Information is analog if it can take on any value in a continuum of values. There are no “forbidden states” in the range of values the information can take. If one were to plot the behavior of an analog variable over time, the resulting graph would have no discontinuities, since the variable can take on any value in the range. Essentially all physical quantities, such as temperature and pressure, are analog in nature. Since voice is a fluctuation in air pressure, it follows that the voice signal itself is analog.
Consider the process of sending analog or digital information over a communications facility. If the signals generated in sending the information can take on any value in a range, the signaling process is said to be analog regardless of the nature of the underlying information. If the underlying information is itself analog, one merely needs a transducer to convert the energy in the analog information to electrical form, and then allow the electrical signal to flow over the transmission facility. An example is provided by voice—the transducer is in the handset of the telephone and the electrical analog of the voice pressure waves is routed over the telephone facilities.
If the signaling process is restricted to analog, and the underlying information is digital, the standard technique for signaling is to provide an analog carrier signal whose properties can be modulated (i.e., varied) to superimpose the digital information. For example, in computer-to-computer communication a modem at the sending end modulates the carrier signal, and a modem at the receiving end demodulates it and extracts the digital information. Modem is an acronym for modulator/demodulator.
In analog transmission, attenuation is overcome by amplifying the signal along the way. Amplifying means making the amplitude greater. Whatever signal comes into the amplifier is boosted in strength, as is, and transmitted. Analog transmission makes no attempt to interpret the signal or separate signal from noise. The only selectivity in the process of analog amplification is that the amplifier amplifies only those frequencies required to carry the signal. This means two or more signals containing nonoverlapping passbands can be carried independently on the same medium (provided the medium can pass all of the frequencies involved). This becomes important when we discuss multiplexing techniques.
Analog signaling procedures vary from the very simple to the very complex. A particularly simple technique is frequency shift keying (FSK), which uses one carrier frequency for a one, and a second for a zero. A somewhat more complex procedure is quadrature phase shift keying (QPSK), which shifts the phase of the carrier by ±45 degrees or ±135 degrees to convey information. The four possible phase shifts in QPSK enable this procedure to send a digital signal that takes on four discrete values. Some very complex modulation techniques modulate several parameters at once; for example, techniques that modulate both phase and amplitude are common.