Questions about splitters and db ratings - AVS Forum
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post #1 of 8 Old 04-26-2004, 12:07 PM - Thread Starter
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I have been doing custom installs for a couple of years now and I've done many cable break outs. But there are still some questions that I have about how to get the best possible result:

My first question is about db ratings when working with cable distribution. All splitters have a db rating but is this a measurement of the amount that is being lost or how much is allowed to pass? What db rating would be considered acceptable or should be my goal?

Also splitters have a measurement of MHz or GHz. Is this saying how much information the splitter will allow to pass? What is considered a quality splitter? Does 1 GHz mean the total amount for the whole splitter or each port?

If you have an 8 way splitter and only 3 wires are hooked up to it does it have the same strength if you had all 8 ports connected?

What do you guys prefer channel amps with multiple ports, or single inline amps that then go into a splitter.

What does it mean when a splitter has DC passing?



I know that this is a lot of questions and I'm sure that none of you have the time to sit down and answer all of them. Any Info that you may have on any of these questions would be appreciated.
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post #2 of 8 Old 04-26-2004, 12:51 PM
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Quote:
Originally posted by novo2

1) My first question is about db ratings when working with cable distribution. All splitters have a db rating but is this a measurement of the amount that is being lost or how much is allowed to pass? What db rating would be considered acceptable or should be my goal?

2) Also splitters have a measurement of MHz or GHz. Is this saying how much information the splitter will allow to pass? What is considered a quality splitter? Does 1 GHz mean the total amount for the whole splitter or each port?

3) If you have an 8 way splitter and only 3 wires are hooked up to it does it have the same strength if you had all 8 ports connected?

4) What do you guys prefer channel amps with multiple ports, or single inline amps that then go into a splitter.

5) What does it mean when a splitter has DC passing?
Okay, since I can answer all of these questions and I'm not a custom installer, maybe I need to go into business for myself.

1) db ratings usually indicate the amount of loss. 3db is required (a perfect splitter will send 1/2 the signal to each outbound port and db is a logarithmic scale where 3db = a doubling of strength.) A quality one should be no higher than 3.5db.

2) The Mhz or Ghz question deals with the highest frequency the splitter will pass. Cable systems typically go all the way to 1000Mhz (1Ghz) but channel 69 is the highest broadcast frequency, or 806Mhz.

3) An active splitter doesn't distribute it's load over the connected ports - it splits it over all the ports.

4) Assuming the same quality, there's no reason not to get an amplified splitter over an inline + a splitter. The cable loss from the extra connection would more than cost you what a dedicated amplifier would gain you.

5) Certain products send DC (direct current) through coax. These include satellite receivers and preamplifiers. Most splitters don't pass the current, meaning that you have to have a dedicated connection between the indoor unit feeding the power and the outdoor unit receiving it. A splitter that passes DC, however, will allow you to send the current up the cable through the splitter. This is only really useful for satellite receivers, though, since you would need indoor preamp units at both ends of the split line to make it worthwhile.

Hope this helps! Feel free to correct my facts, anybody who knows more about this stuff than I do.
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post #3 of 8 Old 04-26-2004, 09:45 PM
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The dB listing on each port of a splitter indicates loss for that port. If a 8-way is used and only 3 ports are connected to outlet's the loss per port is the same as if all ports were connected to outlet's. Digital cable boxes (DCT's) require 1000Mhz (1gig) splitters. If it is 500Mhz the digital music will not come in. Digitap in my opinion are the most durable splitters. Adelphia uses them here in Colorado Springs. A home/dwelling can have a 8-way splitter without loss (substantial picture quality loss). As far as lie amps, use as last resort. Adjustable ones are best. As a general rule if the aerial feeder line or underground feed to the demark is over 200' then the signal may benefit from a amp.

http://www.trainingdept.com/html/videos.html has videos on detail pertaining to coaxial cable installation and theory.

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post #4 of 8 Old 04-27-2004, 08:01 AM
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OK, You guys are making sense.

Now I've got Charter Digital Cable (waiting on HD availability) and a nice new Sony RP LCD TV. There's about 100 ft. from my underground street conn. to my attic. And I have additionally, a modem splitter, splitter @ the Sony, etc.

I installed what appears to be a quality Electroline 4-way Drop Amp in my cable system.

My problem is that my lower channels do not have near the quality of my upper channels, like especially the premium movie ones.

So I'm reading around, and beginning to believe that I need an Drop Amp/Equalizer to balance the signal between high and low freq. channels. What's your opinion?

Or, Do you think this problem is more a function of digital (high channel) vs. analogue signal coming in?

I've also read that the optimal dB strength for incoming signal is lower for digital than analoque. What's the desired values?

Thanks for any replies.
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post #5 of 8 Old 04-27-2004, 12:56 PM
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9 times out of 10 if this is your issue: My problem is that my lower channels do not have near the quality of my upper channels, like especially the premium movie ones. Replacing your incoming line (aerial or underground) will rectify this. Lines are exposed to weather and wildlife. It is the responsibility of the cable provider to replace cable feeder lines. If they bark, email the FCC.

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post #6 of 8 Old 04-27-2004, 01:01 PM
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Quote:
2) The Mhz or Ghz question deals with the highest frequency the splitter will pass. Cable systems typically go all the way to 1000Mhz (1Ghz) but channel 69 is the highest broadcast frequency, or 806Mhz.
Channels should not be tagged to frequencies because any channel can be allocated to any frequency. Therefore, channel 69 may not at 806Mhz everywhere. I don't know if that was being used as an example or not; but the highest analog channel and frequency will vary by location.

Quote:
I've also read that the optimal dB strength for incoming signal is lower for digital than analoque. What's the desired values?
Digital db strength will be approximately 10db's lower than analog. Your desired signal strength at the TV/ VCR/ etc should be around 0db. 0-4db is an ideal range. But this is somewhat unimportant unless you have a signal meter to check. And a SLM is not inexpensive.

In regard to specific brands of splitters, amps, etc, it is tough for the average consumer to get good quality products on a consumer level. The best stuff is generally used by the big cable companies and is not a available to average Joe's. As a general rule, buy the best quality you can get. Get a heavy, well sealed and solid feeling splitter. If you squeeze it right in the middle you should not feel any give. Forget about "gold plating". Regarding amplifiers, adjustable gain doesn't equal a better amp. And don't be concerned with a lot of db's. If you need to amplify more than 15db's, then you've got a bigger problem than an amp is going to solve. A powerful amp will likely make the picture worse. And always amplify at the closest point to the source of the cable as possible.

If you are having a problem on the low band (ch 2-6) or some other area of your channel lineup, there are a number of possible explainations. And guessing is not very productive. But if the problem is isolated to the low channels, it is very often the cable jumpers behind the TV. They will often allow ingress and result in bold white lines or white dots that move quickly. Get the best possible cable jumpers and never use push-on cables. Always screw on.
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post #7 of 8 Old 04-27-2004, 03:02 PM
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Quote:
Originally posted by brvheart
Channels should not be tagged to frequencies because any channel can be allocated to any frequency. Therefore, channel 69 may not at 806Mhz everywhere. I don't know if that was being used as an example or not; but the highest analog channel and frequency will vary by location.
Ah, the wonder of cable!

I was, of course, referring to Over-The-Air channel numbers, which are the same frequency all across North America.
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post #8 of 8 Old 04-27-2004, 03:03 PM
 
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Cable channels are standardised as well....otherwise, cable ready tuners wouldn't work.
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