For example, in analog
transmission, say a telephone system, an electric current or the reproduction of patterned
sound waves are transmitted through a wire and into the telephone receiver. Once this is
completed, they are then converted back into sound waves.
As an example of digital
transmission, in a type of digital telephone system, coded light signals produced by a
rapidly flashing laser travels through optical fibers (thin strands of glass) and are then
decoded by the receiver. When transmitting a telephone conversation, the light flashes on
and off about 450 million times per second. This high rate enables two optical fibers to
carry about 15,000 conversations simultaneously.
information from the computer user into binary code in a process called digital encoding.
Letters can be encoded by replacing every letter with its numerical position (1-26) in the
alphabet, and then converting these decimal numbers into binary equivalents. A sound can
be encoded as a series of numbers that measure its pitch and volume at each instant in
time. An image can be encoded as a sequence of numbers that represent the color and
brightness of each portion of the picture. The computer is able to decode information by
converting the numbers back into letters, sounds, or images.
A modem is a
device used to convert between analog and digital signals. They are often used to enable
computers to communicate with each other across telephone lines. A computer sends digital
signals, which are converted by the modem to analog signals that can be transmitted
through telephone lines. When the signal reaches its destination, another modem
reconstructs the original digital signal so as to enable the receiving computer to process
the data. To convert a digital signal to an analog one, a modem generates a carrier wave,
and modulates it according to the digital signal. The kind of modulation depends on the
application and speed of operation for which the modem was designed. For example, many
high-speed modems use a combination of amplitude modulation (where the amplitude of
carrier wave is changed to encode the digital information) and phase modulation (where the
phase of a carrier wave is changed to encode the digital information). The process of
receiving the analog signal and converting it back to a digital signal is called
demodulation. In fact, the word modem is derived from its 2 basic functionsmodulation
|Analog & Digital|
Top of Page