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# TONE DECODING USING AN AUTOMATIC FREQUENCY TRACKER

IP.com Disclosure Number: IPCOM000008101D
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2002-May-17
Document File: 5 page(s) / 138K

Motorola

## Related People

Wolfgang Meier: AUTHOR [+2]

## Abstract

The main focus of the idea is to build a tone decoder designed for optimal performance in a DSP environment. This technology gets more important in future communication applications.

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MOTOROLA Technical Developments

TONE DECODING USING AN AUTOMATIC FREQUENCY TRACKER

by Wolfgang Meier and Norbert Roettger

INTRODUCTION

The main focus of the idea is to build a tone decoder designed for optimal performance in a DSP environment. This technology gets more important in future communication applications.

The decoder described here is based on a "one tap frequency Tracker" which consists of a one tap adaptive FIR filter. The adoption algorithm is based on a least mean square (LMS) adaptive filter. The typical LMS approach uses a fixed adaption gain constant, whilst the proposed implementation uses an exponential decaying gain factor.

The principle of the LMS adaptive filter is described in [ljto [6] below. .

Y(n)=A(n)*x(n)

A(n+l)=A(n)+ p*(R-y(n))

A(n+l)=A(n)*[l-p*x(n)]+p*R

Y(n)=A(n)*x@)

log [A (n + l)] = log [A(n)] +

[II PI

[31

r41

{log [RI - log [A (n) l x (n)l}

PI

161

P l

log[A(n+l)]=log[A(n)]*(l-~)-

log Lx (0 RI

P l

Figure 1 shows the principle block diagram of the tone decoder.

Fig. 1 One Tap Frequency Tracker with modified LMS algorithm (w = f(t))

S(n) is the input to the tone decoder. The func- and the Frequency of the input signal (s(n)) in case tional description of the above LMS algorithm is of processing the algorithm at a sampling frequency shown in [l] to [6]. The dependency between W(n) of 8kHz is shown in Figure 2.

0 Molomla, 1°C. 1997 168 March I997

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MOTOROLA Technical Developments

@

W = f(Frequency)

4000

3500

3000

2500

2000

1500

1000

500

Frequency

0

-1 0 1

W

Fig. 2 W(n) as a function of the frequency of S(n)

The frequency tracker is a frequency to voltage method used for tone decoding is that the decoded converter (see Figure 2). Figure 2 shows the linear frequency can be directly computed out of th...