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Single mode audio spectrum inverter question

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neazoi

neazoi

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Hi I attach two images. One is a single more audio spectrum inverter and the other one is a dual mode (two stages).
As far as I am awave the dual stage should suppress the carrier and the unanted sideband.

However the single mode is attractive due to it's simplicity.
1. I need to ask, what is the purpose of the transformer in the single mode circuit? Why one couldn't just use a coupling capacitor and leave the other output unconnected?
The ne602 is a balanced modulator and the carrier is suppressed, so why to include an output transformer is a mystery.

2. I think that an opamp lpf should also provide the necessary unwanted sideband suppression.
 

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  • Build a Voice Scrambler or Descrambler Circuit Diagram.png
    Build a Voice Scrambler or Descrambler Circuit Diagram.png
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  • Simple Voice Scrambler Disguiser Circuit Diagram.png
    Simple Voice Scrambler Disguiser Circuit Diagram.png
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NE602 has a balanced output and the transformer is a simple way to convert to single-ended for the audio amplifier. If you only use one NE602 output I do not believe it will work, you would need an op-amp instrumentation amp to reject the large common-mode (carrier+sideband) signal. A LPF even with say 40dB attenuation is still small compared to 70-80dB attenuation with transformer.

I recognize pics from Radio Electronics Jan. 1993
More info here: https://www.nutsvolts.com/magazine/article/build_a_voice_changer
 
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    neazoi

    neazoi

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I made a voice scrambler that used a MC1496 balanced modulator IC similar to the NE602 IC. It inverted the audio frequencies and had a suppressed carrier. Transmitted voices were completely unintelligible but decoded voices were perfect. The decoder circuit is almost the same as the encoder circuit. I used a switched capacitor lowpass filter IC to suppress the unwanted sideband.
 
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    neazoi

    neazoi

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Audioguru said:
I made a voice scrambler that used a MC1496 balanced modulator IC similar to the NE602 IC. It inverted the audio frequencies and had a suppressed carrier. Transmitted voices were completely unintelligible but decoded voices were perfect. The decoder circuit is almost the same as the encoder circuit. I used a switched capacitor lowpass filter IC to suppress the unwanted sideband.
Click to expand...

At the time I was writind this post, I found about the MX/FX128 IC. This seems perfect and with a few peripheral components and available in DIP. Crystal stability and embedded capacitor filter.
The only problem is that it is expensive.

I am not aware if there are other such ICs out there.
 

The two largest electronic parts distributors in the USA do not have the FX128 IC. I guess it is available from the manufacturer if you buy a few thousand of them. Hardly anybody needs a voice scrambler.
 

Audioguru said:
The two largest electronic parts distributors in the USA do not have the FX128 IC. I guess it is available from the manufacturer if you buy a few thousand of them. Hardly anybody needs a voice scrambler.
Click to expand...

Well yes and no.
I am trying to make two DSB transceivers communicate. The technique I have thought of (and tested in software) is to shift the frequency of one of them to receive only one sideband of the other and then invert the detected audio spectrum to recover the original information.
So such a device will be used to do it in hardware.
I am not using it for security but just for recovering the inverted sideband information.
 

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