Waveform 08 FPD/Digital Display Primer

[HD Library]

Originally published 10/21/2004, HDTV Magazine, editor Dale Cripps

Updated 08/05/2005

By Richard Fisher

Digital display technology requires another issue of terms and explanations. These displays are also called FPD, fixed pixel displays. Direct view CRT is also an FPD. If you recall projection CRT does not have pixels. A projection CRT consists of a red green or blue phosphor evenly applied to the complete face. Projection CRT has what is called beam spot size and that determines the ultimate capability of the display based on how small it is which determines how many different spots it can create horizontally. A 1080P projector can morph to any lower scan rate by simply increasing the size of the spot which happens naturally going left to right. What can happen though is scan lines become more visible which again can be offset by defocusing the colors. Bottom line nothing is fixed in terms of resolution and is only limited by how fast it can scan and beam spot size.

FPD is what it says, fixed pixels and that means it cannot change to any other scan rate than what it was designed for. While direct view CRT can do this to a degree it still has a fixed amount of pixels on the screen and that will ultimately determine the maximum resolution and what line counts it can reproduce accurately hence why projection CRT is king. Digital displays suffer from this anomaly even more because unlike a CRT that simply excites those phosphors that the electron beam hits, each individual pixel is controlled to try and simulate this kind of response. Taking the common 1280X720 720P display here is what we have to work with. We can do 1280, 640, 320, 160, 80, 40, 20, 10, or 5 vertical lines and get an accurate response. Bear in mind this is a measurement of horizontal resolution based on 1 pixel off and 1 pixel on or 2 pixels on and 2 pixels off, etc. If we feed this display a 1920X1080 1080I signal we have a big problem. The 720P cannot do 1920 lines and that will have to be scaled somehow to 1280 and this will cause artifacts in the image. Dropping that 1920 to half or 960 we still have problems as this does not exist on the 720P display. If we reverse the circumstances using 1920X1080 display we can do 1920, 960, 480, 240, 120, 60, 30, and 15. As you can see these line counts do not match 720P and if we attempt to display a 720P signal on this 1080P FPD display we have the same problem we had displaying a 1080I signal on the 720P display. What all this means is CRT is king since it does not have this problem and can do any line count we wish. Will you detect the flaws created by fixed pixel displays on a regular basis? Based on experience with real images, probably not. What this does mean though is quality scaling of other scan rates will be directly related to image quality and the internal scalers provided by manufacturers still have flaws, so, as a videophile you should always budget in an external scaler in case you are not happy. There are some FPDs that are not native to anything such as 1024X1024 in which case any signal must be scaled to have an image and you cannot bypass the scaler with a native HD scan rate.

FPD has also suffered from fill factor. Fill factor is all about how much space there is between the pixels and the less the better. Plasma has very poor fill factor which is why it will not meet the 3 screen heights requirement for viewing distance, the pixels are plain as day. The new generation flat panel LCD on the other hand has great fill factor meeting this specification. LCD projection has just recently met this requirement with some of the new models. DiLA and LCoS both meet this requirement. DLP is very close and with some tricks from your local ISF calibrator this can be overcome. DLP also now uses a technique called wobulation in the new series of display chips from Texas Instruments. This technique resolves fill factor issues completely for DLP.

Another aspect that most FPDs have had in common is poor black level. The blacks simply cannot turn off enough of the light so many of these displays will not work correctly in a dark room. Bias lighting is often times necessary to overcome this which will then get you very deep blacks. Another point is the light source of the display. Those that use a lamp have this problem because you cannot turn off the light, only block it or deflect it somewhere else to create black. CRT is king because brightness and light are creating by literally controlling the intensity of the electron beam that excites the phosphor allowing us to actually turn it off if we desire.

Regardless of the technology all FPDs have a limited range of operation. While the display will offer contrast and brightness settings these are not similar in anyway to CRT brightness and contrast due to the way the products operate. While both have the same type of effect on your image the net results are quite different. With an FPD you are taking the video signal and turning into a digital signal for the FPD panel that has dynamic range which means how wide apart the black level, brightness, and peak white level, contrast, settings are and that is determined by the bit structure of the process. If they are too far apart then we will exceed the amount of bits available, the dynamic range will be exceeded, and the bright parts and dark parts of the picture will actually disappear at these two extremes and this called black or white crush. If they are too close then we will use less bits in the processing which can also cause us to lose correct variations in brightness through out the range and induce noise and artifacts into the image. What this means is that the brightness and contrast are setup on FPDs for maximum dynamic range and the ambient light in the viewing environment is setup with a bias light for the kind of output we obtained after this adjustment. While brightness and contrast can be adjusted a little bit we will not have nearly the range we can get with CRT. Most CRT displays provide a wider range for these controls to match the viewing environment without inducing problems into the image and so again CRT remains king. That said there is a modification that can be performed on RPTV versions of FPD by a few select ISF calibrators to decrease the over all light output of these displays so they will work in dark rooms using an ND, neutral density filter. This is not considered a normal part of ISF calibration and most calibrators will not perform this as it requires a fair amount of disassembly of the product to install. Hopefully in the future we will have an iris installed in the optics of consumer digital RPTV. Sony has utilized this approach on their new Qualia display. Like a camera uses this technique to reduce light input an iris can also reduce light output which has been great for front projection as well in reducing black levels.

If you recall the dynamic range there is yet another important variable associated with digital processing related to the bit structure. Digital video itself is based on an 8 bit signal. If we process it at that level there will nearly always be problems with noise, blocking, changing IRE levels and other artifacts. To process it in 10 bit is better with 12 bit preferred. This is not a typical specification you will able to find. Call the manufacturer.

Finally, FPD digital displays typically work with digital signals. If you feed them an analog video signal it will have to be converted to a digital signal for the unit to display it. If your source is analog then no harm done. If your source is digital such as DVD and DTV then artifacts will be introduced by the process of converting it to an analog signal on the output of your source which will then have to be reconverted back to digital for the display. For this reason DVI/HDCP or HDMI digital video connections are preferred on the display and your digital sources.

Next week comes a run down of FPD displays.

KEEP IT SIMPLE

FPD offers a wide range of choices for videophiles or casual viewers depending on your application.

FPD has reached a point where some products work well for critical viewing applications and you should contact a professional for a consultation.

FPD requires quality scaling for consistent performance with different scan rate sources.

Use DVI/HDCP or HDMI digital video connections with digital sources when possible.

If you have any feed back to this article please feel free to post your comments now by clicking on POST reply. All emails received concerning this article are open to being posted and published along with the article unless the sender requests otherwise.

Respect Our Copyright! DO NOT copy and paste this complete review or article to post or distribute. That is a violation of US copyright. Please feel free to quote small passages and please provide a link to the full review. Thank you!