Bearcat synthesiser
The Bearcat is a lo-fi MIDI synthesiser inspired by the sound chips of vintage computers and game consoles. The hardware is finished, but the firmware still needs work (and probably always will). This webpage is under construction.
Quick overview
The Bearcat features two ATMega328 microcontrollers clocked at 16 MHz (to maintain easy compatibility with the Arduino bootloader, which I am currently using). Each chip outputs 8 bit, 32 KHz digital audio using a PWM pin. The output from the two pins is mixed using a standard virtual-ground opamp mixer. The mixed signal is then passed through a Sallen-Key 2nd order active low-pass filter to remove the high frequency PWM noise. The corner frequency is approximately 16 KHz, corresponding to the maximum frequency which can be synthesised without aliasing.
The 8 bit output of each chip is the combination of 4 software-mixed 4 bit channels. Each channel corresponds to a DDS digital oscillator with an ADSR envelope. This gives a total of 8 oscillators, which can be combined in a number of ways.
Power input is 9V DC via 2.1mm center-negative jack. This is regulated down to 5V for the microcontrollers, and all opamps are run from the 9V single supply. Audio signals are referenced to 4.5V instead of ground as is typical. MIDI in and MIDI through are provided by standard 5 pin DIN sockets. Audio output is via 1/4 inch mono phone jack.