Recall that any wave has three basic properties: 1 Amplitude — the height of the wave 2 Frequency — a number of waves passing through in a given second 3 Phase — where the phase is at any given moment. There are different strategies for modulating the carrier wave. First, a user can tweak the height of the carrier.
This is called amplitude modulation or AM. Frequency of an input signal can also be changed. If this input signal is added to the pure carrier wave, it will thereby change the frequency of the carrier wave.
In that way, users can use changes of frequency to carry speech information. This is called frequency modulation or FM. These two strategies can be combined to create a third scheme. In fact, any strategy that combines an input signal with a carrier wave to encode speech or other useful information is called a modulation scheme. Modulation schemes can be analog or digital. An analog modulation scheme has an input wave that varies continuously like a sine wave.
Voice is sampled at some rate and then compressed and turned into a bit stream — a stream of zeros and ones — and this in turn is created into a particular kind of wave which is then superimposed on the carrier. In the continuous-wave modulation, a high frequency sine wave is used as a carrier wave.
This is further divided into amplitude and angle modulation. If the amplitude of the high frequency carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal, then such a technique is called as Amplitude Modulation.
If the angle of the carrier wave is varied, in accordance with the instantaneous value of the modulating signal, then such a technique is called as Angle Modulation. If the frequency of the carrier wave is varied, in accordance with the instantaneous value of the modulating signal, then such a technique is called as Frequency Modulation. If the phase of the high frequency carrier wave is varied in accordance with the instantaneous value of the modulating signal, then such a technique is called as Phase Modulation.
In Pulse modulation, a periodic sequence of rectangular pulses, is used as a carrier wave. This is further divided into analog and digital modulation. In digital modulation , the modulation technique used is Pulse Code Modulation PCM where the analog signal is converted into digital form of 1s and 0s.
As the resultant is a coded pulse train, this is called as PCM. There are more than a dozen or so channels on the radio at any time, each with a given frequency: Each channel gets a certain range usually about 0. Modulation makes it all possible, because it allows us to send voice and music which are essential baseband signals over a bandpass or "Broadband" channel.
A sine wave at one frequency can be separated from a sine wave at another frequency or a cosine wave at the same frequency because the two signals are "orthogonal". There are other sets of signals, such that every signal in the set is orthogonal to every other signal in the set. A simple orthogonal set is time multiplexed division TDM -- only one transmitter is active at any one time. Other more complicated sets of orthogonal waveforms—Walsh codes and various pseudo-noise codes such as Gold codes and maximum length sequences—are also used in some communication systems.
The process of combining these waveforms with data signals is sometimes called "modulation", because it is so very similar to the way modulation combines sine waves with data signals. From Wikibooks, open books for an open world.
A high energy wave is characterized by a high amplitude; a low energy wave is characterized by a low amplitude. Amplitude modulation or AM is the method of varying the instantaneous amplitude of carrier signal accordingly with instantaneous amplitude of message signal.
The frequency of a wave is the number of waves passing a point in a certain time. FM or Frequency modulation is the process of varying the in instantaneous frequency of Carrier signal accordingly with instantaneous amplitude of message signal.
PM or Phase modulation is the process of varying the instantaneous phase of Carrier signal accordingly with instantaneous amplitude of message signal.
Digital modulation is similar analog except base band signal is of discrete amplitude level. For binary signal it has only two levels, either high or logic 1 or low or logic 0. This type of modulation transmits data by varying the frequency of the carrier. In this modulation, the carrier has two predefined frequencies. Problem: Certain types of data such as voice, video, and bit streams from computers have lower frequency bands and therefore they cannot travel very far wirelessly because they cannot propagate through space.
Solution: When using modulation, the carrier wave which has a much higher frequency is used to carry the signal, meaning it can travel further and more quickly through space.
Problem: Antenna length would be impractical for voice, video and bit streams without modulation.
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