7.10.9. STRUDEL-09 — ADSR & Envelope Shaping

An ADSR envelope shapes a note’s amplitude over time:

Attack	time from key-down to peak amplitude
Decay	time to fall from the peak to the sustain level
Sustain	held level (0..1) while the note is on
Release	time to fade to silence after key-up

daslang’s strudel uses tempo-aware defaults for ADSR. Instead of fixed numeric defaults, each field defaults to -1 — a sentinel meaning unset. At playback time a resolver fills in sensible values based on which fields the user actually touched:

Nothing set	held tone — sustain = 1, tiny attack/release
Only `decay` set	percussion — sustain auto-zeroes
`sustain` set explicitly	always wins

The same idea applies to delaytime: it defaults to -1 and is derived from delaysync (in cycles) and the current cps so the echo locks to tempo automatically.

7.10.9.1. Part A: default envelope — held tone

With no ADSR setters at all, the resolver picks a held-tone envelope: the note rings for its full scheduled duration, instead of decaying to silence in 50ms as in older defaults.

let pat <- note("c4 e4 g4 c5", "sine")

This is the unset → held branch of the resolver.

7.10.9.2. Part B: pluck — decay only

Set decay alone and the resolver assumes a percussive envelope: sustain drops to 0 so each note plucks then decays away. This is the idiom for short, rhythmic synth blips:

let pat <- note("c4 e4 g4 c5", "sine") |> decay(0.15)

No need to spell out sustain(0.0) — the resolver does it because “only decay was set”.

7.10.9.3. Part C: pad — explicit ADSR

Setting all four fields gives full manual control. A pad-style envelope uses a long attack so the note swells in, plus a long release so it fades out gradually after key-up:

let pat <- note("c3", "triangle") |> attack(0.5) |> decay(0.2) |> sustain(0.6) |> release(0.5)

Because sustain was set explicitly, the resolver leaves all four values untouched.

7.10.9.4. Part D: tempo-aware delay defaults

The “unset sentinel + resolver” idea also applies to delaytime. Plain delay(0.6) without delaytime(...) derives delaytime from delaysync (3/16 cycle by default) and the current cps. This gives a dotted-eighth feel that automatically tracks tempo changes:

let pat_slow <- note("c4 ~ e4 ~ g4 ~", "sine") |> decay(0.15) |> delay(0.6)
play(pat_slow, 0.5lf, 4.0lf)

let pat_fast <- note("c4 ~ e4 ~ g4 ~", "sine") |> decay(0.15) |> delay(0.6)
play(pat_fast, 1.0lf, 4.0lf)

Same pattern, two different cps — the echo timing follows the tempo. Setting delaytime (or delaysync) explicitly bypasses the resolver.

7.10.9.5. Part E: velocity via gain

gain multiplies the envelope output. Use it for a velocity curve over a pattern: alternating loud and soft hits while the envelope shape stays constant.

let pat <- stack([
    note("c4", "sine") |> decay(0.15) |> gain(0.9),
    note("e4", "sine") |> decay(0.15) |> gain(0.4),
    note("g4", "sine") |> decay(0.15) |> gain(0.7),
    note("c5", "sine") |> decay(0.15) |> gain(0.3)
])

Each voice keeps the same plucky envelope but a different peak level.