Can I get cable internet along with my DirecTV?

You're most likely correct that a sat./OTA diplexer would prevent "interference" between the cable internet and satellite signals and I stand corrected on that point.

But almost as bad instead it may prevent proper reception by the modem of the cable internet signal for DOCSIS 3.0 standard signals.

As for a a similar DSL "training signal" phase for cable internet service I doubt that since whereas DSL uses a point-to-point type topology, DOCSIS for cable internet systems is a point to multipoint type where the CMTS at the CATV headend chooses the particular downstream and upstream frequency that many subscribers in a group use on a time-shared basis.

Therefore if one of those subscribers assigned to a group with a given down/upstream frequency and is unable to receive the downstream signal because of a SWiM sat/OTA diplexer in line with the modem which blocks it, then that sub. will simply be SOL, and just not able to access the network.

I know of no provision in the DOCSIS standard for the CMTS to make a special accommodation and change the downstream frequency just to suit one subscriber in a group with a reception problem caused by a component on the customer's premise. .

 

It would make for an interesting experiment to see what happens if you add a diplexer (used simply as a band pass filter as an experiment, without anything connected to the SAT side) to a DOCSIS 3.0 system configured to use frequencies above 900 MHz and see if it causes problems. Then try rebooting the modem and see if it clears the problems.

What you say makes sense, but I wonder how well it would perform in practice to assume that all channels will work for all customers on the same node, given that the cable company has no control over how screwed up the cable plant inside people's houses is? It doesn't seem like it would be too difficult for a training phase to test the modem to head node connectivity for each modem when it is started up, and block the channels that have problems from being used for data destined for that modem. It is extra work, but when compared with everything the head node is doing already to manage connectivity for hundreds of attached modems it isn't that big of a deal.

However, perhaps it isn't really a problem in the real world and that extra work isn't required. If the only time it would matter is if someone has a filter inline with their cable modem it isn't something they'll bother doing, they'll remove the filters instead. So I'd agree with you that diplexers shouldn't be used in this situation unless it could be proven/demonstrated that DOCSIS will work around the filtering.

 

You might be surprised on how well tech can conceal these cables.

 

Not necessarily. You said that your house is pre-wired for cable. Is there a cable outlet in the room where you want the cable modem? If so, and the pre-wire was done properly, the cable installer should be able to run the new cable to the exact same location where the beige wire already runs. The beige wire likely runs to a centralized distribution point where the various runs to the other rooms begin. The cable installer would just need to connect the new coax with the coax that runs to the room that you want the modem to be.