Transmission of data for short ranges does not need any complex procedures.
Example an audio frequency of range 20 to 20,000 Hz when transmitted over long distances some techniques are used to transmit the information without any loss.
“For long-distance broadcast, the low-frequency baseband signal (input signal) is superimposed onto a high-frequency radio signal by a process called modulation”. (Tamil Nadu Text Book Corporation #)
Carrier signal or radio signal which is a high-frequency signal is employed to carry the baseband signal in the modulation process.
Carrier signal’s frequency is very high as a result it can be transmitted very long with less attenuation.
The carrier signal is mostly a sine wave.
In the Modulation process, three characteristics of the carrier signal are modified by the baseband signal.
Frequency of radio waves and its characteristics are mentioned below:
Those three characteristics of radio waves or signals are Amplitude, Phase, Frequency.
Amplitude Modulation (AM)
“If the amplitude of the carrier signal is modified according to the instantaneous amplitude of the baseband signal, then it is called amplitude modulation.” (Tamil Nadu Text Book Corporation #).
In AM, the phase and frequency are constant and amplitude is modified.
AM is used in Radio transmission and also in TV broadcasting.
Frequency of radio waves
Advantages of AM
Transmission and Reception is easy
Requires lesser bandwidth
The operating cost is less.
Disadvantages of AM
High noise level
The operating range is small
Low efficiency
Frequency Modulation(FM)
In FM, the frequency of the carrier signal is modified as per the instantaneous amplitude of the baseband signal.
In FM the amplitude and phase of the carrier signal remain constant.
When the baseband amplitude has increased the frequency of the carrier also gets increase and vice versa.
Advantages of FM
The noise level is decreased and increases in signal-noise ratio.
The operating range is high.
As all the transmitted power is used, the efficiency of the transmission is high.
FM reproduces better audio quality as the FM utilizes the entire frequencies in the human audible range.
Disadvantages of FM
A wider channel is required for FM
The transmitter and receivers of FM are expensive
The area coverage is less compared to AM
Phase Modulation
“The instantaneous amplitude of the baseband signal modifies the phase of the carrier signal keeping the amplitude and frequency constant is called Phase Modulation”. (Tamil Nadu Text Book Corporation #).
The phase modulation is similar to FM, except the phase of the carrier signal is modified.
The frequency shift in carrier wave frequency subsists in phase modulation as well.
The frequency shift depends on the amplitude of the modulating signal and the frequency of the signal.
Advantages of PM
FM signal produced from PM signal which is very stable.
It is extremely stable.
Phase modulation provides high transmission speed on a given bandwidth.
