Analog transmission is atransmission method of conveying information using acontinuous signal which varies inamplitude, phase, or some other property in proportion to that information. It could be the transfer of an analogsignal, using an analogmodulation method such asfrequency modulation (FM) oramplitude modulation (AM), or no modulation at all.
Some textbooks also considerpassband data transmission using adigital modulation method such as ASK, PSK and QAM, i.e. a sinewave modulated by a digital bit-stream, as analog transmission and as ananalog signal. Others define that asdigital transmission and as adigital signal.Baseband data transmission usingline codes, resulting in a pulse train, are always considered asdigital transmission, although the source signal may be a digitized analog signal.
Analog transmission can be conveyed in many different fashions:
There are two basic kinds of analog transmission, both based on how they modulate data to combine an input signal with a carrier signal. Usually, this carrier signal is of a specificfrequency, and data is transmitted through its variations. The two techniques areamplitude modulation (AM), which varies the amplitude of the carrier signal, andfrequency modulation (FM), which modulates the frequency of the carrier.[1]
Most analog transmissions fall into one of several categories.Telephony and voice communication was originally primarily analog in nature, as was most television and radio transmission. Early telecommunication devices utilized analog-to-digital conversion devices called modulator/demodulators, ormodems, to convertanalog signals todigital signals and back.
The analog transmission method is still very popular, in particular for shorter distances, due to significantly lower costs with complex multiplexing and timing equipment that are unnecessary, and in small "short-haul" systems that simply do not need multiplexed digital transmission.[2]
However, in situations where a signal often has highsignal-to-noise ratio and cannot achieve source linearity, or inlong distance, high output systems, analog is unattractive due to attenuation problems. Furthermore, as digital techniques continue to be refined, analog systems are increasingly becoming legacy equipment.[2]
Recently, some nations, such as the Netherlands, have completely ceased analog transmissions (analog switch-off) on certain media, such as television,[3] for the purposes of the government saving money.[4]