rchao@well.sf.ca.us (Robert Chao) (12/18/90)
I've been having a problem with cable reception:on local channels, there are slight ghosts and a "bar" about 3" wide near the right of the 20"screen. In my case 2,4,5,7, and 9 have the problem but 3(HBO), 6(ESPN) and 8 don't. Two cable people came out and said it was because the cabling on the outside of my apartment building was old, though they certainly checked and tightened all my connections and replaced some. The problem is because the local stations in the air "leak" into the cable stations - so you get ghosts. For reasons I can't go into right now, I don't think they'll be able to rewire the outside wires. But my question is this: why do slight, subtle things I do with my equipment tend to make the problem noticeably better or worse? For example, at one point I tried to put on a converter box to see if it would help (it didn't). When I reconnected everything like before, the ghost bar was far less distinct than before on one channel. Another time, there was a speaker stand pressing against the cable from the wall. I pushed the stand away and the problem seemed to be slightly less. Today, I put an unused bulletin board against the wall behind the TV. The problem got slightly worse on one channel. However, plugging and unplugging and moving things around didn't help! Neither did moving the bulletin board! So I ask in general: Why are these things happening? If the "leak" is outside, what diff should it make when I make slight changes? The cabling is all Belden cable that the cable company uses. The TV and VCR are only two years old. Should the positioning of the cables make any difference: Whether they're longer or shorter, taut or sagging? Should normally shielded cabling be affected by things touching them - that could include other cables of the same kind, a speaker wire, power wires, the wall, the floors, furniture? Obviously there is some delicate factor involved that I don't recognize. Note: on certain times in the past, certain channels had the problem with the TV but much less when I tune with the VCR and use direct input to the TV. Would that mean there was a "leak" at that time just in the cord between the TV and VCR and not between the wall and VCR? You folks have helped me a great deal in the past - I hope to hear your suggestions. Thanks to all. It's certainly an annoying problem! Yours, Robert Chao
whit@milton.u.washington.edu (John Whitmore) (12/19/90)
In article <22229@well.sf.ca.us> rchao@well.sf.ca.us (Robert Chao) writes: >I've been having a problem with cable reception:on local channels, there >are slight ghosts and a "bar" about 3" wide near the right of the 20"screen. >In my case 2,4,5,7, and 9 have the problem but 3(HBO), 6(ESPN) and 8 don't. >Two cable people came out and said it was because the cabling on the outside >of my apartment building was old... This is a classic description of multipath reception. Not only are you picking up local TV stations through the air, but you are getting them (from the cable company) a short time later. The slowness of light integrated over the two paths gives two different arrival times. (Trust me, integrating slowness over the path gives a time delay.) Probably the cable box is sensitive to RF on its output cable or AC wiring, in addition to the cable input The time delay, as you describe it, is about 50 microseconds. Any multiple of 63 us may be added to this delay to produce a direct-signal ghost in about the position you describe. >Note: on certain times in the past, certain channels had the problem with >the TV but much less when I tune with the VCR and use direct input to the >TV. Would that mean there was a "leak" at that time just in the cord between >the TV and VCR and not between the wall and VCR? Probably it means that the VCR has less stray signal RF pickup (it's a smaller box, so has less antenna effect from its internal wiring), and the built-in amplifier in the VCR's RF section rejects RF from the TV and power line better than the amp in the cable box. The only good solution to this problem is to get a cable box with an alternate input (some have A/B switches built in, and wired for normal remote control), or to put an antenna switch in so that you can switch the TV from rabbit-ears to cable. Keeping stray signals out is difficult; rejecting the cable signal is much more reliable a treatment. The problem only shows up (in my vicinity) on strong local stations, so an antenna is good enough (you don't need cable for local TV.) John Whitmore
cgordon@vpnet.chi.il.us (Crash Gordon) (12/19/90)
>Author: [Robert Chao] >I've been having a problem with cable reception:on local channels, there >are slight ghosts and a "bar" about 3" wide near the right of the 20"screen. The bar is a classic symptom of reflections on your cable. Check every connector in the system; somewhere something's not quite right. Also look for sharp bends in the cable. (Radius should be at least six inches!) Cable companies don't necessarily use the highest-quality connectors, tools, and/or personnel :-) I've seen three situations which had the ghost/bar syndrome: In two of them the connector was poorly crimped at the back of the TV set, and in the third case the bad connector was at the wall jack. A quick-and-dirty fix (which may or may not work) is to put an attenuator just before the cable hits your video equipment. This _may_ drop the reflections below your detector threshold, but leave the signal OK. The best way to solve the problem is to find where the reflections are coming from. (Maybe there's a piece of furniture on the cable?) ----------------------------------------------------- Gordon S. Hlavenka cgordon@vpnet.chi.il.us Disclaimer: Yeah, I said it. So what?
