To better support legacy analogue systems, MOTOTRBO radios are able to support simple Frequency Inversion Analogue Scrambling. During the 1990s a number of vendors produced (plugin or solder-in) option boards for a number of makes of radios. Some of these vendors no longer exist or do not produce option boards anymore so customers need a solution that will allow them to continue using their existing analogue fleet until they can fully replace with DMR.
Two base frequencies are supported: 3,29 and 3,39kHz. Here is a nice article on Wikipedia which explains how Frequency Inversion (a.k.a. Voice Inversion) Scrambling works.
Frequency Inversion Analogue Scrambling only provides a very basic level of protection. Because the audio is processed, some fidelity is lost.
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