modulation is scheme under which the the signal is first 
modified to suitable form(compress or amplied)and mixwd 
with the carrier for trassmission

some basicneeds are:-
               (1)multiplexing
   (2)practiability of anteena

Need of Modulation:

To transfer the message signal from one site another site 
without any loss for that we are using modulation.

Modulation classified in three types.

1.Amplitude modulation.
2.Frequency Modulation.
3.Phase Modulation.

modulation is neede basically to 

i)to increase the bandwidth of the signal
ii)to multiplex the signals
iii)to reduce the interferencemade when we transmit the 
signals with nearly same frequency in the audio frequency 
range (20-20k)hz.
iv)to favour the complexity of the transmission system

for example if a person is sitting at the bus to go to hyd.
there is a bus which will go to the hyd. it is not the 
matter that if the bus goes to the hyd he will also go.
he will also go to the hyd only when he sit in the bus.
here the bus is the carrier & the man is the information.
from this it is clear that modulation is why because the 
information which is having less power can't travel large 
distances that will put on the carrier which is having 
large power that it can travel large distances.

NEED FOR MODULATION
1) TO REDUCE THE ANTENNA HEIGHT
2) TO MULTIPLEX THE MORE NUMBER OF SIGNALS
3) TO REDUCE THE NOISE & DISTORTIONS
4) TO NARROW BANDING THE SIGNAL
5) TO REDUCE EQUIPMENT COMPLEXITY

Low freq signals can't be transmitted for long distance. 
Thats why we are modulating the informtion signals.

1)in shortly it improves the signal strength . means the low
frequency signal can't transmit long distance here low
frequency signal means low strength (weak signal) ,when we
modulate that signal it can be transmitted long distance.
while it transmit long distance i.e., nothing but it's
strength s improves. 
2) reduces noise or distortion.
3) multiplexing means we r using frequency translation theorem.
4) we transmit large amount of information at long
distances.i.e., narrow banding the signal.
5) reduce antenna height.
6) we can change the information from one form to another
form. i.e., encryption of data. thats whys we hiding the
information without any hackings.

Modulation is a process in which the characteristics of a 
signal circuit is caried in accordance with the information 
signal which is to be transmitted.
 The modulation signal,which is to be transmitted isknown 
as Modulating signal.
 Need for modulation:
i)multiplexing
ii)practicability of antennas height
iii)frequency conversion

BASIC NEED FOR MODULATION IS:
1.TO REDUCE ANTENNA HEIGHT
2.TRANSMIT THE INFORMATION TO MORE DISTANCE WITH OUT 
INTERFERENCE
3.TO REDUCE BAND WIDTH
EX:SUPPOSE IF U WANT THROUGH A PAPER HAVING INFORMATION TO 
YOUR OPP APPARTMENT FROM YOUR APPARTMENT(EX:D=500cm)IF U 
SIMPLY THROUGH A PAPER IT WILL NEVER REACH TO THE OPP 
APPARTMENT BUT IF YOU USE A STONE.i.e COMBINELY WE CAN 
THROUGH BECAUSE STONE HAVE WEIGHT LIKE WISE CARRIER HAVE 
MORE POWER THAN BASE SIGNAL THAN WE CAN TRANSMITE THE 
SIGNAL.

Modulation is needed in a communication system to achieve 
the following basic needs.such as
1)multiplexing
2)practicability of antennas

where the multiplexing helps in transmitting a number of 
messages simultaneously over a single channel and therefore 
the number of channels needeed will be less.This reduces 
the cost of the installation and maintainance of more 
channels.

Disclaimer: I don't know much about this. I am just
elaborating my understanding in a better way, after reading
the explanation from Radhar, credits to him/her.

Say I( for Information;-) needs to travel from Point T
(transmitter) to Point R (Receiver).
There can be several medium through which I can travel.
1. Air
2. Water
3. Road
4. Rail

There are several type of Carriers vehicle I can choose to
travel in the Medium.
1. By Train 
2. By Bus
3. By Aircraft
4. By a Boat
5. By a Bicycle/Walk

Now the question is how to go? What carrier to choose?
1. How fast I have to travel. ( Certain carriers are faster
than others )
2. How far I have to travel.  ( Certain carriers makes I
less fatigued (Information loss))
3. How much does it cost me. ( Carriers with more bandwidth
are generally less costly )
4. Availability of the Carrier (Bandwidth, clearly ships
have more space than buses on trains, but are slower)
5. What medium do want to travel ( Certain carriers are
better suited to travel in a particular medium than others)

Now you guys do rest of math based on this analogy.

1.reduction of antenna hieght.
2.improvement in noise performance.
3.reduction in number of repeaters used.
4.to strengthen the weak signals.

Problems will occur in the following areas if the base band
signal(information signal)is transmitted as it is,without
modulation.
1.Antenna Height
2.Narrow Banding
3.Poor radiation and penetration
4.Diffraction angle
5.Multiplexing

So Modulation is necessary for the transmission of low
frequency information signal from one site to another in
good quality.

TO Transmit the signal over the line without loss .   low  
freqency signals are converted into high freqency signals, 
that is why the transmission speed is increased.

•	, modulation can be used as a technique to conceal 
the contents of the message being transmitted (for privacy 
reasons). It is the case of the scattered spectrum 
communications, which is relatively widely used in the 
military communication appliances. The concept of scattered 
spectrum came first during the 2nd world war, when the 
spies in the French Resistance used multiple frequencies to 
broadcast their messages to the Allies. The frequencies 
were used according to a previously agreed protocol. This 
way, their communication was less prone to be intercepted 
by the Nazi's.

need of modulation....
1.sme time signal is too small due to reseiver nt get that
signal...
   so by the process of modulation we uses carrier waves
which modulate the signal so that it will not effected by
environment condition,,,it reduces
1,cost of antenna ,,instead of using large anteena we can
use here small anteenas

Need for modulation
1.Antenna height
2.Mutual interference
3.Poor efficiency

reduce antenna height

To reduce the antenna height for which we cant construct 
the antenna for that height if we dont mondulate.And it 
cant propagate such a long distance due to low frequency.

modulation is a process in which the lower frequencey 
signal called message signan and higher frequencey signal 
called carrier signal is mixed to achive effectively 
radiated signal.The need of modulation in communication 
system is to achive the followig :-
1)multiplxing 
2)practicability of antinnas.

modulation-it is the process of varing a periodic waveform. 
for eg:a tone in order to use that signal to carry a 
message. periodic waveform is a sine wave which repeats 
itself. normally a high frequency sino sidal waveform is 
used as a carrier signal. the three key parameters of a 
sine wave are amplitude,frequency,n phase all of which can 
be modified in accordance with the low frequency 
information signal to ontain a modulated signal.

well uhh....i dont know what is it's need plz check 4m d 
above.

Modulation is needed because:-

1) At low frequency there is very high attenuation.
2) we increase the frequeny and thus decrease the 
attenuation of signal, thus making it viable to be 
transmitted over lon distance.

Reduce antenna height
Avoid Interference
Reduction in Noise
Multiplexing is Possible

modulation is used to convert analog siganl to digital 
signal when we want send information from sender to 
receiver.