Snack 'N' Hack

The more-or-less maximum frequency of a standard 22 fret guitar is around 1,100 HZ.

Therefore, for Analog to Digital conversion, a low-pass filter of most signals above say 1,300 HZ would be essential to reducing random artifacts and noise.  The Biscuit and Colin’s Effect Pedal prove that very standard, low end PICs can be used for high-quality 8-bit audio applications.  Both also employ low-pass filters at the beginning of their signal chains to reduce random noise and artifacts.

In the following diagram two op-amps are combined to form a 4-pole Butterworth low pass filter with a cutoff frequency of 22-kHz.  This is NOT my work, it is from another PIC guitar effects pedal.

To get accurate A/D conversion from audio, it is ESSENTIAL that some sort of input buffering and filtering be performed.  For me, this is one of the hardest parts of the process, but I plan on having the filter built very soon.

Just another note: Guitar harmonics above the sampling rate will cause noise in the signal.  Therefore, some harmonic loss is an inherent part of this process.  You wouldn’t be building a bit-crusher if you were looking for audio purity.