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Friday, May 13, 2011

Communications .. fundamentals .. Part 1

(  You may also like to read  :
 Know basics of Rocket Science
Communications .. fundamentals .. Part 2
Basics of PSK Modulation, db scale, Harmonics etc.  )

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Surprised by the image on the right? Yes, we are going to learn basics of communications from the scratch viz with the help of a simple Lip-to-Ear communication.

How do we hear? The sound falling on the ear drum transfers the vibrations through a chain of bones to the Cochlea within which these mechanical vibrations get converted into sensory nervous signals which we call as "Hearing the sound".

Let's go a little deep into this process. The sound vibration energy received through air creates vibrations into the eardrum. Now there is a minimum power which these sound waves should posses .. if the power is less then the drum will not vibrate and we will not hear. This minimum required power is the threshold of hearing. Hearing threshold is a subject in itself and we will not go into that.

The point is that there is a minimum power that is required for us to hear.That is  indicated in Fig 1 .  The ear can't hear the sound with level B because it is below threshold but can hear A which is above threshold.

Here we assumed that the surroundings are totally noise free ( like mid of the night in a very quite location away from unwanted noises). What will happen if there is a surrounding noise  indicated with N in Fig2 ? ( say rumblingsing of a digging machine nearby ) .. then a whisper will not be heard although its sound power will fall on the drum because the major power falling on eardrum is the sound polluting deviceas shown in N>S part of Fig 3  and one has to shout above that noise if you want it to be heard . 2nd part of Fig 3 .

Also notice one more phenomenon: You can hear to the whisper if the speaker brings his mouth near the ear .. if the mouth is not very near to the ear then the other person will not hear the whisper.. you have to speak louder and as you move away from the other person you have to talk still louder. This is because the sound intensity decreases as it travels more distance.

That is the reason when you want to address many people you require Microphone-Amplifier-Loud Speaker arrangement.

Here again we note the following two possible arrangements:

In the first method there are few high-power speakers. Here the speakers emit large volume sound because the sound has to be audible upto the last row which is far off from speakers. But this is deafening to the persons seated next to the speaker while the person at back can't hear because the sound is very less by the time it reaches  him. This leaves more people RED with anger than a few who are happy.

In the second arrangement there are many low-power speakers. This distributed power emission makes everyone get a pleasent volume of sound energy because the speakers are operating at low volume but are near to every person in the audience.

Moral of the story: for avoiding large pollution put many low power transmitters spread over the area.

Does it sound familiar?

Yes, that precisely is the arrangement used in Cell Phone technology.

There are several small towers operating at safe RF power emissions, each serving about a 5-6 Kms radius. By this arrangement each tower has to transmit a small power ( and more importantly ) the cellphone does not have to transmit a large power .. just sufficient to reach the tower which is less than 6 kms away. If the towers were at a larger distance then the cell phone which is touching the ear would have to transmit a larger power which would have caused damage to brain tissues.

And since each of this 5-6 kms circle coverage area is called a CELL, the name Cellular Phone Technology !!

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Now we translate the above narration in technical parlance applying communications terminology:

1. In a communication system the receiver has a minimum detectable level known as Receiving Threshold. For the receiver to detect a signal it is necessary that the signal level at the front end is above this threshold.

2. The receivers are rarely noise free. The noise could be inherent to the front end or the noise level in the vicinity of the receiver. For the receiver to detect the signal the signal has to be more than the noise level by a minimum level called as a Threshold Signal to Noise Ratio ( S/N Ratio or SNR ). If the SNR is below this threshold then the detected signal will be noisy. The more the SNR better the detected signal quality. ( this statement is true only upto a certain value. Beyond a certain value the signal quality remains constant even if SNR is increased.)

3. The signal level decreases as the distance between Transmitter and Receiver is increased. It follows inverse square law.. the signal level fall is proportional to the square of the distance between transmitter and receiver.

3. The kind of signal that we have illustrated above is an Analog signal .. we will see the Digital signal in a subsequent post.

4. Also we have seen a simple transmitter and receiver system. Such systems were in use till not very long ago. A simple manual telephone switching systems that were in use wherein one had to call a operator to connect to other party in a small premises used this type of direct connection between two users and the signal looked like this .. words seperated by gaps of silence between them as shown in the first portion of image below.

Such systems are rarely used now a days. Almost always a Modulation is involved whrerein the sound is modulated on a carrier ( like the 2nd part of image above wherein there is a continuous carrier but its characteristics are changed over a period of time ) and the carrier is transmitted and received by carrier receiver and demodulated to retrieve the original signal.

Also read subsequent writeups :

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