Analog and Digital Signals


In general, there are two types of telecommunication transmission--Analog transmission and Digital transmission.


Analog Transmission

Analog transmission uses signals that are exact replicas of a sound wave or picture being transmitted. Signals of varying frequency or amplitude are added to carrier waves with a given frequency of electromagnetic current to produce a continuous electric wave. The term "analog signal" came about because the variations in the carrier waves are similar, or analogous, to that of the voice itself.

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.


Digital Transmission

In digital transmission the signals are converted into a binary code, which consists of two elements—positive and non-positive. Morse code and the "on and off" flashing of a light are basic examples. Positive is expressed as the number 1, while non-positive is expressed as the number 0. Numbers that are expressed as a string of 0s and 1s are called binary numbers. Every digit in a binary number is referred to as a bit and represents a power of two. For example, in the binary number 101, the 1 at the right represents 1 x 2º; the 0 in the middle represents 0 x 2¹; and the 1 to the far left represents 1 x 2². The decimal equivalent of 101 is (1 x 2²) + (0 x 2¹) + (1 x 2º) = 4 + 0 + 1 = 5. In a standard code used by most computers, the letter "A" is expressed in 8 bits as 01000001.

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.

Digital format is ideal for electronic communication as the string of 1s and 0s can be transmitted by a series of "on/off" signals represented by pulses of electricity or light. A pulse "on" can represent a 1, and the lack of a pulse "off" can represent a 0. Information in this form is very much easier to store electronically. Furthermore, digital transmission is usually faster and involves less noise and disturbances as compared to analog data transmission.


Translating Information

Computers translate 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.

In the 1960s, computer scientists discovered how to translate audio and video information into computer data, by expressing every point in a color video image and every instant of sound as a string of 1s and 0s. TV programs and movies that have been digitized in that way can be held in the memory of a computer as easily as textual documents. However, encoding TV images required a huge number of 1s and 0s (or bits). One TV signal sent digitally meant 90 million bits per second, which was highly impractical, since it would take several channels to convey a single digital TV signal. However, with the invention of digital compression in the late 1980s, the pictures could be transmitted in a highly abbreviated form.

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 functions—modulation and demodulation.