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Discussion Starter · #1 ·
Hi, bought my RG-6Q and have my wires almost in. But I thought I understood how things worked, when I came across this issue on another thread and didn't understand the replies/answer.


I want to put in a Channel Plus 5115BID and modulate cameras, Replay, etc. from a headend onto about five modulated channels.


I have digital cable with 3 set-top boxes, which are my primary concern and don't want to affect their quality or functionality. I may or may not move the cable boxes to a headend, but not any time soon--I will experiment later with moving them.


In order to modulate my inserted channels, I understand I need to clear a path with a low-pass filter, such as the Channel Plus LPF-600.


My channels are occupied as follows: Without using a cable box on a cable ready (analog) TV, the channels are visible on on 2 through 78. But on the digital cable box channels 2-155 and 201 to 277 are occupied--there are no cable channels between 155 and 200.


I am not willing to sacrifice any cable channels in large blocks, e.g,
Quote:
Low Pass Filter - Passes CATV Ch. 2--116, Blocks CATV >121 Passes Off-Air Ch. 2-60
Although I don't care about things like QVC, etc. and could find five or so channels to use, they are not contiguous and are spread out and are probably next to channels that I would want to preserve.


This is where I think I am going wrong: It seems to make sense to me that my modulated channels would be best somewhere between the 155-200 range-but I doubt this is possible.


Question 1: Will I lose any of my digital channels with the required low pass filter? In other words, is my plan incompatible with my primary goal to preserve the analog and digital cable channels?


Question 2: If the answer to Q1 is: "Yes it is possible--you won't lose any cable channels" then which low pass filter should I use?


Question 3: If the answer to Q1 is "No, it is not possible" then is the only solution to move all the stuff to the headend?


Thanks in advance.
 

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Quote:
Question 1: Will I lose any of my digital channels with the required low pass filter? In other words, is my plan incompatible with my primary goal to preserve the analog and digital cable channels?
No you shouldn't. It should work just fine. Keep the modulated channels above the top frequency in the cable plant and you'll be fine.

Quote:
Question 2: If the answer to Q1 is: "Yes it is possible--you won't lose any cable channels" then which low pass filter should I use?
That depends on the cable service you subscribe to. One thing is important for you to realize, and that is that talking about "channels" is relative. Digitial channel numbers have NOTHING to do with the frequencies used in an analog system. They are assigned by the network operator, and just because they have high numbers doesn't mean you can compare them with the analog channel numbers listed on the filters. In fact, I prefer to avoid the use of channel numbers when discussing how the cable network needs to be built and filtered. Frequency is what's important. For example, channel 99 is lower in frequency that channel 78! If you look at a channel map, you'll see how the channels relate to frequencies used. In Europe, they use 8 Mhz channels not 6 Mhz as in the US, and that confuses things even more. But all the amps and splitters and filters and such will still work there.


If you want to play it safe, then get a 750 Mhz low pass filter, and just make sure your modulated channels operate above 750 MHz (analog channels > 121) and you can be virtually sure the digital service will be just fine, as US cable systems rarely operate above 750 Mhz, and the digital service will then definitely run under 750 Mhz which yourn network will be distributing just fine. The only problem is some of your older TV's may not support channels above 750 Mhz (analog channel 121).


So don't try and compare the digital channel numbers to the analog channel numbers, and figure out the frequencies that the cable operator is using, and filter everything above that.


I hope this helps...


Thanks,

Mike
 

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Mike,


Good summary, but there's something that most should be aware of. Yes, you can go with a low pass filter that knocks out channels 121 and above, assuring you that you'll likely not squash your digital channels. However, all of the current channel plus modulators only modulate up to channel 125, so you aren't leaving yourself any room to modulate anything by doing so.


I have a similar situation in that I have the 550BID amplifier and the 5545 modulators. The 550BID is good upto 806Mhz, so theoretically, I can pass my digital channels through it. However, I'm using a low pass filter to knock out channels 80 and below, so I'm stuck there. My solution is to use my alternate set of RG6's that run to every room (that's why you run two RG6's to each room ;)) and distribute the digital cable untouched on that set of coaxes. Using the set-top box, I'll use the S-video or composite out to the TV. Then to change between regular cable channels/modulated channels or digital channels, I use the input select on my TV. It's the cleanest solution I've found, but I'm open to better suggestions.
 

