On Friday 28 September 2007, Jay Gagnon wrote: > I have a digital cable box and a cable modem, both on my TV stand, both > plugged into the same cable source via a splitter. The cable comes into > the room on the wrong side so it runs under the rug before it gets to the > splitter. I recently purchased an RC Helicopter that operates at > 72.somethingsomething MHz (I can get the specifics if they are actually > important), and yesterday I decided to fly it in the living room while the > TV was tuned in to a football game. When I turned on the helicopter, > nothing of interest happened, but the instant I turned on the transmitter, > the cable signal cut. I turned the transmitter off and the signal came > back. I turned the transmitter back on for a little while and eventually > the modem started blinking its "where did the cable go" light pattern and a > message appeared on the TV asking me to check the cable connection. As a > final experiment, I tried lifting off, and discovered that if I kept the > helicopter in the upper half of the room the cable signal would come in > fine, but the closer I got to the floor the worse it got.
Interesting. This type of thing is not uncommon. Based on the description I don't immediately suspect EMF, since the cable signal cuts out before the electric motor is started. I'll explain two typical situations that could cause this. 1) The TX frequency of the RC transmitter is in the IF of the cable system 2) Receiver desense in the cable recevier and cable modem More detail: 1) I don't know the internals of the cable receiver or a typical cable modem, but I'd guess that it would be of a superheterodyne design, which is typical in modern RF receivers. These mix down the received signal to a lower frequency (usually referred to as an "IF") which is the Intermediate Frequency that is actually received and worked on. Well, if that IF frequency just happens to be in the 72 MHz range, guess what -- you jam it with the RC transmitter. Last I remember, I believe RC ransmitters send out an FM modulated PWM [Pulse With Modulated] signal, which essentially looks a lot like a signal you'd expect to see from an intentional scrambler. 2) Receiver desense means that the receiver receives a signal that it normally wouldn't because the signal is so strong. Almost everybody has experienced this while driving around and listening to the radio in the car; get too close to a radio station and it "breaks through" the channel you wanted to listen to. RF signal strenth multiplies by four times every time you half the distance between the receiver and transmitter. The same goes for audio -- get in the shower, right under the shower head, and you can't hear someone outside of the bathroom. The shower isn't that loud, but being that close to it makes that signal a lot stronger than the person speaking outside of the bathroom. > So, I have a couple of questions. Is the problem most likely that I'm > interfering with the cable itself as it runs across the room, or the > decoders in the devices? Could be either one; that's something you can experiment with to find out. > Either way, is there anything I can do about it > other than stop flying the helicopter in the lower half of the living room? Depends; if flying the RC helecopter in front of the TV is important to you, then investigate it further. Increasing the amount of shielding in the cable system might help but only if the source of the signal leakage is the cables and connectors themselves. Good RF cables have a double helix braid for the outer shield, but the cheaper cables only have a few thin sets of strands where an outer braid would normally be. [What's the standard cable type, anyway -- maybe RG6? I forget.] -- Chris -- Chris Knadle [EMAIL PROTECTED] _______________________________________________ Mid-Hudson Valley Linux Users Group http://mhvlug.org http://mhvlug.org/cgi-bin/mailman/listinfo/mhvlug Upcoming Meetings (6pm - 8pm) MHVLS Auditorium Oct 3 - Security and Privacy Nov 7 - Django Python Application Framework
