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Friday, November 11, 2011

Communications .. fundamentals .. Part 2

(  You may also like to read  :
 Know basics of Rocket Science
Communications .. fundamentals .. Part 1
Basics of PSK Modulation, db scale, Harmonics etc.  )

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We see this signal daily. The charger lamp changes its colour from RED >> YELLOW >> GREEN. This gives us information about the state of the battery being charged.
Here the same bulb carries different information at different times. We can therefore indicate the flow of information over a time period as shown in the next figure.

The colour of light coming out of the bulb and entering the eye changes to convey the information. Here the light is a medium that is used to carry info and colour is the parameter which is changed.

Here we defined the signal as having three states : 1. No Charge, 2. Charging and 3. Charge complete.

Types of Signals :

Before we proceed further let's just review about what are the different types of information that is transmitted over a communication network.

The information basically can be classified into two categories : 1. Analog and 2. Digital.

A continuously varying signal which can take any value from its positive max value to its negative max value is calledanalog signal as against a digital signal which does not vary continuously but takes only two discrete 'states' , like a switch which is either 'on' or 'off', no in between state.
Therefore the states of a digital signal are indicated by defining the signal to be
ON or HIGH or 1 ( ONE ) in one state and
OFF orLOW or 0 ( ZERO ) another state.

Having defined the types of signals that generally are transmitted in communication process let's now proceed further from where we left our discussion viz. carrying signal over a distance by changing some parameter of the carrier that is Carrier Communication.

We can have an analogy:
suppose we want to send a parcel to a distant place then the parcel is placed on a vehicle and the vehicle travels upto the destination and there the parcel is removed from vehicle ( and we are least bothered about what happens to the vehicle because it was only a medium to carry the parcel).
Our only goal was to send the parcel to the destination without damaging it.

In a similar manner, at transmitter end we make the information to ride the carrier. This process of putting information on a carrier is called as Modulation.

We transmit the modulated carrier through space or a conductive wire or optical fibre.

Then, at the receiving end we extract information from carrier by the process called 'demodulation' and, bingo, our message has reached the destination.
But rarely does the deliverytake place without some damage ( i.e. some distortion in the information) by the time it reaches destination.

The carrier will generally be a sinusoidal waveform specified in a mathematical form as
A*sin(ωt+ɸ) , where:

A, the amplitude, is the peak deviation from its center position.
ω, the angular frequency, specifies how many oscillations occur in a unit time interval, in radians per second
ɸ, the phase, specifies where in its cycle the oscillation begins at t = 0.

By changing any one of the parameters A, ω or ɸ it is possible to convey intelligence from source to destination. This 'change' is called Modulation and accordingly this way of communication is defined as Amplitude Modulation, Frequency Modulation or Phase Modulation respectively.

Amplitude and Frequency modulation is shown graphically in the image on left. The 1st waveform is the 'unmodulated' carrier

The second waveform is the signal ( called 'intelligence' in technical parlance ) which is to be modulated on the carrier.

If we use Amplitude Modulation then the amplitude of the carrier changes as per the amplitude of signal as shown in 3rd figure.

If a Frequency modulation is used then the carrier with modulated intelligence looks as shown in adjoining figure wherein the frequency ( indicated by distance between adjacent cycles ) changes as per the intelligence.

We have saved phase modulation description for a later part of the post.

Notice that the modulating signal in above case was a continuously varying signal taking any value from positive to negative extreme or an analog signal.

( To be completed)

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1 comment:

  1. Shouldn't be the top sinusoidal signal the data signal and the one below the carrier signal ?