What is the need for any kind of modulation in order to transmit signals?
Modulation is, generically, a means of shifting information from one frequency domain into (typically) a higher frequency domain. This provides a number of benefits; among them:
Two or more input signals with matching or overlapping frequency domains may have their information shifted into disjoint frequency domains; if such signals travel together through a medium, having their frequency domains be disjoint will allow them to be separated.
In many cases it's easier to uniformly handle signals where the minimum frequency is a substantial fraction of the maximum, than signals where the maximum is many times the minimum. An audio signal whose frequency content is 20-20,000Hz has a 1,000:1 spread between the minimum and maximum frequencies. If such a signal were amplitude-modulated at 1MHz, the spread would be about 4% [from 980,000Hz to 1,020,000Hz]. Even if there were no other radio communications anywhere in the world, trying to design an antenna which could work well with a 1,000:1 frequency spread (from 20Hz to 20KHz) would be very difficult. Designing an antenna to deal with a 4% spread would be much easier.
Some use cases for modulation exploit both benefits; others only rely upon one or the other.
Modulation is the means to transmit information using a radio signal. The carrier frequency of a commercial radio station is used to place the signal within the allotted spectrum of a particular radio service. For example, AM radio covers the frequency band from 540 to 1640 kilohertz. Let's say that one station has been assigned the carrier frequency of 1010 kilohertz. If it just transmitted that carrier, no information would be available to the listener. By modulating that carrier (in this case amplitude modulation) information can be transmitted. If the amplitude modulation is made to be in step with a song, for example, than a listener can tune his radio to the carrier frequency and the demodulator in the radio will reproduce the song. Since modulation creates sidebands around the carrier frequency, the modulation frequency is limited (to about 5 kilohertz for regular AM radio) so that many stations can be accommodated in the 540 to 1640 kHz band. For amplitude modulation, the bandwidth transmitted by the radio station is twice the audio bandwith of the modulating signal. Thus each AM station occupies 10 kHz of the available spectrum. With 1100 kHz available, many stations can be transmitting at the same time.
Have you got a radio? Have you noticed that you can tune through several stations across any band on your radio? When you tune to one station, have you noticed that you don't pick-up any (or maybe slight sometimes if AM) signals from other stations?
If "yes" to the above then perhaps you'll realize that there has to be a way to differentiate one station from another. If there wasn't a way of doing this, your radio speaker would be just producing an unintelligible mess of a sound.
The way we can do this is by modulation. Every transmitting station produces an audio signal that it "mixes" with the allotted frequency in the radio spectrum the government has allocated to it. OK sometimes governments don't allocate frequencies but a lot of the time they do.
EDIT - The "mixing" is better defined by the word "modulation" and this process allows us the means to tune our radios to whichever channel we want in the radio band in question.
If you are referring to communications. The answer can be broken down into two main answers:
- Matching the transmission characteristics of the medium, and considerations of power and antenna size, which impact portability.
- The second is the desire to multiplex, or share, a communication medium among many concurrently active users.
Below: an example of modulation ...