Coax Cable: quality difference?

[jptaz]

Here is my current setup:

CableCO provided 2 way
-> to cable Modem
-> to Recoton 4 way 10 DB Amp (BestBuy) http://www.buyextras.com/re4diam.html
-> CableCo Provided 4 way -> To 4 Digital Cable Boxes.
-> 32" HD TV Set
-> 27" TV Set
-> Soon will be a PVR 250 or PVR 500.

The question is will the 10 DB Amp actually output +6.5 DB from each of the 4 outputs assuming the 2 way the cableco provided is -3.5 DB? If so will this potentially damage any of the tuners from too much power? IE the PVR 250/500, 32" and 27" TVs. I am assuming that the 4 way splitter provided by the cableco for the digital cable boxes is at a loss of -7 DB each so they are near to the original signal.

P.S. As a side note 3 of my 4 PVR 250s that are now 3 years old will no longer tune and I am wondering if over amplication could have lead to there demise and I would prefer not to have the same fate for my PVR 500 and the one remaing 250. They all failed basically after about 2 years of use and at least a year of that time they were connected to the 10 DB 4 way.

Thanks,
John

[Opus4]

Currently, I have no signal/PQ complaints, but at least you all seem to agree with how I thought the signal strength got reduced at multiple splits. Thanks.

One of these days I might get curious about whether an amp would improve my picture, but it just doesn't seem bad to me at the moment. Of course, I could just have a really bad old TV & can't tell the difference...

My splits go:

In - 
   2 way (-3.5 each)   	 
     |----4 way -- TV + 3 tuners (-7.5 each) (-11 total)	 
     |	 
    2 way (-3.5 each)  	 
      |-- TV (-7 total)	 
      |	 
     3 way (db drop differs)
       |-- modem (-3.5) (-10.5 total) 	 
       |--TV (-7) (-14 total)	 
       |--tuner (-7) (-14 total)

(Provided I did my match correctly.) Due to the house layout & the new TV location, I just added that 2nd 2-way split for wiring simplicity -- the 3 rooms aren't that close to each other. Looking at those numbers, I might be tempted to see what an amp does after all.

- Andy

[Menehune]

TVs are supposed to have a minimum of -10dbm signal at the cable jack. Most Cable Cos try to pump as much signal into the house as they can and try to plan on 0db at the set. I think my house had +10db at the cable co ground block when I measured it. That way if a customer wants to add a few splitters there is still sufficient signal to allow a quality signal to each set in the house.

Assuming 10db in, three 3-way splitters cascaded (-5db outputs) and 1-2 db loss in the coax cable connectors, you should have 10 - 17db = -7 db at the set. The PQ would be near the threshold and may start to get noisy in the high channels.

[Cayars]

Agreed. BTW, if you don't mind doing a bit of work and know how to read your cable modems signal level you can always walk this around the house to measure levels at different points in the house. Of course some TVs and STBs will tell you this also.

[Menehune]

If anyone is interested, a cable design guide is available at Blonder Tongue. Just register and download the "broadband reference guide" 1.5MB pdf file. More data and FCC rules than you can shake a stick at

[jptaz]

Oh yeah... Without the 10 DB amp in my setup I get snow and noise on the channels.

So would this do damage?

Would I be better off with an inline amp ahead the 4 way split?

Thanks,
John

[silkshadow]

I just wanted to say "thanks"! This is an extremely informative thread.

I'd say the easiest, likely cheapest, and probably still near best quality place to get coax is your cable company.

For my money this is the way to go. When I was in college, I shared a house with a bunch of people and was always doing something funky with the cable and TVs. Right before I started the project I would call Time Warner to fix some snow or something dumb. When they got there I would ask the repair guy to hook me up with some cables and splitters and show him the run I was trying to do and explain what I was trying to do. They always were happy to, not only provide the cable, but help me do the run. One guy even pulled out a huge drill from his van and drilled a run from the 1st floor to the 2nd for me which was awsome because I was going to run the cable out a window to the 2nd floor.

[stanger89]

Originally Posted by Cayars

Quality of cable is often overlooked. Use a cable with the least dB reduction. Most "high quality" cables are close to the same. The important thing is controlling the splits as stanger89 pointed out. If you know the dB rating of you cable per hundred foot you can calculate the loss per segment. Take this and add each split inline. So for example if you had a cable segment with 2dB loss and two splits each -6 dB you have 14 dB of loss for that segment.

Yeah, some interesting trivia, I did a little looking and it appears that CATV goes up to about 1GHz, it would appear that the Canare L-5CFB I use drops 7.1dB @1GHz @100ft.

When you guys mentioned before that your cable guy uses a two way splitter coming into the house to split two feeds. One side goes to the cable modem and the other gets split again for the TVs. This is BAD.

Not saying you're wrong, but I'm willing to trust our cableco a bit more than most. First when they set us up, they ran new coax everywhere (actually they gave us some, so we could run it) not trusting any previous cable instalations. And we're running a Cisco uBR900 cable modem, and no we didn't buy it, that's what they provided.

The other part of the single split for the cable modem (maybe more important) is that you only need one filter that way.

[stanger89]

Menehune,

What do you use to measure the signal strength, that's something I'd like to have if it isn't too expensive.

John,

You said without the amp, you get snow? If so, then you probably aren't overdriving your tuners.

[mdmint]

Originally Posted by stanger89

Menehune,

What do you use to measure the signal strength, that's something I'd like to have if it isn't too expensive.

Good question, I wonder if you could use a DMM?

