MIDI
Not connected.
The video mode can change how some features behave (Wave Traveller loop speeds, maybe more) and can only be switched on the NES itself, so the experiments ask for it and save it with every answer. Mode 1 also has less hum. Panic stops every voice; Esc does too.
Play
Click the keys, or type A W S E D F T G Y H U J K (Z / X octave). Hold keeps the note on until you press it again. Notes go to the voice picked above.
Controls
Sliders snap to the MidiNES's real steps (e.g. volume in 16 levels of 8). Values are kept per voice.
Pitch bend
Pulse and triangle only. Follows the last note you played (the note box updates by itself; edit it to bend a different one). The bend for an interval depends on the note (the NES works in timer periods), so this uses Reeve's formula: p0 · 0.5^((n − 36)/12) · (2 − 2 · 0.5^(x/12)), p0 = 3420 (pulse) or 1710 (triangle), in steps of 4.
Software envelope
The way Fat Chip would give the NES slow attacks and releases: volume (CC7) stepped through its 16 levels while the note sounds, and the note-off held back until the release has faded. Pulse and noise only.
Triangle release
The triangle has no volume, so this fakes a release (Reeve's plugin trick): when the note is let go, its pitch drops quickly and only then does the note-off go out. A new triangle note cuts a drop short.
The triangle's 11-bit timer can't go below about 27.3 Hz (MIDI 21). A drop that aims lower wraps round to a very high pitch (e.g. −24 from G#1), so by default the drop stops at that floor and the note-off goes out there. Defaults: −24 over 250 ms, slow at first (Reeve's pick).
Experiments
Set the NES video mode in the MIDI card first; each answer remembers which mode it was heard in (worth trying the interesting ones in both). Each one plays a short sequence on the MidiNES. Listen, then mark what happened and add a note if anything was odd. Answers stay in this browser; Copy results puts them all on the clipboard to paste into the chat.