Modulation:  If we need to transmit a signal from one place to another for that we need to strengthen the signal so that it can be traveled to long distance.

Analog modulation:  We have three types of modulation.
Frequency modulation.
Amplitude modulation.
Phase modulation.

Amplitude modulation:  It transmits sound by changing signal strength.
Invented in the year 1906.
Frequency is constant amplitude varies.
Radio stations are 16,265.
Frequency bands long waves 153.279 k hz.
                             medium waves 531-1,611 k hz.
                             short waves     2.3-26.1 m hertz.

Advantage:  Talk radios.
 News programming.

Frequency modulation:  It was invented in the year 1933.
Amplitude is constant frequency varies.
Frequency band is 87.5 to 108.0 m hertz.
Radio stations are 28,693.

Advantages:  Music radios.
Public radios.