From: smw@sage.cgd.ucar.edu (Steve Welch)
Subject: Re: Will this work?
Organization: Scientific Computing Division/NCAR Boulder, CO
Distribution: usa
Date: Fri, 16 Oct 1992 01:00:59 GMT

In article <BvvIpE.9t5@ssesco.com>, elmquist@pez.ssesco.com (Chris Elmquist) writes:
|> I'd like to build this filter/combiner circuit to eliminate A/B switching
|> ahead of a cable ready TV...

[stuff deleted]
 
|> Has anyone done this or know of an off-the-shelf solution to this apparently
|> simple (and annoying) problem...

Well, I'm pretty sure this is not exactly what you had in mind, but I think 
it may give a resuly you could use...

Radio Shlock makes a unit they call a Multiple Video Distribution System,
catalog number 15-1290.  It takes the channel 2,3, or 4 signal and converts
it to UHF (NOT CABLE) channel 65, tuneable plus or minus about 3 UHF channels.
This unit also will take a baseband video signal (simultaneously) and "modulate
it" to channel 14-18 (UHF again).  I have one of these units, and the quality
of the modulator is as good as I've seen for less than $2000.  Certainly
beats whatever my local cable company is using!  I can't vouch for the Channel
3 conversion, since I don't really use it, but when I was checking it out,
it looked fine.

The problem in your application is that most TV's don't make it easy to switch
from Cable frequencies to Broadcast frequencies...  If you have an XBR Sony or
some other TV with two RF inputs, and you can set one to Cable and the other to 
Broadcast, that might work...  I have two cables (at least) running around
my house--one is the Cable Companies, and the other is from my antenna.  It
makes me crazy (sometimes) to see the lousy signal of the local stations on
Cable caused by Jones' shitty equipment...  Especially when my $50 antenna
is putting out a perfect picture.

Hey, I bet those UHF freqs trnslate to Cable channels...  Let's see....
Yes!  Ok, maybe this would work for you!  UHF 14-18 correspond to Cable
65-69, and UHF 63-68 correspond to Cable 119-124.  No problem, at least
as long as your cable company doesn't use all of those channels.  There is 
some bandpass filtering in the RS unit, because the instructions tell you to
try to leave "at least one unused channel between the AUX 1 output channel
and any UHF broadcast channel."

I'd give it a try--RS will take it back if it doesn't work for you, or so
they claim.  The tuning is analog, and a bit tricky with some digital AFC TV's.
Seems pretty stable, though.

Here's what your setup would look like:


                                             -------------------
                                            |                   |-Out1
                   -------------------------|  Radio Shack      |
                  |                         | Multiple Video    |-Out2
CATV from  -------*                         | Distribution      |
street            |                         |   System          |-Out3
                  |      ------------       |                   |
                   -----| CATV tuner |------| Chan. 2,3,or 4 in |
                        | box        | Aux1 |   Chan. 65 out    |
                         ------------       |                   |
                                            |                   |
                    Aux 2 Baseband Video ---|                   |
                                            |    Aux 2          |
                    Aux 2 Audio Left -------|   Chan. 16 out    |
                                            |                   |
                    Aux 2 Audio Right ------|                   |
                                            |                   |
                                             -------------------

The modulator doesn't appear to be stereo, BTW.  They claim each of the
outputs can drive a two way splitter, so the thing is a Distribution Amp,
too.  All for $60, not bad.

Good luck!

---
Steve Welch    <smw@sage.cgd.ucar.edu>    Voice: 303-530-2661
National Center for Atmospheric Research, Boulder, CO  USA
also: Complex Systems Research, Niwot, CO   Fax: 303-581-9820
Newsgroups: rec.video.tv,rec.video.cable-tv