synthesizer 32 viz the 6666 hz signal is coupled via a low pass filter 39 having a cut off frequency of 8 8 khz to the second input of mixer 38 the output of mixer 38 is filtered through a 6500 hz low pass filter 40 and comprises the modulation signal carrier vfmo having a pseudo randomly varying frequency lying in the range from 4 khz to 5 6 khz the remaining output of frequency synthesizer 32 is coupled through a mode switch 41 during scramble mode to the input of an lc oscillator 42 shown in fig8 the output of oscillator 42 is coupled through a two pole low pass filter 43 the output of which comprises the pilot signal fig4 illustrates the single side band modulation processing section of the scramble unit 14 as seen in this fig the modulation signal carrier vfmo is phase shifted by 90 u00b0 by an rc unit 51 and the 90 u00b0 phase shifted output from unit 51 is boosted by amplifier 52 prior to limiting in a limiter amplifier 53 in order to provide a constant amplitude 90 u00b0 phase shifted modulation carrier signal as a first input to a mixer 54 the.