Copyright © 2003 Texas Instruments. All rights reserved. DSP C5000 Chapter 17 DTMF generation and detection Dual Tone Multiple Frequency

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 2 Learning Objectives  DTMF signaling and tone generation.  DTMF signal generation  DTMF tone detection techniques and the Goertzel algorithm.  Implementation of the Goertzel algorithm for tone detection on DSP

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 3 Introduction  Dual Tone Multi-Frequency (DTMF) is a widespread used signalling system: telephone services use commonly key strokes for options selection  DTMF is mainly used by touch-tone digital telephone sets which are an alternative to rotary telephone sets.  DTMF has now been extended to electronic mail and telephone banking systems  It is easily implemented on a DSP as small part of the tasks.

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 4  In a DTMF signaling system a unique combination of two normalized frequency tones  Two types of signal processing are involved:  Coding or generation.  Decoding or detection.  For coding, two sinusoidal sequences of finite duration are added in order to represent a digit. DTMF Signaling

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 5 Dual tone Generation A key stroke on « 9 » will generate 2 added tones, one at 852Hz low frequency and one at 1477Hz The 2 tones are Both audible.

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 6 Tones Generation  Dual tone generation can be done with 2 sinewave sources connected in parallel.  Different method can be used for such implementation:  Polynomial approximation  Look-up table  Recursive oscillator  DTMF signal must meet certain duration and spacing requirements  10 Digits are sent per second.  Sampling is done via a codec at 8Khz  Each tone duration must be >40msec and a spacing of 50ms minimum between two digits is required

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 7 2 and 9 digit signal sequence

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 8 DTMF generation implementation  Tone generation of a DTMF is generally based on two programmable, second order digital sinusoidal oscillators, one for the low f l the other one for the high f h tone.  Two oscillators instead of eight reduce the code size.  Coefficient and initial conditions are set for each particular oscillation Z -1 Z Z y(n) Low freq Highfreq

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 9 Digital oscillator parameters  2 pole resonator filter with 2 complexe poles on the unit cercle (unstable) Output signal: Y(n)= -a 1 y(n-1)-y(n-2) Initial conditions: Y(-1)=0 Y(-2)=-A sin(   )    f 0 /fs f 0 is the tone freq fs is the sampling freq

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 10 C55 sine generator code   Frames of data stream of 120 samples (15msec) long contain either DTMF tone samples or pause samples.   The encoder is either in idle mode, not used to encode digits or active and generates DTMF tones and pauses   The sine equation is implemented in assembly language: Mova 1 /2, T1; coded in Q15 Mpym*AR1+,T1,AC0;;AR1 y(n-1); AR1+1 y(n-2) sub*AR1-<<#16,AC0,AC;AC1= a1/2*y(n-1)-y(n-2) AddAC0,AC1; AC1= a1*y(n-1)-y(n-2) ||delay*AR1;y(n-2)=y(n-1) Movrnd(hi(AC1)),*AR1;y(n-1)=y(n) ; output signal pointer is AR1

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 11 Oscillator parameters at fs=8Khz

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 12 DTMF Tone Detection  Goertzel algorithm is the more efficient detection algorithm for a single tone.  To detect the level at a particular frequency the DFT is the most suitable method: The Goertzel algorithm is a recursive implementationThe Goertzel algorithm is a recursive implementation of the DFT, 16 samples of the DFT are computed for 16 tones16 samples of the DFT are computed for 16 tones See DTMF.pdf file for a complete description DTMF.pdf

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 13 Goertzel Algorithm Implementation  To implement the Goertzel algorithm the following equations are required: The only coefficient needed to compute output signal level is Cos(2  f k /f s )

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 14 Goertzel Algorithm Implementation Get N input samplex(n) Compute recursive part: W k (n), n=0 to N-1 For 8 frequencies Calculate X 2 (k) for 8 freq Tests:MagnitudeHarmonic Total Energy Output Digit

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 15  The value of k determines the tone we are trying to detect and is given by: Goertzel Algorithm Implementation  Where:f k =frequency of the tone. f s =sampling frequency. N is set to 205.  Then we can calculate coefficient 2cos(2*  *k/N).

