I recently tried a 11$ radio shack one that had a 3.5 db loss on each leg..
My cable modem worked just as well as before, but I could not get a tv signal.
I also don't pay for cable tv and was just curious to see if they installed a filter on my line
or not.
There is a lot of information out there though, cable modem splitter as a google search will yield more info than you could need.
The ideal splitter from what I read would not have any db loss on the leg going to the modem..
Need to order one from online from what I saw around here locally..
If you can't find or don't want to pay for something like that the 3.5 db loss should still work like mine did, but don't go above that. It depends a lot on how the wiring is as well.
My cable wire comes right from the road around the house; connects to one wire and drops into the house to the modem..
Best advice I read online was to stop a cable guy and see if you can get one off them.