projet mostapha (1)

8/2/2019 projet mostapha (1)

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Bdak tzed 2wal saf7a w lfahras w list of figures

IntroductionHistory records that 1876 was the year when Alexander Graham Bell patented the firstdevice able to transform voice into an electrical signal that could travel across a simple

wire. As this invention gathered momentum it became quickly obvious that a single home

could not be connected to every other home with a dedicated wire (although the first

devices were sold in pairs and ranchmen used barbed wire in the US to communicate

using the newly invented telephone).

Thus the first switchboards were soon deployed, with human operators physically wiring

callers with called people by plugging cords into jacks. Each place with a phone line was

given a number to identify it. The line and its associated telephones were connected to a

local switchboard. These switchboards were themselves interconnected all over the

country. When someone wanted to call another place, a long process started: the caller

picked up the phone, triggering an alarm on the local switchboard where an operator

plugged a headset to speak with the caller. The caller then requested to be connected with

a given phone number. The local operator checked whether the destination number was

attached to the local switch or had to be reached through a distant switchboard. In the

latter case, she called the destination switchboard, talked to its operator to ask her to

make the called phone ring. If the called user picked up the phone, the distant

switchboard operator connected a cord between the called line and the incoming line ofthe calling switchboard. The calling switchboard operator then also connected a cord to

the caller's line, eventually establishing a full voice path between the calling and the

called parties. In order to overcome this huge effort signaling is introduced.

The aim of this project is to introduce and discuss the Dual tone multi frequency

signaling.

The project is divided into two chapters:

The first chapter introduce the concept of signaling and discuss the main idea behindDcadic and DTMF signaling.

The second chapter explain the DTMF signaling, its component and the way its work.

Chapter two also contains some simulation that shows the way DTMF works
http://en.wikipedia.org/wiki/Alexander_Graham_Bellhttp://www.texasescapes.com/DelbertTrew/Barbed-Wire-Telephones.htmhttp://www.texasescapes.com/DelbertTrew/Barbed-Wire-Telephones.htmhttp://en.wikipedia.org/wiki/Alexander_Graham_Bell


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Chapter 1Signaling

1.1-IntroductionThe communication is the act to establish relations with somebody. To communicate, it

is essential that there are a transmitter (or sender) and a receiver (or recipient), and that a

visual or sound message (signals) is delivered first towards the second. Among the

multitude of signals, certain signals are used by all according to international conventions

(ex: measurements). They have a fixed significance. One of these conventions is DTMF.

In this chapter we will introduce telephone signaling. This chapter also explains the

principal of both Decdiac and DTMF signaling

1.2- Telephone signalingWhen the phone is used to place or receive a call, it must communicate with the

telephone company .signaling does this. The two types of signaling discussed are

supervisory signaling and Address Signaling.

Supervisory signaling is the means by which the caller and his telephone company notify

each other of call status. The different types of supervisory signaling are on-hook, off-

hook and ringing.

When allowing the handset to rest in the cradle (on-hook) opens the switch hook andprevents the current from flowing through the phone. Only the ringer is active when the

handset is on-hook. Lifting the handset off the cradle allows current to flow through the

phone, alerting the phone company that a subscriber is requesting to make a call. The

phone company, in turn, returns a dial tone to the phone to indicate that it is ready. When

someone is making a call, the telephone sends voltage to the ringer. The phone company

also sends a ring back tone to the caller, alerting the caller that it is sending voltage to the

recipients phone.

Address signaling can be one of two types, pulse or dual-tone multifrequency, DTMF. A

phone number can be dialed using two completely independent methods: Tones and DialPulses.

1.3-Decadic system and DTMFBefore DTMF was created, telephone networks used a dialing system called Decadic

(also known as Pulse Dial). The Decadic system was used extensively in modern

telephone networks to dial numbers, which were entered by the telephone companies


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users. The Decadic (Pulse Dialling) system used a series of clicks (which could be heard

through the speaker of the phone) to dial the numbers which were dialed via a keypad or

rotary dial. The clicking sounds were actually the connection of the phone line being

connected, disconnected, and reconnected again in a certain pattern. The Decadic (PulseDialing system was very useful, but was limited to the local exchange connections,

requiring an operator to connect long distance calls.

DTMF was being developed for the future of electronic telecommunications switching

systems, as opposed to the mechanical crossbar systems, which were currently in use at

the time. After DTMF was created, Decadic dialing was made pointless to continue, it

made no sense to continue using that particular dialing system in the equipment circuits

which the telephone exchanges were using at the time. Plans were then made to begin the

manufacture of DTMF controlled switching systems in the communications exchanges

and later standard customer owned telephones were upgraded to using DTMF circuits

rather than Decadic (Pulse Dial). After various tests were performed on the DTMFsystem throughout the 1960s (when DTMF became known as Touch-Tone), DTMF was

made official, and was then used as the main telecommunications dialing and switching

system, and remains that way to this day.

