Well, I was referring to Motorola's Eavesdrop Reducer Technology, which is a very basic analog inversion scrambler of sorts available on their FRS/GMRS radios (like the T6220 model). Voice inversion is an archaic and simple form of encryption, but not entirely effective. The FCC concluded there was just enough intelligence in the transmission that it wouldn't be considered illegal, but a casual listener is not going to be able to follow the conversation that well. I know because I've tried. This form of inversion is quite old, simple, and can be found in many higher end FRS/GMRS radios. Let's make this quite clear, analog inversion of this nature is not the same thing as private tone, PL tone, CTCSS, or any other control tone or digital control tone scheme. It is a processing of the audio signal that alters the content of the audio intelligence, not just adding a low frequency tone to the mix. Digital filters and trunking are something entirely different also. There are also a whole host of public agencies that can and do use some form of encryption, usually digital. DES is supposed to be only for the Federal Government use, but you will find DES units at hamfests on a regular basis. There are also a number of industrial two way communications using DVP encryption, which is perfectly legal.

Here's a site that talks a bit about basic encryption. http://www.fordyce.org/scanning/scanning_info/encrypt.html

I used to do a lot of work on DES encrypted radios for the Feds, and we had some more complicated analog encryption add-ons for a couple of metro drug and undercover units that we put together for local police departments. I also recall adding a couple of split double inversion sweep tone encryption circuits to some industrial radios, but that was way back in the 80s. Stepping into the wayback machine, I saw some pretty interesting encrypted stuff in the Navy, but I was a spook back then, and the Russians were trying everything they could think of to keep our nose out of their business.

If I had my druthers, I would use spread spectrum transmissions for all my communication needs. Who needs encryption when your signal is bouncing over a 100 Mhz band unpredictably?