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Old 12-31-2009, 11:55 PM - Thread Starter
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I recently added a new tuner and need to split a cable signal to 3 devices - Cable Modem, TV, TV Tuner. I changed my current 2-way splitter into a 3-way splitter made by "Signal Vision" and connected my cable modem to the output labeled with "3.5db". The other twos are connected with "7.5db" output. I assume it splits the signal twice internally.

I noticed more internet outage than before. Could it be due to the splitter?
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Old 01-01-2010, 06:57 AM
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Yes. A 3 way has more loss than a 2 way. A 2 way splitter will cut the received signal in half per port. A 3 way with cut the received signal by 66% per port. Remember each 3 db halves the signal. Putting two 2 way splitters in series with each other will drop the original signal by 75% on the two ports of the second 2 way splitter. But the opposite is true with an amplifier. Each 3 db will double the signal.

One thing you might want to consider using instead of splitters, use taps. That is what the cableco's use to keep loss down. A tap will have a 5db loss per port. The tap works where you have a main trunk line and the tap, taps off the main line. You won't get a tap at Radio Shack, but you can get them at speciality TV shops and off the net. They are couple bucks each. Taps are what I use so I don't have to worry about adding all those losses up. I know what the gain is at the head of the trunk and I know each tap will be 5db lower than that with just a little more loss due to the cable run. Less amplifiers too so less cost!

When analyzing signal loss, you have to add all the splitters loss and any amplifier gain to determine total system loss/gain. If your antenna is a 15 db gain antenna, and you have 12db of attenuation in the line due to splitters, the signal that reaches your TV is only 3db above the original signal presented at the antenna. Not a good result. If you need to split, find amplified splitters or use low gain amplifiers (10-15db MAX) ahead of the splitters to keep the gain unity through the system. The last thing you want is to overload an amp down stream. Amp overload will cause the signal to be distorted to the point, the receiver will not decode correctly and more than likely not decode at all. A good rule of thumb is you want the loss/gain ratio at the receiver to match the gain of the antenna. This will make sure you don't run the risk of overloading the receiver. Also, if your antenna location is within a mile or so of cell towers and such, your system can also be overloaded with high power gains as well. That is another reason to stick to the antenna gain figure at the receiver as well. Sometimes you still have to experiment if you live next to a cell tower or other high power transmitting site, like FM transmitter sites.

Yes, there is a little bit of black magic in this, but 98% of it is just using good common sense and the 3 db/unity gain rule.

Good luck.

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Old 01-01-2010, 07:21 AM
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Instead of a passive unbalanced 3-way splitter, you should use a cable distribution amp with a decent return path. That 3.5 dB loss on the cable modem side is cutting your modem signal coming and going.

Examples of 4-port cable amps that would work are the Antennas Direct CDA4, ChannelMaster CM 3414, Radio Shack 15-2506, and probably many more.
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Old 01-01-2010, 08:16 AM
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There are two kinds of 3-way splitters: the ones that divide the signal into thirds (-5.5 dB per port) and the ones that are actually a pair of splitters, putting out one half (-3.5dB) on one port and one fourth (-7dB) on the other two, like the one you have.

Cable companies prefer that an internet modem be on half of a two-way split, because it tends to even out the strength of return signals throughout their system, which make is easier for them to manage them.

What you have done should theoretically have no affect on the cable modem, so it is likely that the outages you perceive are unrelated to the insertion of that splitter, but it is always possible that your new splitter might be having trouble passing the low frequency, return signal, so you can put in your old splitter to restore your circuit to its original form, and if you are then satisfied with your internet performance, get another two-way splitter for the two TVs.

The cable company usually furnishes you with enough signal for two TVs and a modem, but sometimes they don't and sometimes you need a little more because of long cable lengths. Before spending any more money and making any more trips to the parts store, you should try connecting the two TVs to one single port of the two way splitter that you can re-install. If you don't have a second, two-way splitter handy, then you can use the three way you have in that application, connecting the longer TV wire to the stronger (-3.5dB) port. If the TVs go from acceptable to unacceptable, then you will either need a small amplifier or will need to have the cable company otherwise slightly boost your input signal.
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Old 01-01-2010, 08:19 AM
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Is this the splitter?
http://cableandwireshop.com/sign...?category_id=0

Although correct in regard to a -7 dB loss on two of the ports, using the -3.5 dB port on a 3-way splitter for the cable modem is no different than using a 2-way splitter.

