Why does this cable splitter screw up my modem? What can I do about it?

Um, yes, you are stealing cable TV service. If all you want is local channels, get a small antenna for the TV and run the cable directly into the modem.

Jeez, cable companies know that people get basic cable on internet-only subscriptions, I first heard about it maybe 10 years ago. They don't care; it's not stealing.

Your question is too generalized for the specific answers I would give. ( A splitter with an unterminated port versus a splitter with a terminated port will behave differently. It's complex. Add to that the fact that the modem signal is bi-directional and while I've dealt with designing amplifiers that work on cable/internet distribution systems, I've never considered a splitter scenario. ) So.... just unplug the cable going from the splitter to the TV and see if it makes a difference. May take a day or two, but a test is a lot better approach than guesses from out here with no instrumentation. If the problem goes away, hook it back up and see if it returns. If you can make the problem go and come at will, you have the basis for a new question that can get resolved.

My understanding is that the FCC only recently gave the cablecos permission to encrypt OTA content,http://www.broadcastingcable.com/article/489863-FCC_Votes_to_Lift_Ban_On_Digital_Basic_Tier_Encryption.php Most cablecos only recently sent out notice of the coming change, and access will be lost sometime this month. There was no theft of service under Clear QAM, active or passive, if you were only getting OTA channels.

It sounds like your splitter may be faulty - if the channels you are getting are simply the local ones and not basic cable, those are readily available anywhere and there's no reason you wouldn't be able to draw a cable signal and an internet signal through that splitter. That's what it's designed for. You can buy a splitter at a local tech store for very little money and swapping them out is quite easy. I'd start there before contacting the cable company.

More details: my splitter says it is 5-1002 MHz, and all of the output plugs are marked as -7db. What should I look for in a splitter?

If it is for regular digital cable, you don't need anything higher than 1 Ghz. Don't spend more than a couple of dollars on this.

radio shack sells a signal booster/splitter, if it comes to that point; it has to be plugged in, but I used it in an old house with beat up cables and it made a big difference.

You need to replace the splitter for sure - a -7db drop is typical in a coax splitter with 3 or more legs, but a simple 2-way splitter should only have a -3.5db drop. If your splitter *is* a 3 or more way, one of the legs *should* be marked with only a -3.5db drop - you can try hooking the cable modem up to that leg. (These things do vary, hence *should*). That being said, if eliminating the splitter altogether eliminates the problem? Then some combination of low signal / high noise is your issue (possibly owing to other upstream splitters between you and the tap out on the pole / in the lockbox / in the ped / wherever. As this is an old apartment, your drop (the piece of coax that connects you to the tap) could be the issue. Is it RG59 coax? (it should be RG60, or might even be RG11 for the drop itself and RG60 internally). And yes, you're stealing basic cable by hooking up a TV. This is possible because in most instances, the first 23-or-so channels (typically referred to as "basic cable") are broadcast unencrypted, and thus your tv is perfectly able to decode and display them. Your installer should have put a trap (an in-line filter) on the line to block those unencrypted channels. That he did not (either by choice or omission does not justify the theft, but it explains why it is possible. I'm skeptical that a radio shack style signal amplifier would solve the issue - there's almost certainly enough noise on the line that amplifying it would just make the problem worse (at least, that's my gut feeling, without being able to hook up my wavetek and see the levels myself.) /spent multiple years as a CATV splicer.