Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!cs.utexas.edu!usc!wuarchive!emory!gatech!prism!mailer.cc.fsu.edu!uflorida!cs.fau.edu!terryb From: terryb.bbs@shark.cs.fau.edu (terry bohning) Newsgroups: sci.electronics Subject: Re: Audio Jamming Message-ID: Date: 16 Feb 91 23:44:57 GMT References: <8071@exodus.Eng.Sun.COM> Sender: bbs@cs.fau.edu (Waffle BBS) Organization: Florida Atlantic University Lines: 78 Summary of possible ECM against offensive audio sources: This summary is for informational purposes only. Intentionally interfering with licensed broadcasts or broadcasting at all without the appropriate FCC license is illegal. For FM and TV broadcasts, a signal generator or other RF source which is tuned to the appropriate frequency and suitably coupled to space (ordinarily via an antenna) may be used. If adequate power is available, broadcasting on an IF frequency would alleviate the need for manually tracking the receiver frequency. Broadcast band FM receivers ordinarily use an IF of 10.7 MHz, TV receivers use a picture IF of 45.75 MHz and a sound IF which is 4.5 MHz below that, at 41.25 MHz. TV receivers built before 1950 or so use a picture IF of 25.75 MHz. Broadcast AM receivers use 455 kHz as the IF freq. Direct audio sources (tapes, CD, albums) are tougher. Because of equipment complexity, size, and power levels, it's difficult to get an audio wavelength propagating. In any event, the offensive source has nothing to act as an antenna except perhaps a long run of speaker wire. This would *really* require a lot of power. A possibility appears to be a CB radio. Consider a common emitter transistor stage in an audio amp which has a lightly bypassed or unbypassed emitter resistor. Then the base-emitter junction (diode) and the emitter resistor in conjunction with the bypass cap (or the capacitance to ground of the subsequent stage) form a classical AM detector circuit. There are of course many other potential AM detectors in an audio amplifier chain, including IC's and FETs. If this method is effective, it can of course also be used against RF sources, since these are ultimately converted to baseband (audio). CB radios are cheap and widely available. Experimentation with power levels may be required. In the mid to late 1970s, linear amplifiers were available from many CB shops. These usually consisted of Horizontal Output Tubes from television receivers of the day connected in a grounded grid configuration. For each tube in the unit, about 50 to 75 W of power was obtained. A lot of older 10 meter amateur gear has been converted to run on 11 meters (CB band = 27 MHz). Many of the aforementioned linears were designed to run on 10 meters, but came with instructions telling which components *not* to change to allow the unit to run on 11 meters. Conveniently, a small bag of said components was also supplied. Some higher power linears (500W up) simply had straps across several turns of the air-core coils in the unit. Cutting these straps retuned it to 11 meters by increasing inductance in the tank circuits. Lower modulation freqs may work best so they don't get bypassed to ground. CB channels have a 6 kHz bandwidth, so the transmitters limit the highest modulating freq to less than 3 kHz anyway, usually with a fairly obvious filter. If the modulation freq were 60 Hz, even a knowledgeable listener might believe it to be an equipment malfunction and perhaps remove the offensive unit for a trip to the repair shop. Voice modulation is ordinarily inadvisable. Lastly, note that very little, if any, CB gear is rated for continuous duty. One suggestion has been to induce transients on the AC line by interrupting a large inductive motor with an SCR. Obviously, if the transients are sufficiently large and/or fast, expensive equipment may be destroyed. Furthermore, such transients would be totally indiscriminate in whose equipment and which equipment would be affected. In addition, it is unclear whether these transients could be coupled through the pole transformer and beyond or not. Also, turning a piece of equipment off does not necessarily protect it from line transients, so the consequences could be very far-reaching. Implementing this action would violate more than FCC regulations, and would leave the implementor open to criminal prosecution and legal action.