1.4DTMF Principal:A DTMF codec incorporates an encoder that translates key strokes or digit information

into dual-tone signals, as well as a decoder that detects the presence and the information

content of incoming DTMF tone signals. Each key on the keypad is identified uniquely

by its row frequency and its column frequency (see Figure 1).

Lfigures lezem ytzabato ykono wad7en w lezem ykon lcaption t7ton


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Touch-Tone Telephone Keypad

The full DTMF encoding standard defines four rows and four columns for a total of 16

two-tone combinations. Besides the numerals 0 to 9,a DTMF dial has *,#, A, B, C and D.

For example, pressing the digit 9(row 3 and column 3)produces 852 Hz and 1477Hz

tones simultaneously, while pressing a 1 produces 697Hz and 1209Hz tones. Each ofthese tones is composed of two pure sine waves of the low and high frequencies

superimposed on each other. These two frequencies explicitly represent one of the digits

on the telephone keypad. Thus generated signal can be expressed mathematically as

follows:

() () ()

Where AH is the amplitude of high frequency signal, fH is the high frequency, AL is the

amplitude of low frequency signal, fL is the low frequency.

The frequencies were chosen to avoid harmonics: no frequency is a multiple of another,

the difference between any two frequencies does not equal any of the frequencies, and the

sum of any two frequencies does not equal any of the frequencies.

1.5- conclusion


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Signaling is essential in modern communication system. It plays a critical role in

initializing communication. In this chapter, we explained the concept of signaling. This

chapter also presented the principal of Decdiac and DTMF signaling.

Chapter2

DTMF Tone Generation and Detection

2.1- Introduction

A DTMF codec incorporates an encoder that translates key strokes or digit informationinto dual-tone signals. These signals are generated by oscillators as well as a decoder that

detects the presence and the information content of incoming DTMF tone signals

(detector).

This chapter presents the DTMF tone generator. It also introduces the mechanism used in

the DTMF detector. Finally a simulation is done to explain the overall mechanism and

the condition where the DTMF works.

2.2 -DTMF Tone Generator

The encoder portion and tone generation part of a DTMF codec are based on two

programmable, second-order digital sinusoidal oscillators, one for the row tone and one

for the column tone. Two oscillators, instead of eight, facilitate the code and reduce the

code size. Of course, for each digit that is to be encoded, each of the two oscillators needs

to be loaded with the appropriate coefficient and initial conditions before oscillation can

be initiated. Since typical DTMF frequencies range from approximately 700 Hz to 1700

Hz, a sampling rate of 8 kHz for this implementation is within a safe area of the Nyquist

criteria. Table 1 specifies the coefficients and initial conditions necessary to generate the

DTMF tones. Figure 2 displays the block diagram of the digital oscillator pair, which

provides some theoretical background and a guideline for determining coefficients andinitial conditions for digital sinusoidal oscillators. Tone duration specifications state the

following: 10 digits/sec is the maximum data rate for touch-tone signal. For a 100-msec

time slot, the duration for the actual tone is at least 45 msec and not longer than 55 msec.

The tone generator must be quiet during the remainder of the 100-msec time slot. The

quiet during are necessary to discriminate between two or more identical digits entered

successively.


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lcaption nafs l7ajem bkel lfigurs w ltabels mesh t3melon sowar.

(Coefficient and initial conditions for sinusoidal oscillators)


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For the following description of the program flow, it is helpful to consult the flowchart

shown in (flowchart eli ta7t maba3rif ra2mo). Essentially, the series of keypad entries are

translated into a series of dual-tones of certain duration that are interrupted by pauses of

certain duration. Later, the dual-tones enable the decoder to identify the associated digits.The program flow, therefore, incorporates two tasks that are swapped after certain time

intervals. One task (the tone task) generates dual-tone samples and the other (the quiet

task) generates pause samples. Each task is assigned a certain duration that is controlled

by a timer variable. At the end of each task, the task has to initialize the timer variable

and the task-name (tone or quiet) for the next task to be invoked. At the end of the quiet

task, one very important component is added: A new digit is retrieved from the digit

buffer and is unpacked. Unpacking means that the digit is mapped to the row/column

tone properties (oscillator coefficients, initial conditions) and pointers are loaded,

pointing to the appropriate locations in the oscillator property table. The entire program

flow is synchronized to the receive-interrupt service routine, which provides a perfectclock for real-time processing and constant sample output. On completion of the

RINT_ISR, the task scheduler is invoked, which determines the particular task (tone or

quiet) that needs to be executed. Both tone task and quiet task check on the timer variable

to determine if the end of the task duration is already reached. If not, a tone or quiet

sample, respectively, is generated. If the end of the task duration is reached, the next task

name and duration is initialized and starts to execute with the completion of the next

RINT_ISR. The quiet task, additionally, unpacks the next digit at the end of its duration.


