Low frequency audio level detector using PIC ADC?

[HankMcSpank]

2. Is this methodology correct....

setup a time to interrupt at a rate of 10khz
in the interrupt routine take an ADC sample into a variable, compare it to the last if it's bigger keep it, if it's smaller ignore it.
run a bit of maths to convert peak voltage to RMS (ie multipy by .0707 using some FP workaround)
Have another timer setup to reset the ADC sample variable every 20ms (ie 50hz the lowest frequency)

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)


Anything else?

[cncmachineguy]

Hank, assuming you have time to spare, why not just sample on each iteration of main? I assume it is running much faster then 20K, And if you can stand the short time it takes to aquire the sample, you will have the best sample rate without going to interupts.

[HankMcSpank]

Hank, assuming you have time to spare, why not just sample on each iteration of main? I assume it is running much faster then 20K, And if you can stand the short time it takes to aquire the sample, you will have the best sample rate without going to interupts.

Hiya Bert ...thanks for the input.

The reason for wanting to use interrupts, is that I can sub-contract out the ADC sampling (precisely) & let it maintain itself without having to integrate the sampling into my main loop, where there's other stuff happening (and a whole heap of pauses within!).

[cncmachineguy]

Well in that case, I would make the rate some easy timer rate. like a 8 bit timer with some prescale, running as fast as you can stand to be interupted.

[HankMcSpank]

Well the concept seems to work, but would you believe the thing that's hampering me is getting the timer1 interrupt frequency set right....I'm chasing an interrupt (ADC Sample) frequency of about 30khz.

I seem to be getting an interrupt frequency of just 1Khz, which means only low frequency sine waves that I'm sampling are stable from an ADC perspective, what am I doing wrong....

here's the top bit of my code (setting up the timer1)

' Timer1 Registers: 
' Prescaler=1:1; TMR1 Preset=65503; Freq=30,303.0303Hz; Period=33,000 ns
T1CON.5 = 0 ' bits 5-4  Prescaler Rate Select bits
T1CON.4 = 0
T1CON.3 = 1 ' bit 3 Timer1 Oscillator Enable Control: bit 1=on
T1CON.2 = 1 ' bit 2 Timer1 External Clock Input Synchronization Control bit: 1=Do not synchronize external clock input
T1CON.1  = 0 ' bit 1 Timer1 Clock Source Select bit: 0=Internal clock (FOSC/4) / 1 = External clock from pin T1CKI (on the rising edge)
T1CON.0 = 1 ' bit 0 enables timer
TMR1H = $FF      ' preset for timer1 MSB register
TMR1L = $DF      ' preset for timer1 LSB register

& below is my (DT) interrupt handler (feel free to suggest better way of writing this ...I'm a kludger at heart & I've not really a clue how to code well!)

zero_cross is just a variable to track whether the signal wave/cycle is above zero (the signal zero point = 128....ie 1/2 my supply of 5V)
signal_in is a variable to store the sample (previous_sample + peak_sample are too!).

'---[TMR1 - interrupt handler]--------------------------------------------------
@ INT_DISABLE  TMR1_INT     ; diable Timer 1 interrupts
T1CON.0 = 0 ' disables timer1
ADC_Sample:
    ADCIN 8, Signal_In                'sample the incoming signal
    IF SIGNAL_IN > 130 THEN           'a level of 128 is the zero crossing point (130 adds a bit of margin for error)
    zero_cross = 1                            'a simple toggle/flag to track whether signal is above or below the 'zero point (ADC val of 128)
    IF SIGNAL_IN > PREVIOUS_SAMPLE THEN previous_sample = signal_in     ; if this latest incoming sample is bigger than the last sample, then it supercedes the previous value
    GOTO INTERRUPT_END
    ENDIF

    if signal_in < 126 and zero_cross = 1 then  'if sample is 128 or less, then we're at the zero cross point, (126 adds some margin for error) therefore we've finished trying to sample peak
    peak_sample = previous_sample               ' our peak sample is therefore whatever value is in the variable 'previous_sample' from above
    previous_sample = 0                         'zero it, ready for when the waveform goes into positive territory again.
    zero_cross = 0                             ' toggle the above zero/below zero flag
    endif
    
    INTERRUPT_END:
    LOOP_COUNT1 =LOOP_COUNT1+1    'this section just allow me to divide the interrupt rate down for onscreen display (else the serial port would bottleneck & PIC would likely restart)
    if loop_count1 = 1000 then                'I'm seeing about 1 update onscreen per second, which suggest the interrupt frequency is 1kHz & not the required 30khz!
    hserout [dec peak_sample,13, 10]
    loop_count1 = 0
    endif
    
TMR1H = $FF      ' preset for timer1 MSB register
TMR1L = $f0      ' preset for timer1 LSB register
T1CON.0 = 1 ' bit 0 enables timer

@ INT_ENABLE  TMR1_INT     ; enable Timer1 interrupts
@ INT_RETURN

end

100hz, 2.5Vp signal - extracted 'peak' sample is circa 255....


100Hz, 1.25Vp signal - extracted 'peak' sample should be circa 192 (remember here that the signal's zero crossing point is ADC=128)


Ok, now you start seeing the problem...up to 300Hz, the ADC readings are fairly stable at +/- 1 sample, but if I now take the audio frequency up to 400Hz, the ADC 'samples' start getting choppy, which suggests the interrupt rate isn't high enough...




