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Originally Posted by Thornhill20
How exactly does rolling code inversion work? I can understand fixed code inversion, but what about rolling code?
Do the radios have a preassigned "codebook" of what frequencies to use, and how often to use them...or does the system send out some kind of command telling it when to switch, or how does that whole thing work?
-M
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Rolling-code has the inversion frequency set by a pseudo-random generator inside the transmitter and receiver. The receiver needs to be synchronized to the transmitter, which happens at the beginning (and during the transmission may be re-sync'ed). The inversion frequency "mutates" during transmissions which is why a fixed inversion-type decoder fails. The pseudo-random sequence is designed to be so long before it repeats that the chances of figuring out anything by decoding what's transmitted, is very remote. From what I understand, voice inversion causes a loss of audio quality (and range) because after inversion at the TX it needs to be filtered, then filtered again after recovery at the RX. The filters are not perfect and the audio is degraded.
I read a whole book on analog voice scrambling (about 15 years back). It mentions that people can be trained (in a matter of days) to understand inverted speech.
Another interesting type is called time-domain scrambling. Think of it like having an electronic "tape" which is cut up in short pieces (less than a syllable) and spliced out of order, transmitted; and only the RX knows how to re-order it correctly. I've heard it on low-band skip (probably military) and it sounds bizarre, human yet non-human at the same time.
Dave