I have a Philips DVDR 3575H. using Comcast's

Try this set up-

Use an RCA audio/video cable for this set up (Yellow/Red/White).

Connect from the Video out (Yellow) connection on the back of the cable box to the Video in on the back of the DVR.

Connect the audio (Red/White) cables from audio out connection on the back of the cable box to the "Audio in" on the back of the DVR.

This should still get you a decent picture but not a true HD picture for your recordings.

When you set the timer to record set it to record either E1 or E2 depending on which input you connected the cable box to and ensure that the cable box is tuned to the channel you wish to record.

*** NOTE: If you keep the HDMI cable connected as you have for the cable box and watch normal TV through the HDMI input on the TV you will get HD programming for your general viewing but still have the option to record with the DVR at a lower quality. ***

Good Luck and please rate this solution....

I too have this unit on comcast basic cable. Connection to the TV is through S-video, which works great.
The good news is that comcast basic cable has 177 digital channels on it (at least here in the Baltimore area) and the Philips has a clear QAM digital tuner (even tho they don't tell you) as well as the ATSC tuner.
Here's how to use it: On the Philips remote select the digital tuner (button labeled "DTV/TV" and do a complete channel scan (analog and digital).
This may or may not address your dark picture problem, but getting access to the unadvertised QAM channels is worth knowing about.