ARMmbed FM Sound Synthesiezer

Simple monophonic FM sound synthesis with carrier, single modulator and low pass filter (wave table sinusoidal oscillators) based on STM32F746G-DISCO board. On board touch screen and additional ADXL345 3-axis accelerometer are used to control the sound synthesis. The synthesised signal is reproduced through the on-board SAI audio codec.

The four controllable parameters in (global variables in main.cpp) are:

float amplitude = 0.5; //numbers between 0.0 and 1.0
float pitch = 400; // numbers between 50 and 5000
float modrate = 0; // numbers between -1000 and 1000
float moddepth = 0; //numbers between -1000 and 1000
float filter = 1.0f; //numbers between 0.0 (full filter) and 1.0 (do not filter)

In the current version the mapping between user input and synthesis are the following (ad hoc scaling coefficients are applied):

the amplitude is set to a fixed value equal to 0.5 when a touch on the screen is detected (otherwise 0).
the pitch is mapped on the horizontal touch position (discrete onto 12 semitones indicated with the piano-like bars on top and bottom of the LCD display).
the filter is mapped to the vertical touch position (continuous).
the modulation rate is mapped to the horizontal accelerometer axis (continuous).
the modulation depth is mapped to the vertical accelerometer axis.

The source code include functions to ease the modification of the mapping/scaling of the user inputs to parameters of the FM synthesizer. The sound synthesis code is written in plain C so that can be ported to other ARMmbed boards or to other platforms.

The ADXL345 module is connected to the STM32F746G-DISCO as it follows: