Audio Masking
cryptomuseum.com
cryptomuseum.com
I like it a lot that many of techniques have a hybrid analog/digital structure that would involve sample-and-hold, sweeps and comparators like the Triple Pulse scheme.
Today I can’t believe you wouldn’t use some digital solution but at that point in time you’d be lucky to be able to use a small IC.
https://www.amazon.com/Great-Wall-Street-Scandal/dp/00701702...
which was a scandal with many dimensions including the author of that book getting busted for insider trading, there was an incident where the management of that company set up bugs in a room where auditors were working so they had some idea of what the auditors were looking at so they possibly trick them.
That web site has some articles where they talk about fabrication techniques the CIA was using for bugs and it seemed they weren't using ICs but rather trying to miniaturize discrete component designs as much they could just as IBM was doing for the digital electronics for the System 360.
I'd assume if you were doing it now you'd use some kind of pure digital scheme with an ADC, encryption and spread-spectrum modulation of some kind. I still have a sweet spot for things like sample-and-hold, PLLs, and such though.
And then you have someone building this thing in 72 with ceramic gold-plated PCBs and mil-spec components: https://www.cryptomuseum.com/covert/bugs/opec/
You need to generate a train of audio pulses separated by silence, and find a band whose activity is being modulated with the same timing.
The great seal bug(The Thing)
https://www.youtube.com/watch?v=NLDpWrwijE8 (Machining and Microwaves)
What I like about this specific video is that the guy actually builds one. And there is a world of difference between a popular article on how the thing worked. and the subtle genius engineering required to get it to actually work.
> As a countermeasure against an NLJD, professional covert listening devices (bugs) of the Central Intelligence Agency were equipped from 1968 onwards with a so-called isolator. An isolator is a 3-port circulator of which the return port is terminated with a resistor. Any energy injected into the bug by an NLJD will be absorbed by the resistor, resulting in no (or very little) reflected energy. An example of such a bug is the CIA's SRT-107.
(or my favorite:)
> A means to hinder isolating a non linear junction is to add inexpensive diodes to places where the NLJD is expected to sweep. This masks the true listening device against a field of false alerts when the many diodes are detected. Such a technique was used in the 1980s construction of the U.S. embassy in Moscow. Thousands of diodes were mixed by the Soviets into the building's structural concrete, making detection and removal of the true listening devices by its American occupants nearly impossible.
One more question: are there any bugs that shut down if there is no chatter in the spectrum... Say, if it's a noisy environment (frequency wise) with many phones and devices, the bug blends in and transmits. If it gets quiet, such as when phones are being turned off or distant, then there's something fishy and the bug suspends its operation?
They aren't usually off-the-shelf. They're custom, and can be as smart as the builder wants to make them. (working within constraints like size and power supply)