About: It is true that, in most cases, an analog video signal displayed on an HDTV will not look as good as it would on an analog set: there are two basic reasons for this:(1) The video processing circuitry in an HDTV upscales the image, which can improve the good aspects of the image. However, at the same time, the video processing also will magnify the defective parts of the image, such as interference and video noise that is not as noticeable on a lower resolution analog set. In this case, the quality of the video processing in the TV can have either a positive or negative effect on the degree of correction for poor analog signals.
(2) If you have upgraded from a 27-inch (as an example) analog television, to an large screen Plasma (lets say 42-inches) – the lower resolution analog image will degrade somewhat since you are blowing the image up to a larger size.
For more details on this topic, please read Robert Silva’s FAQ on HDTV.