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DTMF Encoding and Decoding In C
by Mr. Blue
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Introduction
------------
DTMF tones are the sounds emitted when you dial a number on your touch
tone phone. Modems have traditionally been the device used to generate
these tones from a computer. But the more sophisticated modems on the
market today are nothing more than a DSP (digital signal processor) with
accompanying built-in software to generate and interpet analog sounds into
digital data. The computers sitting on your desk have more cpu power,
a more complex OS, and very often a just as sophisticated DSP. There is
no reason you can not duplicate the functionality of a modem from right
inside of unix software, providing you with a lot easier to understand and
modify code.
In this article I provide the source code to both encode and decode
DTMF tones. There are numerous uses for this code, for use in unix based
phone scanning and war dialing programs, voice mail software, automated
pbx brute force hacking, and countless other legitimate and not so
legitimate uses.
I will not go into depth explaining the underlying mathematical
theories behind this code. If you are of a sufficient math background I
would encourage you to research and learn about the algorithms used from
your local college library; it is not my intent to summarize these
algorithms, only to provide unix C code that can be used on its own or
expanded to be used as part of a larger program.
Use the extract utility included with Phrack to save the individual
source files out to the dtmf/ directory. If you find this code useful, I
would encourage you to show your appreciation by sharing some of your own
knowledge with Phrack.
<++> dtmf/detect.h
/*
*
* goertzel aglorithm, find the power of different
* frequencies in an N point DFT.
*
* ftone/fsample = k/N
* k and N are integers. fsample is 8000 (8khz)
* this means the *maximum* frequency resolution
* is fsample/N (each step in k corresponds to a
* step of fsample/N hz in ftone)
*
* N was chosen to minimize the sum of the K errors for
* all the tones detected... here are the results :
*
* Best N is 240, with the sum of all errors = 3.030002
* freq freq actual k kactual kerr
* ---- ------------ ------ ------- -----
* 350 (366.66667) 10.500 (11) 0.500
* 440 (433.33333) 13.200 (13) 0.200
* 480 (466.66667) 14.400 (14) 0.400
* 620 (633.33333) 18.600 (19) 0.400
* 697 (700.00000) 20.910 (21) 0.090
* 700 (700.00000) 21.000 (21) 0.000
* 770 (766.66667) 23.100 (23) 0.100
* 852 (866.66667) 25.560 (26) 0.440
* 900 (900.00000) 27.000 (27) 0.000
* 941 (933.33333) 28.230 (28) 0.230
* 1100 (1100.00000) 33.000 (33) 0.000
* 1209 (1200.00000) 36.270 (36) 0.270
* 1300 (1300.00000) 39.000 (39) 0.000
* 1336 (1333.33333) 40.080 (40) 0.080
**** I took out 1477.. too close to 1500
* 1477 (1466.66667) 44.310 (44) 0.310
****
* 1500 (1500.00000) 45.000 (45) 0.000
* 1633 (1633.33333) 48.990 (49) 0.010
* 1700 (1700.00000) 51.000 (51) 0.000
* 2400 (2400.00000) 72.000 (72) 0.000
* 2600 (2600.00000) 78.000 (78) 0.000
*
* notice, 697 and 700hz are indestinguishable (same K)
* all other tones have a seperate k value.
* these two tones must be treated as identical for our
* analysis.
*
* The worst tones to detect are 350 (error = 0.5,
* detet 367 hz) and 852 (error = 0.44, detect 867hz).
* all others are very close.
*
*/
#define RANGE 0.1 /* any thing higher than RANGE*peak is "on" */
#define THRESH 100.0 /* minimum level for the loudest tone */
#define FLUSH_TIME 100 /* 100 frames = 3 seconds */
<-->
<++> dtmf/detect.c
/*
* detect.c
* This program will detect MF tones and normal
* dtmf tones as well as some other common tones such
* as BUSY, DIALTONE and RING.
* The program uses a goertzel algorithm to detect
* the power of various frequency ranges.
*
* input is assumed to be 8 bit samples. The program
* can use either signed or unsigned samples according
* to a compile time option:
*
* cc -DUNSIGNED detect.c -o detect
*
* for unsigned input (soundblaster) and:
*
* cc detect.c -o detect
*
* for signed input (amiga samples)
* if you dont want flushes, -DNOFLUSH
*
* Tim N.
*/
/*
* calculate the power of each tone according
* to a modified goertzel algorithm described in
* _digital signal processing applications using the
* ADSP-2100 family_ by Analog Devices
*
* input is 'data', N sample values
*
* ouput is 'power', NUMTONES values
* corresponding to the power of each tone
*/
calc_power(data,power)
#ifdef UNSIGNED
unsigned char *data;
#else
char *data;
#endif
float *power;
{
float u0[NUMTONES],u1[NUMTONES],t,in;
int i,j;
/*
* detect which signals are present.