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 16 Goertzel Algorithm Implementation FrequencykCoefficient(decimal)Coefficient(Q15) x479C x x4090* x4A70* x5EE7* x63FC* x68AD* x6D02* * The decimal values are divided by 2 to be represented in Q15 format (a 1 /2<1). N = 205 fs = 8kHz

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 17 w n = x(n) - w n-2 + a 1 *w n-1 ; 0  n<N-1 = sum1+ prod1 Goertzel Algorithm Implementation Where: a 1 = 2cos(2  k/N) and N=205 This gives 205 MACs+ 205 ADD |Yk(N) | 2 = Q 2 (N) + Q 2 (N-1) - a 1 *Q(N)*Q(N-1) The last computation gives the energy of the tone and is done with: 2 SQRS and one multiplication

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 18 Goertzel Algorithm Implementation void Goertzel (void) { static short delay; static short delay_1 = 0; static short delay_2 = 0; static int N = 0; static int Goertzel_Value = 0; int I, prod1, prod2, prod3, sum, R_in, output; short input; short coef_1 = 0x4A70;// For detecting 1209 Hz R_in = mcbsp0_read();// Read the signal in input = (short) R_in; input = input >> 4; // Scale down input to prevent overflow prod1 = (delay_1*coef_1)>>14; delay = input + (short)prod1 - delay_2; delay_2 = delay_1; delay_1 = delay; N++; if (N==206) { prod1 = (delay_1 * delay_1); prod2 = (delay_2 * delay_2); prod3 = (delay_1 * coef_1)>>14; prod3 = prod3 * delay_2; Goertzel_Value = (prod1 + prod2 - prod3) >> 15; Goertzel_Value <<= 4; // Scale up value for sensitivity N = 0; delay_1 = delay_2 = 0; } output = (((short) R_in) * ((short)Goertzel_Value)) >> 15; mcbsp0_write(output& 0xfffffffe);// Send the signal out return; } ‘C’ code

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 19 C54 assembly programme Goertzel tone detection routine ;Assume input signal x(n) is read through an I/O port at address 100h ;Output level Y(k)2 is sent to a port at address 1001h ;Scratch RAM reservation.bsswn,2;w(n-1) andw(n-2).bssxn,1; input signal xn.bssY,1; tone Energy.bssalpha,1;coefficient storage ; Constant initialisation alphap.word0x68ADh; a2/2 coefficient value at fs=8khz ; (prog memory) N.set205;value of N ;DSP modes initialisation SSBXFRCT;Product shift for Q15 format SSBXSXM;Sign extension during shift RSXBOVA; no overflowmode for A and B RSXBOVB

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 20 C54 assembly programme ;Data pointers Initialisation LD#wn,AR2; AR2 is pointing w(n-1) LD#xn,AR1 LD#Y,AR4 LD#alpha,AR3 MVPD#alphap,*AR3;Move alpha value to data RAM RPTZ#1;Accumulator A=0 STLA,*AR2+, w(0) and w(-1) are set to 0 MAR*AR2-; AR2 is pointing w(n-2)

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 21 Algorithm Core STM#N-1,BRC; repeat block number RPTBloop-1 PORTR100h,*AR1 LD*AR1,16,A; AccH=x(n) SUB*AR2,16,A;A=x(n)-w(n-2) MAC*AR2,*AR3,A;A=x(n)-w(n-2)+alphaw(n-1) MAC*AR2,*AR3,A;A=x(n)-w(n-2)+2alphaw(n-1) delay*AR2;w(n-2)=w(n-1) tap delay STHA,*AR2+;w(n-1)=w(n) tap delay Loop;end of loop

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 22 Energy calculation LD*AR2,16,A;A=w(N-1) MPYA*AR2-;T=w(N-1) B=w(N-1)^2 MPY*AR2,A;A=w(N)*w(N-1) LD*AR3,T;T=alphap MPYAA;A=alphap*w(N)*w(N-1) SUBA,1,B; substract with a left shift to ;obtain 2alphap ; B=w(N-1)^2-2alphap*w(N)*w(N-1) LD*AR2,T;T=w(N) MAC*AR2,B;B=w(N-1)^2-2alphap*w(N)*w(N-1)+w(N)^2 STH*AR4;save to Y PORTW *AR4,101h;copy output level

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 23 Universal Multifrequency Tone Generator and detector (UMTG)   This software module developed by SPIRIT Corp. for the TMS320C54x and TMS320C55X platform   It can be used into embedded devices for generating various telephone services used in intelligent network systems   Or as a simple tone generator for custom applications   It is fully compliant with TMS Algorithm standard rules See SPRU 639 and SPRU 638 AN

Copyright © 2003 Texas Instruments. All rights reserved. ESIEE, Slide 24 Follow on Activities  Application 7 for the TMS320C5416 DSK  Uses a microphone to pick up the sounds generated by a touch phone. The buttons pressed are identified using the Goerztel algorithm and their values displayed on Stdout. The frequency response of each Goertzel filter is given using Matlab.