Hello David,

I would do some sort of correlation on your signal, beginning with a target
frequency that is the first encountered and then raise the frequency target
at the suspected frequency increase rate. Then fail and start again if the
correlator gets out of band during the sweep or if insufficient samples come
in during a given time period.

A 4 MHz Pic should be able to keep up with everything up to 10 kHz and still
do all the rest of the processing provided one brings it in on an interrupt
pin. If you construct your history for the correlator correctly, you should
have good immunity to intervening noise.

Tom

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of dlions@acs.itd.uts.edu.au
> Sent: Thursday, April 08, 1999 7:22 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: FFT on PIC midrange
>
>
> Knowledgeable and experienced PIClisters,
>
> I am 80% finished developing a device which will be used in the
> telephony biz.  So far it is very simple, detect a rising voltage, 3
> pushbuttons, 3 LEDs, ISD speech chip, some latches and delay
> circuits.  So
> far all in discrete components.
>
> But, in order to be useful to more than about 10% of our customers, we
> will have to add two features :
> i) Detect LED start/stop flashing and flash rate
> ii) Look for a certain 'sound' - using FFT (required for that
> pesky last
> 55% of market share)
>
> Part i) is easily solved with a PIC, much experience there, got the
> development tools etc.
>
> Part ii) is our dillemma.  The 'sound' can only be described
> as a 'zzzzip'
> tone, increasing pitch, sort of a ramping-up sound.  I
> guarantee all below
> 10kHz, probably lower.  The system must recognise the 'zip'
> and respond
> (just set a pin low etc) within about 0.5-1.0 seconds.
>
> Our dilemma is whether to use a low range DSP (say $AUS10
> each) or try to
> squeeze it on a mid range PIC ($AUS1-3).
>
> *** Can a mid-range PIC handle this?
>
> ** If we use a DSP, I assume it can light LEDs and read pushbuttons.
> Will we need to use external latches for pushbuttons etc? Can
> anyone tell
> me who is the 'Arizona Microchip' of DSP suppliers? (cheap,
> no-nonsense)
>
>
> Thankyou once again,
> David.
>