Wednesday, December 12, 2007

(HDTV Primer Part 2) HDTV's Demystified


Frank's 2000 Inch TV

Ahhhh... Non one can write songs like that wordsmith, Weird Al!

As the advent of HD television, I am finding greater and greater semi-information coming from the cable companies, the TV companies, and the big box electronic stores.

This post's theme is going to explain exactly what the heck all the TV terms mean. 720i? What the heck is that?

Have no fear friends, I am here to help. If you are a regular reader here, then you know that I am constantly waging an information war against those who would spread lies about new TV technology.

First off, what is High Definition (HD) TV? It is, quite literally, pictures and video that are highly defined. When describing pictures, one uses the term "pixel" a clumsy contraction of the words "picture element". A pixel is, in reality, simply a dot on your screen. This dot has three colour components, red, blue and green. Depending how these colours are brightened or dimmed, gives the pixel an overall colour. Combine a whole bunch of pixels, and you get a picture. Change the colour of the pixels in a controlled manner, and you brain interprets it as movement.

A screen's picture quality is defined by its "resolution" or quite literally, how many pixels it has on its surface. The more pixels, the better the picture.

This is where HD comes in. TV's sold in North America throughout the 70's, 80's, 90's, and early 2000's were what was known as Standard Definiton (SD), 640x480 and multiples there of, also called NTSC (National Television System Committee). This meant that they had pictures that were 640 pixels wide by 480 pixels high.

Larger SD TV's used larger dimensions, but always in the ratio of 3 pixels wide, to two pixels high.

HD content uses a much higher number of pixels to define pictures, so it does indeed look better when displaying high definition content.

HD resolutions use a ratio of 16 pixels wide by 9 pixels high. Official HD resolutions are: HD 720 (1280x720), and HD 1080 (1920x1080). Technically, EDTV 480 (854x480), is not high enough to actually call it High Definition (and is not part of the HD Standard), so it is referred to as Enhanced Definition.

Here is a comparison of various resolutions in scale:


While many TV's rightly claim that they are HD displays, they are not all HD 1080. Make sure you read ALL the numbers, not just the price. There is a reason that giant *HDTV* is so cheap!

Another thing you have to know about is the "scan".

Dear God, more TV lingo? Yes, the "scan" was a term from the days when all TV's were Cathode Ray Tubes (CRT) in which a beam of electrons "painted" the picture on the screen, one line at a time from the top, 60 times per second.

Progressive Scan (PS) means that every line is painted on every scan. This gives the best picture as every line is utilized on the screen in every pass. However the delivery method of the media requires a high bandwidth since more information needs to be delivered. This is indicated as a p such as 720p or 1080p.

Interlaced Scan (IS) means that on the first scan, lines 1, 3, 5... etc. of a frame are painted, then on the next scan lines 2, 4, 6... etc. of the next frame are painted. The benefit is that half the bandwidth is needed at the sacrifice of picture quality, known as "combing". Additionally, IS has a "flickering" effect. Even though you may not be able to see the flicker with your eyes, it is known to cause eye strain and headaches after continuous use. This is indicated as an i such as 720i or 1080i.

If a TV can display progressive scan, it can display interlaced, but not the other way around.

Here is an example comparison between PS and IS. The fourth picture is how non-scanning TV's such as DLP, LCD, and Plasma display interlaced video.

Please note that the flicker shown for IS is exaggerated for demonstration purposes. It really is not that obvious.




This is an example of "combing". This is caused because the odd and even lines are displaying two different frames, so the picture does not line up properly. You can see that the source video was being transmitted by satellite, where bandwidth is a premium, hence the use of interlaced broadcast.



Current TV's use "comb filters" to try clean up the picture, but no filter can replace progressive scanning.

Now finally there is the display technology. While CRT is really the king of picture quality, they are very heavy, very thick, use alot of lead in the construction, and are limited to a one meter (diagonal) screen sizes. Newer TV's are thinner and lighter, saving money on shipping, and saving your back.

The first is Digital Light Processing (DLP) this uses a grid of tiny mirrors to project coloured light onto the screen. Each mirror represents one pixel. Cheaper models use a single DLP chip and a rapidly spinning colour wheel (red/blue/green) to project the picture. Higher quality images are obtained using three DLP chips, each with its own dedicated light beam (again red/blue/green).

While not as thin as LCD or plasma, DLP's are cheaper and more easily repaired when the light source burns out. They are also lighter and immune to "burn in".

DLP may also create eyestrain, are susceptible to the silk screen (ie. the screen surface is noticeable) effect and the screen door effect (ie. the pixels are spaced too far apart, showing a black, grid-like pattern). They may also be vulnerable to vibration, so if you have kids jumping around by the TV alot, the mirrors may be affected.

A Liquid Crystal Display (LCD) creates a picture by using liquid-crystal gels that react to electricity. The back of the display is evenly lit by the backlight and brightens the coloured liquid crystal cells. The cells can then be darkened by applying various voltages to the individual cells. Each gel cell only produces one colour (blue/green/red) so three cells, one of each colour, are grouped together in a pixel.

LCD displays are very thin, but don't produce true black, since they can never be truly opaque. LCD's are also vulnerable to dead pixels, where cells no longer respond to the voltages applied, thus becoming "stuck" on one colour. While negligible around the edge, it is very noticeable in the center area. Cheap models are also susceptible to screen door effect, blurring, and ghosting.

Finally Plasma displays work like millions of neon lights to produce colours in a grid pattern to produce a picture. Each cell contains a gas that is excited by electricity. The cell is coated by a special coating called a phosphor. The excited gas causes the phosphor to display its colour. Each phosphor can only display one colour, (blue/green/red), so three different phosphors are grouped together in each pixel. Plasma displays also have a backlight, to help brighten the image.

While giving a better picture than LCD, Plasma displays are way more expensive than either LCD or DLP and consume much more power. They do produce black better than LCD, but they are vulnerable to screen burn. Screen burn occurs when a single picture is displayed on the screen for a long period of time (such as pausing a movie while the TV is on and forgetting about it for a few hours. This damages the screen, causing that image to lightly be displayed permanently.

Also, LCD and Plasma TV's are not easily repairable. Hence, when it dies, you need to buy a new one. DLP's can be repaired depending on which component dies.

Do here is the big question, which would I choose?

My preference is the DLP, 1080p display. Failing that, I would go with an LCD, 1080p. I just can't justify spending tonnes of money on a plasma screen. I mean, its only television.

Now if they ever come out with SED screens... well, that's another story altogether.

I hope this little article has helped you sort out the quagmire that the big box stores have hoped to drown you in. Now you can go out and make an informed choice as an educated consumer.

Until next time, this is Soundwave saying, keep on having fun!

"bah weep graaagnah wheep ni ni bong"

2 comments:

Sal - www.salciampa.com said...

When you replace the light box in DLP tv's, it's like getting a NEW tv.

There are also reports that you won't notice the difference between 720p vs 1080p. But, for fast action movies where there's alot of movement, it is best to go 1080p.

RadioFreeG said...

Doe this whole debate on TV come down to the more you spend the better, like a lot of things in life?

The eye strain was a good point I never heard about before. Seems like If I'm ever in the market for a TV like this, I'll look for the little "p" and head to that display unit.