Low frequency audio level detector using PIC ADC?

[tony45]

[QUOTE=HankMcSpank;99190]Actually I think my revised method below will be better....

(VCC is 5V, AC signal will sit on 2.5V, therefore positive half of cycle samples @10 bits will be between 512 & 1024, negative parts of the cycle will be between 0 & 512)

1. Setup a timer to interrupt at a rate of 20khz

2. In the timer interrupt routine take an ADC sample into a variable

3. Compare the ADC sample to the last sample - if it's higher keep it, if it's smaller ignore it.

4. If the sample reads below 512 (half VCC - the signal's zero cross), then the highest sample from step 3 is signal 'peak', now zero the variable ready for the next positive cycle (ie samples above 512)

5. Run a bit of maths to convert peak voltage sample to RMS (ie multipy peak sample by .707 using some FP workaround)


i agree

[cncmachineguy]

Hank, I think something is wrong with the settings. The toggle rate seems to represent a word timing out. For instance timer1 is 16 bit, full count is 65535. 65535*.0000005 = 32.7675mSec. that sure looks like your toggle rate!

So I would guess TMR0 is not clearing on a match!

[HankMcSpank]

[QUOTE=tony45;99536]

Actually I think my revised method below will be better....

(VCC is 5V, AC signal will sit on 2.5V, therefore positive half of cycle samples @10 bits will be between 512 & 1024, negative parts of the cycle will be between 0 & 512)

1. Setup a timer to interrupt at a rate of 20khz

2. In the timer interrupt routine take an ADC sample into a variable

3. Compare the ADC sample to the last sample - if it's higher keep it, if it's smaller ignore it.

4. If the sample reads below 512 (half VCC - the signal's zero cross), then the highest sample from step 3 is signal 'peak', now zero the variable ready for the next positive cycle (ie samples above 512)

5. Run a bit of maths to convert peak voltage sample to RMS (ie multipy peak sample by .707 using some FP workaround)


i agree

Tony, Bert - The problem with step 4, is that in the absence of an equal or higher sample arriving (or, as you're proposing, while below zero point) my running peak variable is decremented (remember this code is emulating an analogue peak detect circuit - in such a circuit, the diode only lets through voltage that is higher.....and in the absence of a higher voltage the didoe is reverse biased & the cap starts discharging via a resistor - my running peak variable content decrements in this case).

This will utlimately mean that during the negative portions of the cycle, the running peak I've stored will decrement a fair bit, before the next positive part of the cycle returns & sets the peak high again - this is essentially like half wave rectification vs full wave - the former gives much more ripple, ie half wave ripple....

http://www.m.case.btinternet.co.uk/a...e_Smoothed.gif

...so what I end up with is pseudo ripple (a variable decrementing then being topped up) This is why when the ADC reading goes below zero (ie below 128 on an 8 bit sample), I want to flip the negative cycle up - to be a positive cycle. This greatly reduces any decrementing (pseudo cap disharging)

This diagrams illustrate the end goal in analogue waveform terms...

http://www.eleinmec.com/figures/018_04.gif

The 0V (zero crossing point) in my case is 2.5V (an ADC reading of 128)...I want to flip the negative part of the wave up. It not only reduces ripple...but halfs the amount of time to detect peak etc, leading to less ripple (peak detect fluctuations)

http://images-mediawiki-sites.theful...3847055696.png

I realise 10 bits would be better, but just wanted to keep it all simple while I'm ironing out all the issues.

N8NTA - tks for the tip about measuring the main loop time, I'll do that.

Hank, I think something is wrong with the settings. The toggle rate seems to represent a word timing out. For instance timer1 is 16 bit, full count is 65535. 65535*.0000005 = 32.7675mSec. that sure looks like your toggle rate!

So I would guess TMR0 is not clearing on a match!

Wow Bert - that's a great spot! This from the datasheet...

"The Special Event Trigger output of the CCP occurs immediately upon a match between the TMR1H, TMR1L register pair and the CCPRxH, CCPRxL registerpair. The TMR1H, TMR1L register pair is not reset
until the next rising edge of the Timer1 clock."

As far as I can gather. other than to set the CCP register (to select pin toggle vs special event ADC triggering) ...there's nothing else to be done - not sure how this one can be tackled?!!

[cncmachineguy]

Ok, after some careful reading of the datasheet, I find I am no longer any smarter then I was before I started. I can't find anywhere in the datasheet to say in compare mode timer is reset on a match. It DOES say for the Special event trigger the timer is reset. So I suggest this:

Go back to SET mode (Special Event Trigger).
Enable interrupt on A/D conversion done.
Have the ISR do this:
toggle a pin (for debuging speed)
move A/D results to a variable for later use
Return from INT

With that done, several things are solved for you.

