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# A DTMF TONE DETECTION ALGORITHM WITH LOW COMPUTATION COMPLEXITY

IP.com Disclosure Number: IPCOM000007204D
Original Publication Date: 1994-Jun-01
Included in the Prior Art Database: 2002-Mar-05
Document File: 4 page(s) / 162K

Motorola

## Related People

Hugh Wang: AUTHOR

## Abstract

DTMF tone detection is an essential technique in telephone applications, such as voice mail, auto- matic answering service. To perform DTMF tone detection in real-time, it usually requires dedicated hardware to support, e.g., digital signal processor. The invented algorithm employs only addition and subtraction arithmetic operations, so that it can be implemented in real-time by ordinary micro processor.

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Technical Developments Volume 22 June 1994

A DTMF TONE DETECTION ALGORITHM WITH LOW COMPUTATION COMPLEXITY

by Hugh Wang

By the definition ofs(t), we have: gi(t) = sl(t) + s2(t) - sl(t-Ti) - s2(t-Tr) (2)

Let us define another function dij(t) = gi - gi(t-Vl (3)

If we select i = 1, then Ti = TI. Because Tl is the period of sl(t), so that sl(t) = sl(t-Tl). By (2) we have:
gl(t) = s2(t) - s2(t-TI) (4)

By the result of (2) and (4), we have:

dlJt) = s2(t) - s2(t-Tl) - s2(t-Tj) + s2(t-Tl-Tj) (5)

If we select j = 2, then Tj = T2. Because T2 is the period of s2(t), so that s2(t-T2), s2(t-Tl) = s2(t-Tl-T2). we have:

dl\$) = 0.

As showed in Figure 1, there are four frequen- cies representing rows and four frequencies repre- senting columns. Let us define Trl.. .Tr4 as the peri- ods for the Frequencies representing row 1 to row 4 respectively, and Tel.. .Tc4 as the periods for the hequencies representing column 1 to column 4. So we redefine gi(t) and diJ(t) as:

gi(t) = s(t) - s(t-Trr' (6)

di,,ft) = pi(t) - gi(t-TcIl (7)

If we calculate sixteen djJ(t) (i = 1.. .4j = 1.. .4), the one which i equals the row and j equals the column will result zero.

In practice, the tone signal may be distorted by noise, and the period T ofthe tone signal we used to calculate diJjt) may be inaccurate (e.g. for frequency f = 697 Hz, the period T = Fs/f = 8000/697 =
11.478. However, T must be an integer. The value of

57

ABSTRACT

DTMF tone detection is an essential technique in telephone applications, such as voice mail, auto- matic answering service. To perform DTMF tone detection in real-time, it usually requires dedicated hardware to support, e.g., digital signal processor. The invented algorithm employs only addition and subtraction arithmetic operations, so that it can be implemented in real-time by ordinary micro processor.

1. BACKGROUND

As illustrated in Figure 1, a DTMF tone is a signal with two frequency components and repre- senting a key in a phone keypad. To detect a DTMF tone is actually to detect the combination of two frequencies. Figure 2 is the DTMF signal of row 1 and column 1.

I

Figure 1. DTMF Frequencies

A DTMF signal S(t) can be described as a sum of two signals: s(t) = sl(t) + s2(t). Let fl be the frequency of sl(t) and f2 be the Frequency of s2(t). Then the period for sl(t) is Tl = l/fl and the period for s2(t) is T2 = l/f'2

Now let us define a function g/(t) = s(t) - s(t-Ti) (1)

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Technical Developments Volume 22 June 1994

T should be either 11 or 12). So that diJ(t) may not result zero. In order to overcome this effect, we de&

If the row index is a and the column index is 6,

then A,,b = MIN(A&

another function:

The Figure 3 shows ,the functions of dl l(t) and

i=l...4 j=1...4 (8) d2,l(t). As we see, dl l(t) has very low amplitude, IS0 meanwhile, the amplitude of d2 l(t) i...