Quiz 12: Digital Modulation and Modems
Telephone was a brilliant innovation; they are capable of carrying your input voice from one point in the world to the other point in a very short period of time. This is done by the flowing electricity along a given wire. Telephones are the mastermind behind the internet without which there is no chance of going online. The linkup of telephone along with a computer was done in the 20 th century. Digital technology is used in computers, which involves storing of information, to process it and then transmitting it mainly in the binary form. This is analogous to a telephone, which was somewhat analog. The information is sent down through the lines. They travel as continuous electric signals which vary with time. Question arises as how computers which are digital communicate with telephones which are analog and have the function of carrying speech. Then the modems came into existence. These are the devices that are capable of converting digital information to sound signals and then do the reverse process at the receiving end. If a user wants to connect to his ISP - Internet service provider using a telephone line. then he must have a modem that is used to modulate the signals which are in digital format to analog form. This is done to make the signals travel down the telephone line. After they reach the other end, these signals once again have to pass through another modem that does the demodulation process of turning the signals back to digital form. Therefore a modem is device which has the function (or) which can function as both modulator and demodulator.
Modulation is defined as a process, where a low frequency signal also called as the message signal is impressed to make it able to transmit onto a frequency wave which has higher frequency characteristics. Most commonly, a sine wave is used as a carrier signal in most of the cases. The three main parameters that can be modulated for a sine wave carrier are the amplitude, frequency and the phase. This is because the processes where characteristics of amplitude, frequency and phase of a higher frequency carrier signal are altered with respect to message signal (instantaneous value) is termed as modulation. If varying the amplitude of the carrier sine wave is considered, then the process is known as amplitude modulation. Likewise, if phase is considered, then it is phase modulation and in case of frequency variation, then the process is termed as frequency modulation. Therefore, the parameters amplitude, phase and frequency of a sine wave carrier can be modulated.
In general, the advantages behind using digital modulation is, it offers low bit error rate, high performance rate in conditions of multipath and fading. It offers spectral efficiency which is very high. The three most commonly used digital modulation techniques are the ASK (Amplitude shift keying), FSK (Frequency shift keying) and PSK (Phase shift keying). Amplitude shift keying - ASK comes under the category of AM, where the binary data which is available is represented in the form of changes (variation) in amplitude of a given signal. A modulated signal is often equipped with a high frequency carrier wave signal. Zero values are observed for LOW inputs and carrier outputs are observed for HIGH inputs, when binary signal is modulated using ASK. Frequency shift keying - FSK comes under the category of FM, where the carrier wave frequency changes in accordance with the discrete digital variations. High output is observed for binary input of HIGH and low frequency output is observed for input of LOW. The binary 1s are termed as MARK and the binary 0s are termed as Space frequency. Phase shift keying - PSK falls under the category of PM where the phase of the high carrier wave is varied in accordance to the input message signals at a specified time. Applications where PSK is used include biometrics, RFID and wireless LANs. Depending upon the way phase gets shifted; the PSK is further divided into two types. These are BPSK (Binary phase shift keying) and QPSK (Quadrature phase shift keying).
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