Hi, I have an old DVR-120 from Directv. I now have Charter cable (non-hd) can I hook the dvr into cable and record sd cable? Will it just pass thru the RF to the TV or?

I also have the ability to see/use OTA HD on my TV but its a lttle kluge-ie have to use an a-b switch to jump from cable to ota. I just mention it because I would like to be able to view HD OTA and SD cable with out a lot of manual intervention. I have a Sony XBR 46" and a Samsung 4254. It seem possible on the Sammy because it has 2 RF inputs, the Sony only has one.