[Ioannis]

Hank,

1. why don't you make a full rectifier of your analog signal?

2. is the signal always of the same amplitude?

3. are you after a peak detector or an rms calculator? I am little confused by this thread...

Ioannis

[HankMcSpank]

Hank,

1. why don't you make a full rectifier of your analog signal?

2. is the signal always of the same amplitude?

3. are you after a peak detector or an rms calculator? I am little confused by this thread...

Ioannis

Hi Ioannis,

Ok, I came into this thinking I needed an RMS detector - I don't (despite the title ....which I'm unable to update/edit)....I really want to track 'peak' of a guitar signal. The incoming signal is of very wildly differing amplitude.....but fortunately it only goes up to 1.5khz max (the max fundamental on a guitar...I don't care about the harmonic content here - I'm only wanting to extract magnitude info, not accurately replicate the waveform digitally)

So why not use conventional analogue components? Well I'm *very* pushed for PCB real estate ...the intended PCB will already have a PIC, but the PIC isn't doing much (basic switching, LED indicators etc) ...so that got me thinking, why not extract peak from the incoming analogue signal using the PIC - just like an analogue full wave rectifier into a peak detector (which is what my code above is doing ....'full wave rectifying a signal' but via code inside the PIC) ...if nothing else, my intention was to learn a little more about sampling an AC signal rapidly .....& like most journeys - you discover other stuff enroute. And that has proved to be the case - the special event trigger - it's been a revelation to me at least.

Not only do I save components (PCB space)...I also avoid the forward voltage drop loss that an analogue peak detector suffers from (this is a relatively small signal I'm monitoring)...but it struck me I can also have on the fly varying 'discharge' rates to suit as my peak detector is now working in the digital domain.

The nut has actually been cracked ....all that's happening here now is a bit of a wash up...trying to get to the bottom of a couple of quirks I'm hitting (specifically the calculations wrt sample rate, etc)


Bert,

Again...great ideas...I'll implement this later to get a handle on the sampling frequency (I concur that the datasheet is a bit scant with the 'meat' here!)

[Ioannis]

The first that comes to mind is to use a DSP or dsPIC for the job as there seems to be a lot of maths.

But then since the max frequency is just 1,5KHz then maybe the PIC used is good enough.

Of course it has the speed to convert the input sample, but do you have time to proceess afterwards?

The other matter is the form of the input signal. You mentioned that it is greatly varying. Are you feeding it directly to the ADC? Any pre-conditioning I thing is necessary (filtering, dc-level bias etc.).

Ioannis

P.S. I changed the thread title hoping you do not have any objection.

[HankMcSpank]

The first that comes to mind is to use a DSP or dsPIC for the job as there seems to be a lot of maths.

But then since the max frequency is just 1,5KHz then maybe the PIC used is good enough.

Of course it has the speed to convert the input sample, but do you have time to proceess afterwards?

The other matter is the form of the input signal. You mentioned that it is greatly varying. Are you feeding it directly to the ADC? Any pre-conditioning I thing is necessary (filtering, dc-level bias etc.).

Ioannis

P.S. I changed the thread title hoping you do not have any objection.

Actually, that's the beauty here - there's almost no complicated maths....ie just et the special event trigger grabbing ADC samples at a reasonably high sampling rate (& this is all done in the background without any recourse to interrupts etc), then in the main (fast) loop, just compare the present ADC sample to the last sample...if it is the same or bigger, keep it as the 'present peak'....if it's not the same or bigger. start decrementing the variable linearly.

This approach wouldn't be right, if the PIC was already running intensively....but like I say, in my application the PIC isn't being too stressed.

The signal is preconditioned in that it goes through a buffer.....this puts the AC signal on a DC level of half VCC (necessary to full wave rectify the signal inside the PIC).

At the end of the day, I'm really just using the latent (otherwise wasted) power of the in-situ modest PIC to save on a few components (not for cost, but space) - .I don't want to start involving specific specialised ICs for what is after all a simple enough requirement.

oh, btw I've no problem with the title being changed!

Hank, take a look at the Tips & tricks doc (http://ww1.microchip.com/downloads/e...Doc/01146B.pdf). Pay special attention to chapter3 (CCP and ECCP); tips #7 and #11.

wow...a great doc.....need to digest - thanks!

[languer]

Hank, take a look at the Tips & tricks doc (http://ww1.microchip.com/downloads/e...Doc/01146B.pdf). Pay special attention to chapter3 (CCP and ECCP); tips #7 and #11.