Modem

A modem is a signal modulator and demodulator, a device capable of converting digital signals into analog ones, but also analog into digital.

Explained in a simple way, this input / output peripheral allows different computers to be connected to each other through the telephone or television line, which emits analog signals that are then converted into digital, to access the Internet.

Specifically, the modulator outputs the carrier signal. The modulated signal, which contains the information to be sent, modifies the carrier and a new signal is obtained that includes the content to be transported. Therefore, once the signal reaches the destination, the demodulator removes the part of the carrier and only keeps the modulated information.

Types of modulation

Depending on whether the modem is digital or analog, a modulation of the same nature is used.

There are three types of digital modulation:

• ASK, (Amplitude Shift Keying): The amplitude of the carrier is modulated at levels corresponding to the input binary digits 1 or 0.
• FSK, (Frequency Shift Keying, frequency shift keying): The carrier frequency is modulated by adding or subtracting a shift frequency that represents binary digits 1 or 0. It is the type of modulation common in low-speed modems in which two states of the binary signal are transmitted as two different frequencies.
• PSK (Phase Shift Keying): A type of modulation where the transmitted carrier shifts a certain number of degrees in response to the configuration of the data. Biphasic modems, for example, use 180º offsets to represent the binary digit 0.

In analog channels there are disturbances that the modem must face in order to transmit the information.

These disorders can be listed as:

• Distortions, deformations and echoes
• Random and impulsive noises
• Interference

There are three types of analog modulation:

• AM Modulated Amplitude: The amplitude of the carrier is varied by means of the amplitude of the modulator.
• FM Modulated Frequency: The frequency of the carrier is varied by the amplitude of the modulator.
• PM Phase Modulation. Phase modulation: in this case the parameter that is varied from the carrier is the phase of the signal, mathematically it is almost identical to the frequency modulation. As in AM and FM, it is the amplitude of the modulator that is used to affect the carrier.