BeatPulse — Step Sequencer Drum Machine

Why this project?

Step sequencers are foundational tools in electronic music — from the Roland TR-808 that defined hip-hop to modern DAWs. But most web-based sequencers either require audio samples (increasing page load) or are overly complex. BeatPulse was built to be an instant, sample-free drum machine that runs entirely in the browser using synthesized sounds.

What is it?

A 16-step sequencer with 8 synthesized percussion tracks. Every sound is generated in real-time using the Web Audio API — no samples, no downloads. Features include swing timing, per-track muting, 4 pattern slots, preset patterns, and adjustable master volume.

How does it work?

Each drum sound is synthesized from basic audio primitives:

Kick: Sine oscillator with rapid pitch sweep (150Hz → 40Hz) and exponential amplitude decay.
Snare: Layered triangle oscillator (tone) + filtered white noise (snap) with short decay.
Hi-hat (closed): High-pass filtered white noise burst (7kHz+) with very short decay (30ms).
Hi-hat (open): Same as closed but with longer decay (150ms).
Low Tom: Sine oscillator pitch sweep (120Hz → 80Hz) with medium decay.
High Tom: Sine oscillator pitch sweep (200Hz → 130Hz) with medium decay.
Clap: Multiple noise bursts (simulating hand collisions) with bandpass filter (1-4kHz).
Cowbell: Two square oscillators tuned to inharmonic frequencies (560Hz + 845Hz) with bandpass filtering.

The sequencer uses a lookahead scheduler that checks 25ms ahead and queues notes using AudioContext.currentTime for sample-accurate timing. Swing is implemented by delaying even-numbered steps based on the swing percentage.

How to use it

Click cells in the grid to activate/deactivate drum hits on each track.
Press Play (▶) or hit Space to start/stop playback.
Adjust BPM using the +/− buttons or typing directly into the BPM field.
Add swing with the swing slider — shifts even steps later for a "laid-back" groove (0% = straight, 40-60% = natural swing).
Mute tracks by clicking the MUTE label on the left of each track.
Switch patterns (A/B/C/D) to work on different sections — each pattern has its own grid state.
Load presets for quick starting points — Rock, Hip-Hop, Techno, Breakbeat, Bossa Nova, Half Time.
Clear all active steps in the current pattern with the ✕ button.
Adjust volume with the master volume slider.