Preliminary implementation of software voice scrambler/descrambler

Background

A simple voice scrambler using SSB techniques is discussed. The scrambler relies on unfriendlies not knowing the original scramble frequency. This causes the spectrum to wrap-around itself, causing high frequencies to be represented as low and vice versa depending on where the “secret” scramble frequency is.

From real-world experience, a real scrambler using this technique would not have the CD-quality exhibited here. There are always slight variations in injection frequency, even from one serial number to the next. I have personally installed over one hundred of these scramblers in my teenage years into a variety of two-way radio equipment. I had to open up the radios, and via the schematics, figure out where to properly connect the scrambler for the cases where the particular radio didn’t have an instruction sheet from the scrambler manufacturer.

The Transcrypt SC20-400 is an example product using such a process. Midian Electronics also makes such modules.

Simulation results

Without any real effort to optimize, the MATLAB code runs the voice scrambler/descrambler in about 1.5 seconds on an Intel Core 2 Quad, with the code running on all four cores (the code is vectorized). Note that the decoding process takes place twice within those 1.5 seconds, once for the “good” decode, and once for the “bad” decode.

The above is the voice spectrum, upsampled to 88.2kHz to be compatible with the frequency mixer frequencies used in this simulation (sampling frequency must be no less than twice the highest frequency component of the sampled signal).

The 4096 tap FIR filter has an extremely sharp cutoff of 10kHz.

If you are unaccustomed to the concept of negative frequency, just ignore the spectrum below 0Hz and focus on the positive frequencies only.

The output of the scrambler is very similar to an SSB transmitter set the LSB mode, with a carrier frequency of 20kHz.

Obviously this is too low for on-air transmission, but inside a modern DSP transceiver, this or a similar process occurs in the digital IF of the radio.

The USB has been reduced by more than 80dB via another 4096 tap FIR filter, this time with a cutoff of 20kHz to preserve the LSB only.

This signal would be upconverted to an RF frequency, and sent on-air.

The output of the receiving descrambler, before low-pass filtering to remove the undesired output.

The red spectrum shows the effect of an improper descramble frequency. It is visibly evident that the spectrum has wrapped on top of itself, becoming useless to the “unfriendly” listener.

Categories:

Updated: