Comcast XFINITY → [Signals] Signal strength for modem (can I use a splitter)

Hello,

I'd like to hook up both a set top box and a modem to a single coax jack. Unfortunately, when I tried this using a GE splitter from Walmart, I got very poor streaming video performance through the modem. I'm wondering if there's a splitter or amplifier available that would alleviate this problem. Here are the specs of the modem (SB5100, taken straight from the manual):

Downstream:
-15 dBmV to + 15 dBmV
Upstream:
+8 dBmV to +53 dBmV

Here is what the modem sees when it's hooked up directly to the coax jack:

Downstream
Signal to noise: 35-36 dB
Power level: -9 dBmV
Upstream
Power level: 34 dBmV

And here is what it sees when it's hooked up through the splitter:

Downstream
Signal to noise: 34 dB
Power level: -14 dBmV
Upstream
Power level: 38 dBmV

It seems that even straight from the wall jack my signal strength is on the low end of what the modem can take, and when I hook up the splitter it degrades it to the edge of the operating envelope. What are my options? Should I buy a high quality splitter such as from Monster or CableVision? Should I get some kind of signal amplifier?

Any help would be appreciated.

You need to have a tech come out to fix your signal issue. It's very lopsided.

Are you putting a 2 port splitter on there or bigger?

Your signals are too low (probably, modems are NOT prescion meters and can be off by several points) on the downstream side.
If you can't eliminate a splitter (or 2) by using a different jack call for a tech.
Do NOT waste money on a consumer grade amp, they rarely work well with HSI (they are intend to help TV signals) If an home amp is required CmCast will supply a professional model, more likely they would change your tap value or re-adjust a line amp.
Even more likely is replacing some cables, the splitter(s) or the house drop will fix the problem.

He is wanting to split the coax cable signal. Ethernet hubs don't have anything to do with that.

Just trying to avoid confusion ...

Etherent hubs have NOTHING to do with coax and cable.

Yes, I would get a tech out there.

Any idea if your house is wired with RG59 or RG6 cable?

Thanks for the help guys. I had Comcast send a tech out and the signal downstream has been boosted to 0 dBmV after applying the splitter. It still drops occasionally but much less than before, and at this point I suspect at least part of the problem is outside the scope of Comcast. Thank you all for your help.