Google Patents US5515014A - Interface between SAW filter and Gilbert cell mixer this provided a bit more privacy on ship-to-shore phone calls, but anybody with an analog music synthesizer (the patch cord type, not the later types that were all pre-programmed) could patch together a working descrambler in less than a minute.Įdit: i was going to ask if anybody knew where i could find an EMC-1 synthesizer, but i just realized i could probably put a demo of that scrambler system together using GnuRadio.US5515014A - Interface between SAW filter and Gilbert cell mixer back in the 1980s, some ship-to-shore HF stations used speech inversion scrambling with a twist, the oscillator was frequency modulated by rectifying the envelope of the speech, and using the envelope signal to wobble the oscillator. at the receiving end, passing the inverted speech through an identical modulator and filter restores the original audio. since low frequencies are further from the 3khz frequency of the oscillator, and high frequencies closer to it, the spectrum of the speech is inverted. a common audio use of a multiplier is for speech inversion scrambling, which multiplies the signals from an audio input with an oscillator (usually about 3khz) which creates sum and difference frequencies of the input speech and the oscillator, and the high sideband filtered out. in audio, the balanced mixer or multiplier is often called a "ring modulator" to keep it from being confused with a "mixer". the term "mixer" though is quite common, especially in audio. a simple analog "mixer" such as the two resistors, or using an op amp with a resistive summing node on the input, really should be called a summing circuit, or an "analog adder". a single-ended mixer, such as an RF mixer used in many radios and receivers must have the two original signals, and the unwanted sum or difference signal (in the frequency domain) filtered out. a "balanced mixer", whether a diode ring or multiplier will give suppression of the two original signals. Again, its easiest to see if you do the math, just as crutschow says. A mixer (or any non-linear circuit) does this multiplication. Now, if you multiply sin(x) times sin(y) the result is very different, and you have sin(x), sin(y), sin(x+y) and sin(x-y) all in the result. Why? Well, I was taught this effect by using mathematics to show how when you add one sin(x) plus a second sin(y) all you get is sin(x)+sin(y). If the volume is cranked up very high, the speakers probably begin to operate non-linearly and your ears also start to be non-linear and then you hear harmonics as well as sum and difference products, and the tones (or the music) become a real mess. Here's another way to think about it.when you hear two different tones of sound with your ears and if those tones are pure and they are not too loud, you only hear those tones, you don't hear anything else, right? Take the alternative to an extreme, if simply adding two tones together (again, if the tones are pure and if the volume is not too loud) were to create a sum and difference, how could you possibly listen to any music and not have it sound completely polluted with many many sums and differences? You don't because at reasonable volumes, your ear is operating linearly and not distorting the sound. It takes some non-linearity to cause the distortion. Only when you combine the two while also distorting them do you create new frequencies. Seconding what crutshchow said, when you have two sinusoidal voltages and simply add them together, you simply don't get any new components at any other frequencies.
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