Bastl Instruments KASTLE User Manual

SYNTH

Kastle is a mini modular synthesizer 

with 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 

quite some unique functionality to 

any modular synthesizer systems. 

It delivers the fun of modular 

synthesis at cost and its into 

your pocket so you can play it 

everywhere!

It has unique digital lo-i 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 which 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 irmwares 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 diferent 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 Hordjk. 

It can produce 8 diferent voltages either in random order or in 8 or 16 step looping 

patterns depending on how the BIT IN is patched.

ABOUT

© complex oscillator

© 3 synthesis modes: phase distortion, phase modulation and track & hold modulation

© pitch control with ofset and CV input with attenuator

© timbre control with ofset and CV input with attenuator

© waveshape control with ofset and CV input

© voltage controllable LFO with triangle and square outputs and reset input

© stepped voltage generator with random, 8 step and loop 16 step mode

© 2 I/O CV ports are available and can be routed to any patch point 

© the main output can drive headphones

© 3x AA battery operation with power switch

© open source

© possibility of exchanging diferent LFO and OSC chips

what is in the box

© kastle synth

©10 patch cables

© sticker

© for environmental reasons we are excluding the batteries in the package

FEATURES

A 

Percussive sounds

Connect the LFO PULSE to the LFO RST socket. This results in SAW waveform at the 

LFO TRIANGLE output (because as soon as the TRIANGLE starts rising it receives 

the PULSE and therefore goes to the highest point and starts falling and therefore 

skips the rising stage). Use the SAW waveform to modulate any or multiple of the 

oscillator parameters. On top of that connect the STEPPED output to LFO RATE 

socket. Adjusting the LFO RATE and the RATE MOD knobs will result in different 

lengths of SAW modulation signals and therefore different rhythms.

B 

Different rise and fall times

Connect the LFO PULSE to the RATE MOD socket. Now the RATE MOD  and LFO 

RATE knobs set different proportions between the rise and fall time of the triangle. 

This will also create a swing effect on the STEPPED waveform. You can also use the 

STEPPED output to connect to the RATE MOD socket to change the slope time with 

every new voltage generated at the STEPPED output.

TIPS & TRICKS

C 

Thick sound

Connect the OSC PULSE to the OSC OUT socket to mix both oscillator waveforms 

together. This can result into really thick sound which is great for further filtering.

D 

Sync

The LFO and STEPPED generator on the Kastle can by synced with other gear. 

You can use the LFO PULSE out when you want the Kastle to be the master clock 

or connect clock from another device to the LFO RST when you want it to be the 

slave. Simply connect the I/O jack to you device and patch the L socket to the LFO 

RST or LFO PULSE.

E 

Connect everything to everything! 

There are several connections that might surprise you in exploring the sounds. 

Here is few of the ones you should try out for sure:

Connect OSC PULSE to the BIT IN to create varying random patterns.

Connect OSC PULSE to any of the OSC inputs to see what happens.

Connect any LFO signal to the MODE pin to change the synthesis mode on the go. 

Connect  OSC PULSE to the LFO RST pin to create chaotic modulation waveforms.

Kastl takes some common synthesis methods and implements them with a very 

lo-fi twist which gives it very distinct character full of aliasing artefacts.

I 

Phase modulation

Very similar to FM (frequency modulation) is method pioneered by John 

Chowning and later used by Yamaha for their famous DX synths. Kastle 

implements only two operator sinewave PM with a modulation amount. Pitch sets 

the main oscillator (carrier) , timbre the modulating oscillator (modulator) and 

waveshape sets the amount of modulation.

II 

Phase distortion

Was historically first used in 1984 in Casio synths of the CZ range. It is based on 

two synced ramp oscillators. The synced (higher tuned) oscillator is scanning the 

sine wave wavetable and is multiplied by the inverse of the lower tuned oscillator. 

In practical terms this was supposed to emulate sweeps of resonant filter on 

a sawtooth waveform. The implementation in KASTLE is adaptation of this method 

SYNTHESIS TYPES

See

www.bastl-instruments.com

for more information.

with wider ranges on the oscillators and more formant vocally character. There is 

also the third WAVESHAPE parameter which adds noisier high-pass character to 

the sound.

III 

Track & Hold modulation

This is not so common synthesis method. The main sinewave oscillator is going 

thru a comparator which turns it into variable pulse width waveform which is 

gating a track & hold circuit.The second oscillator is being fed thru the track & 

hold which means when the pulse of the main oscillator is high the signal passes 

and when it becomes low it holds static voltage. The threshold (waveshape) on the 

comparator sets the pulse width therefore the amount of the modulation and pitch 

and timbre set the frequencies of the oscillators.