When I was a kid, prior to cable TV, I would see lines on the television screen when my Mom used the mixer in the kitchen. This would also happen when she vacuumed the floor. After we got cable the interference no longer occurred. Why would a mixer or vacuum cleaner cause interference with a television signal?
Small electrical motors have connections in them to the moving parts which are called “brushes.” In inexpensive motors, such brushes are less closely fitted to the stationary elements. Each time the motor turns each brush connects, and disconnects at least once. The cheaper the motor, the larger the spark which occurs as this happens. (Very powerful motors also do this, even when made very well.) The spark has an electromagnetic frequency, and propagates just like any other energy, although the frequency is not controlled, or modulated. You TV, radio, or whatever receives its signal through the air, and picks up the sparking as well. That is static. Cable signals are shielded by the outer covering of the cable, and are much less prone to pick up static.
Tris
Imagine my signature begins five spaces to the right of center.
Before cable, you had parallel conductors running from the antenna to the TV. These conductors act as an antenna themselves. They broadcast their signal to the environment and pick up interference signals from the environment (interference can be reduced by twisting the conductors around one another). With cable, the conductors are coaxial (same axis, one conductor inside the other). This arrangement (in theory) has no external fields and rejects feilds from the environment.
I have cable, and it picks up interference from my microwave oven. It shows up as two lines of snow which slowly travel up the screen. When I’m heating something at medium (so the microwaves are turned off and on), the snow disappears when the microwaves are off (I can tell because the fan speeds up a bit when the microwave circuit is not drawing power).
So apparently it’s coming from something connected with the microwave apparatus rather than the fan.
I believe I’ve also picked interference on cable from the next apartment’s blinking Christmas lights.
“No,” he replied, and smiled seraphically, as was his wont.
That’s why I said “in theory”. The math says it shouldn’t happen, but the real world is not made up of ideal materials or conditions.
Anyway, I’m betting it would be much worse w/o coax.
PMH, if your microwave is on the smae branch circuit as your television set, you’ll get the kind of interference you describe.
You may have interference propagating through the 115VAC wiring in your walls.
Otherwise, you might consider having the cable company come out and establish an earth ground for your cable system.
–Kalél
TheHungerSite.com
“If ignorance is bliss, you must be orgasmic.”
“Well, there was that thing with the Cheese-Wiz…but I’m feeling much better now!” – John Astin, Night Court
zgystardst, maybe your microwave is leaking?
Your answers above are correct…one area that didn’t get mentioned:
Your TV picture is AM. This is one reason the picture is prone to interference.
This should change around 2006 when North Amercan TV makes the final switch to HDTV, which is digital.
A quick and easy way to fix that noise leaking in through the power cord is to install an RF choke on it. These can be had cheaply at Radio Snack, and just clamp right over the power cord.
Cheap coax cable frequently has a leaking shield and/or connectors. A cheap TV or VCR might have a short unshielded section just inside the chassis, and that’ll pick up all sorts of stuff. IIRC, you can install a choke on coax to keep noise from running down the outside where it will eventually find an opening in the shield.
I lead a boring life of relative unimportance. Really.
Big motors are usually induction type and don’t have any brushes to arc. FWIW motors with brushes are typically called universal motors because they can run from an AC or DC source. They are used in applicances because it’s easy to vary the speed with an inexpensive control. Induction motors are true AC with rotation speed fixed to the AC frequency. Speed can be varied but it’s generally much more expensive to do so than on a universal.
Here’s a good simple overview of Electro-Magnetic Interference or EMI. Of course, if this give you an appetite to learn more, you could always buy a copy of MIL-STD-461 for yourself. There you could learn all about conducted emissions, radiated emissions, shielding theory and aperture leakage. Enjoy, it’s loads of fun.
Just a side note: We had this VERY expensive computer monitor at work (same format as the new digital tv’s) which suddenly developed these strange colored bands across the screen. We thought the tube was about to fail, as degaussing had no effect. Then we realized one of the operators had placed a small electric fan next to the monitor. Moving the fan away from the monitor solved the problem.
This is a different kind of interference. Up till now we’ve been discussion radiated and conducted emissions, but this is a near-field effect, not radiated. Your expensive monitor displays colors by shooting an electron beam at small colored phosphors on the back side of the front glass. Electron beams are deflected by magnetic fields (that’s how the monitor does points it where it needs to go). You fan created its own magnetic field which would deflect the beam enough so that it wasn’t hitting the right color phosphor.
I got lines on my tv & computer monitor once. I also got some minor shocks from the bath tub faucet. Called the electric comp to ask them what’s up? They came out in like only an hour & had to rewire things.
[This is a different kind of interference. Up till now we’ve been discussion radiated and conducted emissions, but this is a near-field effect, not radiated]CurtC
Correct. Magnetic field versus electro-magnetic radiation. I just mentioned it in case anyone was having a similar problem and couldn’t figure out the source. P.S. Don’t store your video tapes too close to the built-in speakers in your entertainment center, if you have one.
Gary
“Master of the uncalled for.”
Homepage: Gary’s Place
I don’t believe that the interference mentioned by toots4fred is the result of radio interference picked up by the antenna, so much as the product of AC line noise (possibly still radio frequency). Here’s what I offer as evidence:
We used to witness the same phenomena at my house, except it did not occur on a battery powered TV. Also, we were one of the firsts to get cable in our neighborhood, but we kept the same old TV down in the basement. The newer TV never experienced the problem (even before cable) and the older TV still experienced the problem (even after cable). I think that newer TVs have better noise immunity and newer motors reflect less noise back onto the AC. That’s my theory and I’m stickin’ with it… at least until someone sez I’m full’a crap…
A guy next to me once was using a 30’ antennae for his ham radio. Made lots of lines on my TV. I called him, got his daughter, she said they have lines on their tv too [duh]. Talked to the guy he said that it is my tv & i should write tv manf’s & tell them to make sets that can withstand incredibly strong ham radio waves. ha.