Analog modulation and Digital modulation

Modulation

Modulation is a process of modifying one or more than one characteristics of a signal on the basis of another signal. Mostly it is used for the transmission of data from one point to another. Technically speaking  during the process of modulation one or more properties of a high frequency periodic signal (which is called carrier signal) are varied with reference to a comparatively low frequency signal (which is called message signal or modulating signal) . The resultant signal is a high frequency signal which carries the information about low frequency message signal and is called modulated signal.

Image courtesy http://en.wikipedia.org/wiki/Modulation

Two main types of modulation schemes used in modern communication systems are

1. Analog Modulation
• Amplitude Modulation (AM)
• Double Side Band Modulation (DSB-AM)
• Single Side Band Modulation (SSB-AM)
• Angle Modulation
• Frequency Modulation (FM)
• Phase Modulation (PM)
2. Digital Modulation
• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)
• Quadrature Amplitude Modulation (QAM)

Difference:

The main difference between analog and digital modulation is the manner in which they transmit data. In analog modulation the message signal is analog i-e continuous time signal while the digital modulation techniques require input/ message signal to be in digital format or discrete time format. As inputs are different so the output signal is also quite different. In analog modulation any value that lies between two peak values is considered to be valid. But in digital modulation 0 and 1 are the only acceptable values. All the other values are treated as noise and rejected at  the receiver end.

The major advantage that digital modulation schemes enjoy is their greater fidelity when compared to analog modulation schemes. Analog modulation techniques are highly prone to noise. any noise or interference that lies with the band of interest get mixed with the signal and causes signal degradation. As digital modulation techniques play with only two values 0 or 1. So theoretically the probability of noise corrupting the signal is almost zero. Practically noise does degrades the digitally modulated signal but not to the extent of analog modulated signal

In short the main points to remember are:

1. Analog modulation takes continuous signal as input message signal while discrete time signals are input for the digital modulation.
2. Analog modulation has range of valid values while digital modulation has only two discrete values.
3. Analog modulation is cheaper to implement than digital modulation.
4. Analog modulation is more prone to noise and interference than digital modulation.
5. Digital modulation produces more accurate output than analog modulation

I shall discuss different types of modulations in detail in future posts