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).

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 mixer twice for 2× 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. channel 9 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 synthesis mentioned already is: 5 square waves are gathered from 4×, 64× and 128× the pitch of channel 8 and 16× and 64× 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

• 10xx: change patch.
• 11xx: set feedback of channel.
• 12xx: set operator 1 level.
• 13xx: set operator 2 level.
• 16xy: set multiplier of operator.
  • x is the operator, either 1 or 2.
  • y is the new MULT value..
• 18xx: toggle drums mode.
  • 0 disables it and 1 enables it.
  • only in drums mode.
• 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, either 1 or 2. a value of 0 means "all operators".
  • y determines whether AM is on.
• 51xy: set SL of operator.
  • x is the operator, either 1 or 2. a value of 0 means "all operators".
  • y is the value.
• 52xy: set RR of operator.
  • x is the operator, either 1 or 2. a value of 0 means "all operators".
  • y is the value.
• 53xy: set VIB of operator.
  • x is the operator, either 1 or 2. a value of 0 means "all operators".
  • y determines whether VIB is on.
• 54xy: set KSL of operator.
  • x is the operator, either 1 or 2. a value of 0 means "all operators".
  • y is the value.
• 55xy: set EGT of operator.
  • x is the operator, either 1 or 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, either 1 or 2. a value of 0 means "all operators".
  • y determines whether KSR is on.

info

this chip uses the FM (OPLL) instrument editor.

chip options

• Ignore top/hi-hat frequency changes: in drums mode, makes the top/hi-hat channels not write frequency since they share it with snare and tom.
• Apply fixed frequency to all drums at once: sets the frequency of all drums to that of a fixed frequency OPLL drums instrument when one note with it is reached.