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From: edhall@rand.org (Ed Hall)
Newsgroups: sci.electronics
Subject: Re: reception problem - need help (cable and local channels)
Message-ID: <1990Dec21.021711.7623@rand.org>
Date: 21 Dec 90 02:17:11 GMT
References: <22229@well.sf.ca.us> <276f1fe8-669.1sci.electronics-1@vpnet.chi.il.us>
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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