Modem is a device that converts between analog and digital signals. Digital signals, which are used by computers, are made up of separate units, usually represented by a series of 1's and 0's. Analog signals vary continuously; an example of an analog signal is a sound wave. Modems are often used to enable computers to communicate with each other across telephone lines. A modem converts the digital signals of the sending computer to analog signals that can be transmitted through telephone lines. When the signal reaches its destination, another modem reconstructs the original digital signal, which is processed by the receiving computer. Voice modem is just a advance functions.
Voice modem is a modem that can be used without sound card and help in voice communication or video & voice communication. Voice modem has also another function i.e. it can store faxes and voice messages when the person is not present near the computer. It also guides the caller to choose between fax or voice message. A numberic keys are specially asigned for this purpose. Some modem have a function that they can receive the fax while the computer is turned off. The fax is stored in modem's memory and when the computer is turned on, it is downloaded in the computer.
How the voice modem works? It converts a digital signal to an analog one, the modem generates a carrier wave and modulates it according to the digital signal. The kind of modulation used depends on the application and the speed of operation for which the modem is designed. For example, many high-speed modems use a combination of amplitude modulation, where the amplitude of the carrier wave is changed to encode the digital information, and phase modulation, where the phase of the carrier wave is changed to encode the digital information. The process of receiving the analog signal and converting it back to a digital signal is called demodulation. The word "modem" is a contraction of its two basic functions: modulation and demodulation.
What is full/half duplex in modem? When both modems are transmitting data to each other simultaneously, the modems are operating in full duplex mode; if only one modem can transmit at a time, the modems are operating in half duplex mode. During transfer of data, most of the data is lost due to disturbance in telephone lines and old copper cables. For voice or video conference a full duplex is strongly recommended especially for the video conference.
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