Measuring audio phase shift through a circuit with a PIC

[HankMcSpank]

Hi Jerson,

Ok, had a bit of time to try & get my head around your method. I'd like to break each of you sentences down a little....

A 16 bit timer is free running all the time. No start/stop sequences. Comparator interrupts too are always enabled. No need to start/stop them.

I'm struggling with why you're letting them free run?

The comparators first - they're gonna be generating a crazy amount of interrupts (to my simple brain, I actually wonder how the code will have time to do anythig else?! lol). Imagine a 1.4khz signal at the comparator pin - that's gonna be an interrupt every 700 us, but wait, there's the other signal presented at the other comparator pin (ie the one I want to measure the phase lag of) & it's generating interrupts too - so that's two interrupts arriving every 700us?!! (potentially the comparator might need to handle much higher frequencies...as high as 5Khz ....my mind boggles at the interrupt rate).The more I ponder this one (I'm into my 3rd night on this & nowhere near a result!), I do woder if there should be some other hardware I shopuld bring into play - to front end some of the work (I was looking at parrallax propellors - http://www.parallax.com/Portals/0/Im...lerBlock-L.jpg .....something like that architecture lends it self more to what I'm trying to do here?!)

Now I'm inexperienced & that might not tax the PIC one iota, but bearing in mind my program already uses other interrupts (Interrupt on Change, INT_PIN etc), and it's to my mind my program is getting a bit interrupt busy!

Ok, next (wrt allowing the timer to roll over) - I just can't get this clear in my head...

Remember Timer always counts up and rolls over, so CurrTimer nearly always is > PrevTimer. On rollover, the result of CurrTimer-PrevTimer is still valid since the bits are limited to a word size. So, 1000-500 is still same as 465-65500 (+ 1) when you do the 16 bit subtract.

I'm sure this is becuase I've not grasped the intracacies!

If you reference my schematic above, once again - I'm measuring two signals for phase lag - what happens if if the first signal comes in before the timer rolls over, but by the time the lagged signal come in, the timer has rolled over?

How does the maths work out for that scenario?

Ok, to your code ideas for me to using...

On comp1 interrupt
FreqCount = CurrTimer - PrevTimer ' timer ticks between 2 edges of the same input give time proportional to the frequency

Bearing in mind the source signal is a guitar - the frequency is forever moving (with periods where it's static), so I can't grasp how a moving target so to speak will work with that code approach?

PrevTimer = CurrTimer ' This is the new reference for phase measurement

This is about the one line I can grasp (!) - ie we need to put a stake in the ground to start timing until the second (phase lagged) signal arrives.

On comp2 interrupt
PhaseCount = CurrTimer - PrevTimer ' gives the time between non shifted and shifted waves

Again, what if the phase has moved (becuase the incoming frequency changed) - couldn't you have a situation where the current Timer hasn't rolled over, but the previous timer (with the stored value) did?

BTW - I really do appreciate you taking the time to explain to a dunderbrain like me!