IF YOU have not gone shopping for a new television set for quite a while, enough has changed to require some serious thought. Your correspondent has finally given in to family pressure to create a dedicated media lounge. Given the limited resources, this is unlikely to be some 24-seat viewing room with a silver screen, curtains and digital projectors to rival the home theatres created for the likes of Steven Spielberg or Larry Ellison. The good news is that, with modern television sets, it does not have to be. A spare room, with a couch and a couple of easy chairs, plus a large enough flat-panel television and a reasonable audio system, can more than meet most family’s viewing needs.Before splurging on a fancy new high-definition television (HDTV) set, though, it is worth considering what features make sense and what do not. Start with the viewing angle. THX, a technical standards-setter for the video and audio industries, requires the back row of seats in a home theatre to have at least a 26º viewing angle from one edge of the screen to the other. Seats nearest the screen should have a viewing angle of no more than 36º. These subtended angles correspond to a viewing distance of roughly 2.2 times the screen width at the back row of the seating down to 1.5 times the screen width at the front. Within these limits, viewers should be able to enjoy the most immersive experience.
The question then is how to relate viewing distance to a person’s visual acuity. In other words, what is the maximum distance beyond which some picture detail is lost because of the eye’s limitations? Visual acuity indicates the angular size of the smallest detail a person’s visual system can resolve. This depends on the sharpness of the retinal focus within the eye, and the sensitivity of that part of the cortex that interprets visual stimuli.
Someone with 20/20 vision (6/6 in metric terms) can resolve a spatial pattern (of, say, a letter in the alphabet) where each element within it subtends an angle of one minute of arc when viewed from a distance of 20 feet (six metres). In other words, a person with 20/20 sight should, in normal lighting conditions, be able to identify two points that are 0.07 of an inch (1.77mm) apart from a distance of 20 feet. Twenty feet is taken because, as far as the eye is concerned, it is effectively infinity.
A person who can detect individual elements that make up, say, the letter “E” on the eighth line of an optometrist’s Snellen chart—and thereby recognise that the letter is an “E” and not a “D”—is said to have normal 20/20 eye sight. Someone with 20/40 sight can see objects at 20 feet that those with normal sight can see from 40 feet. In many countries, 20/200 is the legal definition of blindness. Meanwhile, 20/20 vision is not perfect vision; it is merely the lower limit of normal sight. The maximum acuity of the human eye is around 20/8. Some birds of prey are thought to have eye sight as sharp as 20/2.
As far as watching television is concerned, visual acuity represents the point beyond which some of the detail in the picture can no longer be resolved by the conical receptor cells in the retina of the eye. It will simply blend into the background instead of being seen as a distinct feature. Thus, it is a waste to make individual pixels—the tiniest elements in a display—appear smaller than 0.07 of an inch when viewed from 20 feet.
The problem with viewing images on a television screen—especially a high-definition one like the 1080p HDTV sets in use today—is that most people sit too far back. A survey made some years ago by Bernard Lechner, a television engineer at the former RCA Laboratories, near Princeton, New Jersey, showed that the median eye-to-screen distance in American homes was nine feet. At that distance, a 1080p HDTV set (with a screen 1,920 pixels wide and 1,080 pixels high) needs to be at least 69-inch across a diagonal if viewers are to see all the detail it offers.
In practice, the most popular television size in America today is 32 inches. To see all the detail on a 1080p set of that size means dragging the chair forward from nine feet to a little over four feet from the screen. If it were an older 720p television set (1,280 pixels wide and 720 pixels high), sitting six feet from the screen would suffice to see the full quality of the image.
Put another way, viewers cannot enjoy the full benefits of the higher pixel count of 1080p television if they sit any further back than 1.8 times the screen width. At 2.7 times the screen width, they might as well use a cheaper 720p set instead, as the eye cannot resolve the finer detail offered by a 1080p screen at that distance. Unfortunately, while 720p sets offer good value, they are becoming difficult to find. Manufacturers focus all their marketing efforts these days on higher-margin 1080p sets.
As far as screen sizes and viewing distances are concerned, a room measuring ten feet by 12 feet is therefore more than adequate for watching a 50-inch television set, with viewers no further than six-and-a-half feet from the screen. The question, then, is what kind of 1080p set to use—plasma display, liquid-crystal display (LCD) or the latest light-emitting diode (LED) variety?
Plasmas, with their rapid switching and deep blacks, have long been the favourite for sports fans and movie buffs. Apart from their lack of blur and judder when tracking fast-moving objects and their freedom from wishy-washy greys, they can be viewed from wider angles than LCDs without the picture changing colour. They also produce better three-dimensional images, primarily because they generate less ghosting (double images) when using 3D glasses. But plasmas have lately fallen out of favour because of their bulk and thirst for power. More to the point, manufacturers have begun to fix many of the LCD’s faults.
