Electronic devices produce messages like analog baseband signals in the form of audio, video or even messages can be in the form of digital bits from computer. To send these messages we must have some communication channel like wires, co-axial cable, even wireless radio waves, microwaves or infrared. We can easily transmit messages through wires or cables. Voice, Video, bit streams from computer are having lower frequency band and can travel few distance with wires but cannot be sent through wireless media. Voice signal has lower Bandwidth therefore it will not propagate through space and will be attenuated. To transmit voice signal a large size antenna is required as antenna length is proportional to half of wavelength. The size of the antenna will be more than the distance between transmitter and receiver. Again when more than one transmitter is involved all station will overlap in one frequency band. For those above reasons we choose a carrier, which is a high frequency radio wave, can travel long distance without attenuation and as the frequency is high smaller antenna is required. Selecting different carrier frequency for different transmitting stations can eliminate overlapping of frequency band.

Problem:

Voice, Video, bit streams from computer are having lower frequency band
They canntravel few distance with wires but not cannot propagate through space
Antenna size is half of wavelength thus antenna length for Voice, Video, bit streams would be impractical
Assume we transmit Voice, Video, bit streams over an imaginary antenna but being in the same frequency range all channels will overlap
Solution:

carrier signal is used to carry signal to long distance
Modulation is used with a selected carrier frequency signal to mix baseband with carrier
carrier frequency is in higher frequency radio wave length and thus antenna size would be smaller
Radio waves can travel longer distance with very less att
Radio wave has a wide range of frequencies to select individual non-overlapping channels
Established in 2000, the Soukacatv.com (DSW) main products are modulators both in digital and analog modulators, amplifier and combiner. We are the leading communication supplier in manufacturing the headend system in China. Our 16 in 1 and 24 in 1 now are the most popular products all over the world. For more, please access to https://www.soukacatv.com/.

Modulation:

Now we have to develop some way to send the information of message signal via this carrier signal. The carrier signal is a high frequency sinusoidal signal represented by amplitude, frequency and phase. We can vary one of this parameter accordingly with the message information.

What is Modulation?

Modulation is an operation of varying amplitude or frequency or phase of carrier signal according to the instantaneous amplitude of the baseband signal/modulating signal.

Here baseband signals comes from a audio/video or computer. Baseband signals are also called modulating signal as it modulates carrir signal. career signals are high frequecy radio waves it generally comes from a radio frequency oscillators. These two signls are combined in modulator. Modulator takes the instantenious amplitute of baseband signal and varies amplitute/frequency/phase of career signal. Resultant signal is a modulated signal. It goes to an RF-amplifier for signal power boosting and then feed to antenna or a co-axial cable.

There are two types of modulation analog and digital. Analog modulation delas with the voice, video and regular waves of base band signals. Where as digital modulations are with bit streams or symbols from computing vevices as base band signals.

DeModulation:

Demodulation is the opposite process of modulation. Modulator is a part of signal transmitter where as demodulator is the receiving side. In broadcast system radio tranmitting station does to modulation part. A radio receiver acts as a demodulator. A modem receives signals and also transmits signals thus it does modulation and demodulation at the same time. Thus the name modem has been given. A radio antenna receives low power signal. A co-axial cable end point can also taken as an signal input. An RF amplifer boosts the signal amplitude. Then the signal goes to a demodulator. demodulator does the reverse of modulation and extracts the backband signal from career. Then the base band signal is amplified to feed a audio speaker or video moitor or TTL/CMOS signal levels to match computer inpts.

What is De-modulation?

Demodulation is the opposite process of modulation where the varying amplitude, frequency or phase of carrier signal is extracted to construct the original the message signal.

Analog Modulation:

Analog modulation refers to the process of transferring analog low frequency baseband signal, like an audio or TV signal over a higher frequency carrier signal such as a radio frequency band. Baseband signal is always analog for this modulation.

There are three properties of a carrier signal amplitute, frequency and phase thus there are three basic types of analog modulations.

Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase modulation (PM)
Digital modulation:

Analog modulation refers to the process of transferring digital low frequency baseband signal, like digital bitstream from computers over a higher frequency carrier signal such as a radio frequency band. Digital modulation in somewhat similar to the analog modulation except base band signal is of discrete amplitude level. For binary signal it has only two level, either high or logic 1 or low or logic 0. The modulation scheme is mainly three types.

ASK or Amplitude shift Key
FSK or Frequency shift key
PSK or Phase shift key
ASK or Amplitude shift Key:

When the carrier signal's instantaneous amplitude is varied in proportion to message signal m(t). We have the modulated carrier m(t)coswct where coswct is the carrier signal. As the information is an on-off signal the output is also an on-off signal where the carrier is present when information is 1 and carrier is absent when information is 0. Thus this modulation scheme is known as on-off keying (OOK) or amplitude shift key.

Application:

Used in our infrared remote controls
Used in fibre optical tranmitter and receiver.
FSK or Frequency shift key:

When Data are transmitted by varying instantaneous frequency of the carrier, we have the case of frequency shift key. In this modulation carrier has two predefined frequency wc1and wc2. When information bit is 1 carrier with wc1 is transmitted i.e. coswc1 and When information bit is 0 carrier with wc0 is transmitted i.e. coswc0

Application:

Many modems used FSK in telemetry systems
PSK or Phase shift key:

The instantaneous phase of the carrier is shifted for this modulation. If the base band signal m(t) =1 carrier in phase is transmitted. If m(t)=0 carrier with out of phase is transmitted i.e. cos(wct+П). If phase shift is done in 4 different quadrants then 2bit of information can be sent at a time. This scheme is a special case of PSK modulation known as QPSK or Quadrature Phase Shift Key.

Application:

Used in our ADSL broadband modem
Used in satellite communication
Used in our mobile phones
Established in 2000, the Soukacatv.com (DSW) main products are modulators both in digital and analog modulators, amplifier and combiner. We are the leading communication supplier in manufacturing the headend system in China. Our 16 in 1 and 24 in 1 now are the most popular products all over the world. For more, please access to https://www.soukacatv.com/.


Source: http://www.equestionanswers.com/notes/modulation-analog-digital.php