Measuring audio phase shift through a circuit with a PIC

[HankMcSpank]

Excellent Job. Congratulations!!!

Thanks...I've just hit a problem. While larger phase shifts are being measured well, using this method (feeding into 2 x PIC comparators & counting using DT's interrupts) it seems small phase shifts can't be measured accurately. I made a short video to illustrate the problem....

(better to maximise the video to full screen & select 720p towards the right of the youtube video play bar)



Here's the video's accompanying text with the video explaining what's going on....

"Here you'll see two square waves feeding into the two comparators on a PIC...the green signal trace is feeding into comparator 1 (comp1 count is the count on the left & has a static count of about 9992) - the yellow trace is a 'phase lagged' signal feeding into the PIC's comparator 2. (the comparator 2 count is the green text on the right) What you see on screen is me reducing the phase lag of the yellow trace - as I reduce the phase lag, you can see the comp2 count decrement (as expected) BUT ...as the phase lagged signal gets into the small lag region, the comp2 count won't decrement below 336 - this suggests using PIC comparators & interrupts isn't workable for measuring small phase shifts?"

The audio signal frequency in that video is about 500Hz...and due to the Comp2 count not decremeting below 336, it means any phase lag below 12.5 degrees can't be measured At just 1400Hz (Comp1 count = 3,600, comp2 = count 336 minimum) , it means phase lag under 33 degrees can't be measured


Here are the two Comparator simple interrupt handlers...

'Comparator1 Interrupt Handler++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 
Comp1_Int:
        Comp1Time.Lowbyte = TMR1L  'Store away the timer1 count - this  will be the non lagged signal 'period' count.
        Comp1Time.Highbyte = TMR1H    
        TMR1H = 0                     'Set timer1 value to 0
        TMR1L = 0                     
@ INT_RETURN

'Comparator2 Interrupt Handler++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 
Comp2_Int:
        Comp2Time.Lowbyte = TMR1L    'Store away the timer1 count again (ie once the lagged signal leading edge arrives - this is the lag count)
        Comp2Time.Highbyte = TMR1H    
@ INT_RETURN

EDIT: Following on from the first blurry video I linked to, I re-made the video in HD & uploaded again (the new URL is edited in above)...be sure to view in 720p on youtube! (else the onscreen text will be blurred)