Back in the bad old analog days the bad guys had to use a "monitor" radio. A tuneable multiband portable transistor radio with a slide-rule dial on the front. You had to tune in the local police while they were transmitting, but once it was tuned, you could eavesdrop.
The selectivity of these radios was not great, and if a nearby channel was also transmitting, bleedover would screw up the audio. My boss installed an "all call" button on the PD's radio console that activated all three adjacent repeater channels at the same time. The dispatcher was instructed to flip that switch for anything they didn't want the bad guys to hear. With all three repeater channels transmitting at once, the cheap monitor radios would only produce garbled audio.
In a year or two scanners got cheaper and more plentiful. They were built to be more selective, so this trick no longer worked.
But it was the cheapest "scrambling" setup I ever heard about.
73
The selectivity of these radios was not great, and if a nearby channel was also transmitting, bleedover would screw up the audio. My boss installed an "all call" button on the PD's radio console that activated all three adjacent repeater channels at the same time. The dispatcher was instructed to flip that switch for anything they didn't want the bad guys to hear. With all three repeater channels transmitting at once, the cheap monitor radios would only produce garbled audio.
In a year or two scanners got cheaper and more plentiful. They were built to be more selective, so this trick no longer worked.
But it was the cheapest "scrambling" setup I ever heard about.
73