Posted Fri Jun 26, 2009 at 12:00 PM PDT by Joshua Zyber
Editor's Note: Each Friday, High-Def Digest's own HD Advisor will answer a new round of questions from our readers. If you have home theater questions you need answered, send an email to [email protected].
Answers by Joshua Zyber
Q: I own a Panasonic 50" TV and have viewed many movies on it over the years. What bothers me is that no matter what the aspect ratio is (whether it be 1.85:1, 1.78:1 or even 1.66:1), the screen never seems to have any black bars below or beside the image (with the exception of course that it's a really wide image like 2.35:1 or a really slim TV show image of 1.33:1). That may be fine for most people, but I can't help but feel like I'm missing some of the picture. After months of searching for an answer I discovered that TVs have something called "overscan" built in where the image is purposely cropped off around the screen image so you can't see the edges. So my question is, if the movie has 1080 lines of resolution and my TV has 1080 lines how can the pixels not line up exactly?
A: As you've noted, almost all consumer televisions have some degree of overscan built in. Overscan means that a small amount of picture information will be cropped from all four edges of the image. The exact amount will vary by model. Around 5% is common, with some particularly bad sets losing up to 10%.
With older CRT televisions, the scan gun sprays the picture across the back of the tube. As seen from the viewer's perspective in front, the television's bezel (the frame around the screen) is built to obscure the edges of the tube. In other words, the gun literally scans the image beyond the parts of the screen that the viewer can see. The picture's there; it's just been blocked from view.
Overscan was initially implemented back in the 1930s due to variations in manufacturing tolerances between different television sets. Covering the edges of the screen with the bezel helped to disguise the curvature of the tube a little bit, and also blocked the viewer from seeing parts of the broadcast signal not intended for their eyes.
Although modern digital televisions have much less need for overscan, the process has continued in order to ensure compatibility with older transmission signals. If you were to watch an analog NTSC broadcast with no overscan on your television, you'd be liable to see signal noise in the blanking intervals around the edges. That noise was actually digital data such as Closed Captioning, time codes, and test signals. Of course, analog NTSC broadcasts officially ceased this month, making this an even less relevant concern. Nonetheless, up until now, it served some purpose.
The way overscan works on a digital TV is different than it worked on analog sets. Digital screens have a native resolution; in the case of a 1080p set, that would be 1920x1080 pixels. Generally, all of those pixels are visible to the viewer's eye, with none obscured by the bezel. If the TV implements overscan, the scaling chip inside the set zooms the picture to crop off the edges. Therefore, a 1080p input signal may not be 1:1 pixel matched to the 1080p screen.
Overscan is not and has never been needed (or wanted) during playback of DVD or Blu-ray video. Regardless, if the TV has overscan by default, it may apply it to those as well. Some HDTVs are programmed to automatically disable (or at least lessen) the overscan setting if fed a progressive scan or HD input signal. In that case, the overscan only comes into play with 480i SD content. Unfortunately, other sets are not that smart, and simply apply overscan to everything.
It sounds like your TV is one of the latter. To determine exactly how much picture you're losing to overscan, pick up a calibration disc such as 'Digital Video Essentials' or the 'Spears & Munsil HD Benchmark'. These discs will contain test patterns to measure where your screen cuts off.
24 fps on a 720p Projector
Q: Last year I purchased an Epson EMP-TW700 projector. It is not FullHD, only HD-Ready. According to its manual, it supports the following signals: NTSC / NTSC 4.43 / PAL / M-PAL / N-PAL / PAL60 / SECAM/ 480i / 480p / 526i / 526p / 720p / 1080i / 1080p. Now I would like to purchase a Pioneer 51FD Blu-ray player and connect it via HDMI to this projector. On different forums, I've read that this projector won't be able to properly play Blu-ray discs because of its lack of 24p feature. So, as they write on the forum, in case of this configuration I cannot watch Blu-ray 24p discs without flickering or jumps? Is there any alternative setting in the Pioneer BD player which can fix this problem? It would be lousy if I had to watch High Def Blu rays with continuous jumps.
