I'm not exactly sure but I think it has to do with the fact that a monitor's minumum refresh rate is usually about 60 Hz. Anything above that number can display progressively without our eyes noticing much flicker or any problems that are associated with too slow a refresh rate.
In fact, you can output an interlaced signal from your HTPC if you want.
Progressive scan (480p) is considered to be better than interlaced (480i) when output to a CRT type device. The De-interlacing is normally done by the DVD player. With a fixed panel display (LCD, DLP, etc), those devices are progressive by definition.
In this case, the de-interlacing is done either in the player or in the display. Some of these de-interlacers/scalers are actually very good, but most are usually so-so.
So people turned to scalers/doublers to get a better image than you would get from the built-in de-interlacers/scalers. The best thing is to get the most control over the image.
Normally, you would need a high end scaler to give you this level of control. Some of the good scalers out there cost upwards of $4000. An HTPC will give you the same results for less than $1K.
An HTPC is regarded as better for DVD playback when compared to your STB DVD player because it will allow you to output a signal that matches your display device wrt resolution and scan/refresh rates.
Additionally, with the post-processing tools available for free, you can really get the most out of a DVD.
I don't know how long you've been in computers.. But back in the day it was a big deal "upgrade" to get a "non-interlaced" monitor (I still recall the 14" NI monitor I got with my first Gateway 386DX33). Not long after that all PCs pretty much went to non-interlaced displays. "Progressive Scan" I believe is the same "non-interlaced" concept by another name.
When a computer monior has a refreshrate of 60hz, it paints the whole screen 60x a second. For a TV at 60hz, it does every other line 60x a second (effective 30fps).
It doesn't have anything to do with the refresh rate. A progressive scan device is one that outputs a complete frame each time the screen refreshes. Each time a interlaced TV outputs a frame it only consists of every other line the following frame will contain the missing lines. Therefore it takes two refreshes of the screen to show one complete frame on an interlaced display.
Understanding the basic theory of Cathode Ray Tubes (CRT's) is the key to understanding the difference between progressive and interlaced displays. So here we go...
Cathode Ray Tubes are comprised of four key parts:
Phosphors:Certain substances, when bombarded by electrons, glow. The inside of the flat part of a CRT is covered in these. (This is perhaps a bit innaccurate, but I'm not going into aperature masks, etc. here. We can simplify things a bit for expediency here.)
Cathode Ray Gun: It may sound like something Flash Gordon would have strapped to his hip, but this sucker is actually pretty common. It's sole purpose is to spew forth a thin stream of electrons just big enough to illuminate one pixel on a screen. This is typically located at the very back of the CRT in the pointy bit. (You typically won't see this unless it's a particularily old TV or you've taken the outer casing off of a newer one.)
Electromagnets: The stream of electrons coming out of the Cathode Ray Gun can be bent by magnetic fields. By using two sets of magnets oriented perpendicularily to each other we can control the path of the electron beam precisely.
The Vacuum Tube:The entire enchilada is wrapped up in one big vacuum tube. (This is why TV's typically implode rather than explode, except in really bad movies.) This is to allow for a nice coherent electron beam.
So you have a beam that you can point at any point of the front of the CRT. Wherever you point it, the phosphors will glow. So how do we draw a picture? In CRT video displays the beam is moved systematically, back and forth horizontally and sweeps down the screen vertically, snakes and ladders style. This vertical sweep is called a pass. (e.g. A monitor that does 60 vertical passes per second is said to be running at 60 Hertz.) If we perform sweeps fast enough it will stop looking like a single dot moving back and forth and become a cohesive picture. To work, this relies on the fact that the phosphors continue to glow for a short time after being stimulated, and also on the fact that our eyes tend to perceive extremely short events as being longer than they actually are. (Film works on a the same principle.)
And now, the money shot:
Progressive displays cover every single horizontal line of phosphors on every pass, refreshing the entire image each time. Interlaced displays cover every other horizontal line each pass, taking two passes to hit every phosphor on the screen. The advantage of an interlaced display over a progressive display of the same resolution is that, because the electron beam covers only half the screen each pass, the magnet and controls only have to be half as fast, and are therefore easier to produce and cheaper.
As others have stated, HTPC's are not necessarily progressive scan devices. They can output interlaced resolutions if so required. This is how consumer PC displays started out in fact. The first "video cards" weren't really "cards" at all, but devices that allowed PC's to miraculously display output on a consumer television. While displaying a few lines of blurry interlaced text on a TV screen may not sound that hot, it beat the heck out of teletypes, card punchers, or, god forbid, output LED's! CRT displays were previously the exclusive domain of elite mainframe users. (Going "online" with a mainframe, even over a teletype was once quite a privelage. Most people who worked on computers in those days punched holes in cards, submitted their cards to be run in batch jobs, and got the results back hours or days later, not unlike drycleaning batches today.) As PC's penetrated the market deeper and deeper, it was only natural for purpose-built displays to take over. Progressive scan monitors with higher resolutions are a lot easier to work on after all. Pretty much all modern PC's are capable of outputting progressive signals, and likewise, almost all computer displays are capable of displaying progressive signals. That's the what and why of it.
Consumer CRT televisions, on the other hand, are typically limited to interlaced signals only. Television is broadcast in interlaced NTSC after all. Why build a TV that is needlessly more expensive? Laserdiscs, HDTV, and, to a much larger extent, DVD's of course. DVD's are really the first home video format superior to interlaced NTSC to actually "catch on" to a large extent. Televisions are one of those home appliances that have been built to the same specs for decades. Change doesn't come fast to all markets. Fortunately, we are finally starting to see the result of this pressure on the consumer video display market. DLP and LCD devices, both inherently progressive devices, are making broad inroads into the scene in the form of RPTV's and, to a much smaller extent, projectors. Many RPTV's are still CRT based, but are also being made capable of progressive display modes to meet HDTV standards. As a result, we are starting to see set-top DVD players that are capable of generating progressive output to take advantage of these new displays.
Ian - Thanks for the link, but it?s raised a question that has been really bugging me.
I have a HTPC using TheaterTek going to an OLD XGA projector that I have borrowed from work to test if I really want to spend the $$$ for my own projector. I normally love it, but it some times it drives me insane. I assume that the projector is progressive scan since (I think) all "computer" monitors have been progressive for many years. However, some times it looks like an interlaced image, it appears to be "Combing" error from reading that link. Some movies are HORRIBLE, while others don't have a problem. I have read that TheaterTek is not the greatest at doing the 3-2 pulldown. But how can I minimize or eliminate the combing errors?
A forum community dedicated to home theater owners and enthusiasts. Come join the discussion about home audio/video, TVs, projectors, screens, receivers, speakers, projects, DIY’s, product reviews, accessories, classifieds, and more!