Why and Why not use modulation to transmite signal ?

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RatanaRETH

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Hello!

Could you explain me "Why we can not use modulation when we transmite signal on transmission line?"

Yours faithfully!
 

Hi

We do use modulation when transmitting over long distances, but it is not worth the cost when distances are short. The simplest example may be the serial transmission from PCs, (RS 232) which uses antipodal modulation. And, I think the parallel port communication doesn't use any modulation.

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Sal
 

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    RatanaRETH

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Hi!

What is appeared when we don't use modulation in long distance on transmission line?


Yours faithfully,
 

If we dont use modulation in long distance through transmission lines the possibility of errors is high ..

Sal
 

Hi!

What are characteristic of line to make possibility errors on transmission line when don't use modulation?

Thank for ur reply!
Yours faithfully,[/i]
 

Sometimes the band of the signal and the communication link do not match.
I that case base band modulation (i,e no modulation) is not possible.

Modulation increases the frequency of the signal and that can then be transmitted.
 

Two type of modulation exist: Digital Modulation and Analog (RF) one.
Digital modulations (such as QAM or BPSK or...) are used to convert digital symbols to signal which is transferable.
Analog modulation as the other mentioned are used to far distance communication. antenna will propagates only if your frequency is high. Unfortunately I cannot remember its formula. I can only tell you the higher the frequency the furthur the distance. (look this sentence is not true for frequency above 10GHz)
 

in general, there are few communication media that can pass low frequency signals (baseband) without distortion, so that's why baseband signal is often "modulated" ('copy' the low frequencies to high frequencies) in order that it may be transmitted.
 

a text book answer to this question is available in a book called antenna and wave propagation by kd prasad.
 

the loss is very high in baseband signal transmission due to this the modulation techniques are used, in digital modulation lots of effort are used to make the BER ( bit error rate) as low as possible.
 

There are two basic reasons why modualtion may be used:

1.antenna size is inversely proportional to (wavelength/4).
due to modulation,freq. increases,wavelength decreases and hence antenna size decreases.For basesband signals you would have to use very long antennas.

2.transmission errors are low and signals can be transmitted over longer distances.
 

Hey RatanaReth

Why

1. As SSP said antenna size
antenna size is inversely proportional to (wavelength/4)
Click to expand...
typing error I think?....(it is directly proportional)
2. The baseband channel is not always in best shape to be transmitted e.g. more often
than not the channel is band pass and signal is low pass so one needs to transfer
the signal to a higher frequency
3. To add error detection and correction facility

Why Not
Increases cost and complexity
Isn't really needed for small distances


Regards
tronix
 

yes,it is directly proportional,not inversely.
thanx 4 the correction,tronix
 

You can use base band transmition in short transmittion line,but when the length of wire extend,the signal fade, and at the same time, distortion will not decrease. At some point, the
signal will be coverd by noise, then you can not
recover the message without error.
 

Hi!

Noise always mixe to information signal, it is not relevant to band freqz.
 

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