Unimplemented sound systems

Keyboard synthesizers

• Roland linear arithmetic (D-50, MT-32, etc): Preliminary support on MAME.
• NEC uPD933 (Casio phase distortion): Only recently marked "good" on MAME.
• Yamaha's pre-FM digital chips (YM1019, YM1104, YM1105, IG10771): No reverse-engineering efforts or adequate documentation, extremely obscure, mostly high-length wavetable chips.
• Curtis CEM3394 and National Semiconductor MM5837 (several analog-subtractive synths, Sente arcade machines): MAME support only recently became mostly complete. Sente games use it more like a 2x SID.
• Yamaha DX7: Monotimbral, meaning only one unique instrument is active at a time, and 16 voices only means 16 instances of the same instrument.
• STMicro M114S (Amstrad CKX100, several European (mainly Italian) synthesizers and organs, Zaccaria pinballs as Mr. Game): Inadequate emulation and only on PinMAME. Its main selling point is wave morphing, which can already be done in the wavetable tab of the instrument editor.
• Yamaha YMW258-F GEW8 (Sega 315-5560 MultiPCM): Almost the same as the PCM part of the YMF278B, and same reason.
• L7A1045 L6028 DSP-A (Akai MPC3000, Hyper Neo Geo 64): Explained down below.

Computers and consoles

• Atari AMY: 64-voice additive synthesizer with complete docs, but no emulation material.
• NEC uPD1771c (Super Casette Vision): An MCU with an embedded ROM. Imperfect emulation on MAME. Pull request to be merged.
• Yamaha SCSP (Sega Saturn): Too overwhelming to understand or implement.
• Yamaha YMF278B OPL4: 24-voice CD-quality PCM chip with an embedded OPL3. The fork with it was redone, and now under review and tweaks in the pull request (https://github.com/tildearrow/furnace/pull/2017).
• Astrocade PSG: Limited capabilities, and too few potential users.
• Mountain Computer Music System: Barebones 16-voice wavetable synth of 8-bit 256-length waves, as a pair of sound cards for the Apple II. Only used for professional music production software, and easily imitated with an ES5503.
• Signetics 2636: Used in the Century Electronics CVS arcade system and a few small home consoles. CVS games use pulse widths and other quirks.
• Yamaha YMF288 OPN3/EPSM: Same as the YM2608, but without the ADPCM or CSM. Any importers or converters from FamiStudio or other places should target their EPSM data to a YM2608.
• OPP: Differences between OPM and OPP are extremely insignificant.
• RCA CDP1869 (Cidelsa arcade games, Comx-35): The TED, but dumbed-down.

Arcade and pinball

• Stern BSMT2000: A masked-ROM version of the TMS320C15 DSP, used as a 12-voice sample chip. Needs a ROM to work.
• Yamaha YMF271: Same reason as SCSP, and also inferior emulation outside of PCM despite being in the VGM spec.
• Capcom CPS3: Probably the same as QSound, or mostly.
• Galaxian custom sound and derivatives like in Sigma's "Spiders!" and SNK6502 games: Typical discrete netlist circuits consisting of a 4-bit linear counter and several analog circuits that are hardwired to produce specific SFX. Explained at Plogue's blog post. Pong is the only exception to hardwired netlist circuits not being added, and it was added as a joke. Plogue Chipsounds has a few of these systems (Galaxian, Donkey Kong, Zaxxon, Phoenix).
• OKI MSM5205: Planned.
• Williams-Bally DCS sound system: That's a codec, not an actual custom circuitry. The hardware is (initially) an ADSP-2105 processor running an engine that can do up to 4 simultaneous sounds.
• Namco C352, Ricoh RF5C400, Taito ZSG-1, IGS ICS2115, Hyper Neo Geo 64, Yamaha AICA, Eolith QS1000, other >24 voice hi-fi PCM chips: Redundant with the ES5506, but the PlayStation SPU might have features that stand out. The only exception is the OPL4 (and therefore the MultiPCM)
• SNK Wave: Equivalent to the Commodore PET, but the wave is 3-bit instead of 1-bit. Easily imitated with a generic DAC or a certain wavetable chip
• "Exidy SFX" or "Exidy SFX + PSG" (Exidy and Stern games): A netlist PCB that's equivalent to a 3x PC Speaker, via a Motorola MC6840 PIT for the square waves and a 128-bit Motorola MC14562 shift register for the noise.

Other

• Konami K054539, Acorn Archimedes VIDC, Macintosh, Gaelco, Brezzasoft Vrender0, Imagetek I5000: Easily redundant with generic PCM DACs or a YMZ280B, while the Macintosh is specifically redundant with the Amiga Paula and the I5000 is redundant with QSound.
• Software-based sound DSPs (Atari Jerry, N64, Atari STe, Sega 32X): The GBA and ZX Spectrum are exceptions due to having features specific to them, but these are all CPU-driven softmixed audio. The Jerry is said to have "FM, wavetable, PCM" but those are advertised driver functions. The N64 is said to have 100 PCM channels, but that's theoretical and assumes that each output stream takes a maximum of 1% processing load. The Sega 32X is the only other one besides the GBA to probably have plans, either as its own system or as a generic PCM DAC format once a single DAC can have a variable polyphony to simulate softmixing.
• Yamaha SMAF chips (MA-series): An entire community was invested in emulation and reverse engineering projects, with their furthest progress being manual interfaces to the chips. No reverse-engineering was done on the official emulation cores.
• MT32 / LA32, Sound Canvas, other MIDI modules: Furnace is not intended to be a tracker for any MIDI devices, rather being intended for being able to directly drive sound chips. There are multiple issues with implementing devices like these, such as lack of control over what the hardware does, along with requiring copyrighted data and code.
• PICO-8, Voxatron, Picotron, TIC-80: Lack of control over sound, same as MIDI in this regard.