Speech inversion is a very common method of speech scrambling, probably because its the cheapest. Speech inversion works be taking a signal and turning it 'inside out', reversing the signal around a pre-set frequency. Speech inversion can be broken down into three types, base-band inversion (also called 'phase inversion'), variable-band inversion (or 'rolling phase inversion') and split band inversion.

Base band inversion inverts the signal around a pre-set frequency that never changes. Because of this, base-band inversion is useless!! Because the inverting frequency never changes, running the frequency through another inverter set on the same frequency unscrambles it. Motorolla markets phase inversion scrambling under the trademark "Secure Clear".

Descrambling baseband inversion is simple. Take the scrambled input and re-invert it around the same inversion point used to scramble it. Manufacturers often have a stock inversion point.

Variable-band inversion inverts the signal around a constantly varying frequency, making decryption possible, but not bloody likely. Variable band inversion can be identified by the burst of modem noise at the beginning of the transmission (its a 1200 bps carrier) and the repeated clicking sounds as the inverting frequency changes. Descrambling variable band inversion would be a chore for the amateur eavesdropper, as the inversion point changes every fraction of a second. Professionals however would likely have little trouble extracting clear speech.

Split-band inversion is another method for making inversion more secure. Split band inversion divides the signal into two frequencies and inverts them (baseband) seperately. Some split band inversion systems provide enhanced security by randomly changing the frequency where the signal is split at given intervals. It far from perfect, but better than vanilla baseband.

Speech Encryption Encryption is a much stronger method of protecting speech communications than any form of scrambling. Voice encryptors work by digitizing the conversation at the telephone and applying a cryptographic technique to the resulting bit-stream. In order to decrypt the speach, the correct encryption method and key must be used.