VI8200

Voice inverter option card

    One of widely used method for scrambling voice in radio transmissions is voice inverting. For example here in Czech Rep. all of state police transmissions are scrambled. The pricipe of inverting voice is simple and is explained on my picture below. Audio signal  is modulated on carrier (with frequency range varying from 2.4kHz to 6.15kHz for VI8200) and the products are two side bands. Upper band is AF spectrum added to carrier frequency (4000 + 300 = 4300), lower band is AF spectrum subtracted from carrier (4000 - 300 = 3700). Uper band is useless because is only shifted up and still intelligible. After processing trough lowpass filter, remaining only inverted and shifted voice frequency spectrum.

   It is possible to decode inverted transmissions with computer and proper software. Probably the best one is realized by IZ8BLY. It is available on his homepage as other interesting programs. The other way is to use special IC designed for this purpose. For example FX128 from Consumer Microcircuits. They can be obtained from RS-Components.

   Schematic diagram of original VI8200 module: