We received this email, so I post the reply here so all can benefit from it:
Love the design for the pedal pi. Great concept.
Just wondering why you chose to use the lo fi 12 bits? is this a limitation of the raspberry pi? What would be needed to mod it for higher bit rates?
There are several reasons why pedal-pi is 12 bits:
Parts Availability: Raspberry Pi (contrary to Arduino) does not have ADC integrated, it is challenging to find cheap, easy-to-find, through-hole (we try not to use SMD parts because most of DIYers don't like them)
and with a high sample rate to be able to deal with audio. The microchip MCP3202 was the only part that works at 3.3V (same as pi-zero), fast and easy to integrate (in software and hardware). The downside is that it is 12 bits.
Another reason is that the output stage is generated with 2 PWM signals in parallel. Again, we use PWMs instead of DACs because it is cheaper, it makes the software and hardware simpler and no additional parts are needed. If you use more than 12bits, the PWM frequency start to slow down together with the audio quality. So using 12 bits is not a bad idea after all.
The pedal-pi could have more bits resolution and higher quality, but at the expense of having a more complex hardware (more difficult to troubleshoot), more expensive and at the end of the day less appealing. The whole idea is to make the project accessible to everyone, so you can learn about signal processing, audio, hardware design and digital music.
The topics I describe above are a bit complex (I hope I am not putting you off), you can check this to learn bit by bit: