[SOLVED] why we do modulation?

[hitman1988]

if we have very short distance and agnoring all the other physical difficultes such as the stareable antenna and other things , why we modulate signal before we send it? and why we can't send a digital signal under any conditions without modulate it?

 

[lostinxlation]

What short distance are you talking about ? 50 cm or 50m ?
you don't want to send digital signal unless you are doing it within a board or so, since digital signal has extremely high frequency components which will attenuate.

 

[tony_lth]

yes, you can do so, and use LPF to filter harmonics, accurately control phase and amplitude at RF, etc. If you ignore physical difficult, it should be no problem.

 

[hitman1988]

yeah the short distance is about 50cm and less

 

[Ravipratap]

yes, it's true that the baseband signal can be transmitted directly but it has certain limitations and these limitations can be overcame using modulation. In the process of modulation the baseband signal is translated i.e shifted from low frequency to high frequency, the frequency shift is proportional to the frequency of the carrier. this process has following advantages :-
1). Reduction of the height of the antenna:- for the transmission of signal, the antenna height must be a multiple of quater of its wavelength. as wavelength is inversely proportional to frequency, so for low frequency signal the hight of antenna is very high and impractical while for a modulated signal the frequency is high the height get reduced.
2). Avoid mixing of signals:- If the baseband sound signals are transmitted without using the modulation by more than one transmitter than, then all the signals will be having same frequency range. Therefore all the signals get mixed together and a receiver cannot seperate tham from each other. so, if each signal is used to modulate a different carrier then they will occupy different slots in frequency domain and no mixing will occur.
3). Increase the range of communication:- the frequency of baseband signal is low and the low frequency signals cannot travel a long distance when they are transmitted. they get heavily attenuated. The attenuation reduces with increase in frequency of the transmitted signal and they travel longer distance. the modulation process increases the frequency of the signal to be transmitted. Hence it increases the range of communication.
4). Multiplexing is possible:- Multiplexing is a process in which two or more signals can be transmitted over the same communication channel simultaneously. this is possible only with modulation. the multiplexing allows the same channel to be used by many signals. so many TV channels can use the same frequency range without mixed with each other or
different frequency signals can be transmitted at the same time.
5). Improve quality of reception:- With frequency modulation, and the digital communication techniques like PCM, the effect of noise is to a greater extend. this improves reception.

 

[tony_lth]

1). Reduction of the height of the antenna2). Avoid mixing of signals3). Increase the range of communication4). Multiplexing is possible5). Improve quality of reception:-

The poster said the distance is only 50cm, so it's possible. (1)only some PCB trace antenna can do so, no need the big antenna, and current the baseband signal exceed 1Gb/s. (2) In the shield box, no mixing (3) the range is only 50cm, only near field (4) Maybe no need for multiplexing, OR properly allocate the freq