Basic Communication Technologies : Signal Types

Telecommunication technology involves the transfer of information signals through wires, fiber, or through the air by the by means of electrical or optical signals. Communication signals are usually characterized by their intensity (voltage and current) and frequency (cycles per second). To allow information to be transferred using communication signals, an information source (audio, data or video) is either represented by the signal itself (called the baseband signal) or the information slightly changes the wave shape of the communication signal (called the broadband signal). The information is imposed on the carrying signal (called the carrier) by varying the signal level or time changes (frequency shift).

Signal Types
There are two basic types of signals: analog and digital. Many communication systems receive analog signals (e.g., audio signals), convert them to a digital format, transport the digital signals through a network, and reconvert the digital signals back to their analog form when they reach their destination.

Analog
An analog signal can vary continuously between a maximum and minimum value and it can assume an infinite number of values between the two extremes.

Figure below shows a sample analog signal created by sound. In this example, as the sound pressure from a person’s voice is detected by a microphone, it is converted to its equivalent electrical signal. Also, the analog audio signal continuously varies in amplitude (height, loudness, or energy) as time progresses.



Digital
Digital signals have a limited number of discrete states, usually two, in contrast to analog signals that vary continuously and have an infinite number of states. Digital signals transfer discrete signal levels at predetermined time intervals. Digital signals typically have two levels: on (logic 1) and off (logic 0). The information contained in a single time period is called a bit. The number of bits that are transferred in one second is called the data transfer rate or bits per second (bps). Because many bits are typically transferred in 1 second, the data rate is typically preceded by a multiplier k (thousand) or M (million). For example, if the data transfer rate is 3 million bits per second, 3 Mbps would indicate this. Bits are typically combined into groups of 8 bits to form a byte. When the reference is made to bytes instead of bits, the b is capitalized. For example, 10 thousand bytes is represented by kB. Figure below shows a sample digital signal. In this example, the bits 01011010 are transferred in 1 second. This results in a bit rate of 8 bps.



The earliest form of digital radio communication was Morse Code. To send Morse Code, the radio transmitter was simply turned on and off to form dots and dashes. The receiver would sense (detect) the radio carrier to reproduce the dots and dashes. A code book of dots and dashes was used to decode the message into symbols or letters. The on and off pulses or bits that comprise a modern digital signal is sent in a similar way.

The trend in communication systems, just as in other types of electronics products such as compact discs, is to change from analog systems to digital systems. Digital systems have a number of important advantages including the fact that digital signals are more immune to noise. Unlike analog systems, even when noise has been introduced, any resulting errors in the digital bit stream can be detected and corrected. Also, digital signals can be easily manipulated or processed in useful ways using modern computer techniques.