Communication of information involves use of signals, which are classified on the basis of their origin and nature.
The sound produced by human being in conversation or photograph are converted into continuously varying electrical analog signal. But in modern electronic communication systems, these are converted into discrete form, which has finite values at different instances of time and zero otherwise
Information can be packaged in both analog (or continuous) and digital (or discrete) forms. For example, speech is an analog signal which varies continuously with time. In contrast, computer files consist of a symbolic discrete-time digital signal.
In the digital format, signals are in the form of a string of bits, each bit being either ON or OFF (1 or 0). The binary system refers to a number system which uses only two digits, 1 and 0 (as compared to the decimal system which uses ten digits from 0 to 9).
Communication systems can be either fundamentally analog, such as the amplitude modulation (AM) radio, or digital, such as computer networks. Analog systems are less expensive than digital systems for the same application, but digital systems are more efficient, give better performance (less error and noise), and greater flexibility.
The most crucial parameter in communication systems is the signal bandwidth, which refers to the frequency range in which the signal varies. While analog bandwidth measures the range of spectrum each signal occupies, digital bandwidth gives the quantity of information contained in a digital signal. Analog bandwidth is expressed in terms of frequency, the digital bandwidth is expressed in terms of bits per second (bps).
Human speech has bandwidth of nearly 4 kHz. The bandwidth is about 10 kHz in amplitude modulated (AM) radio transmission and 15 kHz in frequency modulated (FM) transmission. However, the quality of signal received from FM broadcast is significantly better than that from AM. The compact discs have bandwidth of 20 kHz. The bandwidth of a video signal is about 4.2 MHz and television broadcast channel has bandwidth of 6 MHz.