# 5.2 Representing symbols by bits  (Page 2/2)

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Let us assume that we represent a symbol ${x}_{n}$ , with probability ${p}_{n}$ , by ${l}_{n}$ bits. Then, the average number of bits spent per symbol will be

$\langle L\rangle =\sum_{n=1}^{N} {p}_{n}{l}_{n}$
We see that this equation is equal to the entropy if the code words are selected to have the lengths ${l}_{n}=-\lg {p}_{n}$ . Thus, if the source produces stochastically independent outcomes with probabilities ${p}_{n}$ , such that $\lg {p}_{n}$ is an integer, then we can easily find an optimal code as we show in the next example.

## Finding a minimal representation

A four-symbol alphabet produces stochastically independent outcomes with the following probabilities. $({x}_{1})=\frac{1}{2}$ $({x}_{2})=\frac{1}{4}$ $({x}_{3})=\frac{1}{8}$ $({x}_{4})=\frac{1}{8}$ and an entropy of 1.75 bits/symbol. Let's see if we can find a codebook for this four-letter alphabet that satisfies the Source CodingTheorem. The simplest code to try is known as the simple binary code : convert the symbol's index into a binary number and use the same number of bits for each symbol byincluding leading zeros where necessary.

$↔({x}_{1}, \mathrm{00})\text{}↔({x}_{2}, \mathrm{01})\text{}↔({x}_{3}, \mathrm{10})\text{}↔({x}_{4}, \mathrm{11})$
As all symbols are represented by 2 bits, obviously the average number of bits per symbol is 2.Because the entropy equals $1.75$ bits, the simple binary code is not a minimal representation according to the source coding theorem.If we chose a codebook with differing number of bits for the symbols, a smaller average number of bits can indeed be obtained. The idea is to use shorter bit sequences for the symbols that occur more often , i.e., symbols that have a higher probability. One codebook like this is
$↔({x}_{1}, 0)\text{}↔({x}_{2}, \mathrm{10})\text{}↔({x}_{3}, \mathrm{110})\text{}↔({x}_{4}, \mathrm{111})$
Now $\langle L()\rangle =1\frac{1}{2}+2\frac{1}{4}+3\frac{1}{8}+3\frac{1}{8}=1.75$ . We can reach the entropy limit! This should come as no surprise, as promised above, when $\lg {p}_{n}$ is an integer for all $n$ , the optimal code is easily found.

The simple binary code is, in this case, less efficient than theunequal-length code. Using the efficient code, we can transmit the symbolic-valued signal having this alphabet 12.5%faster. Furthermore, we know that no more efficient codebook can be found because of Shannon's source coding theorem.

## Optimality of the ascii code

Let us return to the ASCII codes presented in . Is the 7-bit ASCII code optimal, i.e., is it a minimal representation? The 7-bit ASCII code assign an equal length (7-bit) to all characters it represents. Thus, it would be optimal if all of the 128 characters wereequiprobable, that is each character should have a probability of $\frac{1}{128}$ . To find out whether the characters really are equiprobable an analysis of all English texts would be needed. Such an analysis is difficult to do. However, the letter "E" is more probable than the letter "Z", so the equiprobable assumption does not hold, and the ASCII codeis not optimal.

(A technical note: We should take into account that in English text subsequent outcomes are not stochastically independent. To see this, assume the first letter to be "b", then it is more probable that the next letter is "e", than "z". In the case where the outcomesare not stochastically independent, the formulation we have given of Shannon's source coding theorem is no longer valid, to fix this, we should replace the entropy with the entropy rate, but we will not pursue this here).

## Generating efficient codes

From Shannon's source coding theorem we know what the minimum average rate needed to represent a source is. But other than in the case when the logarithm of the probabilities gives an integer, we do not get any indications on how to obtain that rate. It is a large area of research to getclose to the Shannon entropy bound. One clever way to do encoding is the Huffman coding scheme.

where we get a research paper on Nano chemistry....?
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
I only see partial conversation and what's the question here!
what about nanotechnology for water purification
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
how did you get the value of 2000N.What calculations are needed to arrive at it
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