The Moog filter: how it has changed electronic music, Spice simulation and practical design

What is the Moog Filter ? Why is it so famous even a historical revolution for the electronic music ?

A circuit to help experimenting with various VCF topologies

A voltage-controlled filter (VCF) is an electronic filter whose main characteristics (cutoff frequency, order and resonance) can be set by an input control voltage. VCF are widely used in analog music synthesizers, in our case the Themis.

Saving music calibration data in a flash memory

Today, the aim is to save music calibration data in a flash memory. EEPROM are very useful to keep data and use them when we actually need them without filling the flash memory of our microprocessor. Moreover, memory in microcontroller are easily lost when we rebuild the program. EEPROM will keep our data in stock even when it is no longer powered. In our case, we will use the EEPROM to keep the music calibration data when we don’t use them in the EEPROM and to be able to load them in the microcontroller when we need them. In order to do that, we will use a STM32L432KC and an EEPROM, in this case will use the 24C04 EEPROM.

Writing the VCO calibration algorithm

The Voltage Controlled Oscillator (VCO) is a device whose output frequency depends on a control voltage (the so-called “CV” on vintage synths frontpanes!). This device (associated with others like a Voltage Controlled Filter — VCF — and a Voltage Controlled Amplifier — VCA) can be used to create rich musical signals. This device however — as many analog devices — does experience temperature and humidity drifts and as such, requires precise calibrating if we want to play musically accurate notes!

What is Themis

Analog electronics has gained renewed interest in the last decade in the stimulating field of music synthetisers due to the onset of renowned brands like Arturia or Dave Smith Instruments. The initial impetus was to enrich the sound created by the instrument and make it more “human” by inserting analog chips at specific location in the signal path. These chips are non-linear by essence, and sensitive to temperature and humidity in a way that is too complex to model in a microprocessor.