Cable Splitting (-7db vs. -3.5db)

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 When my cable was originally installed they hooked it up to a 3 way
splitter. 2 of the outputs are listed as -7db (1 going to analog tv
bedroom and the other -7db to modem [30mb service at that time]. The
-3.5db to the HDTV in living room.  I have my doubts that it was done
correctly to begin with.

 I have a couple of different questions on the subject. Which is the
stronger signal and how would it be best to break it down to 4
outputs? If I split a -3.5db would it make two -7db or would
-7db make two -3.5db? Which is stronger? What is sufficient? I know
what a decibel is relating to sound but I have no idea how it relates
to data transfer speed. If someone could briefly explain the

 I want most of my bandwidth to go to my modem and my HDbox in living
room. The other 2 outputs I'm just using 2-99ch analog. As it stands
now my internet connection doesn't seem to perform as well as it
and I get some occasional freezing on my HD signal. Everything in my
hard wired network checks out fine. I have checked all connections
inside and I am using high quality cable. I also had the cable
come out and check the line outside and they even installed a new
box. I am also concidering an inline booster, or even a powered one
it would make a noticable difference.

If there is anyone with an electronics degree out there that could
show me the light I would be most grateful.
I basically just want to make sure I'm doing everything from the wall
to the devices as efficiently as I can possibly do it.

Re: Cable Splitting (-7db vs. -3.5db)
On Thu, 24 Mar 2011 18:06:52 -0700 (PDT), PostMaster

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Since these are negative numbers, (referenced to 0dBmV), if you split
a -3.5dB signal in half, each half is approximately -7dB.

TV's are usually pretty forgiving of signal strength and are able to
work over a wide range. Some TV's have an on-screen signal strength
meter, sometimes available via the Info button on the remote. Those
meters aren't calibrated or standardized in any way, but they can give
you a rough idea of where your signal levels stand.

Cable modems, on the other hand, are usually specified to work over a
range of +15db to -15db, and because signal levels can fluctuate over
time due to various external factors, some people like to play it a
bit safer and want their cable modem to work within a range of +10dB
to -10db.

There is no relationship between signal strength and data transfer
speed, assuming the modem is working with an acceptable signal. Many
modems have an internal web server that will show you, among other
things, your Upstream and Downstream signal levels and your Downstream
Signal-to-Noise Ratio. Try browsing to

You might want to draw a map of your entire cable network, showing the
incoming cable, each splitter (and its effect on the signal), ending
with each TV or modem. For planning purposes, assume the incoming
cable has a signal strength of 0dB, then just subtract 3.5dB for each
leg of a 2-way splitter or subtract 7dB for each leg of a 4-way
splitter. 3-way splitters can either be balanced, (the same amount of
loss on each leg), or unbalanced, (-3.5dB on one leg and -7dB on the
other two legs). The unbalanced type is more common, and is
essentially a 2-way splitter with one output leg feeding into a second
2-way splitter. Likewise, a 4-way splitter can be thought of as a
2-way splitter with each leg feeding an additional 2-way splitter.

Once you've drawn the map and have taken all splitters, amps (if
applicable), and your cable modem's signal level into consideration,
you're ready to decide if changes need to be made. Changes could be as
simple as rearranging your existing splitters, combining multiple
splitters into fewer splitters, asking the cable company to adjust the
level of your incoming signal, or as a last resort, adding a 'drop


Re: Cable Splitting (-7db vs. -3.5db)
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You pretty much gave me all the info I need to achieve my goal.
Much Thanks.

Re: Cable Splitting (-7db vs. -3.5db)
On Sat, 26 Mar 2011 00:03:44 -0700 (PDT), PostMaster

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You're welcome. Good luck!


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