net/forum/viewtopic.php?start=585&t=386