 above reference imbalance between wire pairs in a cable. noise dBrn. A 1000Hz tone having a magnitude of -90dBm will give 2. Poor control of frequency response in an analogue multiplexer a reading of OdBrn regardless of weighting, but all other measure- system caused by defective filters or poor filter design. ments must specify the type of weighting. 3. Non-linear performance in analogue multiplexer systems. 10.9.2 Equivalent noise resistance There are two types of crosstalk: In a circuit using valves or transistor devices shot noise is produced. Intelligible crosstalk. where at least four words are intelligible For noise calculation purposes this shot noise can be thought of as to the listener from extraneous conversations in a seven second the thermal noise produced by an equivalent resistor'. period. For a triode the hypothetical resistor is inserted in series with the 2. Unintelligible crosstalk, which is crosstalk resulting from any grid and its value is given by Equation 10.33, where is the mutual other form of disturbing effects from one channel to another. conductance of the triode. Intelligible crosstalk causes the greatest problem to the listener. (10.33) This is caused by the fear of loss of privacy or the fact that the listener has great difficulty in differentiating between the party to which he is connected and the interfering circuits. 10.9.3 Noise temperature 10.8.8 Impulse noise The thermal concept can be applied to other types of noise sources Impulse noise consists of short spikes of power having an approxi- such as diodes and noise tubes. The noise temperature of such a mately flat frequency response over the spectrum range of interest. device is the temperature of a thermal noise source which produces 10/12 Signals and noise the same available noise power as the device under consideration using a psophometer, which is an instrument for measuring channel regardless of the actual physical temperature of the device. noise and includes a weighting network. For most purposes it is The concept of