17 Describing Rmdorri Signals 408 random process A random process B Figuse 17.2: Randoiri processes A and I3 with a diffwirig distribution between thc indlviduitl smiple furletions Taking the positive square root of the variancr giwt.; the standard deviutzorr a z ( t ) Variitricte and standard deviation are measures for the spread of the amplitudr around the linear averitge for a randnnl signal The rttlldo~xrprocess A in Figure 17.2 deariy has a lower variarice tlrrari random pro( The linear average arid the variance are by far the most frequently used firstordcr pxprctcd values When combined with second-order exp sufficknt to chara rise many common random signals For general raridorn signals it is possible to define inore first-order expected wliicv by choosing a different f ( x ) in (17 3) E x ~ 17.2 ~ ~ A general characterisation with firsbordcr expected values is obtained by pntting j ( z ) = E ( @ - ) into (17.3) and determining tbr expected valuc dependent 011 tht threshold We l