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Quote:
Good summary, but there's something that most should be aware of. Yes, you can go with a low pass filter that knocks out channels 121 and above, assuring you that you'll likely not squash your digital channels. However, all of the current channel plus modulators only modulate up to channel 125, so you aren't leaving yourself any room to modulate anything by doing so.
Well, that's true for Channel Plus, which is my favorite vendor for modulators too, but Channelvision's whole line for 2002 goes all the way to 135, which should have a fair amount of headroom, and they are a quality vendor. Cabletronix also has one that goes to 135, though I don't have experience with it.


As MSO's continue to upgrade their systems to 750 Mhz, I would expect pretty much everyone to go to at least channel 135 in their modulator products.


BTW, even if the modulator will go to 125, on a 750 Mhz system, sometimes MSO's push things a little above 750 with their digital signals, if the amps and such will let them get away with it, so it's really a good idea to modulate from 125 to 135.


In any case, it's still pretty likely that the plant might be 550 Mhz, in which case it's really easy to do with pretty much everyone's gear.


Thanks,

MikeSM
 

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Discussion Starter · #5 ·
As a result of the previous responses, I sent an email to my cable operator (Adelphia--they are really good compared to ATT, etc.) And I was shocked to get a quick answer:

Quote:
Hello, I received your email. The answer to your question is.

Our system is 750 mhz. You need to use a channel above

channel 116.
So, I use a 750 filter as was suggested. But as I look at the Channel Plus website, all their modulators seem to stop at 125. An earlier response says the 2002 line goes to 135--how do you know which modulators/550BID, stuff to buy?


I now know what others were saying about their website being a little out of date.


Thanks in advance.
 

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Quote:
So, I use a 750 filter as was suggested. But as I look at the Channel Plus website, all their modulators seem to stop at 125. An earlier response says the 2002 line goes to 135--how do you know which modulators/550BID, stuff to buy?
Actually, you'll want a filter that peaks at 756 Mhz (one channel up) or so, as the low pass filters we use for this type of application don't cut off precisely at 750 Mhz. Both Channel Plus and Channelvision make excellent low pass filters that should do the trick for your application, as will running the input from the cable company directly into an amplifier, though this usually means using another amplifier down the line to boost the signals coming in from the modulator. Amplifiers used in residential applications are unidirectional above 50 Mhz, so no signal would be able to be passed back through them to the cable company system.


While I like Channel Plus gear a lot, and use them in my home, they still don't go above 125 as far as I know. Channelvision, another excellent company, does go to 135 however, and they work well. I live in an AT&T 550 Mhz area, so going to 125 only works fine for me. Most of the larger dealers like Worthington Dist or Hometech (both if which I have had good luck with) carry both.


I'm glad you reached someone in the MSO who actually knew the answer to the question. Most of the time you get service reps who know very little about the actual plant in service.



Thanks,

Mike
 

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OK. Several issues here:


"Long in the tooth" is a kind description of our website . Next week the new site goes up.


Digital cable typically uses 550MHz - 750MHz for downstream transmission. (Of course, there are now 1GHz systems out there also.) You cannot safely remove any of this spectrum. So how do you use a modulator? Here are the choices:


1) Use one digital cable box for the whole house and modulate the output for the less important viewing areas. Then you may remove the entire digital spectrum with a filter and have lots of places for newly modulated signals. (Rarely is this satisfactory in a house with kids.)

2) Run the modulated signals above 750MHz. You may not need a 750MHz filter if the cable spectrum is clean up there. (But you probably will.) Issue here is the small number of modulated channels you get. (See below for the above channel 125 issue.)

3) Use a notch filter for the low UHF. Our NF-469 opens up channels 64-70 for modulator channels and should not affect the digital signals.

4) Run a second coax (ala Robertmee). The modulator channels can share a satellite LNB coax if the multi-switch has a built-in diplexer.


Yes, our modulators stop at 806MHz (channel 125). When last we looked, few TVs would tune to anything higher. Be sure you can use channel 130 before you decide this is a solution. Also, to legally use a modulator , you must limit the modulator signal to about 1/8 picowatt at any antenna connection! And the higher the frequency, the harder this is to accomplish.
 

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Rfoye, thanks for participating. Channel Plus builds great equipment, and I think it's great that you guys participate in the forums.


