The small series of the articles about Scramblers by MSM Group. Direct Inversion.

Direct inversion.
Final Part III.
Direct inversion of speech spectrum.

Author: SergUA6 6.0
Band Width 2700 - 3400 Hz, it can vary, depends of tasks.
Low Range 100 - 300 Hz, it also can vary.
RX mode All types of modulation.

Sonograms

pic.1 Spectrum of inverted speech

pic.2 Spectrum of normal speech

1. The example of inverted speech, point(frequency) of inversion is 3600 Hz.

2. Exactly restored inverted speech.

Direct (simple) inversion - is the direct(simple) inversion of spectrum. It is one of the most widespread sorts of speech's masking. It is easily recognised by ear (in the case of absence of the strong interferences) and by the form the spectrum: it is well-visible on the spectrum, that the main energy is concentrated in the upper band of the frequencys, what is unusual for speech. Those who saw a normal spectrum of speech and inverted atleast once , will unmistakably define inversion further, even if it is got mixed up with other features. For today this sort of masking Makes impression unless on absolutely trustful people. Direct inversion is easily defined, easily removed, and in general it is more suitable for creation of insignificant minor problems , than for masking.

Nevertheless, the direct inversion is still widely spread today, by virtue:

• of it's simplicity
• of the absense of synchronization (the weak place of all synchronized scramblers)
• of the posibility to work in conditions of the strong interferrences

And anyway the function of the fast hiding of information is realized, althought the direct inversion isn't a barrier even in real-time.

The inverted spectrum is very easily formed by following steps:

• the frequency of inversion usually around 3000-4000 is selected
• after multiplication of an initial signal (speech) with this frequency is occuring
• then the lower lateral is selected from resulted DSB signal. This lateral is the inverted spectrum of the initial signal

The modern tools allows to do it very qualitatively. The same manipulations with the signal are used for restoring.