Q. Analog communication systems?

An analog message is a continuum of possible amplitudes at any given time, and analog signals are continuous in time and in amplitude, such as audio and video signals. When the message to be sent over a communication system is analog, we refer to the system as analog. The transmitted waveform must be some function of the message so that the receiver could decipher the message. Usually the transmittedwaveformis the result of varying either the amplitude, phase, or frequency of a basic signal called a carrier. Combinations of amplitude, phase, and frequency variations are also possible.

The carrier usually is sinusoidal of the form A_c cos (ω_ct + φ_c), where A_c, φ_c, and f_c = ω_c/2π are the carrier's amplitude, phase, and frequency, respectively.WhenAis varied as a linear function of the message, amplitude modulation (AM) occurs. In phase modulation (PM) a phase term that is a linear function of themessage is added to the carrier.When the added phase is a linear function of the integral of the message, the result is known as frequency modulation (FM). Note that the carrier's frequency in FM is a linear function of the message, because instantaneous angular frequency is the time derivative of instantaneous phase. Thus, FM and PM are closely related. Every communication system has a modulator at the transmitting station to structure the transmitted waveform, and a demodulator (detector) at the receiving end to recover the message from the received signal. Radio (AM and FM) and television broadcasting are the most familiar forms of communication through analog signal transmission. The FCC in the United States regulates the carrier-frequency assignments in order to minimize the interference between nearby stations. Commercial AM radio broadcasting utilizes the frequency band of 535 to 1605 kHz for the transmission of voice and music. The carrier-frequency allocations range from 540 kHz to 1600 kHz, with 10-kHz spacing. Each station can occupy a channel bandwidth of only 10 kHz centered on its carrier. Even though the basebandmessage signal is limited to a bandwidth of about 5 kHz, the AMbroadcasting system adequately meets the need for low-cost mass communication and general audio entertainment, in spite of lacking high-fidelity behavior.