Some points:

Quote:
Digital cable typically uses 550MHz - 750MHz for downstream transmission. (Of course, there are now 1GHz systems out there also.) You cannot safely remove any of this spectrum.
This is a little misleading. The majority of homes in the US are not served by 750 Mhz plant. Most new rebuilds are built to support 750, but this is a pretty expensive upgrade, as MSO's have to not only install new amplifiers, but the location of the amps and power supplies also tends to have to change, so many MSO's have deferred a 750 upgrade in the past, sometimes even when doing a fiber upgrade. A lot are served on 750 Mhz plant, and many more at 550 and 450 Mhz, most of which also support digital service. You do not have to have a 750 Mhz plant to run digital cable. My system, an AT&T broadband system, is a 550 Mhz plant, and has many digital cable channels served on it.


In fact, if 750 Mhz plant were ubiquitous, you probably would have built modulators that went to 135 or above some time ago. Modulators that top out at 125 work fine for me and many many others on 450 and 550 Mhz plant.

Quote:
2) Run the modulated signals above 750MHz. You may not need a 750MHz filter if the cable spectrum is clean up there. (But you probably will.)
It never acceptable to allow modulated signals to escape the home - the can lead to various problems, many of which will get you in hot water with your cable company. If you don't use an amplifier as the filter, you really need a low pass filter, and a good one. I would also reccommend not modulating below channel 120 in any circumstances if the CATV network is a 750 Mhz plant, to make sure the filter isn't leaking some of the signal due to roll off at the band edge. Filters don't usually have a very steep rolloff in this price range, and you want to make sure the anything you modulate is getting maximally attenuated by the filter.


Quote:
3) Use a notch filter for the low UHF. Our NF-469 opens up channels 64-70 for modulator channels and should not affect the digital signals.
This can work in certain circumstances, but with MSOs changing channel lineups periodically, and reallocating spectrum to digital services, I wouldn't do this. Ideally, you want to install a configuration that won't fail years later when the person who did the install isn't around to remember how it was done.

Quote:
Yes, our modulators stop at 806MHz (channel 125). When last we looked, few TVs would tune to anything higher. Be sure you can use channel 130 before you decide this is a solution. Also, to legally use a modulator , you must limit the modulator signal to about 1/8 picowatt at any antenna connection! And the higher the frequency, the harder this is to accomplish.
I don't believe this has been true for some time. High end TV's from some time ago have sported 181 channel tuning, and even cheaper TV's in recent years support this. Given that modulators are often used in higher end installations, this is a poor assumption to make. But it is a very valid point that the user should make sure their TV's can recieve channels above 121 if they intend to modulate on those frequencies.


The power level of the modulated signal injection into the network can be the subject of a very long discussion, but I'm not sure what the frequency range does to complicate this issue. Injecting a modulated signal into the input of a amplifier at much higher power than the cable signal's level can be a problem, but it's easily taken care of with attenuators. If you do a simple loss calculation on all the components in the CATV network in a home (which ideally should be configured for unity gain at the various TV inputs), it's pretty easy to keep the modulator's signal where it needs to be.


I love Channel Plus gear - as I said, it's what I use in my own house. But I am puzzled as to why you guys don't go up to 135 when Channelvision and a number of your competitors do. You are a higher end vendor, so I would have expected this from you.


Thanks again for participating!


Mike
 

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Hey Mike!

Some interesting points.

1) From what our installers told us, we believed that most cable operators upgraded their plant before adding digital cable. I probably believed too much in the hype of gigahertz systems being the new wave. Also, sitting here in So California it is often hard to understand what is the norm for the middle of the country. Do you know where the bottom of the digital band is for your 550HMz system?

2) I certainly didn't mean to suggest that pumping a signal backwards into a cable plant is a good idea. To us the filter is a spectrum clearing strategy and preventing signal infusion is the job of a gain stage. (Of course the -50dB passband of these filters will prevent infusion in the attenuated band just fine.)

