Low frequency audio level detector using PIC ADC?

[HankMcSpank]

Per the errata, they have fixed it for all current stuff. So unless your supplier has old stock, you are prolly good. I didn't think to look at the date on the errata, that may shed some light on if it will work. It sounded like it will either work or not. It didn't sound like it could be interminent.

And remember to change your ccpr values to reflect the faster clock. - unless you want to double the samples

Thanks Bert - Actually, I think we're done here would you believe! (Bert sighs in relief!)

I've aborted any attempt to confirm the sampling rate & have reasonable faith in our calculations! I've got the PIC Internal Oscillator running at 16 Mhz, and a special event trigger frequency of of 16Khz (ADC sample rate ), ie Timer1 matching against this....

CCPR4H = 0
CCPR4L = 250

(16Mhz clock, 4Mhz Instruction = 0.00000025sec period, therefore 250 x 0.00000025 = 0.0000625 sec .....so 1/0.0000625 sec= 16Khz)

....I've integrated the 'peak detect' code above into the final destination main loop - I've scoped a pin toggling within my my main loop @50khz which means it's looping at 100khz (so plenty of time to do other stuff in the main loop if I so desire). I don't use any interrupts except for switch presses,...and when a switch is pressed, I'm not worried about losing ADC samples.

...just connected it up to my guitar (vs the sig gen I've been testing with)....it's working as expected

A *very* happy Hank...thanks for all your input.

[cncmachineguy]

Thanks Bert - Actually, I think we're done here would you believe! (Bert sighs in relief!)

No sigh at all Hank, I really enjoy these kinds of threads. Lots of new things learned by all. I don't seem to have the imagination or desire to make such projects, but I LOVE thinking about them.

So Thank YOU and I look forward to your next one.

BTW, I agree you should consider making a WIKI of this. Your coding is fine, as Dave said you underestimate yourself. I do get the time aspect of it.

[Ioannis]

Thumbs up from me too.

Good idea with just a few components and clever programming. To the Wiki absolutely!

Ioannis

[ronsimpson]

Back at post #42 there is a picture of a square wave, (not so square wave) and a comment about how bad it looks. The wave looks like this because it has just passed through a capacitor. (A high pass filter) (DC blocking capacitor)

What are you trying to do? Measure the output of your electric guitar in peak response mode?

For long before I was born, audio has been measured in two modes.

Average mode: Full wave rectify. Then use a 2hz low pass filter. The response time is set to about the spoken syllable rate. (half wave produces little error)

Peak mode: Full wave rectify. Use a filter with an attack time of 1.7mS and a decay time of 660mS. You need the 1.7mS attack time or any very small peak will peg you meter but you ear can not hear that small peak.

Do you want RMS response?

[HankMcSpank]

Hi Ron,

Post #52 above explains what I was trying to achieve.....thankfully, it's al sorted now & seems to work ok.

[ronsimpson]

If you search for VU meter you will find examples of what you are working on.
With out a PIC the two attached pictures show an analog way of getting average and peak readings.
The rectifier is:
if MSB is 1 then shift left to remove that bit.
if MSB is 0 then compliment number +1 then shift left.
The peak detector (analog version):
The meter moves up at a rate set by R18 & C5. (fast)
The meter moves down at a rate set by R17+R18 & C5. (slow)
In software:
If the output of the rectifier > than the meter reading then (meter-rectifier)/x add to meter.
If rectifier is < meter then meter=meter-y.
Where y is a small number that will move the meter from full scale to 0 in about 600mS.
Where x sets the response time. (moving up)

There are several companies (THAT CORP and National see LM3915/16.) that have applications notes on the response of of VU meters. A typical test is to inject a 5khz tone for 10mS then stop. The meter should reach near full scale in 1.7mS and decay in 660mS.

My first try at a VU meter is attached. R2,3 set the input at 1/2 supply.
C1 blocks DC. 1/(2*2.14*50k*C1)=20hz set lowest frequency of intrest.
R1 & C2 makes a low pass filter to block information above 20khz.
R1 is to limit the current going into the PIC if the signal gets above 5 volts or below 0 volts. Could be 1k.

There is several better ways of setting the DC input of the ADC to 2.5 volts. More later.

[HankMcSpank]

In software:
If the output of the rectifier > than the meter reading then (meter-rectifier)/x add to meter.
If rectifier is < meter then meter=meter-y.

Hi Ron

If you glance at the final code I knocked together - that's essentially what I'm doing ...... 'x' being the AC signal coming in on a PIC pin, 'y' being my pseudo (sw) cap discharge rate (albeit I'm not driving VU meters & I require different attack/release times).