ion channel often includes many speech or data channels that are multiplexed into the same cable pair or fiber. The principle of multiplexing is explained later in this chapter. 4.1.1.3 Receiver The receiver operates on an output signal from the channel in preparation for delivery to the transducer at the destination. Receiver operations include filtering to take away out-of-band noise, amplification to compensate for Transmission transmission loss, equalizing to compensate for distortion (different attenua- tion of frequency components), and demodulation and decoding to reverse the signal processing performed at the transmitter. 4.1.1.4 Noise, Distortion, and Interference Various unwanted factors impact the transmission of a signal. Attenuation is undesirable because it reduces signal strength at the receiver. Even more seri- ous problems are distortion, interference, and noise, the last of which appears as alterations of the signal shape. To decrease the influence of noise, the receiver always includes a filter that passes through only the frequency band of message frequencies and disables the propagation of out-of-band noise. 4.1.2 Signals and Spectra Electrical communication signals are time-varying quantities such as voltage or current. Although a signal physically exists in the time domain, we can also represent it in the frequency domain where we view the signal as consisting of sinusoidal components at various frequencies. This frequency-domain description is called the spectrum. Any physical signal can be expressed in both domains. In the time domain we draw the amplitude along the time axis and in the frequency domain we draw the amplitude (and phase) along the frequency axis. Although both of them give a perfect description of the signal, both presenta- tions are needed for easier understanding of the different phenomena. The time-domain signal is the sum of the spectral sinusoidal components. Fourier analysis gives the mathematical connection between the time- and frequency-domain descriptions. Here w