Telephone > Decadic Pulse Telephone and DTMF Telephone
In a Decadic Pulse Dialing, also called Loop Disconnect Dialing, a Direct-Current Pulse Train, representing each Digit, is produced by interrupting a continous Signal according to a defined Ratio. Figure 1 shows a Decadic Pulse Telephone.
A Decadic Pulse Telephone is equipped with a Rotary Dial with a Finger Plate over it. The Rotary Dial is designed to send Electrical Pulses. Figure 2 shows the Circuit of a Pulse Telephone. The 3 spring contacts: the Impulsing Contact (ICT), the Bypass Switch 1 (BP1) and the Bypass Switch 2 (BP2) are mounted inside the Rotary Dial.
Making a Call:
When the Customer lifts the Handset (Off Hook), the Hook Switch is closed and a DC Loop between the Telephone Exchange and the Customer is formed, Figure 1. Therefore, a Continous Current DC flows through the Loop.
Receiving a Call:
When the Handset is on the Cradle, the Telephone is said to be "On the Hook", or ready to receive a Call, Figure 3. The Hook Switch is opened and the path to the right part of the circuit is disconnected.
The DTMF Telephone, as its name implied, is based on a concept known as Dual Tone Multi-Frequency (DTMF), Figure 1. It generates a combination of two Tones for each Dialed Digit, and sends the Digits to the Telephone Exchange by Hearable Tones instead of Electrical Pulses as in Decadic Pulse Telephone.
The DTMF Telephone is equipped with a Pushbutton Dial, in which 10 Dialing Digits (0 through 9), the Star " " and the Pound "#" symbols are assigned to specific Pushbuttons. The Pushbuttons are arranged in an two-dimensional array with four Rows and three Columns, as shown in Figure 2. Each Row and Each Columns is assigned a Tone of a specific Frequency, the Columns having Tones of higher Frequencies and the Rows having Tones of lower Frequencies. When a Button is pushed, a Dual-Tone Signal is generated. This Signal is a combination of Two Tones of different Frequencies, one from the Lower Frequency Group and the other from the Upper Frequency Group, and it is the reason for calling it "Dual Tone Multi-Frequency". In this way, 7 (4 + 3) Tones of different Frequencies are used to generate 12 (4 x 3) combinations. For example, pushing the Button "5", the Tones of 770 Hz and 1336 Hz are transmitted together to the Telephone Exchange. This Signal is decoded by the Telephone Exchange in order to determine which Digit was Dialed.
Making a Call:
When the Customer lifts the Handset (Off Hook), the Hook Switch is closed and a Circuit connection between the Telephone Exchange and the Customer Telephone is formed.
Generation of Signaling Tones
Receiving a Call:
When the Handset is on the Cradle, the Telephone is said to be "On the Hook", or ready to Receive a Call. That is, the Hook Switch in Figure 3 is opened and the path to the right part of the Circuit is disconnected.
Whenever there is an Incoming Call, An AC Ringing Signal from the Telephone Exchange is transmitted to the Telephone. The Ringing Signal is generally 10 mA AC Signal, with a Frequency between 20 and 25 Hz, that activates a pair of Electromagnet which operates a small Hammer to strike the Bells.
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Telephone > Decadic Pulse Telephone and DTMF Telephone