To lick the motion problem, LCD set-makers have developed special circuitry for estimating and compensating for any rapid movement within a scene. This increases the screen’s frame rate from the 60 hertz of traditional television to 120 hertz and even 240 hertz. A few manufacturers have begun offering sets with refresh rates of up to 480 hertz, with 960 hertz on the horizon.
Unfortunately, the motion-compensating circuitry can make filmed content appear like a cheap video—a glitch known in the trade as the “soap-opera effect”. The source of the problem is the way film shot at 24 frames a second has to adjust to television's refresh rate of 60, 120 or even 240 frames a second. One way of doing this is to analyse first one frame of film and then the next, and calculate an average of the two. This interpolated frame is inserted between the first and second frames, and the process repeated for each successive frame of the film. The interpolation process is good at removing blur and judder, but it can make the motion appear unnaturally smooth and disconcerting. Be warned, 240-hertz sets are the worst offenders.
Lastly, there are the LED sets. Manufacturers would have you believe these are a new form of display. They are not. They are simply LCD televisions that use LEDs for backlighting instead of the usual fluorescent tubes. The LEDs can be either along the edges of the screen or spread as an array behind the whole of the display. Edge-lit displays have problems with uniformity of brightness as well as a limited viewing angle.
Screens that use a full-array of LED backlights are much better. Apart from giving more uniform brightness, they allow the screen to be dimmed selectively in places where a scene needs to be dark. The effect is to make the LCD’s blacks appear almost as dense as a plasma’s. Only top-of-the-range LCD sets from Sharp and Sony currently have this feature. Expect to pay dearly for it.
So, what to choose? That depends on budget and personal preferences. All things being equal, plasma televisions are about two-thirds the price of their LCD equivalents, which are themselves up to a third cheaper than LED sets. Meanwhile, the premium that 3D sets once commanded has all but vanished. They are now worth buying, not so much for their ability to show 3D content, but because they display 2D even better than conventional plasma or LCD sets (see “Beyond HDTV”, July 28th 2011).
To lick the motion problem, LCD set-makers have developed special circuitry for estimating and compensating for any rapid movement within a scene. This increases the screen’s frame rate from the 60 hertz of traditional television to 120 hertz and even 240 hertz. A few manufacturers have begun offering sets with refresh rates of up to 480 hertz, with 960 hertz on the horizon.
Unfortunately, the motion-compensating circuitry can make filmed content appear like a cheap video—a glitch known in the trade as the “soap-opera effect”. The source of the problem is the way film shot at 24 frames a second has to adjust to television's refresh rate of 60, 120 or even 240 frames a second. One way of doing this is to analyse first one frame of film and then the next, and calculate an average of the two. This interpolated frame is inserted between the first and second frames, and the process repeated for each successive frame of the film. The interpolation process is good at removing blur and judder, but it can make the motion appear unnaturally smooth and disconcerting. Be warned, 240-hertz sets are the worst offenders.
Lastly, there are the LED sets. Manufacturers would have you believe these are a new form of display. They are not. They are simply LCD televisions that use LEDs for backlighting instead of the usual fluorescent tubes. The LEDs can be either along the edges of the screen or spread as an array behind the whole of the display. Edge-lit displays have problems with uniformity of brightness as well as a limited viewing angle.
Screens that use a full-array of LED backlights are much better. Apart from giving more uniform brightness, they allow the screen to be dimmed selectively in places where a scene needs to be dark. The effect is to make the LCD’s blacks appear almost as dense as a plasma’s. Only top-of-the-range LCD sets from Sharp and Sony currently have this feature. Expect to pay dearly for it.
So, what to choose? That depends on budget and personal preferences. All things being equal, plasma televisions are about two-thirds the price of their LCD equivalents, which are themselves up to a third cheaper than LED sets. Meanwhile, the premium that 3D sets once commanded has all but vanished. They are now worth buying, not so much for their ability to show 3D content, but because they display 2D even better than conventional plasma or LCD sets (see “Beyond HDTV”, July 28th 2011).
As a sports-loving, old-movie addict, your correspondent’s choice is easy. With the help of a brother-in-law in the business, he has ordered a Panasonic Viera TC-R50VT20, a 50-inch plasma set with all the bells and whistles (arigato, Hiroshi-san). He recommends others read the annual ratings for television sets published in Consumer Reports (March 2011), then go to the nearest big-box store and see for themselves. One rule of thumb: manufacturers’ recommended prices average around $36/inch for plasma televisions and $48/inch for LCDs. Discounts in-store and online should lower such prices by at least 20%. Do not settle for less.
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