APU Envelope

In a synthesizer, an envelope is the way a sound's parameter changes over time. The NES APU has an envelope generator that controls the volume in one of two ways: it can generate a decreasing saw envelope (like a decay phase of an ADSR) with optional looping, or it can generate a constant volume that a more sophisticated software envelope generator can manipulate.

Each volume envelope unit contains the following: start flag, divider, and counter.

$4000	ddLC.NNNN	Pulse channel 1 duty and envelope (write)
$4004	ddLC.NNNN	Pulse channel 2 duty and envelope (write)
$400C	--LC.NNNN	Noise channel envelope (write)
bit 5	--L- ----	APU Length Counter halt flag/envelope loop flag
bit 4	---C ----	Constant volume flag (0: use volume from envelope; 1: use constant volume)
bits 3-0	---- NNNN	The reload value for the envelope's divider (the period becomes N + 1). Used as the volume in constant volume (C set) mode.
$4003	LLLL.Lttt	Pulse channel 1 length counter load and timer (write)
$4007	LLLL.Lttt	Pulse channel 2 length counter load and timer (write)
$400F	LLLL.L---	Noise channel length counter load (write)
Side effects		Sets start flag

When clocked by the frame counter, one of two actions occurs: if the start flag is clear, the divider is clocked, otherwise the start flag is cleared, the counter is loaded with 15, and the divider's period is immediately reloaded.

When the divider outputs a clock, one of two actions occurs: If the counter is non-zero, it is decremented, otherwise if the loop flag is set, the counter is loaded with 15.

The envelope unit's volume output depends on the constant volume flag: if set, the envelope parameter directly sets the volume, otherwise the counter's value is the current volume.