Modulate the signal

Why people say that "When we want to send information (transfer from one pint to another) we modulate the signal below or above the center point (reference point)?

1) By modulation we mean what?
2. Why it is necessary to modulate signal for information center.
3) Why more the modulation more informative can be send?

rana_m_zeeshan
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Oct 1st, 2007
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Electrical Engineering

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Eng. Maged saber

Jan 17th, 2013

Modulation methods used because of when I need to transmit signal with low frequency it need long antenna as frequency inverse proportion with wave length and Wave length is in direct proportion with length of antenna. So to avoid long antenna we use signal with high frequency as carrier to minimize the height of antenna and carry the signal with low frequency to far receivers.