jaugu...@verizon.net wrote:
> I never made a NF (Notch Filter), but I am thinking about making a "stub"
> (I think that's the term) NF for CATV to "block" a single "channel" coming in
> on the digital TV cable system. Note: This "channel" consists of ANNOYING
> ads/promos that you can not avoid or "turn off" when you use the "On Demand"
> feature to look at a list of "on demand" available movies, TV shows, music
> videos, etc.
> I know the frequency, which by the way is between 200 and 300Mhz.
I think you need to look up building a better tin-foil hat, the one you are
using isn't working.
I used to mess around with cable notch filters back when the systems were
all analog and I can't see where you came up with this idea to work on
digital systems. It's just not the same thing.
The analog systems used the same 6 mhz channel bandwidth as the terrestrial
broadcast systems so you narrowing down to the 200 to 300mhz range isn't
much better than saying "it's in the cable somewhere".
On digital systems the channels simply don't exist, unless they are analog.
It's data like ethernet. Your cable box is more akin to a computer than a
cable converter. What's worst is, some systems like fios and uverse, the
"cable" ends at that box out front or in the alley and your converter is
just telling what to send down the copper from the box to your house. Is how
they feed the 500 channel universe into a pair of 50 year old abestos
covered wires.
Even if it is an analog channel that you can pin down exactly (i.e.
63-69mhz), the filter is quite a bitch to work properly. If you look at (I'm
sure there is some still for sale on ebay, I think the big one was ARCOR)
one professionally made, those things are a precision made, highly shielded
peice of hardware.
The easiest thing to mess with to experiment is something that may be
difficult to get because radio shack sold it. The device was an FM radio
trap for TV use. Besides coming in a nice metal box that was easily opened,
it had both an in/out F connector and a circuit board with non-waxed coils
you could spread open and close, changing the frequency and depth of the
notch. If you were trying to get rid of something below 150mhz, it probably
could be beat into submission.
Of course having a $20,000 Wavetek rf sweep generator with the 75 ohm option
and a scope with a decent pre-scaler to see where and how the notch is
working is somewhat mandatory for that $4.99 fm trap. Luckily I had that
stuff laying around.
But the bigger problem is, besides getting your notch deep enough and on the
right freq, the major challenge is getting everything above and below it to
pass though. This was a major problem back when most cable systems were
limited to 450 or 550mhz total bandwidth but being 2ghz quality cable is
being used and channels/carriers up in the nose-bleed section above 1 gig, I
doubt if it could be done correctly.
Even those high dollar arcor filters the cable companies used to manually
block channels "leaked". Maybe they did a good job scrubbing out channel 18
or something, but at least 2 or 3 others up the way from it were also
weaken. Snow, herrbones, colored snow.
But the point is, if it's a digital system, whatever you think is a
"channel", isn't. It's like trying to come up with a notch filter for your
internet ethernet to filter out all the porn.
It just doesn't work like that.
I dunno why you find that advertising so annoying, if it's the one I'm
thinking of on the selection screen, but a better solution that is more
practical is just get a black peice of paper and tape it over the PIP box,
mute the audio and you are all set. When you find something to watch, take
the paper off and unmute the audio.
-bruce
b...@ripco.com