3) You are right about using a notch filter to open up the low UHF for new channels. It is only a temporary solution. Unfortunately in our business, wherever we have placed channels on a piece of coax has been a temporary solution. The cable companies have had a habit of taking more and more spectrum or providing set top boxes that will not tune to anything they did not transmit. The only clean solution is to use another piece of coax that the cable companies will never touch. (Ty Cobb talked about hitting them "where they ain't". Babe Ruth reponded that he would "hit 'em where they'll never be".) Of course, few houses have this second piece of coax. We try to have lots of possible solutions so installers can select what makes sense for them. Some installers use the low UHF approach. Most go above as you prefer.

4) The 806MHz limit: About 6 months ago, my product manager said we ought to re-design our modulators to reach channel 135. I asked the same thing "Are you sure that TVs actually tune up there?" We wandered around the factory and tested everything we could find, including brand new Toshibas from the show booth and very recent SONYs in my lab. We found exactly none that went beyond 125! OK, we'll look again. I guess that for a hobbyist, the message is still the same: be sure you can use this as a solution. But with cable companies talking 1GHz, this is again a temporary solution.

5) I guess I was a little obscure regarding the higher the frequency, the tougher comment. The real technical challenge does not come from adding to a cable company's spectrum, but in connecting to a terrestrial antenna. Not only do you get wildly differing signal strengths, but the FCC imposes the limit that I referred to: 3uV max for any modulator signal that finds its way to the antenna. Designing gain stages that have the required reverse gain required is rather tricky. And the higher the frequency, the trickier it is.


I've rambled on enough. We'll look at the the 860MHz modulator issue again. (Boy, am I tired of redesigning modulators.) And we'll canvas our installers about just what is actually being installed for digital cable. Thanks for the comments.
 

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Rich, thanks for the quick reply.


I see what you mean about capturing off-air signals and distributing them on the inhouse plant. I don't think this is as common anymore given local channels being broadcast by the DBS vendors. I think this was the primary reason people who might be using modulators would want off-air signals on a local distribution system in their house, and so I think this need is getting less important.


Even in that case though, why wouldn't you use a pre-amp on the antenna feed? It would help boost signals and totally prevent emission of any inhouse channels. A couple of low pass filters in series would probably work fine too, but the pre-amp is the cleanest, and may help get the signals boosted to a level that could be similar to the modulators.


On the TV's, I guess my experience is a little different, but then I haven't been doing this for very long. I just scanned a bunch of manufacturer web sites to determine the specs of the tuners in modern TV's, so that's probably more comprehensive than looking at what's in your lab. But in any case, you can expect a lot of your target market to be using higher end equipment, hence the extended range on the tuners is probably there for a lot of folks. But I totally agree with you the user needs to check here.


And Channelvision clearly thought this was important, as did a couple other manufacturers, and have moved their whole line up to channel 135 capable systems. So it does seem the market is already going there. Don't be caught behind - I like your stuff, but if I had 750 Mhz service, I'd have to go elsewhere.


I live the SF Bay Area, which is mostly AT&T. These guys take their sweet time in rebuilding their systems, and I think they just use about 20-30 Mhz here on the top of their 550 systems for digital. But I have heard of them deploying digital even in 450 Mhz systems, and shifting a lot of analog channels to digital to be able to support a better diversity of programming. I think the even are supporting digital on the crappy Palo Alto system, which has no fiber in it and is an old dual cable system that runs at 350 Mhz if I recall (these city owned co-op systems can be such a joke). It appears that rather than rebuilding it soon, they are shifting a lot of programming into the digital tier to enable them to carry a lot more channels. So digital sometimes gets deployed in really bad plant too.


You guys in Orange county get to enjoy Cox service, which is really good. They do an excellent job compared to AT&T and many others, and almost their whole footprint is running at 750...


Thanks,

Mike
 

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Hi again.

We do recommend using a gain-stage on CATV and antenna lines for just those purposes. (Not sure how I gave you the impression we don't.)


I'll look at the channel135 TV issue once I'm back at the factory. I'll send an email if I find anything interesting. (Rather than continuing this ever further off topic thread.)


FWIW regarding our competition: several years ago we offered 860MHz modulators under the Cablevision name. These were built in the same Taiwanese factory as those other guys. We did this to explore a lower price market without involving the ChannelPlus nameplate. (We ended this experiment a while back, but still have a few models in stock.) It was our understanding that the Taiwanese units were designed for 860MHz because of the European market. Having them tune to 126-135 for the US market was just a bonus.


You are right about Cox. They do seem to lead in the technical side of things.
 
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