I've always believed in quality cabling. FWIW right now my Sage setup is being fed from analog cable.
Cable outside feeds directly to CE Labs AV 6001 0-20dB Amp http://www.cable-electronics.com/html/6001f.html, which feeds to Monster Cable 2-way splitter http://www.monstercable.com/productPage.asp?pin=1892, one side goes about 100' using cable from Comcast to Living room into 4-way Monster splitter which feeds PVR250 in HTPC, TV & 2xVCR, the other side from 2-way connects to 8-way Monster splitter which feeds cablemodem, 4xPVR250 in Server & Radeon AIW in my workstation (AIW soon to be removed since who needs it now anyway, will eBay it to help pay for 6600GT that's on it's way for HTPC, ATI 9600 now in HTPC to my workstation) - unused splitter outs terminated. All cable connects using Audioquest/CinemaQuest VDM-Xf cables (except 100' line to livingroom) http://www.hiwaylaser.com/cms/genera...ctid=75&catid=

I don't have the CE amp anywhere near max gain and don't know how exactly much gain being used, just "eyeballed" it.

[Menehune]

I borrow a CATV meter from work, a SAM 3030, IIRC. I can't recall the manufacturer (wavetek?). I work on a College campus and I'm responsible for the campus wide cable system.

I don't think you can use a DMM since the signal is RF up to 800MHz. I've never seen any of the cable co engineers use one to test the power output. They just pull out their tester. The meters are expensive around $750 and up.

This meter from Leader is $650.
You may be able to use this RF meter but you would have to convert from mW to db and there are no built in attenuators.

[Menehune]

Originally Posted by jptaz

Oh yeah... Without the 10 DB amp in my setup I get snow and noise on the channels.

So would this do damage?

No. If the signal is too high and overloading the CATV inputs I usually see ghosting, but snow is typically a symptom of too low a SNR. I place an amp at the very begining of the splitter chain so I boost the signal before it is split. If you put the amp at the end of the chain, you may be amplifiying the noise and the signal even though it is strong signal, will have strong noise. One of my campus buildings has 32 cable drops and I have a 50db 700MHz amp hooked up the the cable input. The output of the amp goes to a 4 way splitter (-7db out) then to an 8 way (-24db out) then typically to 150 feet of rg59 (-5db) to the cable jack on the wall.

Since I only have +7db coming into the building, there is no way I would be able to get a usable signal into the rooms unless I compensated for the splitter losses at the begining of the chain.

[mdmint]

Originally Posted by Menehune

The output of the amp goes to a 4 way splitter (-7db out) then to an 8 way (-24db out)

That's why I used the Monster splitters. The Monster 8 way only -12db loss per output with digital ready 5MHz to 1000MHz bandwidth, though your 4 way used better loss rated @ -7db vs Monster's -7.4db. And totally agree amp before 1st split.

[stanger89]

Originally Posted by Menehune

I place an amp at the very begining of the splitter chain so I boost the signal before it is split. If you put the amp at the end of the chain, you may be amplifiying the noise and the signal even though it is strong signal, will have strong noise.

A very good point there, the absolute noise level never really goes down in an analog system only up. So when you go through a split, the signal will drop, but the noise will stay the same. However when you amplify the signal everything gets amplified including noise. So by amplifying at the beginning, you maintain the highest possible SNR.

[mdmint]

Originally Posted by Menehune

This meter from Leader is $650.
You may be able to use this RF meter but you would have to convert from mW to db and there are no built in attenuators.

The 2nd link "cheaper" one does list CATV as on of it's applications. Uh, but "cheaper" still being $462 + shipping! Guess I'll stick with eyeballing the result.

[Menehune]

I usually assume 0db coming in and if the taps add up to more than 10 db loss, then I start to worry. I have seen some TVs work down to -20db without noticable noise in the picture. maybe the next time you call the cable tech, have him write the power level on the wall next to the ground block (usually the cable co demarcation point) so you have a reference level. I do that at work and it helps when I go back for trouble calls to see if the signal has noticably dropped.

[rsagetv99]

Very informative thread, I have a question though. Do most of the amplified splitters report the net gain or gain before loss due to the split? In other words, if it is a four-way amplified splitter that claims 8db gain at each output, it is really only a 2db boost, since 6db are lost due to the split?

[Menehune]

The commercial amps I have at work list the power leaving the output connector. I would assume the consumer models are the same-net gain.

I don't have a multi-output amplifier on hand to verify this.

[JasonJoel]

Hah! I have about -15 dB coming into my house again (-19 dB at my cable modem after one 2 way splitter)... I have to call Charter to come look at it for the 3rd time... BLAH.

Cable TV actually looks good, except the digital channels drop out right and left. Oh, and forget about trying to watch an HD channel at that signal level... However, since the latest drop in strength (dropped ~ another 5 dB the other week) my cable modem is in and out.

Just thought I would share.

Jason

Originally Posted by Menehune

Since I only have +7db coming into the building, there is no way I would be able to get a usable signal into the rooms unless I compensated for the splitter losses at the begining of the chain.

[Menehune]

Welcome to wonderful world of digital error correction. Error correction works fne until it cannot correct the data loss then the data disappears.

During training for my OC3 radios at work, the trainer reduced the power level of the radios and we could see on the test gear that the number of corrected errors was increasing with no decrease in channel quality (i.e. full bandwidth) until the correction threshold was reached then the radio thruput went from OC3 to OCzero (no data)

At least with analog systems you could see an increase in the noise and still get a usable picture (or data) thru the system. IIRC, that's why aircraft radios operate analog AM.