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Lezim el caption yotzabbat :Flowcharts of the DTMF Encoder Implementation

2.2 -DTMF Tone Detector:The task to detect DTMF tones in an incoming signal and to convert them into actual

digits is certainly more complex than the encoding process. The decoding process is by

its nature a continuous process, meaning it needsto continually search an incoming data

stream for the presence of DTMF tones.


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2.2.1- Collecting Spectral InformationThe Goertzel algorithm is the basis of the DTMF detector. This method is a very effective and

fast way to extract spectral information from an input signal. This algorithm essentially utilizes

two-pole IIR (infinite impulse response) type filters to compute DFT values effectively. It is,thereby, a recursive structure (always operating on one incoming sample at a time), as compared

to the DFT (or FFT) which needs a block of data before being able to start processing. The IIR

structure for the Goertzel filter incorporates two complex-conjugate poles and facilitates thecomputation of the difference equation by having only one real coefficient. For the actual tone

detection, the magnitude (here, squared magnitude) information of the DFT is sufficient. After a

certain number of samples N (equivalent to a DFT block size), the Goertzel filter output

converges towards a pseudo DFT value vk(n), which can then be used to determine the squaredmagnitude. See Figure 4 for a short mathematical description of the algorithm.

Hyde lezem terja3 tenkatab

The Goertzel algorithm is much faster than a true FFT, as only few of the set of spectral linevalues are needed and only for those values are filters provided. Squared magnitudes are needed

for eight row/column frequencies and for their eight-second harmonics. The second harmonics

information later enables discrimination of DTMF tones from speech or music. Table 2 containsa list of frequencies and filter coefficients. The choice of N is mainly driven by the frequency

resolution needed, which sets a lower boundary. N also is chosen so that (k/N)fs most accurately

coincides with the actual DTMF frequencies (see Table 1) assuming ks are integer values and fs

is a sampling frequency of 8 ksps.


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Modifications to the Goertzel Algorithm

The coefficients of Table 2 reflect the coefficients needed to recursively

compute the true DFT. The evenly spaced frequency bins of a true DFT

present an inherent drawback in the DTMF tone-detection process. The

DFT frequency bins mostly deviate from the true DTMF frequency by

an amount in the range of up to 2% off center frequency. To be able to

meet the acceptable bandwidth specifications, a modification of the

algorithm departs from the true DFT and tunes frequency bins exactly

with the DTMF tone frequencies. This modification gives up the DFT

property of evenly spaced frequency bins; and with that, takes two

calculated risks: (1) A frequency bin is possibly moved inside of the

mainlobe of its neighboring frequency bin. Therefore, neighboring

frequency bins can affect each other. Note that the mainlobe of the

continuous magnitude spectrum of a rectangular windowed sinewave(window is N wide) is exactly the distance of 2 DFT frequency bins. (2)

This is especially true when column frequencies and 2nd harmonics of

row frequencies lie close to one another. Note that column frequencies

and 2nd harmonics of row frequencies share the same frequency band.


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2.3-Simulation

We have done the simulation of the DTMF with the Matlab Program.

2.3.1 Components

The DTMF simulation is composed of 3 basic elements. The DTMF generator, the medium or the channel

where the signal is transmitted and the DTMF receiver.

In the channel we can control the noise power. The output of the receiver is connected to a display so we

can visualize the values detected.

We note that the channel is connected also to a spectrogram so we can visualize the frequency spectrum

of the signal transmitted.

2.3.2 Procedure

In this figure that is snipped from the simulation on Matlab, we generated the values:

[ 1 2 3 4 5 6 7 8 9 9 0] with the noise power set to 0.05 dbm. The DTMF receiver detects all the values

without errors as it appears on the display. The frequency spectrum is signal generated is illustrated in fig

2.3.b


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establishment between a user and the central office. Furthermore, researchers are working

to develop these techniques and techniques of signaling in the core network

References:

Lezem tzed references w t7ot 2r2am 2elon

[1]Dtmf tone generation and detection using the tms320c54x

[2]J. Nagi*, S. K. Tiong, K. S. Yap, S. K. Ahmed

Department of Electronics and Communication Engineering

College of Engineering, Universiti Tenaga Nasional

[3]

DTMF Detection and Generation Virtual

Peripheral Module

Application Note 41

August 2000

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