The characteristics of the disturbances affecting the paper web are discussed with particular reference to grammage variation, wet press vibration, and calender barring. Some of the difficulties of investigation are pointed out. It is shown that the usual assumptions of stationarity and linearity may not hold, so that frequency domain analysis must be applied with caution.
The key to any investigation lies in the comparison of corresponding spectral components in signals from different points in the system. Two contrasting approaches are compared and discussed: that adopted in the conventional two channel analysis of random data, and that of considering the interrelationship as a vector function of time. The latter approach, embodied in the Vector Correlator, gives considerable insight into the underlying relationship between the variables. Some practical examples are given.