A: The EMP-TW700 is a 720p projector. Any video content you feed it will be scaled to the projector's native 1280x720 resolution for display. While 720p is obviously inferior to 1080p in terms of resolution, both are considered true high definition formats. The TW700 should be able to play Blu-ray discs just fine. It just scales the 1080p input signal to 720p first.
Some 720p projectors are able to accept a 24 fps signal and display it at an even multiple of that frame rate without 3:2 Pulldown judder. I used to own a Mitsubishi 720p projector that could display Blu-rays at 48 Hz. In my searching, I can't find a conclusive answer as to whether the TW700 will accept a 1080p24 signal. Those specs you cite from the manual would suggest not. A couple of the online reviews I've found state that it will, while a few others say that it doesn't.
In a worst case scenario, you'll still be able to watch Blu-rays at the projector's native 720p resolution and 60 Hz playback rate. The "jumps" you refer to would be the judder introduced by 3:2 Pulldown. However, as I wrote in my What's the Big Deal About 1080p24? article, the difference between 24 fps and 60 Hz is very subtle. Most people don't notice it at all. Since you've owned this projector for a year now, I'm going to assume that you've been watching DVDs and/or TV content on it during that time. If the 60 Hz rate hasn't bothered you with those, then it won't bother you with Blu-ray either.
Q: I understand that when viewing BD/DVD movies on a player, we should calibrate our HD screens to 6500K (D65) as the disc has been mastered for this color temperature. Does this also apply to HDTV shows (other than movies) we get over the air/sat/cable? What about other material on BD/DVDs such as concerts, documentary, TV series, etc. that are shot on HD video? Are the discs mastered to D65? Comparing the D65 mode to the standard mode on my screen, the D65 mode is less bright, close to what I get in the movie theatres. Viewing a 2 hour movie in this mode (vs. the Standard mode) puts less strain on my eyes! Also, I find that the whites on D65 are kind of yellowish, is this normal? The whites on Standard mode appear purer.
A: I'm going to provide a general answer to this question, but please keep in mind that I'm not a video engineer. To start, know that there is actually a distinction between the terms "D65" and "6500k." I'll be honest, the technical difference is beyond my ability to explain. This page and this page do a better job than I can. (Don't feel bad if some of what's written on those pages goes over your head. Frankly, from an end user's perspective, a lot of that argument is semantics.)
When it comes to HDTV calibration, I believe in the "Set It and Forget It" philosophy. I don't want to be changing my video settings between shows. In practice, I realize that some content has more "pop" when watched at different color temperature settings. Sports, in particular, may look more vivid at higher color temps. Nevertheless, for accuracy, it's better to leave everything calibrated to the same standard. But if you get more enjoyment out of something by switching to a different setting, I'm not going to stop you.
As for why the "D65" mode on your television makes whites look yellowish, be aware that the factory presets in most HDTVs are rarely accurate. In fact, what's labeled as "D65" or "6500k" (or whatever the manufacturer chooses to call it) may be nowhere near that actual measurement. For best results, you should hire a professional calibrator to bring in color analyzer equipment and fine tune your display to mathematical precision (or at least as close as the set's inherent limitations will allow).
Some questions that the HD Advisor receives are best answered by a consensus of opinions from our readers. If you can help to answer the following question, please post your response in our forum thread linked at the end of this article. Your advice and opinions matter too!
Toslink Switcher Recommendations
Q: I'm very much in need of an Optical/Toslink audio switcher (not splitter). Researching them on the internet you could imagine my surprise when I found there are barely any trusted, functional ones out there! From what I've seen and read, every one out there is made of cheap bulky plastic that falls apart or begins having issues after 6 months or sooner, at least according to the majority of reviewers. I got my HDMI switcher for around $40 and I was hoping to find an optical audio splitter for about $20-$40 too. I've seen set-top boxes with multiple inputs for HDMI and optical audio cables but those run well upwards of $100. I'm trying to avoid adding a whole other set top box to my system. You'd think there'd be a reputable, reliable switcher on the market by now, is there something I'm missing?
Check back next week for another round of answers. Keep those questions coming.
Joshua Zyber's opinions are his own and do not necessarily reflect those of this site, its owners or employees.
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