furnace/doc/7-systems/opll.md

# Yamaha YM2413/OPLL

the YM2413, otherwise known as OPLL, is a cost-reduced FM synthesis sound chip, based on the Yamaha YM3812 (OPL2). thought OPL was downgraded enough? :p

OPLL also spawned a few derivative chips, the best known of these is:
- the famous Konami VRC7. used in the Japan-only video game Lagrange Point, it was **another** cost reduction on top of the OPLL! this time just 6 channels...
- Yamaha YM2423, same chip as YM2413, just a different patch set
- Yamaha YMF281, ditto

# technical specifications

the YM2413 is equipped with the following features:

- 9 channels of 2 operator FM synthesis
- A drum/percussion mode, replacing the last 3 voices with 5 rhythm channels, with drum mode tones hard-defined in the chip itself, like FM instruments. Only pitch might be altered.

  - Drum mode works like following: FM channel 7 is for Kick Drum, which is a normal FM channel but routed through mxier twice for 2x volume, like all drum sounds. FM channel 8 splits to Snare Drum and Hi-Hat. Snare Drum is the carrier and it works with a special 1 bit noise generator combined with a square wave, all possible by overriding phase-generator with some different synthesis method. Hi-Hat is the modulator and it works with the noise generator and also the special synthesis. CH9 splits to Top-Cymbal and Tom-Tom, Top-Cymbal is the carrier and only has the special synthesis, while Tom-Tom is basically a 1op wave. 
  - Special syntheis mentioned already is: 5 square waves are gathered from 4x, 64x and 128x the pitch of channel 8 and 16x and 64x the pitch of channel 9 and they go through a process where 2 HH bits OR'd together, then 1 HH and 1 TC bit OR'd, then the two TC bits OR'd together, and those 3 results get XOR'd.
 
- 1 user-definable patch (this patch can be changed throughout the course of the song)
- 15 pre-defined patches which can all be used at the same time
- Support for ADSR on both the modulator and the carrier
- Sine and half-sine based FM synthesis
- 9 octave note control
- 4096 different frequencies for channels
- 16 unique volume levels (NOTE: Volume 0 is NOT silent.)
- Modulator and carrier key scaling
- Built-in hardware vibrato support

# effects

- **`11xx`**: set feedback of channel.
- **`12xx`**: set operator 1 level.
- **`13xx`**: set operator 2 level.
- **`16xy`**: set multiplier of operator.
  - `x` is the operator (1 or 2).
  - `y` is the multiplier.
- **`18xx`**: toggle drums mode.
  - 0 disables it and 1 enables it.
  - only in drums chip.
- **`19xx`**: set attack of all operators.
- **`1Axx`**: set attack of operator 1.
- **`1Bxx`**: set attack of operator 2.
- **`50xy`**: set AM of operator.
  - `x` is the operator (1-2). a value of 0 means "all operators".
  - `y` determines whether AM is on.
- **`51xy`**: set SL of operator.
  - `x` is the operator (1-2). a value of 0 means "all operators".
  - `y` is the value.
- **`52xy`**: set RR of operator.
  - `x` is the operator (1-2). a value of 0 means "all operators".
  - `y` is the value.
- **`53xy`**: set VIB of operator.
  - `x` is the operator (1-2). a value of 0 means "all operators".
  - `y` determines whether VIB is on.
- **`54xy`**: set KSL of operator.
  - `x` is the operator (1-2). a value of 0 means "all operators".
  - `y` is the value.
- **`55xy`**: set EGT of operator.
  - `x` is the operator (1-2). a value of 0 means "all operators".
  - `y` determines whether EGT is on.
- **`56xx`**: set DR of all operators.
- **`57xx`**: set DR of operator 1.
- **`58xx`**: set DR of operator 2.
- **`5Bxy`**: set KSR of operator.
  - `x` is the operator (1-2). a value of 0 means "all operators".
  - `y` determines whether KSR is on.