An analogue signal is an electro-magnetic waveform that continuously varies its amplitude over time. It differs from a digital signal in that small fluctuations in the amplitude of the signal may convey information. The word analogue reflects the fact that the signal is often an analogy of some real-world input to the system. For example, there is a direct relationship between the variation in the voltage of an electrical signal on a telephone line and the pattern of sound waves entering the microphone mounted in the telephone's handset.
An analogue system uses some physical property of the signal to convey information. In telecommunications systems, the property most commonly used is voltage, which is made to vary in response to some physical input. This is achieved using a transducer. A transducer is a device that converts energy from one form to another (e.g. heat energy to light, sound energy to an electrical signal, etc.). A clock with hands is said to be an analogue device because the time is represented by the constantly changing position of the clock's hands (although for many clocks the movement of the hands around the clock face occurs as a series of small, discrete increments, rather than a smooth and continuous circular motion).
In on of the oldest types of microphone, sound waves striking a thin diaphragm cause it to vibrate. Carbon dust inside the microphone, used to conduct an electrical current through the device, rapidly changes in density as the vibrating diaphragm compresses, and then releases it. The small changes in the density of the carbon dust alter its electrical resistance, varying the amount of current that can flow through it. Since the resistance of the telephone wire itself does not change, and since, for a given value of resistance, voltage varies in direct proportion to current, these small changes in current can be seen as changes in voltage across the telephone line.
A typical analogue signal
The main disadvantage of an analogue signalling systems is that, because the signal is continuously varying (as opposed to the two or three discrete levels used in digital systems) any unwanted signals (noise) introduced into the system are often difficult to detect and to filter out of the signal. Furthermore, the effects of noise get worse the further the signal has to travel, because the signal is attenuated. Essentially, this means that the signal becomes weaker the further it travels from its source, whereas the level of noise, both inherent and external to the system, remains relatively constant. As a result, the signal-to-noise ratio (SNR) decreases steadily, and at some point the signal will become indistinguishable from the noise. A signal may, of course, be amplified at one or more points along the transmission path in order to compensate for attenuation, but the noise in the signal will inevitably be amplified as well. The effects of noise can be mitigated by using suitable cable and connector types to screen out external interference, but there is no way of eliminating the so-called Gaussian noise (or thermal or white noise) which is due to the random movement of electrons in a conducting material.
The range of levels in an analogue signal can be said to be infinite, because any two points on the waveform, however adjacent, will have different values. The relative distance between the two points can theoretically be halved, and halved again an infinite number of times, without producing two identical values, since an analogue signal has no discontinuous points and follows an unbroken curve for its full duration. In principle, therefore, it would seem that an analogue signal should be able to represent some real-world dynamic entity, such as the sound of the human voice or a symphony orchestra, far better that a digital signal that essentially consists of only two or three discrete voltage levels. Indeed, when it comes to the subject of the reproduction of music, there is much debate over the relative merits of analogue and digital recording techniques. When it comes to telecommunications, however, the problem becomes one of maintaining signal integrity over long distances.
The signal can, of course, undergo amplification at various points along the transmission path to ensure that the signal-to-noise ratio is maintained above some predefined threshold. Some of the inherent or injected noise can probably be filtered out of the signal. Unfortunately, the very nature of an analogue signal (i.e. constantly varying) means that it is usually not possible to completely separate the original signal from the noise, particularly in view of the fact that the inherent Gaussian noise is present across the entire frequency spectrum supported by the physical medium. Hence, when an analogue signal undergoes amplification, any noise that cannot be removed from the signal is amplified along with it, in equal proportion.
The effects of noise can be reduced in analogue telecommunications systems using appropriate design, engineering and installation techniques. Such techniques would include the use of suitable transmission media, which could dictate the use of shielded cabling, and careful selection of cabling routes to avoid potential sources of electromagnetic interference. Analogue signals have been used successfully for decades to carry relatively low-frequency voice signals through the public switched telephone network, and are still widely used in the local loop of the telephone network (the connections between telephone company subscribers and their local exchange). Until relatively recently, analogue systems were also used for radio and television broadcasting. The advent of the Internet and the proliferation of computers in commerce, industry and the home have fuelled the development of digital communications systems capable of carrying virtually any and all kinds of digital data. Despite the digital revolution, however, an understanding of analogue signalling techniques is still crucial to a study of telecommunications systems.