So what am doing wrong wrt getting the timer1 interrupts to interrupt at a 30Khz?!!!

[cncmachineguy]

(if it helps, I'm using a PIC16f1828 ...max internal oscillator of 40Mhz)

I don't see how to set this up for 40Mhz. At any rate, what speed are you running?

Do you have a free pin to toggle each interupt to see exactly what the frequency is? While I agree the seeming like 1K is a far cry from 10K, but for me I just like to know for sure to fine tune it.

Also as a suggestion, DT pointed me to TMR2 and the use of the PR register. The advantage is no reloading of the preload. it also has pre and post scaling to turn your 8 bit counter into a slower counter.

Now for more confusion, your preload will give you 32 instructions before overflowing. I think you have that in the int handler itself. not that it will matter, but if you are running at 32 Mhz, 4uSec timing, or 250KHz. So maybe you are flooding HSEROUT?

[HankMcSpank]

I don't see how to set this up for 40Mhz. At any rate, what speed are you running?

Do you have a free pin to toggle each interupt to see exactly what the frequency is? While I agree the seeming like 1K is a far cry from 10K, but for me I just like to know for sure to fine tune it.

Also as a suggestion, DT pointed me to TMR2 and the use of the PR register. The advantage is no reloading of the preload. it also has pre and post scaling to turn your 8 bit counter into a slower counter.

Now for more confusion, your preload will give you 32 instructions before overflowing. I think you have that in the int handler itself. not that it will matter, but if you are running at 32 Mhz, 4uSec timing, or 250KHz. So maybe you are flooding HSEROUT?

Hi Bert.

Yeah, I got carried away with the max Osc value this pic can run at - it's 32Mhz.

that said, I'm only running it at 8Mhz (there an errata sheet out that says early versions of this PIC had probs with ADC above 8Mhz clock - so I'm staying on the safe side)

Good call about the toggle - I've put a toggle pin line in the interrupt handler, my scope measures the square wave at 2.66Khz, but I'm assuming the interrupt rate is double that? (half, the time low, half the time high)...therefore I've an interrupt frequency of about 5Khz

I'll have a a go with Timer2, but the PICcalc timers that abound are doing my head in! :-)


Edit:

Just tried timer2...

@ __CONFIG _CONFIG1, _FCMEN_OFF & _FOSC_INTOSC & _WDTE_OFF & _MCLRE_OFF & _CP_OFF & _IESO_OFF & _BOREN_OFF & _PWRTE_OFF & _LVP_OFF
@ __CONFIG _CONFIG2, _LVP_OFF

INCLUDE "DT_INTS-14.bas" ' Base Interrupt System
INCLUDE "ReEnterPBP.bas" ' Include if using PBP interrupts

Osccon = %01110010      'Osc 8Mhz
DEFINE  OSC 8

Now the timer2 settings...

'Timer2 Registers:
'Prescaler=1:1; TMR2 PostScaler=1:1; PR2=66 - Freq = 30,303.0303Hz - Period = 33,000 ns
T2CON.6       = 0 ' bits 6-3 Post scaler 1:1 thru 1:16
T2CON.5       = 0
T2CON.4       = 0
T2CON.3       = 0
T2CON.2  = 1 ' Timer2 on bit: 1=Timer2 is on;
T2CON.1 = 0 ' bits 1-0  Prescaler Rate Select bits
T2CON.0 = 0 
T2CON = %00000100
PR2 = 66            ' PR2 (Timer2 Match value)

Now the interrupt routine...

ADC_Sample:
@ INT_DISABLE  TMR2_INT     ; enable Timer 0 interrupts
    toggle PortC.3
    ADCIN 8, Signal_In                'sample the incoming signal
    IF SIGNAL_IN > 130 THEN           'a level of 128 is the zero crossing point (130 adds a bit of margin for error)
    zero_cross = 1                    'a simple toggle flag to track whether we're above zero or below
    IF SIGNAL_IN > PREVIOUS_SAMPLE THEN previous_sample = signal_in     ; if this latest incoming sample is bigger than the last then it supercedes the prvious value
    GOTO INTERRUPT_END
    ENDIF

    
    if signal_in < 126 and zero_cross = 1 then  'if sample is 128 or less, then we're at the zero cross point, (126 adds some margin for error) therefore we've finished trying to sample peak
    peak_sample = previous_sample               ' our peak sample is therefore whatever value is in the variable 'previous_sample' from above
    previous_sample = 0                         'zero it, ready for when the waveform goes into positive territory again.
    zero_cross = 0                             ' toggle the above zero/below zero flag
    endif
    
    INTERRUPT_END:
    LOOP_COUNT1 =LOOP_COUNT1+1
    if loop_count1 = 100 then
    hserout [dec signal_in,9,dec peak_sample,13, 10]
    loop_count1 = 0
    endif


@ INT_ENABLE  TMR2_INT     ; enable Timer 0 interrupts
@ INT_RETURN

Still just seeing 5khz 'interrupt frequency' on my scope (this is the pin I'm toggling in the interrupt routine)...




AaaaaaarrrrrggggghhhhhHHHH!!!!!!!

[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.

[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.