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TV Modulator Advice

upstatemike

Senior Member
I need some advice on using filters with TV modulators. I currently have 2 four-channel modulators in my house for distributing dvd, camera, and HA displays. I have plain old analog cable so I just put a low pass filter at the cable feed that blocks anything above channel 100 from feeding back to Time-Warner. I then use a block of 16 channels (allowing a for blanks between each modulated channel) above channel 100 for my internal stuff.

My question is this: If I someday decide I want to get digital cable, which uses the frequencies above channel 100, where is the best place to move my block of 16 internal channels and what type of low-pass or notch filter would I use to prevent backfeed to Time-Warner yet not degrade the digital channels?

The "technical" folks at Time-Warner had no clue or advice.
 

Mike

Senior Member
I'm not an expert on this, but I don't think the filter is to block things going back to time warner, but to clear the channels you use internally (you need to modulate onto an unused channel). This does this for you by 'clearing all of them'.

You should look at the channes in the 65-75 range , there are typically unused channels there. I have digital cable now, and use 65 and 69 (I only use 2 channels) and I have no issues. I don't even use a filter.
 

jrfuda

Active Member
I've been considering modulating the output from my receiver (which has the DVD changer, HTPC, Single DVD and other sources connected to it and a zone 2 output). I currentl have a Dish 522 DVR, a dual tuner model that feeds two rooms and has it's own modulator built in. The room it's in (the living room) can be fed via Channel 3, Comosite or S video (using Svideo and RCA stereo to TV, plus SPDIF to receiver), and the distant room via a UHF channel (I use ch 21) and the other connections, if I wanted. Anyway, if I hook the zone 2 of my receiver (composite and RCA stereo) to a modulator, like one of the low-priced (less than $30) modulators that rat shack sells, can I set it to, say channel 23 and have it ride on the same cable that the Dish box uses to send it's signle to the remote location... can I add yet another modulator, on channel 25, and feed the baby monitor into it? I'm assuming, yes, since that sounds like what UpstateMike's doing.

Oh, and UpstateMike, sorry for semi-hijacking your thread.

Sounds like, from what regular Mike is saying, that I can, and you can too.
 

bfisher

Active Member
Yes, this should work. Connect the coax output of the Dish box to the coax input of the modulator 1 (baby monitor). Connect coax output of that to the coax input of Modulator 2 (from your zone 2 amp) and the zone 2 outputs to the modulator input. Connect coax output of that modulator to the cable that feeds your TVs.

Might be easier to buy a multi-channel modulator. Then you won't have to daisy-chain modulators (reducing another potential source of picture degredation).
 

smee

Senior Member
bfisher said:
Yes, this should work. Connect the coax output of the Dish box to the coax input of the modulator 1 (baby monitor). Connect coax output of that to the coax input of Modulator 2 (from your zone 2 amp) and the zone 2 outputs to the modulator input. Connect coax output of that modulator to the cable that feeds your TVs.

Might be easier to buy a multi-channel modulator. Then you won't have to daisy-chain modulators (reducing another potential source of picture degredation).
You can also feed all modulators (and incoming cable if you use it) into a combiner (looks like a splitter, some work as both). You don't need to do it in stages.

Filters:
I don't know if filters will affect digital cable or broadband.

But, even if you don't need a filter to block incoming channels, if you are connected to external cable and don't use one your modulated channels will leave your house. Depending on signal strength, etc., your neighbors will be able to see your channels. Not only is this something you may not want to have happen (baby monitor, etc.), it is probably illegal, violates FCC regulations, or violates your agreement with the cable company.
 

rocco

Active Member
smee said:
. . . it is probably illegal, violates FCC regulations, or violates your agreement with the cable company.
. . . or all of the above.

A simple solution to this is a cheap distribution amplifier on your incoming cable feed. A video amp will let the signal pass in only one direction. A video distribution amp may be required anyway if you are doing a lot of splitting and combining. At $10 to $15, it's often cheaper than another filter.
 

