Kastle is a mini modular synthesizer with a headphone output, 2 in/out ports for interfacing other gear and it runs on just 3 AA batteries. It is DIY friendly and ideal for beginners in modular synthesis, but it will add some quite unique functionality to any modular synthesizer system.
It has unique digital lo-fi sound and it can be melodic as well as very noisy and drony, soft or harsh. It is designed to be fun on its own but it is most powerful when combined with other modular gear.
Kastle is an open source DIY project which runs on two Attiny 85 chips that can be reprogrammed with an Arduino (google: “programming Attiny 85 with Arduino”). One chip is dedicated to sound generation while the other handles modulation. Several firmwares for the Attiny chips are available.
The Synth version combines complex oscillator and LFO with stepped waveform generator.
The oscillator section has 3 sound parameters pitch, timbre and waveshape – all under voltage control and with 3 different synthesis modes. It has a main output and a square wave output. Both can be used independently or combined. The 3 synthesis modes are phase distortion, phase modulation (also known as FM) and track & hold modulation. Each mode utilizes two oscillators. The Pitch controls the main oscillator, the Timbre sets the pitch of the modulating oscillator and the waveshape depends on the synthesis mode. The waveshape also controls the pulse width of the square wave output from the main oscillator.
The voltage controllable LFO has a triangle and square output and a reset input. The stepped waveform generator is inspired by the Rungler circuit by Rob Hordijk. It can produce 8 different voltages either in random order or in 8 or 16 step looping patterns depending on how the BIT IN is patched.
User manual for Kastle 1.5