MEGAfm Teardown

Here is a look inside the Twisted Electrons MEGAfm. It is a nicely constructed boutique synth built around Sega Genesis sound chips.

An upper PCB containing LEDs and audio/MIDI jacks has been removed in these photos. The dual YM2612 chips are clearly visible. The microcontroller is visible in the bottom right. The other ICs on the board are TI multiplexers, an 8-segment LED controller, MIDI optocoupler, and an extra 512k EEPROM.

The microcontroller is an Atmel ATmega 1284P. It is a 20 MHz, 8-bit RISC CPU with 128k of program memory, similar to those used in the Arduino series and in Mutable Instruments Shruthi. Its role is mostly to convert MIDI controller data into YM2612 control signals. The internal EEPROM is only 4 KB, so I suspect an extra EEPROM was necessary for user presets. Just to give an idea, this is the kind of computing power we're talking about.

The YM2612 has a built-in DAC (known for its "ladder effect" at low volumes). I'm not sure if the analog audio output of the YM2612 chips is summed on the board or if their signals make a round trip to be mixed in the ATmega. If I had to guess, I'd say no. The volume knob is throttled to prevent zippering.

32 sliders and 14 potentiometers – FM synthesis doesn't get more hands-on than this.
Two original Yamaha YM2612 chips for a total of 12 FM voices.
The MEGAfm is controlled by an Amtel ATmega 1284P AVR chip.

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