Analog Speech scrambler

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SETEC_Astronomy
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Analog Speech scrambler

Post by SETEC_Astronomy » Mon Aug 20, 2007 9:41 pm

I'm trying to build an analog speech scrambler but I'm having a hard time finding a schematic. All of my web searches pull up circuits that simply clip the audio. Clipped audio is still understandable and from what I can tell irreversible. Is there a moderately simple method to scramble speech and have it reversible (for the most part) so the unscrambled speech is fairly close to the original? I'm trying to shy away from a digital solution since it would be more difficult to make and more expensive to send via radio waves. Any suggestions?

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Chris Smith
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Post by Chris Smith » Mon Aug 20, 2007 10:37 pm

They do have soft ware for the computer.

Pure scrambling I assume, not voice changing?

Also PGP has some method of scrambling the voice using his code.

But I have to assume both use the services of a sound card and a computer.

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Post by SETEC_Astronomy » Mon Aug 20, 2007 11:09 pm

Thanks Chris. I should have mentioned though that I'm looking for a solution that doesn't involve a computer. In my eyes if I have access to a computer with the sound I might as well just encrypt the mp3 or wav file and send it. I have found a few schematics that use special ICs that were designed for cordless phones and that may be the way I have to go unless someone can point me to another method.

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Dave Dixon
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Post by Dave Dixon » Tue Aug 21, 2007 7:21 am

Years ago I built a circuit to descramble the narc detectives I could hear on my police scanner. The signal was merely inverted. They sounded like sideband with the BFO not adjusted (Donald Duck), and a simple circuit did the job. The same circuit worked on the input to my CB (yeah I used to play with them), inverting my audio as well. I'll dig around and look for it in my old Pop. Electronics collection. I remember several diodes in a chain, and a Mouser audio transformer. Don't remember if it had any active components or not (chips).
In the meantime, you might do a search on audio inverter circuits, or what not :)
Best of luck,
Dave

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Post by SETEC_Astronomy » Tue Aug 21, 2007 2:33 pm

I've been continuing to do research on this and the standard theory seems to be to invert the audio with an opamp at a set interval. Most of what I'm reading says the frequency used for inversion should be around 3.5KHz. For some reason though I'm getting better results with a much lower frequency, somewhere between 800Hz and 1200Hz. I'm ok with it but I must be missing something if that frequency is the best for others when used. The higher the frequency the more you can understand cause the inversion is so rapid it seems non existent, all a higher frequency seems to do for me is add an annoying whistling sound. I have tried recording a scrambled conversation and then running back through to unscramble it and it works but barely. To effectively descramble you need to sync with the freq/signal used when the sound was scrambled. Right? Does anyone know if this is indeed true and if so how is this typically accomplished? I figured I could send a tone (dtmf?) before each start of the audio and use that tone to sync the inversion signals. Once the tone is sent and received the inversion signals could be set to goto a known state after a set period of time. That's the only method that I can come up with but I don't see it having much of a chance of working.

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haklesup
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Post by haklesup » Tue Aug 21, 2007 2:49 pm

Depends on how scrambled you want it. Just about any method that distorts the analog audio can be descrambled by a determined snoop but if you want strong encryption, you'll have to go digital first.

Another analog method would involve mixing noise with the audio then filtering that out on the other end. Maybe use a stereo encoder, send the audio plus noise on one channel, send the noise only on the other and subtract the two at the receiver. That's just a theoretical idea, I don't have a practical circuit to suggest.

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Post by SETEC_Astronomy » Tue Aug 21, 2007 2:56 pm

Thanks for the tip. I'm not to concerned with the audio being descrambled. I'm just aiming to weed out the casual listener. I've been reading about some very detailed and complex methods of inversion using real time shifting frequencies, split phase inversion and alternating band inversion. Cool stuff I say...

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Chris Smith
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Post by Chris Smith » Tue Aug 21, 2007 8:16 pm

What radio freq are you on, and is side band enough?

Jumbling can occur many ways.

