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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: rec.video,sci.electronics
Subject: Re: reception problem - need help (cable and local channels)
Message-ID: <13241@milton.u.washington.edu>
Date: 19 Dec 90 01:27:06 GMT
References: <22229@well.sf.ca.us>
Organization: University of Washington, Seattle
Lines: 39

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