jpc@briar.Philips.Com (John P. Curcio;6442;3.34a;) (12/19/90)
>>I've been having a problem with cable reception:on local channels, there >>are slight ghosts and a "bar" about 3" wide near the right of the 20"screen. >>In my case 2,4,5,7, and 9 have the problem but 3(HBO), 6(ESPN) and 8 don't. >>Two cable people came out and said it was because the cabling on the outside >>of my apartment building was old... On a similar note, I have been having trouble with my CATV reception, but in a slightly different manner. On the pay-TV channels which are scrambled (Showtime, Sportschannel, Sportschannel America), I will get interference when vehicles pass by the front of the building. I live in a six-unit apartment building along a fairly busy road, so this interference is frequent enough to be obnoxious. The first thing the cable company did was replace the drop from the pole, saying that the old single drop couldn't drive all six apartments. This fixed the problem on one of the channels, but not on the other two. Any ideas where the problem might be? I assume that the descrambler for these channels uses some sort of feedback loop, and the cable picking up interference from the outside may be upsetting this. -JPC -- John P. Curcio Philips Laboratories jpc@philabs.philips.com 345 Scarborough Road (914) 945-6442,(914) 945-6345 Briarcliff Manor, NY 10510
edhall@rand.org (Ed Hall) (12/21/90)
In article <276f1fe8-669.1sci.electronics-1@vpnet.chi.il.us> cgordon@vpnet.chi.il.us (Crash Gordon) writes: > >>Author: [Robert Chao] >>I've been having a problem with cable reception:on local channels, there >>are slight ghosts and a "bar" about 3" wide near the right of the 20"screen. > >The bar is a classic symptom of reflections on your cable. Check every >connector in the system; somewhere something's not quite right. Also look >for sharp bends in the cable. (Radius should be at least six inches!) Unless he has cable runs of several thousand feet running around his house (i.e. ~27us worth of delay), any such problems with "reflections" are going to be far away from his control, and furthermore will affect hundreds if not thousands of subscribers. >Cable companies don't necessarily use the highest-quality connectors, tools, >and/or personnel :-) I've seen three situations which had the ghost/bar >syndrome: In two of them the connector was poorly crimped at the back of >the TV set, and in the third case the bad connector was at the wall jack. Indeed, bad connections are often the source of the problem. However, the problem has nothing to do with reflections. The problem is reception of the off-the-air signal mixed in with the off-the-cable signal. If the shield of the cable is poorly connected at some point, the cable in effect becomes a long-wire antenna. The reason for the bar is that the off-the-air signal is about 10us *earlier* (or some multiple of the horizontal scan period plus 10us) than the off-the-cable signal. Thus the horizonal sync period of the weaker signal falls inside the visible area of the raster. Local reflections might cause smear or color imbalance if they are strong enough, but the bar-in-the-picture is a classic sign of either cable/antenna mixing or channel crosstalk. >A quick-and-dirty fix (which may or may not work) is to put an attenuator >just before the cable hits your video equipment. This _may_ drop the >reflections below your detector threshold, but leave the signal OK. The >best way to solve the problem is to find where the reflections are coming >from. (Maybe there's a piece of furniture on the cable?) This is absolutely the wrong thing to do, for two reasons: 1) It adds yet another potentially poor connection, and 2) It makes the cable signal weaker, but may not make the shield-carried "antenna" signal any weaker. Of course, if connections get tightened as a result it might solve the problem anyway. One other thing that's important: avoid the slip-on cable connectors available from places like hardware stores and Radio Shack. These may be OK for connecting a VCR, but generally have too poor a shield connection for use with cable television. Use only the screw-on connectors, and have as few of them in the path as possible. -Ed Hall edhall@rand.org