AVS Forum banner
Status
Not open for further replies.
1 - 15 of 15 Posts

·
Registered
Joined
·
3,139 Posts
Discussion Starter · #1 ·
I am reviewing my (coaxial) cabling to ensure that I am getting the best reception possible, given that I have alot of stuff pulling power out of my cable feed. I have a couple of 3-way coaxial splitters in use and I notice that 2 of the outputs are labelled "8db" and the third is labelled "4 db". So, my question is, which is the stronger output, the 8 or the 4? (Does "db" stand for decibels in this context?)


In case anyone would like to take this a step further and offer any other advice, here's how I currently have things wired:


The cable feed is split three ways at the source:

split1: short cable feeding one ReplayTV box

split2: short cable feeding one TV

split3: 100 foot cable...


The 100 foot cable wraps around the house and is then split as follows:


split1: feeds cable modem

split2: feeds digital cable box

split3: feeds into a powered signal amplifier, which feeds:


2 ReplayTVs

2 VCRs

1 analog cable box



I can't use a regular signal amplifier any earlier in the line than where it is possitioned because they don't work with either the cable modem or the digital cable box. Is there such a thing as an amplifier that does work with these devices?


Would there be any advantage in replacing the first 3-way splitter with two 2-ways? The idea being that the 100 foot cable gets 50% of the power, then a second 2-way splitter could be used to feed the two other units that get their feed from the source? (Note: those two lines are not heavily used).
 

·
Registered
Joined
·
6,497 Posts
1. The db figures are the "loss" for that tap. So the 4db is the stronger signal. You should use that tap for the 100' run.


2. While you can get amplifiers with greater bandwidth 1.2GHz and up, the cable modem needs bi-directionality, which presents a problem. The solution here is to run a dedicated cable from the first split to the modem. The cable box should do OK with a 1.2GHz amp.


3. My approach would be to use a two-way spitter first, with one leg going to the cable modem, the other leg into an amp with a minimum of 3 outputs. 2 of which feed the current first leg clients, the third feeding (after a 100' run) your second tier devices via your existing amp. You may or may not need an attenuator or tilt corrector on this "third" feed.


Note that these are guidelines only, YMMV and specific existing conditions may greatly influence the results you get.
 

·
Registered
Joined
·
202 Posts
Quote:
The cable box should do OK with a 1.2GHz amp.
I have an amp on my digital cable box. I know the installer would not put it on the line because they said it would not work. After they left I installed it and it works like a charm.
 

·
Registered
Joined
·
436 Posts
To reduce loss for those distances use a good coax cable. At 1GHz you get the following loss per 100 feet of cable.

RG-59 21.5db

RG-6 6.1db

RG-11 5.6db


RG-6 is the best choice based on availibility and cost.
 

·
Registered
Joined
·
2,584 Posts
Quote:
Originally posted by bpratt
...RG-6 is the best choice based on availibility and cost.
RG-6 makes a big big difference -- and rg6 quad shield is better than rg-6 regular so you should use that as long as you're running new cable.


The cable you use makes such a huge difference that I found that I needed an amp before I switched the cable but I didn't need it aferward.


If you do go with an amp, you should get one of the high quality low noise ones like channelvision. I found that the RS specials made things worse rather than better.


Channelvision makes an amplified splitter that you might consider.


Finally, note that the splitters themselves can be high or low quality and the loss figures (4db etc.) don't tell the whole story.
 

·
Registered
Joined
·
3,139 Posts
Discussion Starter · #7 ·
Thanks for the feedback guys.


Bas, I can't run a dedicated line to the modem as it's located on the other side of the house (ie, at the end of that 100 foot run). It sounds like it might be a good idea to go with two 2-way splitters right at the beginning to maximize the signal strength going into the 100 foot run.


Just FYI, I just replaced the 100 foot cable with brand new shielded RG6 cable and I think I'm using good quality splitters.


The cable source is in an external location, so I guess if I were to try using a bi-directional amp here, I would need a wall-mounted box of some sort to mount it in. The cable company supplied box is falling apart, so that won't do. I did a quick Google search and found this Skyvision Box , anyone got any other suggestions for such a box? Is this the sort of thing you can get at a hardware store?
 

·
Registered
Joined
·
6,497 Posts
Jimre's link to the new amps is right on the mark. If it fits inside the Skyvision box AND you can get power to it you should be home free. Maybe you can find at a Home Depot, doubt that a mom & pop HW store would have one. But I could be wrong....


An option not explored yet: a picture is worth.... and no new holes or cables!
 

·
Registered
Joined
·
3,139 Posts
Discussion Starter · #9 ·
Quote:
Originally posted by BaysideBas
An option not explored yet: a picture is worth.... and no new holes or cables!
Yeah, but the one piece of info I left out is that the first two splits go to different floors. Look ing at the house from the rear (see attached), the cable comes in on the right. It feeds a Replay in the half-basement and a TV on the upper floor, then the third feed goes around to the left side of the house where it feeds the modem in the full-basement and the HT system on the main floor. (The house is a 60's split level).


So, if I put the amp indoors, I would need to send a feed up through the ceiling into the room above, or run it back outside. That being said, it might not be a bad idea.
 

·
Registered
Joined
·
1,023 Posts
I'm right in the middle of a complete house re-wireing myself. While no expert, I did a lot of research on this before I started.


Yes "modem cable capable" two way amplifiers are indeed easy to get. Believe it or not, Radio Shack sells a nice unit as one of their store stock "on the wall" items. It's bi-directional, has four(4) outputs each providing 7 db gain. It's relatively low noise and seems to be a re-badged high quality item. Pick up the box and you see by it's weight that it's very well built. They also have a 15 db (bi-directional) one output amp.


I would not put ANY spitter before the amp. Typical loss for a 2-way is about 3 db. Depending on your cable provider the input at your house could be anything from 7 db to 13 db. The FCC has rules that no TV input point should be any higher than 15 db. If you are typical, then you most likely have about 10 db at the input to your home. Discounting any line loss or filters, a two way splitter would give you around 7 db at each port. The signal to noise level at that low signal would most likely introduce some noise and also a poorer picture at higher frequencies.


If you are starting from scratch, the best thing would be a high quality amp from 7 to 15 db at the front end. The amount of amplification will be depending on what you are running "downstream". Best thing to do is to first have your cable company come out and measure the signal into your house. Then you can draw up a diagram .. adding and subtracting db's along the way. You want to shoot for at least 10 db at each TV. If using a large screen (projection or even a 32 plus inch CRT) then it wouldn't hurt to provide it 12 to 13 db.


Make sure the system is "balanced". Add attenuators at various TV inputs where the signal would be too strong. Be sure to "cap" any unused outputs of splitters with terminating resisters.


Ideally you want to develope what they call a "home run" system. That is where each viewing device has a direct connect to the distribution point.


If at all possible try not to put any "splitters" downstream from the distribution point. This is a pain (like in my case) where I'm working with fully finished house (after the fact) and had to do a lot of drilling and snaking of multiple runs of wires. Use RG-6 for all interior runs.


Here's a hypothetical example which will show you how to diagram this. It all starts with the signal at the street input to your home. (by the way, you can get meters to measure that signal across bandwidths (like 50 mHz to 1.3 Ghz) But they are usually hundreds of dollars. My cable company came out and measured my signal at no cost. (which I might add they are sorry for doing as they found out the signal was very low (top @ 7.5 db)


Starting with your "known" signal from the street.


Street to house 12 db signal

50 foot run RG6 to distribution panel (- 1.5 db) = 10.5 db

Notch Filter (-1.5 db) = 9.0 db

Two-way amplifier with one output (+15) = 24 Db

8 way splitter (-9 db) = 15 db

--------- (Out of spitter) -------------

line 1 has 3db attenuator and 30 ft RG6 (-1db) = 11 db at Cable Modem

line 2 has 100 feet RG6 (-3db) provides 12 db at TV #1

line 3 has 1.5 db attenuator plus 50 feet RG6 (-1.5 db) = 12 db at TV #2

line 4 has 30 ft RG6 (-1/2db) and 2x splitter at end (-3 db) = 11.5 at each of two devices. (better to have no splitters upstream and each device with it's own feed but this will work if you plan ahead and have enough signal to that line.

lines 5 through 8 figured same way with unused outputs terminated with 75 ohm terminators.



Or say you had the aforementioned RS amplified splitter with 4 outputs.


Street to house 12 db

50 foot run RG6 to distribution panel (-1.5 db) = 10.5 db

Amplified splitter ( 4 outputs each at + 7 db) = 17.5 db at each output


line 1 has attenuator at TV # 1 based on line loss to keep TV signal at 10 to 12 DB (IE 17.5 db - line loss + attenuation) should equal 10 to 12 db.


Ditto other lines


The line for your cable modem should also be calculated to insure you don't provide too high a signal to that device.


The "notch" filter by the way is something you'll need to do if you are providing your own signals for distribution in addition to the Cable company. For example I have an unused block of cable channels here that that run from channels 72 to 82. I have a "modulator" that's being fed by a switchable video source. (two out door video cams and a couple remotely operated ReplayTVs) This modulator is on channel 77. So my "notch" filter is cut for center frequency of channel 77. Why? Well this stops my personal channel 77 from being distributed back vial the cable company to my neighbors. ;) And using a remote switch and viewing channel 77 on any tv in the house, I can watch either RPTV box or any of the cameras. I also have the output of my main computer's video being fed to that remote switch and modulator so I can monitor the PC (email etc) by simply switching to channel 77.


The big deal is to know what you are starting with as far as signal. And strive to provide at least 10 db signal at the input of each TV or device.


Put splitters after the amp. Put attenuators at device inputs. Use good quality RG-6 and ground everything properly. HTH .. Rich
 

·
Registered
Joined
·
3,139 Posts
Discussion Starter · #11 ·
Hey Rich,

I've never heard of "notch filters" before but it sounds like they would be useful to have. I'm using a ChannelPlus modulator to convert the A/V output from my two Replay boxes to an RF signal that I send to to another TV (on channels 65 and 70), but as I'm not mixing the original cable feed back into the coaxial, those are the only channels that are viewable on that TV.


Could I use a notch filter to mix the cable feed with the modulator output so that channels 65 and 70 would be from the modulator and everything else would be the regular cable channels?


Btw, do you have part numbers for the Radio Shack amps?
 

·
Registered
Joined
·
1,023 Posts
Well actually you only need some kind of filter to block that signal from going out over the cable lines to your neighbors.


If you have an AMP then AFTER the amp, you put a combiner/splitter and one input of the splitter is fed by the amp output, ie the feed from the street. The other input goes to your modulator. The output of the splitter feeds the distribution block. This way every device in your system that is hooked up to the distribution block can view your "custom" cable channel. So you are just "mixing" your channel (example 77) with the rest of the cable channels


But if you DON'T have a cable modem and do NOT need the bi-directional amplifier, then it's easier. Because the amp in that case would be one way only (from the street in) then anything you place on it's output will stay within your own system. Ergo you don't need any kind of filter.


But if you need the bi-directional capability to allow two way cable internet modems to work then you have to somehow "filter" the new channel you are generating yourself from the rest of the world by putting a filter in front of (before) the amp.


Keep in mind the following.


The filter works best with a lower signal. Ergo it should be BEFORE any amps, off the line coming from the street before any spitting or amplification.


Also the filters as I understand it are do not have steep skirts. In other words if you had one that had x amount of db attenuation at say channel 75, then as you get further away from that channel the attenuation drops off. So you may have for example (and I don't know the "real" numbers) 30 db at 75, 15 db at 74 and 76, 5 db at 73 and 78, and minimal loss below 73 and above 78. That's one thing you have to consider, that it's a sloping drop off of attenuation that might affected the adjacent 1 or 2 channels.


These are "notch" filters which block a specific frequency. There are also High Pass filters that only pass above a certain frequency and Low Pass that only pass below a certain frequency. It gets even more complicated when you start dealing with premium services like HBO etc., that are on additional frequencies. I'm also dealing with only analog so I can't say how a digital feed would be taken care of. But most digital signals are above the normal cable tv analog signals as I understand it.



The purpose I'm using it is strickly to block my modulated channel 77 from getting out to the neighbor's cable systems.


I have to use a "notch" filter because the standard band-pass filters they sent me blocked my cable Internet modem signal. By the way, I haven't received the replacement notch filter yet and I'm wondering if my neighbors are enjoying my nightly ReplayTV commercial-less viewing! :p


The better modulators have adjustable video levels so you can "fine-tune" it's RF signal that is feeding the combiner. Ideally you want the two signals at the combiner at the same level. Make sure the combiner is not just a splitter and will work both ways (splitter or combiner) and make sure it's top end frequency is high enough to pass all your signals.


When I get home this afternoon I'll follow up with some part numbers for ya.
 

·
Registered
Joined
·
1,023 Posts
The part number of the RS 4 output amplified two-way splitter was 15-1169. This will provide a plus 7 db (over the input signal) to each of the four outputs and is "bi-directional" to work with Cable Internet Modems.


Be warned .. BECAUSE it is by-directional it could pass any modulated signal you are creating locally back out to the cable system in your neighborhood.
 

·
Registered
Joined
·
1,023 Posts
Well I've learned one thing for sure. Many of the sales and support people don't have a clue about anything other than part numbers and prices. What a run-around I've had with Cox Cable, just trying to get some simple facts. What frequencies are the up-link and down-link for their cable Internet service and what is the signal coming off the street to my house.


I also had to spend some time at various manufacturer's web sites checking their engineering specs and product information as the people selling the stuff don't know the products. Sad.


Anyway you can get a single channel filter that DOES have steep enough skirts to block only a single channel. But the cost is 2 to 5 thousand bucks! (yes .. that's a 2 with three zeros before the decimal) Ouch. Obviously the cable TV service providers must be the only people buying that stuff.


However for our use the typical notch filter will be 4 to 6 channels wide and generally cost around 40 bucks. I found the one I needed which blocks channels 75 thru 80 centered at 77 (the exact channel I'm modulating. It's from Leviton and is a 40 buck filter. Part number 47690-NFA.


One company had a kind of "mickey mouse" notch filter. They provide it by paralleling a high pass and low pass with the over-lap being the notch frequency. Unique but I would think this would add some loss and noise as in order to parallel them they have to be between two splitters.


As my Dad would say .. "Kind of Smokey Stover". Anybody remember "Smokey Stover"? Back in the days when Sunday's newspapers had "real" comic strips .. :D
 

·
Registered
Joined
·
4,546 Posts
Rob,


I'm on DSL now but I had good results with a ChannelVision bi-directional amplified splitter. I now have both their 4-way & 8-way splitters and my old cable modem worked fine with the 4-way.


I bought both items from Worthington Distribution, great company that I've ordered from several times. You'll probably have to call them, their online store ain't the greatest. You can get the part numbers from the smarthome page.

http://www.worthdist.com


this page at smarter homes has the same items, but for a lot more money:

http://www.smarthome.com/7750A.html
 
1 - 15 of 15 Posts
Status
Not open for further replies.
Top