Iowa Public Television

Friday, December 7, 2007

How Big A Screen Do I Need?

Some of the first questions I get asked regarding the digital television conversion come from people who are considering the purchase of new DTV televisions. They generally want to know whether they should buy plasma or LCD. I tell people before they even go to a store to look at the television they should first layout the room where the TV is going to be. Figure out where the TV will sit or hang and where the primary viewing location will be. Then use a tape measure to determine the distance from the front of the screen to the where the eyes of the viewer will be and don’t worry too much about being exact, if you’re within a foot or so of the actual measurement you’ll be fine.

So how do you begin to plan your purchase? Using the SMPTE EG-18-1994 which was originally used for movie theater design and has now transitioned to home theater design you want to try and maintain a minimum viewing angle of 30°. This means that if you are seated in the optimum viewing location and look directly at the center of the screen, by rotating your head (or moving your eyes) 15° to the left or right you would be looking at left or right edge of the screen. There is a lot science research that has gone into this, much of it based on visual acuity and the portion of the human eye called the fovea. This area of the retina has the greatest density of photoreceptors in the eye and is responsible for the amount of detail that we see. It is the area that allows us to read, drive and yes, watch television. So by locating the viewing position at a location that meets this minimum viewing angle you essentially fill the fovea region of the eye and have the perfect viewing experience.

Unfortunately, there is also some reality that enters into the calculations. If you adhere strictly to the calculation you realize that in any environment there is only one good seat and this certainly didn’t make sense in the design of movie theaters and doesn’t make any more sense when planning at home, unless you live alone and will never have friends over to watch television with you. Let say that you make the viewing distance measurement and discover that you’re planning on sitting 13 feet back from your television screen. Doing the math, you should have a screen with about a 96 inch diagonal measurement!!! Now this is not impossible, there are front screen projector systems that can easily do this and there are even 100 inch plus plasma and LCD displays available but that is a seven foot wide, four foot tall television, it will weigh in at 500 pounds or more and will set you back somewhere in the neighborhood of $70,000.00. So clearly there has to be an acceptable compromise.

What has become the generally accepted practice is to specify viewing distance in terms of screen height or width. I have heard people expound that you should measure the height of the screen and sit three times that distance back for the best experience which from the example above would indicate that you either need to move your chair about a foot closer to the screen or try to find a bigger display which I personally believe approaches insanity. If you want to reference the screen, you are better off going with somewhere between three and six times the picture width. In my example with the 13 foot distance I can pretty quickly figure out that I want a screen that is at least 37” diagonal. That puts me pretty close to the furthest distance I would want to be from the set so if I can afford to go larger I will.

I have attached a grid with some of the most common screen sizes and their corresponding optimum and maximum viewing distances. The grid includes both standard (4x3) televisions and wide screen (16x9) televisions to show how the shape of the screen also impacts the viewing distance. Once you figure out how large a screen you want, then you can start looking at what technology you want to buy and find the best match between acceptable screen size and the cost of the set.

Friday, November 30, 2007

Is This Trip Really Necessary?

Greeting all and welcome to the inaugral Iowa DTV Answers blog. I had originally considered writing an introduction of myself and my qualification and history for my first entry but I have decided against that in favor of the simple introduction. My name is Bill Hayes and I am the Director of Engineering and Technology for Iowa Public Television and I am charged with overseeing the conversion of IPTV's facilities for analog to digital. What this means is that I am taking a perfectly good analog operation that is still working very well and I am disrupting it with a lot of "new digital" technology that like all thing that combine the term "new" and "digital" still has some bugs in it. My staff and I are in a race to have all of our analog stuff removed from service and all of the digital stuff in service to meet the looming deadline of February 17, 2009. Not since the Y2K scare has there been more countdown clocks running that all remind us of this crucial date because on that day, we pull the plug on the analog television service that we know so well and we rely solely on the new digital television service that still has many known and unknown issues.

That day in February concerns you as well because until then, all of the hiccups and oddities that we have been dealing with at the station have had very little impact on what you as a television viewer see and hear while watching our station. There have been the occassional issues that may have momentarily interupted a program that you were watching but for the most part you have been shielded from the vast majority of issues that have come up and believe me there have been some huge ones. Problems so large and scary that lawyers have had to sort through the rubble and vendors have fallen on their swords. The road to digital television is strewn with wreckage and I'll bet every television engineer working through the transition can share with you scary stories about some incident they confronted that made them wonder if this stuff was ever going to work. And some, are still wondering....

Now on this well laid foundation comes your part in all of this. On February 18th, 2009 when you turn on your TV, will you see anything or will you feel like the late Heather O'Rourke from the movie Poltergeist, sitting in front of a television display noise saying "They're here!" Well, before you end up in flooded swimming pool surrounded by unearthed coffins trying to flee into the night, let me just say that the "They" in this case are myself and my staff at IPTV and we are indeed "here" but our mission is to take what we know about television and what we've learned about digital television and help insure that when television becomes exclusively digital on February 18, 2009, you are already there.

During my next few postings I will answer some of the basic questions like what happens to cable and satellite subscribers and how do I receive over the air? But the most important questions to answer aren't the ones that I think you have but the ones you ask. So please let me know if what is on your mind. Remember, there are 10 kinds of people in the world....those that understand digital and those that don't. If you got the joke you're well on your way if not, you'll need help.