of sending, receiving and processing information through space
is called space communication
In space communication, electromagnetic wave carry information
over long distance through space without the use of wires. For
this reason, it is some times called wireless. Radio, television
and satellite communication fall under this category.
radiation of electrical energy is practicable only at high frequencies
e.g. above 20 kHz. Since audio frequencies (20 Hz to 20 kHz)
are small, the audio signal (e.g. music, speech etc) cannot
be sent directly over air for appreciable distance. It is because
the energy of a wave is directly proportional to its frequency
and at audio frequencies, the signal power is quite small. Therefore,
radiation is not possible.
In order to send the audio signal over great distance, we first
convert the audio signal into electrical signal. Then electrical
audio signal is superimposed on high frequency wave. The resultant
waves are known as modulated waves or radio waves and the process
is called modulation. At the radio receiver, the audio electrical
signal is recovered from the modulated wave by the process called
demodulation. The electrical audio signal is then amplified and
reproduced into sound by the loudspeaker.