You may want to double check the coax connector and/or the center conductor. It's quite easy to break or bend when connecting.

Do you notice any picture quality change with the two TV's since adding the 3-way splitter?

Who knows... perhaps the frequent outages are pure coincidence since changing splitters and unrelated.

EDIT:
Wow... you beat me to it Mike!



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Old 01-01-2010, 10:00 AM - Thread Starter
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Thanks folks. The splitter I use is more of the "Traditional" look. Like

http://techtoolsupply.com/v/vspf.../HFS-3D-2T.jpg

with 5-1000MHz printed on it. Last night I have like 10+ outage events based on my cable modem. I usually see 0-5 per day. I tried using two 2-way splitter and I am seeing similar result. The TV signal has been very good before and after using 3-way splitter.
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Old 01-01-2010, 10:30 AM
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"Looks" really doesn't mean anything.

Anyway... if you see the same outage events when using the original splitter, then it may be time to give your cable provider a call.

Just for giggles, have you tried unplugging the cable modem from wall power for 5 minutes? Perhaps a hard power cycle will resolve the "issue".



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Old 01-01-2010, 02:32 PM
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Quote:


Instead of a passive unbalanced 3-way splitter, you should use a cable distribution amp with a decent return path. That 3.5 dB loss on the cable modem side is cutting your modem signal coming and going.

NEVER use a distribution amp with digital cable or a digital modem. that ruins the signal and actually will make it worse. those amps are fine for analog cable, but will ruin a digital signal.

like everyone else has said, the cable modem is picky about the kind of signal it gets. thats why most cable companies will run the modem off of a tap directly from the house's main feed, so it gets the cleanest signal possible. the digital cable box however wont be affected too much by a signal split.
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Old 01-01-2010, 04:26 PM
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Whether analog or digital, distribution amps work fine when appropriate/necessary.

No one stated that a cable modem is "picky" about the signal it receives. Most cableco's use a "regular" two or three way splitter to seperate modem and TV feeds. When a 3 way is installed, the -3.5 dB leg is always connected to the modem.



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Old 01-01-2010, 04:43 PM
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NEVER use a distribution amp with digital cable or a digital modem. that ruins the signal and actually will make it worse. those amps are fine for analog cable, but will ruin a digital signal

This is not a true statement the amp doesn't care what type of signal it amplifies.

In a cable system there can be a number of amplifiers along the path between the node and your home.
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Old 01-01-2010, 05:16 PM
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well i work for a cable company, not an installer but i know a lot of them. we wont install a cable modem in a house that has an amp, it never works correctly. for new installs, we always will tap off the main line and run that to the modem, and the rest of the house off the other half.
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Old 01-01-2010, 05:38 PM
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I don't know what exactly your particular cableco's policy may be nor your expertise, but you are incorrect that "amps never work" correctly.



If you can’t explain it simply, you don’t understand it well enough – Albert Einstein
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Old 01-01-2010, 05:59 PM
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Quote:
Originally Posted by sitlet View Post

well i work for a cable company, not an installer but i know a lot of them. we wont install a cable modem in a house that has an amp, it never works correctly. for new installs, we always will tap off the main line and run that to the modem, and the rest of the house off the other half.

You are confused.

All cableco's use distribution amps as needed. Here are typical examples:

http://broadband.motorola.com/catalo...ith_return.pdf

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Old 01-02-2010, 01:14 PM
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Quote:
Originally Posted by sitlet View Post

well i work for a cable company, not an installer but i know a lot of them. we wont install a cable modem in a house that has an amp, it never works correctly. for new installs, we always will tap off the main line and run that to the modem, and the rest of the house off the other half.

It depends very much on the amplifier. It is very important that the amplifier have some means of getting the return path of the modem back to the cable company (that is signals below analog channel 2).

I am driving my cable modem from one of the outputs of a channel master 3418 video amplifier that has a passive return channel from 5 to 42 MHz, and it is working perfectly.
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