Thursday, April 19, 2007

HDTV Ghosting

Pictures speak a thousand words. Especially when it comes to explaining technology issues. The purpose of this article is to elaborate on the problems that HDTV's have, and why the various display types are imperfect. By understanding the imperfections, you'll be able to decide where you'd prefer to compromise on you purchase, and also understand the newer technologies that attempt to improve these imperfections. Marketing jargon really confuses things a lot, but really, these are things that you've probably already experienced, but not not paid too much attention to:

This is an issue we are used to with our non HDTV's. The analog video signal is produced by RF (radio frequency) and the audio would be produced by FM (frequency modulation, yes the same as the radio). Two separate sources. This is why sometimes, the audio would be perfect, while the picture would be as shown below. Ghosting would occur 1) If you had a bad connection , 2) Interference from other devices near your TV, 3) Nearby objects such as buildings, blocking or reflecting the broadcast signal, so that only part of the signal gets to the TV.

With HDTV's we don't have this specific problem. If any of the above symptoms occur, we will simply see a blank screen. With HD digital signals, its a matter of "yes" you get the signal, or "no" you don't get anything. There's no in between. However, what we do sometimes get is the "impression" of a previous frame in the background that hasn't gone away. The picture has changed, but the faint image of what was previously onscreen a split second ago is still there. The important difference, is that this is more down to the video processing in the TV and the display technology itself rather than the signal. Ghosting of this nature on an HDTV, disappears after a few seconds and is more of an annoyance than anything. Ghosting is NOT however, the trails left by an image, that is called motion blur. I see a lot of inaccurate references to this all over the Internet.