# Beat Offset

> A rhythm timing practice instrument in the browser. Pick a notated pattern, a tempo, a time signature and how the
> click behaves, then tap the rhythm by keyboard, mouse or touch. Every tap is matched to the beat grid and measured
> in milliseconds - early or late, by beat position, by bar, with drift and the tempo actually kept - and drawn on a
> scrolling timeline. A coach model reads the timing profile and prescribes a practice set that loads with one tap;
> every number it cites is checked against the page's own measurements.

URL: https://beat-offset.skillsafe.ai/
API: https://beat-offset.skillsafe.ai/api.html
Model: gpt-terra (SkillSafe alias; resolves to gpt-5.6-terra), 10% publisher markup
Derived from: @onewave-ai/practice-plan-creator (https://skillsafe.ai/skill/@onewave-ai/practice-plan-creator/, github.com/onewave-ai/claude-skills, MIT) - "drills by skill level and time, warm-up, skill work, scrimmage, cool-down", applied to rhythm practice.

## What is free (no sign-in, sound optional)

- Metronome synthesised with Web Audio (square-wave clicks, accented downbeat, optional subdivision clicks), or a
  visual click alone with sound off. Tempo 40-220 BPM; 3/4, 4/4, 5/4 and 6/8 (the 6/8 beat is the dotted quarter).
- 19 patterns over five levels, drawn as rhythm notation: quarter pulse, straight eighths, backbeat, off-beats,
  triplets, swing, Charleston, sixteenths, gallop, reverse gallop, tresillo, triplet skip, sixteenths without the beat,
  only the "a", hemiola, three over two, and three 6/8 patterns.
- Click modes: every beat; two bars on and two off; backbeat only; count-in only.
- Taps: Space, J, F, K, D, a click or a touch on the pad. Event timestamps are used, not handler time.
- Latency calibration (tap along to 16 clicks; the median offset of the settled taps), kept separately for sound and
  visual-only.
- Measurement: each tap goes to its nearest note within half the gap to the neighbouring note (at most 300 ms); a
  second tap for one note is an extra; taps before the first note are count-in taps. Reported: share of notes within
  +-25 ms ("in the pocket"), hit rate, mean, spread (sample SD), median, mean and 90th-percentile absolute offset,
  offset by position (each on-beat, and the e / & / a / trip / let pooled across beats), by bar, drift (least-squares
  ms per bar), felt tempo (from tap spacing alone), first vs second half, bars with vs without the click.
- A timeline (beats as a ruler, early marks above the line in magenta, late below in amber, in-pocket in cyan, missed
  notes as rings, extra taps as diamonds, a moving-average drift curve and the fitted drift line, click-off bars
  hatched), charts by position and by bar, a tempo progression suggestion (+6 BPM at 80% in the pocket and 95% hit,
  -8 BPM under 45% or 75%), CSV of every tap, a Markdown report, history in the browser and the account.
- Three example takes whose coach replies replay at no cost.

## What the coach does (metered, needs sign-in)

One system prompt, two tasks selected by `task`:

- `diagnose`: 2-5 findings, each citing one statistic copied from the take; 0-4 callouts pinned to bars; a practice
  set of 3-5 exercises in session order (warmup, skill, performance with the click dropping out, cooldown), each with
  a library pattern, meter, tempo, bars, click mode, minutes, a cue sentence and why.
- `recheck`: the same, for a new take after practising, plus `progress` (each statistic before and after, with a
  verdict).

The page checks every cited number (ms within 2 ms or 10%, percentages within 1.5 points, counts exact), rejects and
lists any exercise whose pattern is not in the library or not in that meter, clamps tempos to 40-220, drops callouts
on bars that do not exist, and compares each before/after verdict with its own rule.

Inputs carry aggregates plus a clipped tap list: the first 24 taps in order and the 12 furthest off, up to 16 missed
notes. Object fields are JSON strings (the input schema is scalar-only). Runs carry an Idempotency-Key of the form
`beat-offset:<task>:<inputHash>:a<attempt>`.

## Contract summary

reply: {"task":"diagnose|recheck","headline":str,"findings":[{"finding":str,"stat":str,"position":str,"bar":int,"value":num,"severity":"high|medium|low"}],"callouts":[{"bar":int,"text":str}],"prescription":[{"phase":"warmup|skill|performance|cooldown","title":str,"pattern":str,"meter":str,"tempo":int,"bars":int,"click":"full|dropout|backbeat|countin","minutes":int,"cue":str,"why":str}],"progress":[{"stat":str,"position":str,"before":num,"after":num,"verdict":"better|same|worse"}],"next_step":str}

## Pages

- https://beat-offset.skillsafe.ai/ - the instrument
- https://beat-offset.skillsafe.ai/api.html - the API tutorial in eight languages