Digital Modular Synthesizer

 

This is a music synthesizer that follows the principles of subtractive synthesis.

It consists of a set of indepentand modules. It actually mimics the analog subtractive synthesizers. Patch cables are used to connect the various modules. However, the signal that travels across these patch cables is digital.

The basic idea is to replace the analog Voltage with a digital signal that represents this voltage. We can think it as if we sample the voltages at 48KHZ rate and 16-bits resolution (stereo).

Each module has a microcontroller unit (MCU) that reads one or more such signals and produces one signal as output.

 

The digital signal

 

The digital signal follows the Left Justified format with 2 X 16-bits channels and a sampling rate of 48KHZ. It is similar to the I2S format.

There is a time base that produces signals CLK and LRCK that are distributed to all modules. Each module then produces its own data signal in synchronization to the CLK/LRCK signals. These signals can feed other modules.

 

The modules

 

The basic part of each module is a STM32F411 MCU capable of reading 5 (five) and producing 2 (two) I2S-type signals. All signals have a common syncronization with the global CLK/LRCK signals.

 

A second MCU (STM32F103) in each module is responsible of reading the knobs and switches that consist the user interface and send the parameter data to the main MCU through a simple UART interface.

 

 

The required parts

 

There are some parts that are required to support the overall design:

• The CLK/LRCK source
• The DAC module
• A MIDI input interface
• The power supply