Split NES and PSG instrument docs.

doc/4-instrument/README.md

@@ -13,7 +13,8 @@ every instrument can be renamed and have its type changed.
 depending on the instrument type, there are many different types of instrument editor:
 
 - [FM synthesis](fm.md) - for use with YM2612, YM2151 and FM block portion of YM2610.
-- [Standard](standard.md) - for use with NES and Sega Master System's PSG sound source and its derivatives. 
+- [PSG](psg.md) - for use with TI SN76489 and derivatives like Sega Master System's PSG.
+- [NES](nes.md) - for use with NES.
 - [Game Boy](game-boy.md) - for use with Game Boy APU.
 - [PC Engine / TurboGrafx-16](pce.md) - for use with PC Engine's wavetable synthesizer.
 - [WonderSwan](wonderswan.md) - for use with WonderSwan's wavetable synthesizer.

doc/4-instrument/nes.md

@@ -0,0 +1,18 @@
+# Standard instrument editor
+
+The instrument editor for NES consists of these macros:
+
+- **Volume**: volume.
+- **Arpeggio**: pitch in half-steps.
+- **Duty**: duty cycle and noise mode.
+  - pulse duty cycles:
+    - `0`: 12.5%
+    - `1`: 25%
+    - `2`: 50%
+    - `3`: 75%
+  - noise modes:
+    - `0`: long noise.
+    - `1`: short noise.
+- **Panning**: output for left and right channels.
+- **Pitch**: fine pitch.
+- **Phase Reset**: trigger restart of waveform.

doc/4-instrument/psg.md

@@ -0,0 +1,14 @@
+# PSG instrument editor
+
+The instrument editor for PSG (SMS, MSX, and other TI SN76489 derivatives) consists of these macros:
+
+- **Volume**: volume.
+- **Arpeggio**: pitch in half-steps.
+- **Duty**: noise mode.
+  - `0`: short noise, preset frequencies.
+  - `1`: long noise, preset frequencies.
+  - `2`: short noise, use channel 3 for frequency.
+  - `3`: long noise, use channel 3 for frequency.
+- **Panning**: output for left and right channels.
+- **Pitch**: fine pitch.
+- **Phase Reset**: trigger restart of waveform.