Abstract: A band scrambler which processes only time domain samples is described. The band scrambler has the effect of dividing the input signal spectrum into N sub-bands. The N sub-bands are permuted such that the r th band is mapped onto the k.r th band modulo N, where N is a constant of the scrambler and k is the key which is variable in the range 2<k<N-1. The output samples y(n) produced by the scrambler from the input speech signal samples x(n) are defined by the equation: ##EQU1## The down-sampling function s(n'+n(k-1)) determines which of N series of ganged switches is closed and the window function h(n') determines the values of the factors stored in the multipliers. The summation is carried out by the adder in order to produce the required output time samples which are reconverted to an analogue signal via a digital-to-analogue converter. The signal can be de-scrambled by sampling and passing through another identical scrambler operating with a key, k*, where kk*=1 (mod N).