RE: DTMF Decoder

Started by Hocquet Franck March 19, 2002
Hi Shahram,

As I experienced implementation of very well working DTMF
in TI C54x, you can try the following algorithm, which is
a part of my decoder. I have it in assembler if you are interested :

;The accumulation of each filter output is endangered by a possible
;An early overflow is prevented with an additional gain control, which
;the Goertzel filters from strong signal inputs. This gain control operates
on a
;buffer of input samples and estimates the peak amplitude of upcoming
;If the estimation exceeds a desired peak amplitude, the input samples are
;i.e. multiplied by a factor inverse proportional to the peak amplitude
; Description:
; ------------
; Control gain of the input sample x(n)
; On a 8 samples length buffer of input samples, the average peak
; amplitude GAIN_amp is computed using the last 8 samples. The current
; input samples are then attenuated using the foll formulas:
; GAIN_const = (1/2)SQRT(2)
; GAIN_amp = GAIN_const*(1/8)*[abs(x(n))+abs(x(n+1))+...+abs(x(n+15))]
; y(n) = (GAIN_THR/GAIN_AMP) * x(n)
; else
; y(n) = x(n)
; The only thing you have to do then is to set correctly GAIN_THR (depending
; of your input signal caracteristic that you should know)

Hope that helps.

Field Application Engineer
EBV Elektronik
Paris, FRANCE -----Original Message-----
From: shahram heidari [mailto:]
Sent: mardi 12 mars 2002 02:11
Subject: [speechcoding] DTMF Decoder Dear Friends

I am working to simulation and implementation of
multichannel dtmf encoder/decoder on ADSPs. I have
found many documents and codes, which among them, TI's
application notes is more completed. But in DTMF
decoding stage before Goertzel Block the design should
have a gain and amplitude control of input for
preventation of variable in Goertzel algorithm. I
converted this part of the TI's TMSC54x code to c++
but it is not sufficient and some overflow will
happen. If each of you have any experince or idea, I
will appreciate to propose me.

shahram heidari
Field Application Engineer
BBS Electronics Pte Ltd

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