Coax Signal Splitters

Thread Starter


Joined Apr 28, 2004
Comcast came by and installed my internet, television, and phone service the other day. They swapped out my old splitter - a 5 - 1000 MHz 4 port -7.4dB - with a 3 port 1x -3.5dB and 2x -7.4dB splitter with the following layout:

HDTV signal: -7.4dB
Internet: -7.4dB
Telephone modem: -3.5dB

I found out I could use the telephone modem to also connect internet with, so I disconnected my original cable modem. The telephone modem is now on -7.4dB port. Is this an "optimal" configuration? How does one determine the signal needs of the equipment plugged in? Should I move it back to the -3.5dB port?


Joined May 16, 2005
"dB" is a ratio, of course. "dBm" is a signal level. "dBm" is "decibel referenced to one milliwatt."

Regardless of input level, -3.5dB will be a bit less than half the input, and -7.4dB will be roughly 1/5 or 1/6 the input.

If everything works the way they hooked it up, I'd leave it be. Often they troubleshoot from the CO when troubles occur. They might be expecting certain levels from your gear when they troubleshoot.


Joined Apr 26, 2005
Think about this ... you went from a high speed cable connection to a 56k modem connection, and your satisfied with the results?

I certainly would use the telephone port for access by my fax in the computer, but not for everyday internet access. What's next? Using a 300 baud modem for internet?


Joined Jul 17, 2007
He's still on a cable modem. The cable telephone/modem also has an Ethernet connector. He's just reducing the number of components he has to deal with.

Most cable modems have a diagnostic page that you can reach via your browser by plugging in it's Ethernet address, which is usually:
Sometimes, they require a password to access all but the most basic info - or any at all. In that case, I can't help you.
You usually want the downstream signal (from the cable provider) to be between -10dBmV and 0dBmV, with a signal to noise ratio > 30dB.

If your Internet connection seems "choppy" (like when accessing streaming video/audio), your downstream signal level may be too high or too low, resulting in data packet CRC errors and multiple retries. Some cable modems are more sensitive than others. If yours seems to be working well, make a note of what the current signal levels are for future reference.

I suggest that you get an uninterruptible power supply (UPS) and keep your phone/modem plugged into it. Otherwise, in the event of a power outage, you won't have a telephone.