From: rxv9839@ritvax.isc.rit.edu (Bob Vogel)
Subject: Re: Sharing FM and TV Cable
Reply-To: rxv9839@ritvax.isc.rit.edu
Organization: Rochester Institute of Technology
Date: Mon, 21 Dec 1992 07:26:24 GMT

In article <1992Dec21.000237.6699@welchgate.welch.jhu.edu>, peterli@welchgate.
welch.jhu.edu (Peter Li) writes:
>In article <4614@vidiot.UUCP> brown@vidiot.UUCP (Vidiot) writes:
>>
>>The cable-FM frequencies are NEVER the same as the over-the-air frequencies.

never say never, so the saying goes.  Many cable companies have all or part of
their FM channels (stations) on exactly the same frequency as they are off-air. 
The primary determinant is the strength of the particular signal throughout the
area served by the headend (cableese for central transmission facility, the
phone equivalent would be the central office).  If the signal is low enough,
frequency offset is not needed.  In short the cable signal is so strong that it
will overpower the off-air signal.  The reason that any interference occurs in
the first place is because of the time delay introduced by the cable amplifying
equipment and line transmission time.  Most cable FM services include some
stereo re-broadcast of cable stations like HBO, a leftover from the days when
TV's were not stereo.  Stereo simulcasts of HBO concert specials used to be a
big thing.  They sent the FM over a local radio station which cable carried on
the FM or the sub just got the sound offair.

>>Think about it for a moment...yep, that's right, interference.  If you put on
>>cable the same frequency as the over-the-air frequency you can end up with
>>crap.  They will interfere with each other.  So, cable-FM always uses a

The original poster wanted to run in his FM on  the same cable as an antenna (or
cable TV)  While interference is possible in the TV antenna/FM antenna
combination, if the lead lengths between the two antennas are kept the same and
they are co-located they will probably add signal strength.  In the
FMantenna/cable TV combination there is a high probability of interference, on
every channel that is the same on cable as off the air.  If there is nothing in
the FM band, then there would be no problem.  It may well be illegal for one to
hookup anything to the cable line and if no isolation is provided you could
easily wind up broadcasting all the cable channels out over the FM antenna
which well interfer with other airwave users.

>>different frequency.  If you notice, you can still pick up the over-the-air
>>frequency as well as the cable-FM frequency.
>>-- 
>>      harvard\                          spool.cs.wisc.edu!astroatc!vidiot!brown
>>Vidiot  ucbvax!uwvax..........!astroatc!vidiot!brown
>>      rutgers/            INTERNET:vidiot!brown%astroatc.UUCP@spool.cs.wisc.edu
>>                                   brown@wi.extrel.com
>
>Another naive question:
>
>Why would cable-FM signal interfere with over-the-air signal if the only
>signal a FM tuner sees is the one from the cable feed ?
>Of course, I am assuming the cable company keeps its lines 100% shielded
>so that it won't act as an antenna and let over-the-air signal sneak in.
>Or is this an improper assumption of cable-tv's line quality ?
>
>Peter Li

There are probably about 100+ different versions of coaxial cable such as is
used to wire your house.  Most of the difference is due to the various shields
used.  Cheap discount wire is what is called 40% braid only, and leaks like a
sieve.  It is perfectly suitable for TV antenna lead ins, where leakage is not
a big concern.  At the other end oof the spectrum is quad shield.  It has two
layers of 90% braid and two layers of bonded (glued) aluminum foil shield.
Most older FM tuners are not shield especially well and have twin lead (also
non-shielded) connection wire.  Even the newer tuners with direct shielded coax
inputs have some places where off-air signal can get in.  Shielding is a
relative term.  The big heavy aluminum housings that cable amplifiers use have
in excess of 100dB of shielding.  We have put a standard pocket pager inside
one, terminated it (so no signals get in the cable holes), and can still set
off the pager with the lid firmly closed.  The pager has very good sensitivity
and we happend to be within 1/2 mile of the transmitting tower.  However its
stilla good example of how shielding from RF is relative.  Any given shield is
usually a little better or worse at different frequencies.
Most cable companies use cable that has in excess of 100dB of shielding.  90%
braid and bonded foil is common as is tri or quad shield.  If the FM tuners
(and many are) were as well shielded as the cable line there would be no
problem.