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Voice scrambler

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dr pepper

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We use Cb radios on Fork lift trucks to communicate between drivers and the office, delivery trucks also have cb's, and I'd like to have a secure channel for maintenance use.
Scrambler mic's are available, but they are expensive.
So i'm investigating whether its worth making something, I'm not after something unbreakable, just inaudible when unscrambled, frequency inversion as provided on some handhelds is still audible esp on male voices.
 

Dr_Doggy

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what about an additional modulator/demodulator extension patched right in to the tx mic/rx speaker? usually walkies have plugs for external mics/spkrs we could jack right in to
 

dr pepper

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Yup thats what I meant, the rigs have the usual 4 pin mic socket and 3.5mm mono ext speaker sockets, I'd have to implement an external audio amp & speaker but thats no problem.

I found this:
http://www.swtpc.com/mholley/PopularElectronics/Mar1970/PE_Mar1970.htm
The I found this (google translate convert sit to english):
http://digilander.libero.it/francodpx/filetesto/Tex-scrambler.htm

The former is from the 70's and requires experienced operator adjustments, the latter is a similar circuit but not using fiddly transformers, and it has a nifty pll locked oscillator so the rx oscillator remains in phase with the tx carrier oscillator.
Listening to youtube vids voice inversion still seems to still be legible to a degree after encryption, so I'm not sure if this circuit will be good enough, but one thing usefull is the pll carrier extarction circuit, I'm sure I could use that with a 'duino.
I had thoughts of using a msgeq7 spectrum analyser with a 'duino, then sending pcm codes at 3k baud or so for each of the 7 bands over the air, then synthesising those 7 bands to reproduce the audio at the rx end in a similar fashion to vocoder using voltage controlled filters and a noise source, 3k baud would give 50 samples/sec, I dont know have naff that would sound.
 
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audioguru

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I made a suppressed carrier single sideband scrambler for wireless microphones in a teleconference system. The voice frequencies were inverted and were completely non-understandable but the demodulated voices sounded perfect.
I used an MC1496 modulator-demodulator IC (surface-mount ones are still made), a crystal oscillator and switched -capacitor lowpass filter ICs.
 

dr pepper

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The second link I posted uses a ne602 to do the same thing, I think the '1496 uses the same gilbert cell as the '602.
A good thing in that circuit is the carrier is generated by the encoder and some of it is transmitted with the audio so the decoder carrier oscillator can be easily phase locked to the encoder meaning nearly any old oscillator can be used including the one in the ne567, a transfo on the o/p of the '602 would improve suppression further, the whole thing is fairly simple.
I have watched a few more youtube vids on inversion and I must have watched a poor example as just about every other has illegible audio, just goes to show not every vid is good.
 
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audioguru

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My suppressed carrier is non-existent and the non-inverted sideband is totally eliminated by the switched-capacitor lowpass filter IC leaving the inverted sideband with excellent fidelity when a carrier is added and when the single sideband is decoded to be normal.
 

dr pepper

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Yep I get that, do you have any issues with carrier sync?, I spose a few hz drift and resultant pitch change isnt going to be so bad.
Transmitting some carrier also makes it harder to eaves drop, and the pll totally cancels it in the decoder.
I knocked togther a circuit on breadboard, not a decoder just an encoder, a 555 and '602, yes it makes a right mess of speech, I dont think lorry drivers and bosses would be able to tell we are calling them names over the air.
I spose if I were to record the scrambled audio then played it back through it would descramble.
 

BobW

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Is it legal to transmit scrambled audio on CB?
 

dr pepper

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I dont think so, its illegal to transmit music, but thats only because you need a public performance licence for that.
And at 400mw in a sparsely populated area it isnt going to upset too many.

I was thinking the phase locked carrier could be usefull for other projects, being able to sync two devices with an audio frequency that is cancelled out at the rx and has minimal effect on the audio is kinda nifty.
 
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