Guide
Writing music for the Neo Geo’s YM2610
The Neo Geo’s sound chip gives you four FM channels, three SSG channels, six
ADPCM-A voices and one ADPCM-B channel. That distribution — not the raw
count — is what makes the console sound like itself: melody and bass on FM, real
recorded drums on ADPCM-A, and one channel left over for something streamed.
The voice budget, and what each part is for
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4 × FM. Four-operator FM synthesis. This is where the leads,
pads and basslines go. Four is fewer than it sounds, and the envelope carries most of
the character — on a YM2610 the envelope is the timbre.
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3 × SSG. The square-wave side, inherited from the AY-3-8910
lineage. Useful for arpeggios, noise, chiptune texture and anything that would waste an
FM channel.
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6 × ADPCM-A. Sample playback, and in practice the drum kit. Six
simultaneous voices means a full kit plus a couple of one-shots. This is why Neo Geo
percussion sounds recorded rather than synthesised.
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1 × ADPCM-B. A single higher-quality sample channel with hardware
looping. Vocals, a solo instrument, or a whole streamed track — whatever you spend
it on, you spend it once.
Samples live in the V-ROM, and they cost cartridge
FM and SSG cost nothing but channels: they are synthesised on the fly from a handful of
register values. Samples are different — they occupy the V-ROM, and the V-ROM is
finite. Every drum hit, every voice line and every streamed second is cartridge space you
are not spending on graphics.
ADPCM-A material is mono and modest by design; sound effects are normalised to a mono
18519 Hz stream before encoding. ADPCM-B is the better-sounding path and the more
expensive one. A streamed soundtrack in a real Neo Geo ROM is entirely possible —
within what the cartridge’s sample budget holds, which is the whole art of it.
Drums are not a track, they are six voices
The most common way to overrun the chip is to write percussion as if it were one
instrument. It is not: each simultaneous hit takes one of the six ADPCM-A voices, so a
busy fill with a crash still ringing underneath can quietly exceed the budget and drop the
quietest hit.
Writing on a grid that shares its time axis with the melody helps more than any rule of
thumb, because you see the collisions rather than hear them later. In
Neo Studio the drum strip stays pinned under the piano roll and
shares its columns, and at build time the drum tracks fold onto the chip’s six real
voices.
Composing on the chip rather than near it
The traditional route is a tracker plus a converter: write in one tool, export, run a
command-line packer, load the result into the game, discover that a patch you liked does
not survive the trip. It works, and a lot of excellent Neo Geo music has come out of it.
The alternative is to have the editor drive the real chip while you write, so what you
hear is what will ship. That is the approach in Neo Studio: a full piano roll with a
playhead and a stereo meter, tracks grouped by voice family, one instrument per track, an
FM patch editor you can shape live, and a built-in analog drum kit. If you already have
material, a Furnace .fur song or a Standard MIDI file imports as playable
lanes with GM percussion mapped onto the kit; hand it a WAV or an MP3 and it is encoded to
ADPCM-B and looped by the hardware itself.