It's not clear from your message whether or not you are also a Comcast TV subscriber. If you are a Comcast TV subscriber, depending on your situation, you may be qualified to rent a set top box (or two) for a year or two. The box will take the cable channel and convert it to one of several possible formats for your TV (or computer), such as NTSC (analog) Channel 3 or 4, composite video, or component video. I don't know if the standard-def cable boxes come with a HDMI port. If you elect to go with a HD package, the HDMI port will come on the HD set top box.
Analog cable channels are likely disappearing since the FCC required cable carriers that had hybrid cables to continue providing analog channels until 2012. (I lost my local analog cable channels in October 2002.)
The FCC has agreed to permit cable carriers to encrypt local content, having decided that maybe the five largest cable providers in the United States have a case for encrypting all channels: it will reduce cable theft, which reduces unauthorized splitters that weaken the signal for all using the cable, including the paying subscribers. So Comcast has been both dropping analog channels and encrypting digital channels. See the thread: Are All Your QAM Channels Scrambled?
So, once Comcast has encrypted all your local channels, the only way to receive a TV signal is to subscribe to Comcast Xfinity TV, and have either some form of set top box from them, one per display device that you plan on using, or rent a CableCARD from them and use it in your CableCARD-compatible equipment (such as a TV with a CableCARD slot, or the appropriate adapter on your computer, but that part is outside of my personal knowledge). Be sure to check with Comcast before going out to buy a TV with a CableCARD slot because with the mix of equipment Comcast has, particularly from acquisition of older cable companies, there's no guarantee that all areas could be handled by CableCARD.
Another possibility is that Comcast didn't suddenly kill both analog and Clear QAM channels on the same day, but somehow filtered the feed to your residence to allow just the broadband Internet. They aren't suppose to do that if you subscribe to both Internet and TV service through them, but sometimes a technician will accidentally put a filter on the wrong line. (It had happened to me once.)
I was doing ok with an old analog TV and a pair of VCRs for time-shifting shows ... until October 2012 when Comcast killed our local analog channels. Well, I knew that the days of the VCRs were numbered because there was no local store where I could buy a VCR (though I could buy a VCR/DVD player combo without tuner for a reasonable price), and that TV was already having issues, for the past couple of years displaying a huge orange halo on the right of any bright object on the screen, so I played those VHS tapes in a third VCR (and later a VCR/DVD player combo unit) that I had hooked up to a HDTV. So when we lost our analog channels, I visited our local Xfinity store, got the "Digital Starter Package" and rented a HD DVR from them, which allowed me to record two channels at the same time while playing a third, and on the way home purchased a HDTV for the HD DVR to feed, and that setup is in my bedroom. I knew I was going to do that some day, but the dropping of all the analog channels pretty much set the date of doing that which I was already convinced I would someday do.
Edited to add: The HD DVR I rent from Comcast has a CableCARD serial number as well as a DVR serial number, and for "activating" the DVR, I had to have them activate the CableCARD serial number.
Added: It took just two days, and then I wondered why I didn't start renting a HD DVR from Comcast a lot sooner! It's far more convenient than VCRs and no worries about having to schedule changing the VHS tapes so I don't run out of tape before running out of programs to record, and no more of this of having to watch all the programs on one tape before I can reuse the tape for another six hours! And HD sure looks a whole lot nicer than good SD, and several orders of magnitude better than the dying TV with the big orange halos I was using in the bedroom!
And none of this affected the broadband Internet service and the broadband modem I am renting from Comcast, other than there is a package deal for having both Internet and TV through them.
Edited by Mark12547 - 11/19/13 at 9:01pm