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The word analog (French, analogue 1826; Greek Analogos) means similar to something. In electrical physics (1948) it has come to mean set of variable physical quantities that represent or correspond to another set of variable physical quantities.
The loudness of a person's voice depends on how far they push their own diaphragm (below the chest), and the pitch of their voice depends on how much they stretch or relax their vocal cords. Thus, we can say that a person's voice is an analog of the force exerted by their diaphragm, and of the tension of their vocal cords.
When we turn a doorknob, we are applying a certain amount of force. The knob turns in the direction in which we twist our hand, and turns in a speed proportional to the force we applied to it. The movement of the knob is thus an analog of the force we applied to it.
Many of the electric appliances we have used in the past, and still use today can be called analog. To illustrate, consider the examples of the record player, the tape player and the telephone.
In the record player, the surface of the record is divided into tracks. The surface of each track contains bumps of varying sizes that correspond to the sound that was recorded. When the sound was loud, the bump are high, when the sound is low, the bumps are low.
When a needle passes over those bumps it move up and down in proportion to the height of the bumps. Just as we feel the vibration of the wheels of car if we drive over road bumps placed before a toll station, there is a diaphragm that vibrates with the vibration of the needles.
Now the bumps (high or low) on the record's surface and the vibration of the diaphragm are said to be an exact analog or similar to the sounds recorded (high and low).
When the vibration of this diaphragm is amplified, we can hear the music.
In the case of the tape player, the sound information is recorded as quantities of magnetism on the surface of the tape. These quantities, when read by the reading head, are converted into electrical current that is an analog of the recorded sound. When amplified and converted to vibration inside the speakers, we can hear the music.
Similarly, telephones convert the vibration caused by our sounds in a carbon chamber to electric current.
The intensity of this electric current again is proportional to the loudness of the original voice. When the electric current travels over the telephone wire and reaches its destination it is converted again into amplified vibration, which we recognize as the voice of the speaker.
There are two important disadvantages of analog devices:
1. Analog devices are never faithful to the original.
Examples of analog devices distorting the input:
2. Analog devices add noise to the output.
The word "digital" comes from digit, which means finger. Because we are a species that counts on its fingers, digits came to mean numbers also. Therefore, when we say digital, we are referring to something that uses numbers to store and manipulate information (as opposed to analog or mechanical).
Computers are digital, calculators (a limited computer) are also digital. CD's are digital, because the information (e.g., music) to be recorded on the CD is encoded as numbers.
As you know by now, when the music is recorded, it is turned from sound into an analog electric signal. An electronic circuit called the Analog-to-Digital (A-to-D) converter measures the electric signal and produces a numeric value (in zero's and one's) that is proportional to its level. At very small intervals (micro or Nano-seconds) the computer reads those numbers and stores them. These numbers are then turned into pattern on an aluminum surface, which is then encased in plastic, in what we call a CD.
When a CD is played, the CD player reads the pattern of pits on the aluminum surface using a laser beam (so there is virtually no contact with the surface.) It then uses a Digital-to-Analog converter that reads in all those numbers and converts them back to an analog electric signal. This signal is then converted into sound.
We can see that the CD is not worn down, no matter how many times it is played. Nor is the master recording worn down, when it is used to make thousands or millions of copies. Nor does it contain or add any background noise.
A good thing to remember is that not only sound, but also text and video can be stored in digital form. Just the right dish for computers.
Now can you guess which invention can be considered as the first digital device? (Next section)
Over a century and a half ago, the Telegraph enabled people to send messages over very long distances in only a few minutes. To do so, the telegraph operator would have to convert text into a code consisting of dots and dashes. Electrically speaking, the circuitry of the telegraph uses a switch which the operator turns on and off, and produces a click sound. If you push and release the switch quickly, you hear a short click (which would stand for a dot.) If you push the switch and hold it down a little longer, the receiver would hear a longer click (a dash.)
Thus were letters and words, emergencies and vital information, encoded into electric signals, very much in the same way computers use 0's and 1's. In other words, the telegraph is digital, transferring information in combinations of two distinct values, instead of a continuously varying signal.
The encoding scheme (or code) of choice became one devised by Samuel Morse and named after him.
As Professor Comer puts it, "the world suddenly changed."
As a predecessor of the Internet, the telegraph:
This is the same case as what we have today with the Internet and computer networks in general. The difference of course, is in the quality and quantity of information we can transmit over the Internet. That is just about anything excepting physical objects (we need quantum printing and Teleportation for this).
One problem with the telegraph was that people had to rely on telegraph operators. They had to put up with how skilled those operators were (the best could send over a dozen words per minute). Privacy was also compromised to a certain extent.
The telephone was a major improvement on both counts (having to deal with an operator's speed, and the privacy.)
Digitizing the telephone took place in stages. Most significantly, before using dials, operators would have to manually reconnect wires on big switchboards to reach the proper destination. Later on, AT&T introduced a dial mechanism that enabled customers to dial their destination's number directly. Thus, there were two wires for each telephone line. One analog to transfer voice and the other digital to transfer the telephone numbers.
Later, the two converged, and today most telephone systems transfer telephone calls completely digitally.
Some facts about transmitting electrical signals over wires:
The reason I am mentioning these facts is to give a sense of the considerations that go into designing networks, and that have taken place throughout the history of computing and computer networking.
As usual, scientists came up with a nifty idea to transmit human voice over telephone lines with little distortion. Their idea was to include, in the telephone set, a device called modulator that generates a basic electric signal that oscillates or changes continually between two values of intensity. In other words, this signal is a modulation. This signal is called a carrier. The carrier is what we hear when we pick up the telephone. When the speaker's voice is turned into an analog electric signal, the modulator uses this signal to change the carrier signal.
On the receiving end, a demodulator which is set to expect a carrier, measures how much the incoming signal deviates from the normal value of the carrier. The receiver of the telephone then recovers the human voice.
Computers still use the same principle to transfer digital data. The device they use can send and receive at the same time by using two independent sets of wires. To be able to do that they have to use a device that contains both the modulator and the demodulator. This device came to be called the Modulator-Demodulator, or the MODEM.
Modems are also equipped to use telephone lines for transmission.
As we have seen with the telegraph, a method was developed to interpret letters in terms of a series of ON and OFF states for the electric current. Because computers also deal with ONLY two distinct states of current. Anything to a computer is a set of combinations of the two digits, zero and one. That is why we call computers binary machines.
The single smallest unit of information in a computer is one binary digit, otherwise known as the bit.
We use a code to represent characters, where each character is stored in the computer as a sequence or set of eight bits (including one bit for error detection).
Each set of eight bits is called a byte.
Multiples of 1024 bytes are referred to in terms of KiloBytes, or KB for short. (Kilo refers to units of thousands)
Multiples of 1024 KB are referred to as MegaBytes, or MB for short. (Mega refers to units of millions)
Multiples of 1024 MB are referred to as GigaBytes, or GB for short. (Giga refers to units of billions)
Multiples of 1024 GB are referred to as TeraBytes, or TB for short. (Tera refers to units of trillions)
The groups of zero and one's that are used to code letters numbers and symbols not only tell the computer which letter it is referring to, but it also contains information that enables the computer to ensure that no errors occurred during the transfer of information.
Another important convention to measure the transfer of information over transmission lines, are as follows:
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