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Merge pull request #1219 from freq-mod/freq-mod-docs 

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11
doc/7-systems/ay8910.md
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@ -6,6 +6,8 @@ it is a 3-channel square/noise/envelope sound generator. the chip's powerful sou
the AY-3-8914 variant was used in Intellivision, which is pretty much an AY with 4 level envelope volume per channel and different register format.
as of Furnace 0.6pre7, AY-3-8910 supports software sample playback, where all 3 channels can play 4-bit PCM samples (at the cost of a very high CPU usage)
# effects
- `20xx`: **set channel mode.**
@ -44,3 +46,12 @@ the AY-3-8914 variant was used in Intellivision, which is pretty much an AY with
- this changes the port's mode to "write". make sure you have connected something to it.
- `2Fxx`: **write to I/O port B.**
- this changes the port's mode to "write". make sure you have connected something to it.
# chip config
## AY derivative modes
AY-3-810 was an absurdly popular chip that was blessed with many third-party clones, licensed or not.
- the AY-3-8914 variant was used in Intellivision, which is pretty much an 8910 with 4 level envelope volume per channel and different register format.
- Yamaha YM2149 was an AY-3-8910 clone released in 1983. it's almost identical to AY with minor differences being: higher hardware envelope step resolution (16 vs 32), half-clock mode when voltage level is low, much stronger DC offset and cleaner, but softer output.
- Sunsoft 5B is YM2149 clone with half-clock mode forced on.

2
doc/7-systems/ay8930.md
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@ -7,6 +7,8 @@ it is best known for being used in the Covox Sound Master, which didn't sell wel
emulation of this chip in Furnace is now complete thanks to community efforts and hardware testing, which an MSX board called Darky has permitted.
as of Furnace 0.6pre7, AY8930 supports software PCM, where all 3 channels can play 5-bit PCM samples (at the cost of a very high CPU usage)
# effects
- `12xx`: **set channel duty cycle.**

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doc/7-systems/genesis.md
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@ -54,7 +54,7 @@ CSM is beyond the scope of this documentation. for more information, see this [b
## DualPCM
DualPCM splits channel 6 into two individual PCM channels. using the console's Z80 processor, these are mixed together in software and streamed to channel 6 in PCM mode. because this generates a stream of data, exported VGM files will be very large.
[info here.](ym2612.md)
## Sega CD

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doc/7-systems/msm6258.md
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@ -1,7 +1,20 @@
# OKI MSM6258
a single-channel ADPCM sound source developed by OKI. it allows max sample rate of 15.6 KHz... with no variable pitch. most prominent use of this chip was Sharp X68000 computer, where it was paired with Yamaha YM2151.
Furnace's implementation is MSM6258V, a CPU driven variant that is unlimited by amount of sample data, being able to be fed from the system's RAM.
# effects
...
# chip config
## chip clock rates
MSM6258 is an extremely basic ADPCM sound codec. it has no variable frequency rate; it depends on clock rate of a chip itself. Furnace supports following rates:
| clock rate | sampling rate |
|--------------------|---------------|
| 4 MHz | 7812 Hz |
| 4.096 MHz | 8000 Hz |
| 8 MHz | 15625 Hz |
| 8.192 MHz | 16000 Hz |

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doc/7-systems/msm6295.md
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@ -1,7 +1,33 @@
# OKI MSM6295
an upgrade from 6258 - it provides 4 ADPCM channels, at max 32 KHz (still no variable pitch though). between late '80s and late '90s, it was one of the most common, if not _the_ most common soundchip used in arcade machines (Capcom, Toaplan, Kaneko, Atari, Tecmo, the list can go on and on...)
an upgrade from 6258 - it provides 4 ADPCM channels, at max 32 KHz (still no variable pitch though). between late '80s and late '90s, it was one of the most common, if not _the_ most common soundchip used in arcade machines (Capcom, Toaplan, Kaneko, Atari, Tecmo, the list can go on and on...). Without bankswitching, the chip supports 256kB of sample RAM and can hold up to 127 samples at once.
# effects
- `20xx`: **set chip output rate.**
# chip config
## chip clock rates
like MSM6258, MSM295 is an extremely basic ADPCM sound codec. it has no variable frequency rate, it depends on clock rate of a chip itself. Furnace supports following rates:
| clock rate | sampling rate |
|--------------------|---------------|
| 1 MHz | 7576 Hz |
| 1.02 MHz | 7727 Hz |
| 1.056 MHz | 8000 Hz |
| 1.193 MHz | 9038 Hz |
| 0.89 MHz | 6742 Hz |
| 0.875 MHz | 6629 Hz |
| 0.9375 MHz | 7102 Hz |
| 1.5 MHz | 11364 Hz |
| 1.79 MHz | 13561 Hz |
| 2 MHz | 15152Hz |
| 2.112 MHz | 16000 Hz |
| 3 MHz | 22728 Hz |
| 3.58 MHz | 27122 Hz |
| 4 MHz | 30304 Hz |
| 4.224 MHz | 32000 Hz |
## chip clock divisor
MSM6295 clock rate could be divided by 132 (resulting sample rates above), or by 165. To get a clock rate using divisor of 165, formula is clock rate (in Hz) / 165. Example: 1 MHz MSM6295 in 165 divisor mode results in output rate of 6060 Hz.

