All HDTV's have one (or a couple) of preferred resolutions at which they will display TV signals. Many HDTVs are what's known as fixed-pixel displays - they can show TV signals (not matter what resolution they are recorded in) at a single resolution on-screen.
Sometimes the stars align and everything just works fine for you. For example, your CRT HDTV displays HDTV signals in a 1080i mode. Lucky you - the input resolution on that favorite TV program you can't wait to watch broadcasts as a 1080i signal. All is well in the world, and good TV is yours to enjoy.
But what if you have a TV, such as a LCD rear-projection model, that displays HDTV at 720p, not at 1080i! Law and Order: Special Victims Unit might not measure up to your viewing expectations.
What magic will enable you to turn a 1080i signal into a 720p display? A brainy bundle of chips and software called a scaler or video processor gets involved. A scaler converts one resolution to another, using various mathematical techniques to figure out what the video signal should look like at a different resolution.
Going up or going down?
If a scaler is converting a lower resolution to a higher one, the process is called upconversion. The opposite process is called, unsurprisingly, downconversion.
The main job of a video-processing system is to up- or downconvert an incoming signal to a different resolution. This process for a scaler working with your HDTV has two main benefits:
- Video signals are matched to the best display resolution for a particular model of HDTV, regardless of the signal's original resolution.
- Standard-definition video signals are upconverted to a higher resolution to look closer to HDTV than standard-definition.
Don't buy anyone's marketing spiel or sales pitch telling you that his or her scaler can make any TV source into HDTV. Good scalers can produce something very pleasing to the eye, which looks close to HDTV. But you can't create something from nothing - real HDTV signals can have six or more times as many pixels as standard-definition. In other words, in an upconverted video stream, five out of every six pixels could be "made up" by the scaler. Even the best scaler can't create something as good as the original recording!
De-interlacing your video
The simplest video processors - which actually pre-date HDTV - are those devices known as line doublers. High-end home theaters have used line doublers for years, in conjunction with fancy (and expensive) front-projection systems.
You may also hear line doublers sometimes called deinterlacers.
The job of a line doubler is pretty simple - it scales the video by converting an analog (480i) video signal into a progressive scan (480p). By doing this simple trick (effectively doubling the scan lines in a CRT TV - hence the name), a line doubler greatly smoothes out the picture and reduces the subtle flicker that you normally get from an interlaced picture.
You also hear about devices call line quadruplers, which not only deinterlace video, but also interpolate in between the lines to create a picture that's the equivalent of 960p (twice the resolution of 480p). Line quadruplers are really high-end devices for use with the most expensive CRT front-projection HDTVs.
Getting fancy with scaling
Fixed-pixel HDTV displays - plasmas, LCDs, DLPs, and LCoS - have created a need for something beyond just a simple line doubler (not that a line doubler is all that simple!). Many fixed-pixel displays actually have rather funky native resolutions that require all incoming video signals to be scaled.
This demand has led to the development of scalers that convert any incoming video signal (well, any standard incoming video signal) into a specified output resolution.
Choosing scalers
If your HDTV needs a scaler to operate (and most need at least a deinterlacer/line doubler), then it already has one built in.
Internal scalers do have some limitations, however. Sometimes an external scaler offers advantages. Here are three scenarios that may call for an external scaler:
- Some HDTVs have internal scalers that accept signals of only certain resolutions. For example, some HDTVs may accept only 480i, 480p, and 1080i inputs. If your external HDTV tuner puts out only a 720p signal (admittedly rare), you're out of luck.
- Some HDTVs (CRT tube-based TVs, either direct-view or projection systems, usually) contain an internal scanner that deinterlaces analog video sources (meaning it converts 480i to 480p) but doesn't upconvert analog signals to a higher resolution. There's nothing wrong with this lack of upconversion - 480p probably looks better than analog's ever looked! - but the picture doesn't use the full capacity of your HDTV.
- You may just be a person who wants the best! The scalers built into most HDTVs are good enough for most owners. But, if you demand those last few percentage points of picture perfection from your system, you may want a fancier external scaler.
