The idea of anaQuad is to use an analog-quadrature source (such as the encoder disks used in an old "ball-mouse") to achieve significantly higher resolution than could be achieved by treating that quadrature-signal digitally. While being relatively immune to analog noise, calibration-error, etc.
It does-so, reliably, by *not* looking at analog *thresholds*, but instead looking at analog *crossovers*. This is well-explained in these pages.
There are several potential sources... Some quadrature-encoders output an analog signal rather than digital (again, e.g. looking at the output of the photo-transistors in a computer's ball-mouse). Some other sources include the hall-effect sensors used in BLDC motors (these are usually 120 degrees out of phase, rather than 90, as in quadrature, but anaQuad120 can handle that).
To *use* this system... Currently it's a software-only approach:
Two analog-to-digital converters are necessary for each encoder.
(Though, theoretically this could be implemented in hardware with a few comparators and op-amps, or maybe even voltage-dividers!)
anaQuad (the software) implements a non-blocking function-call to test the current state of your ADCs and determine the current state (and therefore update the "position")...
In all, it executes only a handful of instructions to detect a change in position, or lack thereof. So as long as it's called often-enough (faster than changes can occur), it puts little strain on your system. The same could be accomplished with a non-blocking digital quadrature-encoder routine in only a few fewer instruction-cycles. This could be called in a timer interrupt or from your main loop (if everything therein is non-blocking, and it loops fast enough).
Interestingly, anaQuad appears capable of "resynchronizing" if a few "steps" go undetected. So, e.g. if your main loop is slowed for some reason during one cycle, and a few anaQuad "steps" are missed, as long as the next few main-loops are faster, the system won't lose any steps! (Again, probably smarter to use a timer interrupt if you're not certain).