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doc/7-systems/sms.md
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@ -13,3 +13,19 @@ the original iteration of the SN76489 used in the TI-99/4A computer, the SN94624
- `y` controls whether to select noise or thin pulse.
- `0`: thin pulse.
- `1`: noise.
# chip config
## SN7 versions
SN7 was extremely popular due to low cost. Therefore, it was cloned and copied to no end, often with minor differences between each other. Furnace supports several of these:
- SN94624, can only produce tones as low as 100Hz, and is clocked at 447 KHz.
- SN76494, which can play notes as low as 13.670 Hz (A -1). It has a different noise feedback and invert masks.
- SN76489, identical to SN94624, just without a clock divider
- SN76489A, identical to 76494, just with a /8 clock divider
- SN76496, literally identical to former. Why is it even here?
- SN76496 with a Atari-like short noise. The chip of many legend and rumours, might be a result of inaccurate emulation.
- Sega Master System VDP version has a different, characteristic noise LFSR.
- Game Gear SN7, identical to the above, but with stereo
- NCR8496, different noise invert masks
- PSSJ3, literally identical to the former, it just swaps "high" and "low" signals in the output, which results in no audible difference
TODO: all these checkboxes

6
doc/7-systems/ym2203.md
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@ -99,3 +99,9 @@ several variants of this chip were released as well, with more features.
- `5Dxx`: **set D2R/SR of operator 2.**
- `5Exx`: **set D2R/SR of operator 3.**
- `5Fxx`: **set D2R/SR of operator 4.**
# system modes
## extended channel 3
in ExtCh mode, channel 3 is split into one column for each of its four operators and feedback 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.

8
doc/7-systems/ym2608.md
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@ -2,7 +2,7 @@
like YM2203, but with twice the FM channels, stereo, an ADPCM channel and built-in drums ("rhythm")!
it was one of the available sound chips for the NEC PC-88VA and PC-98 series of computers.
it was one of the available sound chips for the NEC PC-88VA and later models of PC-98 series of computers.
the YM2610 (OPNB) and YM2610B chips are very similar to this one, but the built-in drums have been replaced with 6 sample channels.
@ -99,3 +99,9 @@ the YM2610 (OPNB) and YM2610B chips are very similar to this one, but the built-
- `5Dxx`: **set D2R/SR of operator 2.**
- `5Exx`: **set D2R/SR of operator 3.**
- `5Fxx`: **set D2R/SR of operator 4.**
# system modes
## extended channel 3
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.

8
doc/7-systems/ym2610.md
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@ -2,7 +2,7 @@
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 in a single package!
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!
# effects
@ -97,3 +97,9 @@ its soundchip is a 4-in-1: 4ch 4-op FM, YM2149 (AY-3-8910 clone) and 2 different
- `5Dxx`: **set D2R/SR of operator 2.**
- `5Exx`: **set D2R/SR of operator 3.**
- `5Fxx`: **set D2R/SR of operator 4.**
# system modes
## extended channel 2
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.

6
doc/7-systems/ym2610b.md
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@ -96,3 +96,9 @@ it is backward compatible with the original chip.
- `5Dxx`: **set D2R/SR of operator 2.**
- `5Exx`: **set D2R/SR of operator 3.**
- `5Fxx`: **set D2R/SR of operator 4.**
# system modes
## extended channel 3
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.

3
doc/7-systems/ym2612.md
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@ -1,6 +1,7 @@
# Yamaha YM2612
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. as of Furnace 0.6pre5, Furnace offers DualPCM, which allows 2 channels of software-mixed 8-bit PCM samples at 13750 Hz.
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.
as of Furnace 0.6pre5, Furnace 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.
# effects

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doc/7-systems/zxbeep.md
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@ -2,7 +2,10 @@
rather than having a dedicated sound synthesizer, early ZX Spectrum models had one piezo beeper, controlled by Z80 CPU and ULA chip. its capabilities should be on par with an IBM PC speaker... right?
not really - very soon talented programmers found out ways to output much more than one square wave channel. a lot of ZX beeper routines do exist, but as of 0.6 Furnace supports only a Follin/SFX-like engine with 6 channels of narrow pulse wave and click drums.
not really - very soon talented programmers found out ways to output much more than one square wave channel. a lot of ZX beeper routines do exist, but as of 0.6 Furnace supports two engines:
- a Follin/SFX-like engine with 6 channels of narrow pulse wave and click drums.
- QuadTone: PWM-driven engine with 4ch of pulse wave with freely variable duty cycles and 1-bit PCM drums.
# effects