Are you hearing the Same thing on the output channel as well?
I am guessing your pretty close if your able to pick up the input signals of the patrol cars. but maybe the signal is not quite strong enough to decode the modulation.
I usally hear them quite clear on the output . heck I have heard them as far away as the medical center with out any problems decoding the signal.
to my knowledge I have never head them use any encryption. but it is certainly not out of the realm of possiblities.