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By now, you have encountered digital scanning if you've tuned across certain channels used by some federal agencies. Nothing being said is understandable, nor does it even sound like anyone is speaking-just bursts of white noise or basic hiss. Well, even if listeners could copy the digital mode, digital federal traffic is all encrypted, anyway. Sensitive communications on FBI, DEA, Secret Service, and Customs Service frequencies are often sent using digital (properly called DVP, for digital voice processing) scrambling-or what the Feds refer to as the bubble machine. Non-sensitive communications, however, are often sent in clear-voice mode.
In addition, there is analog voice scrambling. This decades-old technology was once called speech inversion. It's very simple. At a specially equipped transmitting station, the modulation's high frequencies are reversed with its low frequencies. To casual listeners, such transmissions are unintelligible gibberish. At properly equipped receiving stations, the process is reversed; and the voice sounds completely normal.
This unsophisticated and relatively inexpensive method achieves a useful degree of communications privacy. It's popular with public service and industrial licensees. Scrambled 46/49-MHz cordless telephones all use analog methods. I have monitored analog systems on VHF-FM commercial maritime channels and also on some TV newscrew assignment desks.
It's possible to understand scrambled channels-as well as to scramble channels oneself. CTP offers a nifty combination speech scrambler and descrambler-the Model DS49. It does a fine job of descrambling standard frequency-inversion schemes using 3300-, 3500-, and 3750-Hz pilot tones, as well as all non-standard splitband schemes using pilot tones between 2.4 to 4.7 Hz. Capable of providing analog scrambling, the unit is compatible with most cordless phones and two-way communications systems. If hooked to a scanner, it will also descramble such signals. Be sure to check communications laws and regulations in advance to see if they relate to your intended usage.
The DS49 is available in several versions. For instance, the DS49-CU is a stand-alone self-contained unit with a built-in speaker and an AC adapter. It plugs into the extension speaker or earphone jack and costs $89.95. Other models require four easy solder connections to fit any receiver and a +12VDC power source inside the radio. The basic DS49 kit is priced at $39.95; with external controls, $46.95; or wired and tested,...