one of two chips that powered the Sega Genesis. it is a six-channel, four-operator FM synthesizer. channel 6 can be turned into 8-bit PCM player, that via software mixing, thanks to Z80 sound CPU, can play more than one channel of straight-shot samples at once.
Furnace also offers DualPCM, a Z80 driver that splits channel 6 into two individual PCM channels with variable pitch. using the console's Z80 processor, these are mixed together in software and streamed to channel 6 in PCM mode with a mix rate of 13750 Hz. VGM export requires the "direct stream mode" option to be enabled, and resulting files will be very large.
in ExtCh mode, channel 3 is split into one column for each of its four operators. feedback and LFO levels are shared. the frequency of each operator may be controlled independently with notes and effects. this can be used for more polyphony or more complex sounds.
all four operators are still combined according to the algorithm in use. for example, algorithm 7 acts as four independent sine waves. algorithm 4 acts as two independent 2op sounds. even with algorithm 0, placing a note in any operator triggers that operator alone.
## CSM
CSM, or "Composite Sine Mode", involves a timer matching the frequency of the note in the "CSM Timer" channel. each time it triggers, it generates key-on and key-off commands to reset the phase of all operators on channel 3 and force their envelopes to restart at the release point. this can be used to create vocal formants (speech synthesis!) or other complex effects. outside this chip's specific implementation, the technique is known as "oscillator sync".
working with CSM is beyond the scope of this documentation. for more information, see this [brief SSG-EG and CSM video tutorial](https://www.youtube.com/watch?v=IKOR0TUlnWU).
## DualPCM
thanks to the Z80 sound CPU, DualPCM can play two samples at once! this mode splits channel 6 into two individual PCM channels with variable pitch. these are mixed together in software and streamed to channel 6 with a mix rate of 13750 Hz. VGM export requires the "direct stream mode" option to be enabled, and resulting files will be very large.
## SSG-EG
SSG-EG is short for "Software-controlled Sound Generator – Envelope Generator". it is the AY-3-8910/YM2149 envelope generator applied to each individual operator. it makes the operator's envelope play through attack, decay, sustain, and decay 2 until it reaches zero amplitude, at which time SSG triggers. according to the shape of the SSG envelope, the operator's envelope may then either loop or hold, and either of these can be set to invert the envelope (attack decreases and decay increases) when triggered.
a full guide to SSG-EG is beyond the scope of this documentation. for more information, see this [brief SSG-EG and CSM video tutorial](https://www.youtube.com/watch?v=IKOR0TUlnWU), this [detailed technical explanation](https://gendev.spritesmind.net/forum/viewtopic.php?t=386&start=106), and this [chart of tunings](https://docs.google.com/spreadsheets/d/1HGKQ08CnLGAjA1U0StJFldod3FkQ3uq86rYy1VBIuZc/).
