Understanding Telephone Basics

Islamic University of Gaza Faculty of Engineering Communications Engineering I (Lab.) Electrical Department

Experiment # (2) Understanding Telephone Basics

Experiment Objectives:  Understanding Telephone Basics.  Study Pulse dialing and Tone dialing "Dual Tone Multi-Frequency"  Use MATLAB to generate DTMF signals, decode DTMF signals

Experiment theory: A telephone typically consists of the following components:  Handset containing a transmitter and receiver.  Switch hook.  Dialer (either rotary or touch-tone)  Ringer.

A typical telephone does the following:  Requests service from the network.  Performs dialing functions.  Performs a notification function (it rings)  Provides answer and disconnect supervision.  Converts outgoing speech to electrical signals, and vice versa.

Understanding Telephony Signaling: For a telephone call to be completed, several forms of signaling must occur:  Access signaling  Station loop signaling  Address signaling

1. Access Signaling The first type involves access signaling, which determines when a line is off hook or on hook. When the handset is on its cradle, the phone is referred to as being on hook. When a telephone is on hook, the two wires do not touch, so the circuit (loop) is open and no current flows. When the handset is out of its cradle, it goes off hook. The wires touch, closing the loop, allowing current to flow through the two conductors that connect the phone to the network, sending an "off-hook" signal to the switch. To place a call, the phone must be off hook. To receive a call it must be on hook.

Figure 2.1 Access signaling

2. Station Loop Signaling: When initiating a call, the telephone is placed off-hook. The connected network switch senses the off-hook condition (current flow) and provides an audible Ready signal known as 'Dial Tone'. The tones used in the telephone system are:  Dial tone : consist of a summation of two low distortion sine waves. Precise dial tone consists of 350 and 440Hz.  Busy tone: Precise busy signal consists of 480 and 620 Hz.  Call waiting: 440 Hz.

3. Address Signaling In response to the audible prompt of the dial tone, the caller can request connection to another telephone by transmitting the address (telephone number) of the requested telephone (sometimes referred to as the called party identification number) to the network. This is known as address signaling. Telephones generally use two basic types of address signaling: pulse and tone.  Pulse dial (rotary dialing): Rotary dial phones represent the digit being dialed by momentarily stopping the current flow when the user turns the circular dial. For example, the circuit is broken three times, which creates three pulses in the current, to dial the digit "3." , and for zero, the line is "disconnected" ten times.

Figure 2.2 Pulse dialing

 Tone dial (dual tone multi-frequency, or DTMF; the method used by pushbutton telephones). DTMF is the most commonly used signaling system today. Touch-tone telephone dialing is an example of every day use of Fourier analysis.

Telephone signaling is based on encoding keypad digits using two sinusoids of different frequencies, hence the name DTMF. Each digit is represented by a low frequency and a high frequency sinusoid.

The DTMF technique outputs distinct representation of 16 common alphanumeric characters (0-9, A-D, *, #) on the telephone. The lowest frequency used is 697Hz and the highest frequency used is 1633Hz, The Column corresponds to the high frequency, with the row corresponding to the low frequency. The pound digit, #, would be transmitted as the sum of a 1477 Hz sinewave and a 941 Hz sinewave.

So, the DTMF signal can be defined as:

Where Wl and Wh are the low and high frequencies of the sine waves being used, A and B are the amplitude of the signals and φ is the initial phase shifts.

The tone generated by "1" button is shown in figure 2.3:

Figure 2.3 Time domain and spectrum of "1" button

Recording of an11-digit telephone number is shown in figure 2.4:

Figure 2.4 11-digits telephone number

Figure 2.5 shows the magnitude of the FFT, finite fourier transform, which is the key to determining the individual digits.

Figure 2.5 The spectrum of the 11 digits in figure 2.4

There are seven peaks, corresponding to the seven basic frequencies. This overall FFT shows that all seven frequencies are present some place in the signal, but it does not help determine the individual digits. To determine the individual digits, break the signal into eleven equal segments and analyze each segment separately. Figure 2.6 is the display of the first segment.

Figure 2.6 the display of the first segment (time domain and spectrum)

For this segment, there are only two peaks, indicating that only two of the basic frequencies are present in this portion of the signal. These two frequencies come from the '1' button.

Figure 2.7 DTMF circuits (Generate and Decode)

Note: Instead of driving a speaker, you can drive an infrared (IR) emitter. You would place a IR detector in the TONE IN line of the decoder, or you can drive FM transmitter and place FM receiver in the TONE IN line of the decoder.

Experiment items:

Experiment 1:

1. Read the following files using MATLAB. (dial_tone.wav, busy.wav). 2. Use the command 'sound' to listen the tones. 3. Find the Fourier Transform for each signal. 4. Plot each signal with its magnitude spectrum in one figure. 5. From the plot, determine the frequencies for dial tone signal and for busy tone.

Experiment 2:

1. Use Matlab to generate the 12 DTMF signals of telephone keypad, and plot these signals clearly. 2. Find Fourier Transform for the 12 DTMF signals, and plot the magnitude spectrum for each signal. (From the plot, notice the frequency components for each signal).

Experiment 3: 1. Using Matlab, read the files x1.wav and x2.wav 2. Use the command 'sound' to listen to the tones. 3. Find the fourier transform for each signal. 4. Plot each signal with its magnitude spectrum in one figure. 5. From the plot, determine the button in telephone keypad that generated x1.wav and x2.wav.

Experiment 4: 1. Using Matlab, read the file dtmf.wav. 2. Use the command 'sound' to listen to the tone. 3. Find the fourier transform of the signal. 4. Plot the signal with its magnitude spectrum in one figure. 5. From the plot, determine how many digits does this signal act? can you determine the dialed number from the plot? If yes, explain how you can determine the dialed number.