Yes it can be done but will cost you a lot of time and tests.
Basic idea of analog scrambler is to mix the audio spectrum with a higher carrier, then mix again but with another carrier and obtain a reversed spectrum but in original BW so will be UN-intelligible.
Take a llok on CML website, have circuits that do that already in a small package
**broken link removed**
Otherwise, you should mix your self which isn't a big deal, but filtering will be a huge effort
http://circuitsstream.blogspot.de/2013/06/build-voice-scrambler-or-descrambler.html
For example
300....3000 mixed (use MC1496 or NE612) with fixed 10 KHz will produce two side bands
10300... 13000 (the normal audio, shifterd) <-- filter this out
7000... 9700 (the reversed spectrum) <-- keep this one
Mix again with 6700 Hz and will obtain
300... 3000 <-- scrambled, but in the same audio band
Now the problem is filtering on side band. If you are clever, will mix with something to use a professional crystal filter used by hamradio amateurs in SSB communication (search for SSB crystal filter). For this particular filter (in KHz or MHz range) you will need two carriers that can be obtained by using either proper crystals (hard to find) or some DDS chips.
And finally, all this crap can be easily obtained inside a DSP circuit like dsPIC or something.
Regards,
Why can't the orginal receiver resolve LSB?
Brian.
Ok I got it, the problem is because you can't filter effectively the audio carrier and the upper audio sideband.Yes, you can. This is the idea inside of DSP where is much simpler to implement a LPF with 80dB slope or something.
In real life, you will not be able to cancel out the 3500 Hz carrier which is in the same audio band, so basically will hear a continuous tone along scrambled and de-scrambled signal.
If I position the LO below an LSB signal (assuming my RX is set for USB), I will receive the audio inverted. If I re-invert the audio I can understand the message.Since LSB is reversed you can try demodulate by positioning yourself under the carrier, so will appear like USB for you. But in the same time will receive also LSB from bellow frequencies so will end up in listening a nice mix of 2 stations.
I made an audio scrambler circuit with an MC1496 balanced modulator/demodulator IC and a couple of switched capacitor lowpass filter ICs, about 31 years ago. The demodulator was almost the same and its output sounded perfect. The transmitted signal had a suppressed scrambler carrier and it sounded like high audio frequencies were low and low audio frequencies were high, completely scrambled.
The circuit in post #6 does not invert audio frequencies. Instead it simply adds buzzing modulation then a voice sounds like an old robot.
If the scramble frequency is a little higher than the audio frequency band then the audio might have an upper and a lower sideband, one which could be filtered to make inverted frequencies.
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