*
* return values defined in the include file
* note: DTMF 3 and MF 7 conflict. To resolve
* this the program only reports MF 7 between
* a KP and an ST, otherwise DTMF 3 is returned
*/
decode(data)
char *data;
{
float power[NUMTONES],thresh,maxpower;
int on[NUMTONES],on_count;
int bcount, rcount, ccount;
int row, col, b1, b2, i;
int r[4],c[4],b[8];
static int MFmode=0;
if(bcount == 2) { /* MF */
/* b1 has upper number, b2 has lower */
switch(b1) {
case 7: return( (b2==6)? D2426: -1);
case 6: return(-1);
case 5: if(b2==2 || b2==3) /* KP */
MFmode=1;
if(b2==4) /* ST */
MFmode=0;
return(DC11 + b2);
/* MF 7 conflicts with DTMF 3, but if we made it
* here then DTMF 3 was already tested for
*/
case 4: return( (b2==3)? D0: D7 + b2);
case 3: return(D4 + b2);
case 2: return(D2 + b2);
case 1: return(D1);
}
}
return(-1);
}
if(on_count == 0)
return(DSIL);
return(-1);
}
read_frame(fd,buf)
int fd;
char *buf;
{
int i,x;
for(i=0; i<N; ) {
x = read(fd, &buf[i], N-i);
if(x <= 0)
return(0);
i += x;
}
return(1);
}
/*
* read in frames, output the decoded
* results
*/
dtmf_to_ascii(fd1, fd2)
int fd1;
FILE *fd2;
{
int x,last= DSIL;
char frame[N+5];
int silence_time;
if(x != DSIL && x != last &&
(last == DSIL || last==D24 || last == D26 ||
last == D2426 || last == DDT || last == DBUSY ||
last == DRING) ) {
fputs(dtran[x], fd2);
#ifndef NOFLUSH
fflush(fd2);
#endif
}
last = x;
}
}
fputs("\n",fd2);
}
main(argc,argv)
int argc;
char **argv;
{
FILE *output;
int input;
input = 0;
output = stdout;
switch(argc) {
case 1: break;
case 3: output = fopen(argv[2],"w");
if(!output) {
perror(argv[2]);
return(-1);
}
/* fall through */
case 2: input = open(argv[1],0);
if(input < 0) {
perror(argv[1]);
return(-1);
}
break;
default:
fprintf(stderr,"usage: %s [input [output]]\n",argv[0]);
return(-1);
}
dtmf_to_ascii(input,output);
fputs("Done.\n",output);
return(0);
}
<-->
<++> dtmf/gen.c
/* -------- local defines (if we had more.. seperate file) ----- */
#define FSAMPLE 8000 /* sampling rate, 8KHz */
/*
* FLOAT_TO_SAMPLE converts a float in the range -1.0 to 1.0
* into a format valid to be written out in a sound file
* or to a sound device
*/
#ifdef SIGNED
# define FLOAT_TO_SAMPLE(x) ((char)((x) * 127.0))
#else
# define FLOAT_TO_SAMPLE(x) ((char)((x + 1.0) * 127.0))
#endif
/*
* take the sine of x, where x is 0 to 65535 (for 0 to 360 degrees)
*/
float mysine(in)
short in;
{
static coef[] = {
3.140625, 0.02026367, -5.325196, 0.5446778, 1.800293 };
float x,y,res;
int sign,i;
if(in < 0) { /* force positive */
sign = -1;
in = -in;
} else
sign = 1;
if(in >= 0x4000) /* 90 degrees */
in = 0x8000 - in; /* 180 degrees - in */
x = in * (1/32768.0);
y = x; /* y holds x^i) */
res = 0;
for(i=0; i<5; i++) {
res += y * coef[i];
y *= x;
}
return(res * sign);
}
/*
* play tone1 and tone2 (in Hz)
* for 'length' milliseconds
* outputs samples to sound_out
*/
two_tones(sound_out,tone1,tone2,length)
int sound_out;
unsigned int tone1,tone2,length;
{
#define BLEN 128
sample cout[BLEN];
float out;
unsigned int ad1,ad2;
short c1,c2;
int i,l,x;
/*
* silence on 'sound_out'
* for length milliseconds
*/
silence(sound_out,length)
int sound_out;
unsigned int length;
{
int l,i,x;
static sample c0 = FLOAT_TO_SAMPLE(0.0);
sample cout[BLEN];
x = 0;
l = (length * FSAMPLE) / 1000;
for(i=0; i < l; i++) {
cout[x++] = c0;
if (x==BLEN) {
write(sound_out, cout, x * sizeof(sample));
x=0;
}
}
write(sound_out, cout, x);
}
/*
* play a single dtmf tone
* for a length of time,
* input is 0-9 for digit, 10 for * 11 for #
*/
dtmf(sound_fd, digit, length)
int sound_fd;
int digit, length;
{
/* Freqs for 0-9, *, # */
static int row[] = {
941, 697, 697, 697, 770, 770, 770, 852, 852, 852, 941, 941 };
static int col[] = {
1336, 1209, 1336, 1477, 1209, 1336, 1477, 1209, 1336, 1447,
1209, 1477 };
/*
* take a string and output as dtmf
* valid characters, 0-9, *, #
* all others play as 50ms silence
*/
dial(sound_fd, number)
int sound_fd;
char *number;
{
int i,x;
char c;
for(i=0;number[i];i++) {
c = number[i];
x = -1;
if(c >= '0' && c <= '9')
x = c - '0';
else if(c == '*')
x = 10;
else if(c == '#')
x = 11;
if(x >= 0)
dtmf(sound_fd, x, 50);
silence(sound_fd,50);
}
}
main()
{
int sfd;
char number[100];
sfd = open(SOUND_DEV,O_RDWR);
if(sfd<0) {
perror(SOUND_DEV);
return(-1);
}
printf("Enter fone number: ");
gets(number);
dial(sfd,number);
}
<-->
<++> dtmf/Makefile
#
# Defines:
# UNSIGNED - use unsigned 8 bit samples
# otherwise use signed 8 bit samples
#
