PreenFM2 SMD Version
13th November 2017
This project is a redesign of the open source FM synthesiser PreenFM2 created by Xavier Hosxe.
Improvements from the original version:
- fewer and cheaper components
- single PCB (cheaper)
- 4 separate audio channels (on 2 stereo 6.3mm audio jacks)
- stereo DAC chip PCM5102 can be driven directly by the I2S processor's interfaces (faster);
sample resolution can be increased to 24 or 32 bits - headphones stereo amplifier (up to 2 × 175mW on 8ohm loads) on stereo 3.5mm jack
- one extra encoder and separate encoder switches
- all buttons and encoders are handled by a separate STM32F1 processor so they don't need to be constantly scanned
an interrupt (Srf_Int) is generated when a new surface event is detected and the main processor can then interrogate the slave STM32F1 for changed inputs with just a fast SPI transaction - digital volume control (range is -46dBu to 2dBu).
global volume is controller by encoder5 independently by STM32F1 which also converts rotations to a logarithmic scale
STM32F1 can remember previous volume settings and restore them - MIDI interface compliant with 3.3V specifications
- AUX (second) serial interface
- separate analog ground and analog 3.3V supply
