This will probably be two expletives short of a Pitting, but I seriously hate Comcast. Ever since they changed the Cartoon Network (yeah that’s right… I need my Family Guy!) from Channel 77 to 50, there have been not one, but two “ghost lines” on this one and only channel! A thin horizontal one that stays stationary, and a very thick vertical one that slowly moves from right to left across the screen.
I’m in the NYC/Jersey City area… has anyone else been having this problem? I’ve had three (count 'em, three) cable guys come out, and each time I’m told something different.
Time #1: “Oh I see what it is. This can’t be fixed at your house, this is an internal problem. I’ll head back and take care of it. Give it 48 hours.”
A week later. No change. So I called back and scheduled another appointment.
Time #2: “I have no idea what this is. I’ll head back and have them look into it on our end. Give it a week.”
Six weeks later because I’m a busy guy. Still no change. Now I change the tone in my voice when I call to schedule another appointment.
Time #3: “It’s probably your DVD Recorder. Here, let’s disconnect that and connect the cable directly to your TV–well, that didn’t do anything. Okay well, sir, you’re just going to have to live with it. Don’t you know that unless you’re paying for digital cable, nothing’s going to be perfect? Most customers are a little more understanding that not every channel is going to come in perfectly.”
WHAT?!?? You have got to be kidding me. I’m not asking for perfection here. I’m only asking that all my channels come in equally as well as the next, especially if it’s a channel I want to watch frequently.
I’m going to take a deep breath at this point because I don’t want to turn this into a Pitting, but does anybody have any suggestions on a course of action I can take here?
I wish I could help, DooWahDiddy. I have a similar problem (and with Comcast). Periodically, we fuzzy reception on our “local” channels, but the remaining cable channels are fine. At some point, the fuzziness will go away and everything will be fine. Then, it comes back. I have no clue what causes it, or what causes it to go away.
I can offer one suggestion. Look carefully at the cable as it routes from the pole to your TV. Look for any staples holding the cable down. Measure the distance between the staples. Is the spacing between staples nearly the same? The staples slightly compress the cable changing its impedance. Having several staples with constant spacing can turn your coaxial cable into a single-channel filter. By measuring the distance and using the speed of electrons in a coax you can calculate the frequency of the filter. FYI, the video carrier of channel 50 is at 687.25 MHz.
If this is the case, remove a few staples and make sure they are spaced irregularly. If the cable is crushed, you may have to replace it.
It’s not the speed of the electrons in the conductor you’re interested in, it’s the speed of the electric field which propagates through the space around the conductor. This is dependent upon the speed of light through the insulating medium surrounding the conductor. For typical poly-foam insulated TV coax, this is about 66% of the speed of light in a vacuum. The speed of the actual electrons, called the drift velocity, is MUCH lower–typically a few meters per second which is slower than your average person’s walking speed, and is dependent upon the current through the conductor.
Nitpick and correction accepted. It’s been a few years since I’ve looked at this terminology.
Anyway, using 379.25 MHz and 66% of the speed of light in a vacuum, a half-wavelength is just over 10" (26 cm). This is a very typical spacing for staples. Many installers don’t realize the importance of spacing the staples irregularly and go for a neat look with regular spacing. It’s also better for the cable to be slightly loose in the staple so the cable is not crushed.
I have heard that one of the reasons local channels on cable are fuzzy or staticy is because the TV is getting the channels signal from the cable in addition to picking up the broadcast signal with the cable itself acting as a crude antenna. Of course, this theory is only valid if the local broadcast channel is the same as the cable channel. Where I live, there are four or five broadcast channels, but onle two of them have the same channel on cable as they broadcast on. And one of those two (my NBC) is very fuzzy. The NBC on the cable where I went to college was the same way.
Wow, great advice, Ex_Chemist! I never would have thought of that. Unfortunately though, I checked the entire length of the wire, and while I will admit that it’s long and windy (the cable outlet is in a different room than the TV), the cable guy didn’t use staples to attach it to the wall, so I’m not sure that this is the problem.
He used these bracket things that have a hole large enough for the cable to go through it without being squished. But I still looked at every inch of the wire and no part of it seems to be pinched or damaged in any way.
How would I go about determining the MHz? Can you elaborate on “By measuring the distance and using the speed of electrons in a coax you can calculate the frequency of the filter.”? Thanks!
If there are no obvious problems, you have to do some troubleshooting. Is the TV small enough to move into the other room? If so, locate a short cable, move the TV as near to the outlset as possible, connect the TV to the output using the short cable, and see if the problem still occurs. If not, the problem is in the cable running from the outlet to the wall. I’d just replace it.
If the TV is too large to move, you may want to buy a new cable long enough to reach and run it across the floor (for testing only). If the problem goes away, you know it’s the cable running from the outlet to the wall. Again, I’d just replace it.
If the problem still appears right at the outlet (or with a new, known good cable), the problem is in the wiring in the house. Tracing this could be a real pain. However, the problem is most probably in a visible area (where clamps or staples would be used).
BTW, I gave the results of the calculation in post #6 above. The calculation is
(3.0E10 cm/s) * (0.66) / (379.25E6 /s) = 52 cm
for a full wavelength. So a have wavelength would be 26 cm or a bit over 10".
Some other things to check:
[ul][li]Be sure all the cabling in the house is good-quality ERG-6. Replace any sections using ERG-59, since it’s a lossier cable particularly at higher frequencies.[/li][li]Check all F connectors to be sure they’re tightly crimped on, with no loose strands of braiding sticking out from under the shell. If there are any of those awful twist-on connectors, replace them with properly-crimped ones. The tools and connectors are readily available at electronics shops, such as Radio Shack.[/li][li]If there are any splitters, be sure that any unused outputs are closed with a 75-ohm terminating resistor. If there are any open outputs, you’ll want to pick up some terminators and cap them off, or replace the splitter with one with fewer outputs. And be sure that all connections are screwed on tightly.[/li][li]Ditto the above for any other devices on the line with unused RF outputs, such as a VCR or cable box. Such unterminated outputs can result in multipath interference from reflected signals, which often appears as ghost lines and images.[/li]Very slowly moving lines are often cause by powerline interference, since the refresh rate of the screen (59.94 Hz, IIRC) and the power line frequency (60Hz) are extremely close together. There are power line filters available to help with this sort of problem. You might give one of those a try.[/ul]