originally an arcade board, but SNK shortly adapted it to a rather expensive video game console with the world's biggest cartridges because some people liked the system so much they wanted a home version of it.
its soundchip is a 4-in-1: 4ch 4-op FM, YM2149 (AY-3-8910 clone) and [2 different format ADPCM](https://wiki.neogeodev.org/index.php?title=ADPCM) in a single package!
this chip uses the [FM (OPN)](../4-instrument/fm-opn.md), [AY-3-8910/SSG](../4-instrument/ay8910.md), [ADPCM-A](../4-instrument/adpcm-a.md) and [ADPCM-B](../4-instrument/adpcm-b.md) instrument editors.
in ExtCh mode, channel 2 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.
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/).
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 2 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).
