The standard phrase

Audio features are only comparable under an identical stimulus. This module owns that stimulus.

The spec

PhraseSpec::default() is four notes, ~5.05 seconds, 44 100 Hz, RNG seed 0xE05_F00D:

Pitches are V/Oct offsets from C4. The seed is installed into quiver's thread-local RNG before rendering, so noise and analog drift are bit-reproducible: a patch's features are the same every time it is measured.

Why each segment

The original phrase was three short notes (0.6 s stab, 0.25 s stab, 0.8 s low note), and it was the loop's weakest link. It could not discriminate

• slow pads: a 2-second attack was silent for most of the stimulus,
• anything modulated below ~1 Hz: no register-constant segment long enough to hold a modulation cycle,
• anything above Eb4, its highest note,
• how a patch stacks polyphonically: it was strictly monophonic.

So the grammar could express patches the audition could never reveal, and the taste model was being asked to learn preferences over evidence that was not in $\varphi$. No amount of model improvement fixes that; it is a measurement problem.

The v2 default covers each hole with the cheapest segment that reveals it:

1. C4 held 1.8 s. The attack measurement window (onset → next onset) is now 2.0 s rather than 0.75 s, and the sustain is long enough that sub-Hz modulation completes most of a cycle. held_centroid_std is measured here specifically, which is what makes it register-constant by construction.
2. C5 stab. One octave above the old ceiling. With the compiler's fixed 0.5 keytracking, this is where dark patches reveal whether they speak up high (high_ratio).
3. C4+E4 dyad. A second compiled voice, gate-synced with the main voice, reveals intermodulation (chord_flatness_delta). A dyad rather than a triad because render cost is per voice-second and pairwise intermodulation is the first-order phenomenon.
4. C3 with a 1.1 s release window, kept last. Bass register, and its position matters: the tail measurement is the final 300 ms, so putting this note last is what makes the tail see release length and reverb rather than a truncated chord decay.

Cost: about 2× the v1 render, measured. The dyad's second voice is the difference between wall seconds and rendered voice-seconds.

Chord voices

Note::chord carries additional simultaneous pitches, each rendered by its own compiled voice, gate-synced with the main note.

Two behaviours worth knowing:

• Chord voices start cold at the note's onset, exactly how live voice allocation behaves, so the measurement matches what a player would hear.
• After the shared gate closes they keep ticking until their own output parks on silence. A truncated release tail is a broadband click, and a click would poison every spectral feature in the frame it lands in.

max_voices() reports the largest simultaneous count (2 for the default spec), and the vet gate's peak ceiling scales with it.

The :p2 stimulus tag

Every audio feature name carries a generation tag:

centroid_mean:p2   rms_std:p2   attack_s:p2   …

This is the migration mechanism, not a version comment.

A stimulus change changes what every audio value means, even when the formula is untouched. A slow pad's rms_mean under a phrase that never lets it open is a different quantity from the same field under one that does. The observation log stores raw $\varphi$ by name, and FitSet::build projects old logs onto the current names on the rule same name ⇒ same coordinate.

So tagging the name with the stimulus generation means votes recorded under the v1 phrase:

• keep their structural coordinates, which are stimulus-independent;
• have their old-stimulus audio coordinates imputed as "no evidence" rather than mixed into a standardizer they were never commensurable with.

Bump the tag whenever PhraseSpec::default() changes audibly. Failing to bump it is worse than a wrong number: it is old evidence presented as current evidence.

What the phrase still does not reveal

Stated because the model cannot learn what the stimulus does not show:

• Velocity response. The phrase plays at one velocity.
• Fast passages. No segment tests how the patch behaves in a run.
• Long-term behaviour. Five seconds cannot reveal a 30-second evolving pad.
• Stereo width. The render is summed to mono for feature extraction, and there is no width coordinate in $\varphi$ at all. The chorus module's spec card says so outright in the app.

The intended direction is per-style audition phrases (a discovered bass style picks a bassline, a pad style picks a chord swell), which would make the stimulus adaptive rather than fixed. That is a design note, not shipped code, and the :p2 tag is the mechanism that would let it happen without invalidating history.