Solo Freedom
// status
LIVE at solofreedom.org · solo build, end to end.
// what_it_is
A real-time musical key detection and scale recommendation web app for guitarists. Record audio from your mic or upload a track, the app analyzes the harmonic content using a custom Fast Fourier Transform pipeline, identifies the key, and recommends three tiers of scales to solo over it — from beginner-friendly pentatonics up to full theory-correct modal scales — rendered as both guitar tab and standard notation.
// who_its_for
Guitarists who want to figure out what key a song is in and what they can play over it — without years of music theory or expensive software.
// whats_under_the_hood
This isn’t a wrapper around someone else’s API. The audio analysis is built from scratch:
- Custom Fast Fourier Transform with 4096-sample Hann windows, 2048-sample hop
- 12-bin chromagram mapping FFT bins to MIDI pitch classes
- Harmonic suppression algorithm to reduce overtone bias (subtract 40% of each pitch class energy from its perfect fifth, 20% from its major third)
- Krumhansl-Schmuckler key detection with Pearson correlation against major/minor tone profiles
- Multi-window voting — audio split into overlapping 4-second windows, each votes independently
- Confidence thresholds to refuse low-quality results instead of guessing
- Three-tier scale recommendation engine (Best Sound / Technical / Theory Correct)
- SVG fretboard rendering with color-coded scale positions, fingering numbers, and collision-aware fret labels
- Standard notation rendering with treble clef, ledger lines, accidentals, stems, note heads
// the_business_layer
A real freemium SaaS on top of the audio engine:
- 3 free analyses per device for anonymous users, tracked server-side via deviceId (works in incognito and Safari, no cookies required)
- Atomic MongoDB upsert with $inc + $lt:3 guard for race-condition-safe limit enforcement
- Anonymous-to-signup migration that marks free analyses as consumed at registration to prevent the "log out and reset my count" exploit
- Stripe subscription for unlimited Pro access with Customer Portal for self-serve management
- Resend transactional email for password reset with cryptographic 1-hour expiry tokens
- JWT auth with bcrypt password hashing, NoSQL injection guards, XSS escaping, rate limiting (15 req / 15 min on auth, 5 req / hour on password reset)
// stack
// the_result
A production app with real users, real auth, real payments, and a real DSP pipeline — all built and maintained by one person. Solo Freedom is the proof that "I do the work myself" isn’t just a pricing pitch; it’s how the work actually gets done.
Want one like this?
> RUN /quote