copy SID2 docs from LTVA1/furnace

tweak them a bit for Furnace

doc/7-systems/sid2.md

@@ -0,0 +1,54 @@
+# SID2
+
+a fictional chip created by LTVA. the idea is to fix SID flaws and add more features, but not too much.
+
+unlike SID, it has per-channel volume control, better ADSR envelope which doesn't have bugs, more waveform mixing modes and the ability to play tonal noise waves.
+
+filter cutoff and resonance ranges were extended, as well as the frequency - finally the chip can hit B-7 note with default clock speed!
+
+## effects
+
+- `10xx`: **change wave.** lower 4 bits specify the wave:
+  - `bit 0`: triangle
+  - `bit 1`: saw
+  - `bit 2`: pulse
+  - `bit 3`: noise
+- `11xx`: **set resonance.** `xx` may be a value between `00` and `FF`.
+- `12xx`: **set filter mode.** the following values are accepted:
+  - `00`: filter off
+  - `01`: low pass
+  - `02`: band pass
+  - `03`: low+band pass
+  - `04`: high pass
+  - `05`: band reject/stop/notch
+  - `06`: high+band pass
+  - `07`: all pass
+- `13xx`: **disable envelope reset for this channel.**
+- `14xy`: **reset cutoff**:
+  - if `x` is not 0: on new note
+  - if `y` is not 0: now
+  - this effect is not necessary if the instrument's cutoff macro is absolute.
+- `15xy`: **reset duty cycle**:
+  - if `x` is not 0: on new note
+  - if `y` is not 0: now
+  - this effect is not necessary if the instrument's duty macro is absolute.
+- `16xy`: **change additional parameters.**
+  - `x` may be one of the following:
+    - `0`: attack (`y` from `0` to `F`)
+    - `1`: decay (`y` from `0` to `F`)
+    - `2`: sustain (`y` from `0` to `F`)
+    - `3`: release (`y` from `0` to `F`)
+    - `4`: ring modulation (`y` is `0` or `1`)
+    - `5`: oscillator sync (`y` is `0` or `1`)
+    - `6`: filter mode (`y` is `0` to `7`)
+    - `7`: waveform mix mode (`y` is `0` to `3`)
+    - `8`: noise mode (`y` is `0` to `3`)
+    - `9`: phase reset (`y` is a discarded parameter and does not matter)
+    - `A`: envelope key on/key off (`y` is `0` (trigger envelope release) or `1` (restart envelope again))
+    - `B`: filter on/off (`y` is `0` (disable filter) or `1` (enable filter))
+- `3xxx`: **set duty cycle.** `xxx` range is `000` to `FFF`.
+- `4xxx`: **set cutoff.** `xxx` range is `000` to `FFF`.
+
+## info
+
+this chip uses the [SID2](../4-instrument/sid2.md) instrument editor.