Rough 'n Ready Audio Frequency extraction with a PIC

[HankMcSpank]

Ok, I think I have the loose makings of the necessary code for a rolling 1 second window to check whether the frequency has remained the same...

(the signal to be monitored is fed into comparator1)

INCLUDE "DT_INTS-14.bas"
INCLUDE "ReEnterPBP.bas"
INCLUDE "Elapsed_INT.bas"  ; Elapsed Timer Routines

ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   TMR1_INT,  _timer1_interrupt,   PBP,  yes
        INT_Handler   CMP1_INT,  _comparator_interrupt,   PBP,  yes
    endm
    INT_CREATE            ; Creates the interrupt processor
ENDASM

@   INT_ENABLE  TMR1_INT  ; Enable Timer 1 Interrupts
@   INT_ENABLE  CMP1_INT  ; Enable comparator1 Interrupts 

nochange_count var word   'count number of 'timer overflows' where frequency hasn't changed

comparator_count var word   'a variable to count number of comparator interrupts (frequency detection)

loop:
if nochange_count > 3 then     'ie frequency has remained the same for 1 second
nochange_count = 0 
gosub dostuff
endif
goto loop


'---[Timer1 - interrupt handler]--------------------------------------------------
timer1_interrupt:        'prescaler set for circa 250ms
if comparator_count = previous_comparator_count then   'needs a 5% 'deviation' allowable error to go here!!
nochange_count = nochange_count+1   'count how many times frequency has NOT changed
else
nochange_count = 0 ' if frequency has changed, then zero counter and start over
endif
previous_comparator_count =  comparator_count
comparator_count = 0
@ INT_RETURN

'---[Comparator1 - interrupt handler]--------------------------------------------------
comparator_interrupt:
comparator_count = comparator_count +1
@ INT_RETURN

There may be some syntax errors in there (I'm at work!), but that's the basic framework - what I don't know how to do is the 'deviation' part (bolded in red above).

How can I have an 'if' condition that checks to establish if present count is 'within 5%' of previous count' (plus or minus), therefore....

if comparator_count = previous_comparator_count '+ or -' 5%

Two 'AND'ed ifs?

Many thanks!