Measuring audio phase shift through a circuit with a PIC

[Jerson]

Hi Hank

Is it possible you got your T1 prescaler(T1CON) set to 1:8 ? That will explain the counts you see. 20e6 / 8 / 1000 = 2500 counts. You haven't mentioned which pic you use.

I've never used a resonator on my circuits, but, you could try with HS / XT options. Most likely the XT might work.

Regards

[HankMcSpank]

Hi Hank

Is it possible you got your T1 prescaler(T1CON) set to 1:8 ? That will explain the counts you see. 20e6 / 8 / 1000 = 2500 counts. You haven't mentioned which pic you use.

I've never used a resonator on my circuits, but, you could try with HS / XT options. Most likely the XT might work.

Regards

Thanks for bearing with me....and keep those ideas coming! (to my knowledge there are no prescalers set anyhere in my code)

Ok, I think I know (in part) what the problem is....& I throw myself open to public ridicule.

My signal path is this...

Signal 1 -> Audio preamp->schmitt trigger-> comparator1 input pin

Signal2 -> Audio preamp-> all pass filter-> schmitt trigger-> comparator2 input pin

When dealing with phase shift - it's easier on the eye to work with sine waves on a scope (for me at least)....*but*, doh, I was aligning aformetnioned 'zer degrees phase shift' in sine wave form as viewed on the scope. Alas, it appears that becuase each of the signals is of different amplitude going into the schmitt tirrger, the schmitt triggers are erhm triggering at slightly different times wrt to one another - which translates itself into phase shift! (now I'm pretty sure I was checking at the input to the pic comparators the other night, so I'm bemused how this has only come to light)

I therefore need to modify my circuit to ensure that the signals feeding into the shcmitt are of exactly the same amplitude (preferable amplified to nice sharp edged square wavea) ....this should ensure they trigger at the same point

Ok, something else I've established tonight ....if I feed signal1 into both schmitt triggers & ultimately into both comparators (so, absolutely no phase shift in play whatsoever then!)....I get a constant count of 29....this stays at 29 no matter what the input frequency - would it therefore be safe to assume that that's how many clock cycles it takes for all of the interrupt routines, storing the counts etc to work?


So therefore my new plan of attack is.....rejig my 'feed' circuitry to ensure an identical signal feeds amplitude into both schmitt triggers....and then also subtract 29 from the comp2 count. (at my upper frequency of 1.5Khz...I get a comp1 count of about 3000...therefore 1%, hich in phase terms is an error of about 3.6 degrees - not ideal. )

Would using any other form of interrupts/hardware get this 'floor' count of 29 down? (faster processor, higher frequency oscillator - 40Mhz?)

[HankMcSpank]

Ok, so I impemented my plan (square up the source signa, make them the same amplitude ....then subtract 29 from the final comp2 count).

Works...

(green trace is comp1 input, yellow trace is the phase lagged signal into comp2)








Thanks for all your help........"I have exorcised the demons this code is clear!"

[gadelhas]

Excellent Job. Congratulations!!!

[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)

[sinoteq]

What PIC do you use and how fast do you run it?

[HankMcSpank]

What PIC do you use and how fast do you run it?

I use a 16f690 @20Mhz

The 16F690's Comparator 1 receives the non lagged signal (set to internal VREF to trip at about 1/2 VCC)
The 16F690's Comparator 2 receives the phase lagged signal (set to internal VREF to trip about 1/2 VCC too)

Identical signals are fed into both comparators (bar the phase lag)

DT's interrupts enabled for CMP1 & CMP2.

On the scope the lag time measures about 20us - below which Comp2 will not decrement.

I don't know enough about what's going on under the hood wrt interrupts (or timer1 zero-ing) to shine any more light on why the comp2 count won't go below 336...when clearly it should as the leading edge of the lagged signal gets close to zero lag. So being a bit wet nehind the ears, is there any other possible method I can use that circumvent this problem?