NesEnvelope
StructSource: AudioTypes.h
Description
NES APU envelope unit state for PULSE 1, PULSE 2, and NOISE.
Mirrors the $4000 / $4004 / $400C write side effects (loop flag, constant volume flag, VVVV) and the $4003 / $4007 / $400F length load side effect (start flag). Clocks at quarter-frame rate (240 Hz NTSC mode 0) via the M3 tickAllNesEnvelopes() dispatcher.
The loop flag is SHARED with the length counter (halt bit): tickAllNesEnvelopes() reads lengthCounter->halt to decide whether the decay level should wrap to 15 on underflow (loop) or silence the channel (!loop). The lengthCounter pointer is bound in AudioChannel::reset() so the two sub-units see the same bit.
When envelopeEnabled == true on a PULSE or NOISE voice, the envelope's output (0..15) REPLACES the ADSR envelope.currentLevel as the per-sample volume source. When the envelope reaches 0 and loop is clear, the voice is silenced directly (matches the NES hardware, which gates the channel on the envelope output).
Default state: envelopeEnabled == false on construction, so the quarter-frame dispatch is a no-op and the canonical track PCM remains byte-for-byte identical. Hito 2 M7.