Devices passing multiple signals simultaneously, such as multichannel radio equipment, develop IM products that are SO varied that they resemble white noise. Intermodulation noise may result from a number of causes: Improper level setting. If the level of an input to a device is too high, the device is driven into its nonlinear operating region (overdrive). Improper alignment causing a device to function nonlinearly. 7A harmonic of a certain frequency F can be 2F (twice the value of F), 3F, 4F, 5F, and SO on. It is an integer multiple of the basic frequency. QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS Nonlinear envelope delay. Device malfunction. To summarize, IM noise results from either a nonlinearity or a malfunction that has the effect of nonlinearity. The causes(s) of intermodulation noise is(are) different from that of thermal noise. However, its detrimental effects and physical nature can be identical to those of thermal noise, particularly in multichannel systems carrying complex signals. 3.3.3.4 Impulse Noise. Impulse noise is noncontinuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude. These spikes are often called hits, and each spike has a broad spectral content (i.e., impulse noise smears a broad frequency bandwidth.) Impulse noise degrades voice telephony usually only marginally, if at all. However, it may seriously degrade error performance on data or other digital circuits. The causes of impulse noise are lightning, car ignitions, mechanical switches (even light switches), fluorescent lights, and SO on. Impulse noise will be discussed in more detail in Chapter 10, Data Communications. 3.3.3.5 Crosstalk. Crosstalk is the unwanted coupling between signal paths. There are essentially three causes of crosstalk: 1. Electrical coupling between transmission media, such as between wire pairs on a voice-frequency (VF) cable system and on digital (PCM) cable systems. 2. Poor control of frequency response (i.e., defective filters or poor filter de