If you only want to screw with the government, make up a fake dialog, let them get sucked in, and bleed them dry.

Its worth every second.....[ mine was two years]

Dumb and dumber

The bigger the false story, the more they cant resist.

STROKE, STROKE was my motto 20 years ago.

Where are you feds? I have material.

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jollyrgr
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Post by jollyrgr » Tue Aug 21, 2007 11:07 pm

SETEC_Astronomy wrote:Thanks for the tip. I'm not to concerned with the audio being descrambled. I'm just aiming to weed out the casual listener. I've been reading about some very detailed and complex methods of inversion using real time shifting frequencies, split phase inversion and alternating band inversion. Cool stuff I say...
Casual listeners will be easily thwarted with a simple frequency inversion scrambler based on a ring modulator. This is what Dave Dixson was talking about. It looks like you have already been experimenting with one. Here is a schematic just in case you don't have one.

Simply place your source material on the IN, pick a CARRIER in the pass band of your system. If you are using a two way radio this may explain why you get better results at lower frequencies. Possibly your inversion point is too high in frequency. A scrambled signal will be heard on the OUTPUT. To reverse the procedure, put the scrambled audio to the INPUT, inject an identical CARRIER, listen to the descrambled audio on the OUT. This would require TWO identical circuits. Try to match the diodes as close as possible.

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Post by dyarker » Wed Aug 22, 2007 12:02 pm

"... but I must be missing something if that frequency is the best for others when used." (refering to 3.5KHz)

Try putting the input through a 3KHz low pass filter before scrambling.
Dale Y

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Lenp
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Post by Lenp » Sun Aug 26, 2007 11:30 pm

Years ago I saw a telephone scrambling design using a ring modulator. The carrier was an audio signal that was common to both sides of the conversation, like a network broadcast Sorry, no design info, just planting a seed!

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Post by Dean Huster » Mon Aug 27, 2007 6:04 am

Poplular Electronics had a project for one back in the (I think) early 1970s using (I think) a couple of balanced modulator circuits. They used a common radio broadcast audio source for the scrambling signal as it was more inconsistent than a fixed modulating tone. However, at intervals of dead air time, I think the scrambling went out the window. I suppose that one could mix three radio sources to reduce that problem.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

SETEC_Astronomy
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Post by SETEC_Astronomy » Thu Aug 30, 2007 6:48 pm

Thanks for all the help and suggestions. I tried out the circuit that Jollyrgr posted and it works great but I don't know how to properly use it with my limited electronics knowledge. The sound from the carrier overpowers the audio to be scrambled. Can someone point me in the right direction on how to interface the supplied circuit with line level sources? The audio to be scrambled is line level and the device receiving the sound needs a line level source.

rshayes
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Post by rshayes » Fri Aug 31, 2007 3:03 am

This type of modulator is usually operated at a relatively low level, with the carrier being larger in amplitude than the modulating signal. You might try a sine wave as a carrier to start with. A possible starting point would be a carrier of about 1 V and a signal level in the 100 mV range. You may have to attenaute the audio going in (resistive attenuator) and amplify the modulated carrier coming out (possibly using an op amp). When this type of modulator is used at RF frequencies, the input, carrier, and output ports are usually matched to a 50 ohm impedance. The telephone company uesd to use these in the carrier system used on their long lines. These were usually operated with impedances in the 600 ohm range.

Since you don't need to use a carrier in the RF range, I would suggest using a 600 ohm impedance. Audio transformers for this impedance should be relatively easy to get.

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Craig Kendrick Sellen
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speach or voice scrambler

Post by Craig Kendrick Sellen » Wed Sep 05, 2007 6:19 am

First to JOLLYRGR; In your ring modular I whould use SCHOTTKY BARRIER DIODES. SUCH AS 1N5819 or eqv.

I have a sugustion; how about using a A/D converter. If someone out there knows how to use one. Also I herd of a CODEX chip. You will probley have to get a matched chip set. Ask around the chip makers, to see what thay have. :grin:

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