Coding and decoding
We usually communicate with each other using
words, but they only mean anything if we understand the language. Imagine trying to explain the
ideas of Physics to your pet guinea pig or telling your cat about the origins of the universe. They
are probably not going to understand you. However you may have taught the cat to understand
some simple words such as "no", "mouse" or "fish" spoken in a special way.
Now think
about talking to someone in another country whose language is not English – you will have a
similar problem to the cat although the brain of the other person will be much better than that of
the cat and so you will both be able to learn to understand words in each others
language.
However what of you want to be able to communicate with only selected
people – you will need some sort of code that only you and they understand. We will now have a
look at a few simple codes.
The simplest if probably to replace letters with numbers, two
example are given:
A |
B |
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D |
E |
F |
G |
H |
I |
J |
K |
L |
M |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
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N |
O |
P |
Q |
R |
S |
T |
U |
V |
W |
X |
Y |
Z |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
21 |
22 |
23 |
24 |
25 |
26 |
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L |
M |
26 |
25 |
24 |
23 |
22 |
21 |
20 |
19 |
18 |
17 |
16 |
15 |
14 |
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N |
O |
P |
Q |
R |
S |
T |
U |
V |
W |
X |
Y |
Z |
13 |
12 |
11 |
10 |
9 |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
or letters by letters
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L |
M |
Z |
Y |
X |
W |
V |
U |
T |
S |
R |
Q |
P |
O |
N |
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N |
O |
P |
Q |
R |
S |
T |
U |
V |
W |
X |
Y |
Z |
M |
L |
K |
J |
I |
H |
G |
F |
E |
D |
C |
B |
A |
Or how about letters by symbols:
There are some very clever codes that change
each time you use the letter.
One invented for the resistance workers in World War II
used the lines of a poem such as the two examples shown below.
The love that I have
is all that I have
And the love that I have is yours
The love that I have of the life that I
have
Is your and yours and yours.
This is the morning I will not forget
For then
we stood in awe
And saw the world created in a day
The letters of the poem were put into a grid in order, using each letter only once. When the end of the poem has been reached any letters not used are added to complete the grid. (Z and V occupy the same square).
The grid is then used to encode the message.
The message is encoded and decoded by
taking the letters in pairs from the grid and replacing them with the letters at the opposite corners
of the rectangle formed by the two original letters.
For example in the grid for the
second poem shown here the letters DS would be encoded as FE.
T |
H |
I |
S |
E |
M |
O |
R |
N |
G |
W |
U |
L |
F |
D |
A |
B |
C |
J |
K |
P |
Q |
X |
Y |
V |
So for the example of the second
poem:
HOW ARE YOU TODAY would be first written as
HO WA RE YO UT OD AY
and coded as
OH AW IG QN WH UG PJ
Find out something about the Eureka machines developed by the germans in World War II and how the code used by them was broken at Bletchley Park in England.
Electronic encoding and
decoding
Electronic devices such as computers use the binary system of numbers to
"communicate" with each other. If we use an LED module with four lamps we have a variety of
possible combinations of output, 16 in fact.
These are:
0000 0100 1000
1100
0001 0101 1001 1101
0010 0110 1010 1110
0011 0111 1011 1111
Therefore
you can only send a message of 16 different "words" if you only have four LEDs as your
output.
As an example of this consider the following sentence:
Four output lights
can be used to transmit up to only sixteen pieces of simple information.
If each word is
represented by a set of ones (1) or noughts (0) starting with 0000 the sentence could look
like:
0000 0001 0010 0011 0100 0101 0110 0111 1000 0110 1001 1010 1011 1100
1101 1110
If so what does this set of data mean:
1010 0010 0011
0111
A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB