# Descrambling split-band voice inversion with deinvert

DevFeed: [Descrambling split-band voice inversion with deinvert](<https://devfeed.tech/articles/descrambling-split-band-voice-inversion-with-deinvert-21651.md>)

Original publisher: [Read original article](<https://www.windytan.com/2017/09/descrambling-split-band-voice-inversion.html>)

Author: Oona Räisänen (noreply@blogger.com)

Published: 2017-09-12T20:31:00Z

Content type: tutorial

Language: en

Sources: [Oona Räisänen](<https://devfeed.tech/sources/oona-raisanen.md>)

Topics: [Software](<https://devfeed.tech/topics/software.md>), [Algorithm](<https://devfeed.tech/topics/algorithm.md>), [Tool](<https://devfeed.tech/topics/tool.md>)

Tags: [algorithm](<https://devfeed.tech/tags/algorithm.md>), [communications](<https://devfeed.tech/tags/communications.md>), [radio](<https://devfeed.tech/tags/radio.md>), [security](<https://devfeed.tech/tags/security.md>), [software](<https://devfeed.tech/tags/software.md>), [tool](<https://devfeed.tech/tags/tool.md>), [walkie-talkie](<https://devfeed.tech/tags/walkie-talkie.md>)

## AI overview

This article explains how the deinvert software tool reverses simple voice inversion and descrambles split-band inversion. It covers the filtering and mixing stages, the role of inversion carrier and split-point frequencies, and why split-band audio requires two inversion passes.

## Source excerpt

Voice inversion is a primitive method of rendering speech unintelligible to prevent eavesdropping of radio or telephone calls. I wrote about some simple ways to reverse it in a previous post. I've since written a software tool, deinvert (on Codeberg), that does all this for us. It can also descramble a slightly more advanced scrambling method called split-band inversion. Let's see how that happens behind the scenes. Simple voice inversion Voice inversion works by inverting the audio spectrum at a set maximum frequency called the inversion carrier. Frequencies near this carrier will thus become frequencies near zero Hz, and vice versa. The resulting audio is unintelligible, though familiar sentences can easily be recognized. (HTML5 audio: Inverted speech.) Deinvert comes with 8 preset carrier frequencies that can be activated with the -p option. These correspond to a list of carrier frequencies I found in an actual scrambler's manual, dubbed "the most commonly used inversion carriers". The algorithm behind deinvert can be divided into three phases: 1) pre-filtering, 2) mixing, and 3) post-filtering. Mixing means multiplying the signal by an oscillation at the selected carrier frequency. This produces two sidebands, or mirrored copies of the signal, with the lower one frequency-inverted. Pre-filtering is necessary to prevent this lower sideband from aliasing when its highest components would go below zero Hertz. Post-filtering removes the upper sideband, leaving just the inverted audio. Both filters can be realized as low-pass FIR filters. This operation is its own inverse, like ROT13; by applying the same inversion again we get intelligible speech back. Indeed, deinvert can also be used as a scrambler by just running unscrambled audio through it. The same inversion carrier should be used in both directions. Split-band inversion The split-band scrambling method adds another carrier frequency that I call the split point. It divides the spectrum into two parts that are