Mike

Senior Member
smee said:
bfisher said:
Filters:
I don't know if filters will affect digital cable or broadband.

But, even if you don't need a filter to block incoming channels, if you are connected to external cable and don't use one your modulated channels will leave your house. Depending on signal strength, etc., your neighbors will be able to see your channels. Not only is this something you may not want to have happen (baby monitor, etc.), it is probably illegal, violates FCC regulations, or violates your agreement with the cable company.
Apparently I was quite wrong, I'll have to fix this. My two tivo's are being broadcast in the open!
Plus I had this all wrong. Thanks for the clarification.

One thing to consider, if you get digital cable you get different copies of the channels (depending on your setup) so you may be able to just use a low pass filter and wipe out channels 80-100 (which in light of this new information is what I expect to do, unless I get further guidance here) since the quality is so much worse compared to the digital cable (in my area). In my case I dont think there is anything I watch in that range anyway (aside from the digital versions of the channels that are in other numeric ranges).
 

jrfuda

Active Member
smee said:
You can also feed all modulators (and incoming cable if you use it) into a combiner (looks like a splitter, some work as both). You don't need to do it in stages.
That's what I was thinking too. I'll make a decision based on whether it's cheaper to buy two of the low-priced mods (I think $30 each, so $60 total), plus the splitter/combiner, or to get a 2+ channel capable modulator and let it handle the combiniing.... I don't think I'll be able to find one for $60.

Since I'll probably do it in stages, starting off with just the receiver's zone2 output, since I can see the baby monitor output via a wireless video receiver (though not too clearl) on the remote set, I'll probably use the 1 channel modulators and add one at a time. Also, since the Dish box does it's own moduling, I'll need the combiner anyway to get it's signal mixed with the other modulated signals.
 

upstatemike

Senior Member
So to summarize:

It is not clear how digital cable uses the frequencies above channel 100 so internal channels should probable be lower than that.

A filter IS required to prevent backfeed (especially since my distribution amp DOES amplify in both directions according to the specs).

So I need a notch filter to carve out enough free space space for 17 internal channels (8 with blanks on each side) in an empty area (80s and low 90s?) while allowing the higher digital freqs to pass.

Since lots of people have digital cable and many have modulators, I should be able to go into Radio Shack, tell them the specific channel range I want to notch out and they should have the exact filter I need right on the shelf right?
 

JohnBullard

Active Member
Only if the clerk at ratshack even has a clue what your talking about, you may have to draw him pictures and diagrams so he can call and ask someone else ;)
 

rocco

Active Member
upstatemike said:
A filter IS required to prevent backfeed (especially since my distribution amp DOES amplify in both directions according to the specs).
Mike, look closer at those specs.

Some amplifiers are listed as bi-directional, for use with interactive cable and Internet, but the frequencies are different in each direction. The forward feed (into the house) amplifies frequencies that are 55 MHz or higher. The back-feed (to your cable provider) amplifies frequencies below 55 MHz. These amplifiers have built in filters.

You could not amplify the same frequencies in both directions without creating a feedback loop.
 

upstatemike

Senior Member
Thanks for the clarification. I guess that makes sense when I think about it.

But if I use my amp as a filter then I have to inject my internal channels AFTER the amp which means those channels won't get amplified. Do I need a small amp first to isolate me from the outside world and then a second amp after I inject the internal channels from the modulators?
 

rocco

Active Member
Yeah, it is not easy.

I usually put an amp as the second thing, after splitting off the Internet. That way, 1) I don't need a bi-directional amp, 2) it regains the 4db loss from the splitter going to the cable modem, and 3) it boosts the cable up further, since the cable in my area was always poor. I used a small, 10db, $12 amp.

The hard part is getting the cable and modulated signals to be close to equal in strength. I used four tools: 1) a little math, 2) strategic placement of splitters and combiners, 3) hope, and 4) prayer.
 
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