A baseband or information signal produced by a source is generally not fit enough to be transmitted directly over a channel to farther distances. To facilitate transmission over a given channel and to a certain distance we need to modify some parameters of the signal, for example the frequency of the signal should lie within the range of the given channel. This modification of a baseband or information signal is called Modulation.
Thus, Modulation is the process of varying one waveform in relation to another waveform.
In telecommunications, modulation is used to convey a message. Often a high-frequency sinusoid waveform is used as carrier signal to convey a lower frequency signal.
Any of the three key parameters of the carrier wave - its amplitude ("volume"), its frequency ("pitch"), and its phase ("timing") can be modified in accordance with a low frequency information signal m(t) to obtain the modulated signal. Accordingly, we have three types of modulation- Amplitude Modulation, Frequency Modulation and Phase Modulation.
At the receiver end a modulated signal passes through the reverse process to give the information. This is called Demodulation.
Figure 1 Amplitude Modulation
AM or Amplitude Modulation was the earliest modulation techniques used to transmit voice on radio. As shown in the figure above, In amplitude modulation, the strength or the amplitude of the carrier wave, which is generally a sinusoid, is varied or modulated by the baseband signal before transmission.
Figure 2 Frequency Modulation
Frequency modulation varies the frequency of cycles in relation to the pattern of the original sound information. As shown in the figure above, In Frequency modulation or FM, the instantaneous frequency of the carrier wave is varied or modulated by the baseband signal before transmission.
Digital Modulation methods
In digital modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet).
These are the most fundamental digital modulation techniques:
Phase shift keying: In the case of PSK, a finite number of phases are used.
Frequency shift keying: In the case of FSK, a finite number of frequencies are used.
Amplitude shift keying: In the case of ASK, a finite number of amplitudes are used.
Quadrature amplitude modulation: In the case of QAM, a finite number of at least two phases, and at least two amplitudes are used.