Analog Television signals, both broadcast and cable, as well as VHS, in most cases, will look worse on an HDTV than they do on a standard analog television.
The reason for this is that HDTVs have the capability of displaying much more detail than an analog TV. This results in the video processing circuitry in the HDTV enhancing both the good and bad parts of a low resolution image.
The cleaner and more stable the signal, the better result you will have. However, if the picture has background color noise, signal interference, color bleeding, or edge problems, (which may be unnoticeable on an analog TV due to the fact that it is more forgiving due to the lower resolution) the video processing in an HDTV will attempt to clean it up. However, this may deliver mixed results.
Another factor that contributes to the quality of analog television display on HDTVs also depend on the types of video processing circuitry employed by different HDTV makers, and some HDTVs perform the analog-to-digital conversion process better than others. When checking out HDTVs or reviews of HDTVs, make note of any comments regarding analog signal quality.
Another important point to be made, is that most consumers that are upgrading to HDTV are also upgrading to a larger screen size. This means that as the screen gets larger, lower resolution images look worse, in much the same way as blowing up a photograph until shapes and edges become less defined. In other words, what looked really great on that old 27-inch TV, isn't going to look quite as good on that new 42-inch Plasma TV.
Here are some suggestions:
1. Make sure you have the cleanest analog signal possible - or, better, switch to Digital Cable, HD Cable, or HD Satellite. If you have a high performance HDTV, why waste your money by supplying it with an inferior signal source - you are paying for HD capability - you should reap the rewards.
2. If you have an HD-cable box or HD satellite box, connect them to the HDTV using HDMI, DVI, or Component Video connections (whichever is type of connection is used by the cable or satellite box to transfer HDTV and Digital signals), rather than a standard RF connection.
3. Keep in mind that all over-the-air analog broadcast television signals will end on June 12, 2009, and you may have to switch to digital cable, or HD-cable, at that time anyway.
For more information on what you need to know about HDTV, check out my HDTV FAQ's