What is the need for any kind of modulation in order to transmit signals?
Best Answer
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.
A signal that is phase-shift modulated with some continuous signal f will have the same waveform as one which is frequency-shift modulated with the derivative of f. Likewise, a signal that is frequency-modulated by some signal f will have the same waveform as one which is phase-modulated by the integral of f. Note that in some cases, it may not be practical to take the integral or derivative of f, so some signals can only be meaningfully represent phase modulation and others may only meaningfully represent frequency modulation.
Even though the waveforms of a frequency-modulated signal and phase-shift-modulated signal may look similar, however, there's often another important distinction. Phase-shift modulated signals are often modulated relative to some other reference signal. In some cases, the reference may be a signal sent on another wire. In other cases, the reference may be a signal which was sent at an earlier time on the same wire. In NTSC video, for example, every scan line starts with a few cycles of a 3.579545Mhz reference sine wave. Colors later in the line will be encoded by waves that have a certain phase relationship to that reference wave. Yellow, for example, will be represented by a wave whose phase matches the reference; blue will be represented by a wave 180 degrees out of phase. Red and green are represented by waves +/- 90 degrees out of phase. Note that all solid colors are be represented by the same frequency; the only difference between them is the phase relative to the reference wave.
Modulation just means modifying one signal with another.
In the most fundamental kind of modulation, amplitude modulation (AM), the amplitude of a high frequency sinusoidal 'carrier' wave is made to follow the value of a much lower frequency signal. AM has been around probably longer than any of the others, but since it wastes a lot of power, other modulation techniques have evolved. Variations on AM include suppressed carrier AM and 'sideband', which are less wasteful of power.
Frequency modulation (FM) can be thought of as making the instantaneous frequency of a carrier wave to increase and decrease according to the value of the modulating signal. Phase modulation (PM) similarly adjusts the phase of the carrier. These generally require a bit more bandwidth, but tend to be more immune to noise.
There are plenty of other modulation techniques, more sophisticated than AM, FM and PM, which do a better job of getting more information across in the same bandwidth.
Best Answer
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.