Rough 'n Ready Audio Frequency extraction with a PIC

[HankMcSpank]

Ok, a wet VAc has sorted most of my carpet problems, but going straight to the manual for info about using DIV32 has seemingly brought more jitter to the fore....

First here's what I'm doing....

@ __CONFIG _FCMEN_OFF & _INTRC_OSC_NOCLKOUT & _WDT_OFF & _MCLRE_OFF & _CP_OFF & _IESO_OFF & _BOR_OFF & _PWRTE_OFF

'***********************************************************************************************

'16F690 Pin Connections....
' PIN# NAME     USE & CONNECTION
'  1   Vdd      +5VDC power supply   
'  7   RC3      C12IN3- (external comparator input)     
'  20  Vss      Ground
'*************************************************************************************
DEFINE  OSC 4          ' set Oscillator at 4Mhz.
DEFINE  NO_CLRWDT 1   ' PBP doesn't clear WDT automatically
DEFINE HSER_SPBRG 25   'HSEROUT Stuff.
DEFINE HSER_TXSTA 24h  'HSEROUT Stuff.
DEFINE HSER_CLROERR 1  'HSEROUT Stuff.

txsta = %10100100       'setup the tx register
RCSTA.7 = 1             ' Enable RB7 for TX USART
INTCON.0 = 0            ' clears the RABIF Flag (to 0), COULD be 1 on reset (unique to F690)
ANSEL      = 0    'disable AtoD.
ANSELH     = 0    'disable AtoD.
CM2CON0 = 0   'turn off comparator 2.

'Turn on & Set up Comparator 1
CM1CON0 = %11100011    'Comparator ext op pin disabled (op of comparator avaible internally only), compare against external VREF
VRCON = %0100001       'turn on internal reference
TRISA.2 = 0            'allow the comparator 1 output pin as an output

MyTime       var  word    ' used to amalgamate TMR1 High & Low Bytes.
Frequency    var  word    'used to convert the count to frequency.

' the following is pretty much a straight lift from the compiler manual (DIV32 section)
a Var Word
b Var Word
c Var Word
dummy Var Word
b = 500
c = 1000
          
INCLUDE "DT_INTS-14.bas"     ' Base Interrupt System
INCLUDE "ReEnterPBP.bas"     ' Include if using PBP interrupts

ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   CMP1_INT,  _Comp1_Int,   PBP,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
T1CON = $11               ; Prescaler setting 2us between clocks

@ INT_ENABLE  CMP1_INT     ; enable Comparator 1 interrupts

T1CON.0=0 'stop the timer
TMR1H = 0 'Set the high part of the timer value to 0
TMR1L = 0 'Set the low part of the timer value to 0

MyTime = 0 'clear down Mttime, prior to starting.


'Main body of Code*********************************************************************************************
Main:
        pause 10
        goto Main
        end
   
'Comparator1 Interrupt Handler+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++        
Comp1_Int:
        if T1CON.0= 0 then 'if timer1 is not running...
        TMR1H = 0 'Set the high part of the timer value to 0
        TMR1L = 0 'Set the low part of the timer value to 0 
        T1CON.0= 1  'start timer
        else        'therefore if it is running, stop the timer & calculate the number of 'clock' counts between comparator interrupts....
        T1CON.0= 0  'stop the timer
        MyTime.Lowbyte = TMR1L 'puts the timer's low byte in MyTime's lower 8 bits 
        MyTime.Highbyte = TMR1H 'puts the timer's high byte in MyTime's upper 8 bits

        dummy = b * c '  'a straight lift from the compiler manual (DIV32 section)
        a = DIV32 MYTIME  'use the DIV32 to divide 50,000,000 by the comparator 'time between counts' 
        hserout ["Comparator1_Count = ", dec MyTime, 9, "Frequency = ", dec a,13, 10]       
        endif
        
@ INT_RETURN

....& here's the onscreen hserout data....

(just to remind you 'Comparator1_Count' is the 'number of clocks' counted between successive comparator interrupts - and the frequency is the DIV32 result of 500000 divided by comparator1_count number )

Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1961 Frequency = 254
Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1961 Frequency = 254
Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1961 Frequency = 254
Comparator1_Count = 1959 Frequency = 255
Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1961 Frequency = 254
Comparator1_Count = 1959 Frequency = 255
Comparator1_Count = 1961 Frequency = 254
Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1961 Frequency = 254
Comparator1_Count = 1960 Frequency = 255
Comparator1_Count = 1960 Frequency = 255


(FWIW My sig gen was showing 255.2Hz on its LED display & using a calculator to divide 500000 by the 'average' comparator1_count of say 1960, works out at 255.1Hz)

so as a percentage the 'variance/jitter' on the lefthand figure (Comparator1_Count) is miniscule....but using the DIV32 seems crank up the 'error' when converted to Hertz? (eg 254, 255, 255, 254)

What am I doing wrong?!

[prstein]

HankMcSpank,

I suspect you are expecting to see a rounded final value when in fact your final value is truncated.

1960/500,000 = 255.102, PBP truncates to 255 (same as rounding)
1961/500,000 = 254.972, PBP truncates to 254 (would round to 255!)

If you add "half of a whole value" before truncating it is the same as rounding. You can't add 0.5 to the DIV32 result because we aren't using floating point. So we add it before the division:

1960/500,500 = 255.488, PBP truncates to 255
1961/500,500 = 255.227, PBP truncates to 255 (all is good)

You've may still have some granularity issues but this should get you closer.

I remember hearing a long while back that "floating point math is for weenies who can't think big." 8^)

Best Regards,
Paul

[HankMcSpank]

Hi Paul,

I was aware that the PIC simply threw the decimal portion away, but I'm no mathmetician & just wanted to clarify that I wasn't approaching this incorrectly!

Thank you for your suggestion...but my goal here is simply to detect the frequency remaining the same for a set length of time (ie the guitar player is 'holding' a note for more than x seconds)....so I don't actually even need to convert to hertz - for simplicity, I can just work in counts.

It might be handy one day to detect the actual 'Note Played'. when I think my approach would be along the lines of a monster lookup table (with a 'window' either side of the centre frequency for each note)...

                  Hertz     Clock Count

	 (from ) 	78	6410  
Open	E	82.41	6067
	 (to) 	84.85	5893  
			
	 	84.86	5892
Fret 1	F	87.31	5727
	 	89.90	5562
			
	 	89.91	5561
Fret 2	F#	92.50	5405
		95.24	5250
			
	 	95.25	5249
Fret 3	G	98.00	5102
		100.91	4955
			
	 	100.92	4954
Fret 4	G#	103.83	4816
		106.91	4677
			
	 	106.92	4676
Fret 5	A	110.00	4545
	 	113.26	4415
			
	 	113.27	4414
Fret 6	A#	116.54	4290
	 	120.00	4167

& so on (I guess Hertz could be derived to decimal points using an even bigger lookup table?!)


PS Once you get to the highest note on the fretboard (E on the 24th fret - 1318.5Hz) , the compartor clock count between interrupts comes down to something around 379....so it might be worth messing with the prescaler to give a bit more granularity)