[mythtv-users] Semi-OT: OTA, Comcast business HSI and the single coax

Brian Wood beww at beww.org
Thu Jul 24 16:28:37 UTC 2008


On Thursday 24 July 2008 03:03:54 Peter VanDerWal wrote:
> > Greetings.  This is semi-off topic IMO, but not fully as I'm sure
> > someone on the list may have dealt with a similar situation.
> >
> > I am currently set-up for OTA digital only and my current connection to
> > the world is via DSL.
> >
> > I am switching to Comcast business for my pipe, and would like to make
> > use of a single coax for both.
> >
> > I know that OTA and Comcast television do not mix over the same coax,
> > but what about just data?  Is there a splitter or filter that I can get
> > to allow this to work, or am I going to have to run a new line just for
> > the data?
>
> You want to connect your OTA antenna to your cable??  That probably won't
> work very well.  The Cable already has signals on the same frequencies as
> most of the OTA channels and will swamp them out.

DO NOT DO THIS! It will result in your transmitting cable channels over the 
air. Some of the cable channels use frequencies not used for broadcast, such 
as aeronautical navigation channels. You really don't want a 747 in your 
backyard.

Cable companies are required by law to patrol their systems for leakage, so 
they should pick up your system pretty quickly, but a lot of operators don't 
give this a high priority.

>
> I suppose you could add RF filters to block the cable channels and then
> inject the OTA stuff, but it seems like a huge expense/hassle just to
> avoid running another cable.

Filters good enough to do this well are far beyond the budget of most Myth 
users.

[Interesting anecdote follows, don't read if not interested]

When I worked at WWOR we installed some new equipment, Ampex ADO-2000 digital 
effects units, which were brand new on the market.

Seems some genius of an engineer had chosen 40.5 Mhz. as the clock frequency. 
The third harmonic of this is 121.5 Mhz., which just happens to be the 
international distress beacon frequency.

Imagine our surprise when several folks in Air Force uniforms showed up and 
told us to shut down the gear. Turns out that leakage at 121.5 had been 
picked up by a Russian satellite, and interpreted as an aircraft in trouble. 
The fact that were were quite close to Newark and Teterborough airports 
probably helped this idea along.

We called Ampex, of course, who said that the situation was "impossible". The 
next day a kit of shielding and RF gasket material showed up at the station.

We were told to not open the shielded doors except during the first five 
minutes of each hour. This is the test period for emergency beacons, and 
signals picked up during this period are supposed to be ignored.

You can't make this stuff up :-)

beww


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