Frequency detection (audio)

[HankMcSpank]

I keep revisiting this project, then parking it & then resurrecting it...and guess what I've just resurrected it!

I seek to derive the frequency of a plucked guitar note so I can do lots of funky frequency related stuff to the signal

I've presently got a PIC (16f1828) a comparator setup & I'm looking at the number of counts timer1 has incremented between each DT comparator interrupt.

for now I'm usiing a sig gen with a sine wave (yes I realise a guitar note is way more complicated...but I'll cross that bridge blah blah)

Anyway, by & large it's working OK.....the TMR1 count jitters about a bit, but it's close enough. but what I'm seeing is that with some frequencies the count jitters *way* too much.

For example, heres a stable-ish(!) timer1 count for 1khz into the PIC comparator.....

3208
3203
3194
3183
3161
3163
3163
3156
3150
3152
3148
3141
3139

Ok so it's a little jittery but in percentage terms the jitter is still fairly low, but look what happens when I then feed 800hz into the same same setup....

4955
180
4982
205
5012
176
4936
176
4963
183
4988
207
5001
176
4936
176
4957

...the Timer1 count between comparator interrupts is all over the shop!

Can anyone enlighten me why this might be happening?

here's the interrupt SETUP......

ASM
INT_LIST  macro ; IntSource,    Label,         Type, ResetFlag?
    INT_Handler  CMP2_INT,  _CMP2_Interrupt,  PBP,  YES 
    endm
    INT_CREATE       ; Creates the interrupt processor
ENDASM



CMP2_Interrupt:   ' COMPARATOR2 INTERRUPT.
@ INT_DISABLE CMP2_INT 
hserout [dec TMR1,13,10]
TMR1=0
@ INT_ENABLE CMP2_INT 
@ INT_RETURN

Is my approach wrong here?


Basically I want this type of sequence...

1. Guitar string plucked - PIC detects this (envelope detector)
2. PIC waits about 0.5s (to let the complex harmonics drop out)
3. PIC then attemps to detect frequency for a period of about 0.3s
4. Use a look up table for number of counts
5. Go do stuff with the outcome!


Now step 4 (go do stuff) I don't need help on - it's step 3 that I'm wondering what the best approach would be?

[cncmachineguy]

Hi Hank, just stabbing at this, you second set of numbers is not really all over, every other one is close to the last. I am guessing here maybe timer is overflowing with the lower freq?

[HankMcSpank]

Hi Hank, just stabbing at this, you second set of numbers is not really all over, every other one is close to the last. I am guessing here maybe timer is overflowing with the lower freq?

hi bert,

Very plausible/good idea, but alas, it's not down to Timer1 overflowing (I worked out that with a 16 bit timer @16Mhz oscillator I can go down to about 58hz before worrying about overflows), for example if i take the frequency even lower than 800hz, it goes fairly stable again, here's 500Hz...

3120
3218
3212
3208
3202
3198
3194
3184
3182
3179
3173
3168
3163
3154
3151
3150

[cncmachineguy]

Are my eyes lying to me or are they the same-ish as for 1K? One must ask them selves How can this be?

BTW Nice to be back and looking at hanks fun stuff

[HankMcSpank]

Are my eyes lying to me or are they the same-ish as for 1K? One must ask them selves How can this be?

Good spot!

Here's approx the Timer counts I'm seeing for different frequencies...

82.4Hz (lowest note on a guitar) - 42,900 (ish)
330Hz (top E) - 7,300(ish)
600Hz - 1800 (ish)

650Hz -1200(ish)

& then when I go higher in frequency, the timer1 counts start going all over the place ..

Here's 800 hz...

4924
177
4940
177
4965
179
4987

Oddly, when I dial 1000hz in, yep the counts go to 3,200 again ....so clearly something is wrong with my detection method using interrupts & timer1 counts

(btw I know the frequencies the PIC is handling internally are correct, as I'm outputting the PIC's comparator output to an external pin & attaching a frequency counter to the pin)

Hmm [strokes chin]

[Archangel]

500 hz is the first harmonic negative of 1 Khz. some Pi filters might help or lower the input gain