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
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