I'm going to give it a go anyway (on one pin - combined strings), so to my questions.....

Working on the basis that the highest note on a Guitar (fundamental frequency), is about 1.4kHz, Therefore, this highest (guitar string) note 'cycle' will take about 700us to complete

The lowest note is 82Hz - therefore this lowest (guitar string note) 'cycle' will take 12ms to complete.

I'll work on 32 'samples periods'for the highest frequency as a starter, so to cater for the highest note, I need to carry out an AtoD command every 21us (700us/32)...& to cater for the lowest note, I must continue doing this for 12ms. (the time it takes for the lowest note to complete one cycle of sound)

How can I be sure that such fairly rigid sample times are met?

At a PIC internal clock rate of 4Mhz, would I be right in saying that each clock cycle is 0.25us?

Therefore I need to carry out AtoD every 84 clock cycles?

Can someone help out here to get me started ...please?!!!