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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_Bell8/2/2019 projet mostapha (1)
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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