the telefon

The Telephone The telephone is one of the simplest devices we have in our house. It is so very simple because the telephone connection to our house has not changed in nearly a century. The telephone only contains three parts and they are all simple as shown in figure 1.2.  A switch to connect and disconnect the phone from the network. This switch is generally called the hook switch. It connects when you lift the handset.  A speaker - It is generally in a small size, 8-ohm speaker of some sort.  A microphone - In the past, telephone microphones have been as simple as carbon granules compressed between two thin metal plates. Sound waves from our voice compress and decompress the granules, changing the resistance of the granules and modulating the current flowing through the microphone. 1.2 Telephone Bandwidth In order to allow more long-distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3000 hertz. All of the frequencies in our voice below 400 hertz and above 3,400 hertz are eliminated. That's why someone's voice on a phone has a distinctive sound. 1.3 Digital Telephone The digital button is the latest technique of dialing. It uses the button to give signal for every one digit. The diagram in figure 1.3 shows the Dual Tone Multi Frequency (DTMF) type of dialing. The button on the phone is connected to a set of oscillators which produces a pair of tone on the local line whenever a button is being pressed. The tone will be detected at the main distributor and the digit will be confirmed. The detector circuits in the main distributor will confirm the tone within 33ms. 1.3.1 Progress Tones The various types of tones generated by the exchange to guide the users are : Dial Tone (DT). This is a 33 c/s continuous note and is applied to the line after the subscriber has lifted his handset and the switching equipment has allocated him an available outlet for this call to proceed. There would have been a physical limit on the number of calls an exchange could handle so if all equipment was already in use, the subscriber would not get a dial tone. Busy Tone (BT). A higher pitched note of 400 c/s interrupts to give a cadence of 0.75 seconds on, 0.75 seconds off. Busy tone indicates either that the called subscriber is already off-hook (busy) or that the route to the called subscriber is congested. In later systems, a slightly different cadence was introduced in order to distinguish between these two scenarios. A busy tone is made up of a 480-hertz and a 620-hertz tone, with a cycle of one and a half second on and one and a half second off. Number Unobtainable Tone (NUT). Identical pitch to the busy tone but continuous. This tone is used to indicate that a number is out of service, faulty or that a spare line has been dialed. Ring Tone (RT). A tone of 133c/s which interrupts in the same cadence as the ring current which rings the telephone bell at the called party's end : 0.4 seconds on, 0.2 seconds off.