Project of the Month

A simple speech-inversion de-scrambler.

This project is a simple de-scrambler for the 'speech-inversion' type scramblers. The parts are cheap, the circuit is fairly simple - and it works well. The de-scrambled audio is not as good as a ridgey-didge descrambler, but it is perfectly readable. Using the right speaker helps as well. If you build this circuit in a small, bassy bookshelf speaker, you can't tell it from a store-bought.

The circuit

The four diodes are the heart of it all. The four diodes arranged nose to tail form a 'ring modulator' also known as a 'product detector'. The audio from the radio is fed into two ends of the diode ring, and a mixing tone is fed into the other two. VR1 adjusts the balance of the mixing tone so that it is fed into each end of the diode ring at exactly the same level. When it is balanced perfectly, the only audio that comes out is the mix of the input audio and the tone. If the incoming audio is scrambled, then it already has a tone, so if we adjust the 'Tune' control so the the mixer tone frequency matches the scrambler tone frequency, it cancels it out and you get un-scrambled audio out. Simple eh?

Of course, it works backwards, too. If the incoming audio is not scrambled, this circuit scrambles it. So you have to only plug it in when you hear scrambled audio.

The mixing tone is generated by a NE555 timer and oscillator IC, being used as an oscillator here. The 'Tune' control adjusts the tone frequency from about 2 kilohertz to about 4 kilohertz. Instead of taking the 'square-wave' tone from the output pin of the '555, we sample the capacitor charge/discharge voltage, which is a sawtooth-wave. The sawtooth-wave is a 'softer' waveform, and will give much better audio quality.

The mixed tone and audio from the product detector goes straight into the LM380, a 2.5 Watt audio amplifier, and into a speaker.

You will need:

A radio that has an external speaker socket.

A speaker.

An LM380 audio amplifier IC.

A NE555 timer IC.

Four 1N4148 ordinary silicon diodes.

A 1K ohm linear pot. Any convenient size. And a knob to go with it.

A 3.5mm speaker plug.

A piece of veroboard to build it on.

Some resistors and capacitors and such.

Step 1

Build the circuit. I recommend building it on a piece of veroboard, or stripboard.

The best way to build this is put it inside a speaker box. An old bookshelf stereo speaker, or a communications extension speaker will do fine. Drill a hole in the side of the speaker for the 'Tune' control.

Pay attention to the orientation of the diodes.

Click the schematic diagram for a bigger picture.

Step 2

Setting it up.

Plug the de-scrambler into the external speaker socket of a radio.

Put the 'Tune' control in the centre. Connect power. At first, you should hear a tone coming out of the speaker. Adjusting VR1, the tone should be loudest when the trimpot is at each end, and quiet when it's in the middle. Adjust it till the tone is at it's quietest. If there is no quiet point, disconnect power and make sure the diodes are connected as shown in the schematic. I had to redo mine because I got it wrong, and it's my own circuit!

Tune radio to a channel with active users. Adjust the volume on the radio till the output audio is comfortable. If everything has gone right, you should be hearing people scrambled. That's because the de-scrambler is a two-way circuit. If the received audio is scrambled, it will be de-scrambled, and if it's not scrambled, it will be scrambled.

That's it. Wait till you hear someone on a scrambler, then just adjust the 'Tune' control until you hear them clearly. Don't go too fast on the tune control, or you may miss it. Old sidebanders will get it straight away, it works just like a clarifier.

It won't de-scramble the rolling code type scramblers (like those on CH3 recently), not unless you're pretty fast on that tune control, they change code every second or so.

Final Notes

As noted, this circuit is a simple one. Although it works well, it's missing refinements like - a volume control, a bypass switch to switch between scrambled and un-scrambled, and audio filters to clean up the de-scrambled audio.

For the advanced constructor, get the ducks-guts circuit, here.