Wednesday, July 5, 2006
The HD TV Evolution: Part 5 – Plasma is Hot Stuff
Posted by Felix Torres in "THOUGHT" @ 07:00 AM
Plasma Display Panels are the ‘tweeners of the HD world: stuck in between the high-volume small-size LCD displays and the low volume, large size micro-display rear projection markets, and getting squeezed by both. Some think it is a doomed technology—it isn’t. It's just…misunderstood by both its detractors and its supporters.
Technology-wise, PDPs are an oddity in that their core technology doesn't come from the semiconductor industries like DLP and the various LCD variants, but rather from the analog TV industry. And where the others are transmissive or reflective technologies that rely on separate light sources, PDPs are emissive and produce their own light. Most of the science behind these displays has its roots in the earliest B&W CRT displays. As a result, in the PDP marketing world the term HD doesn't mean the same thing it does elsewhere, particularly when it comes to resolution. In the analog TV era and especially the B&W days, horizontal resolution was not a critical design element; you didn’t design for a given resolution—you took whatever you could extract from the received broadcast signal and what you got is what you got. Even after the advent of color and stripe-based displays, analog TV designers never bothered much with the horizontal resolution of a display - that was strictly a concern of the computer monitor manufacturers, not the designers. As a result, most consumer-grade CRT TVs are rated by their lines of resolution (usually 600 for the low and mid-range, 800 for the better displays) and any attempt to divine the dot-pitch of a TV tube (and hence its horizontal resolution) usually gets blanks stares or is just shrugged off.
PDP manufacturers seem to share this lack of concern over horizontal resolution and their customers don’t seem to mind at all. As recently as mid-2005 over half the PDPs sold were ED, not HD TVs, and of the ones billed as HD, an unspecified but significant (over 50%?) fraction are actually 1024x768 stretched XGA, rather than native HD displays. This does not hamper sales. Indeed, it is not uncommon to hear from PDP proponents that ED image quality is better than HD. (They have a point, too, but not the one they think).
PDP buyers just take what they can get and are happy to do so. Indeed, if you look carefully at recent sales data (2005), it becomes apparent that 90% of all PDP sales are for just two sizes, 42" and 50". If you're looking for HD-native resolutions, you're most likely looking at 50"+, as most of the 42" models are stretched-XGA, not 720p-native.
Which is to say, size-wise, PDPs don't offer consumers much choice.
What they do offer is the most CRT-like image quality among HD displays, and some tend to think of PDPs as "flat CRTs", which isn't technically accurate. PDPs create their images by indirectly stimulating a glowing phosphor layer instead of direct stimulation, as CRTs do—but it does reflect the "flavor" of the displays pretty accurately. For a real “flat CRT” display, we’ll all have to wait for the mythical SED flat panels, if they ever do get to market this decade. Their expected ‘06 arrival has recently been pushed back at least a year.
The CRT-like nature of PDPs is both the source of their strengths and weaknesses. Consider that PDPs generate their images by electrically exciting a low pressure gas into a high-energy state of matter, called a Plasma. This then causes it to emit ultraviolet photons that in turn stimulate a layer of phosphors to emit visible-light photons—somewhat like fluorescent bulbs do—which then go through a colored filter layer to produce a visible, colored pixel. While PDPs are fixed-pixel digital displays at the panel level, the individual pixels are basically analog in nature—the intensity of the ultraviolet glow can be controlled with great precision so that each sub-pixel channel can have a different response curve. A very nice attribute. The price of this? It takes a lot of energy to excite a gas into a light-emitting Plasma; hence PDP's reputation as high power-consumption devices. Current PDPs aren’t as bad as early models, though, and they only average about 300-350 watts. A second side effect is that the phosphors being excited by the hot Plasma degrade with time and that degradation is a function of the power being applied. The higher the stimulation, the brighter the image, the faster the phosphor degrades. Current phosphor formulations have good half-lives (the time it takes for the panel to lose half its initial maximum brightness) of over 10 years of normal usage.
PDPs do offer the best pixel-to-pixel contrast and the best black levels of current (2005-06) HD displays. This is due to the presence of—indeed, need for—dark barrier ribs separating the individual sub-pixel gas cells, that are analogous to the shadow mask that defines the pixel borders in CRT displays. It is these necessary barrier ribs that constrain PDP panel sizes and limit manufacturers’ ability to increase resolution or reduce HD panel size below 42”. With other HD display technologies, there are no physical sub-pixel borders (microdisplays) or the inter-pixel area is very small compared to the lighted screen area (LCOS, direct-view LCD). Not so with PDPs, where the barrier ribs are structural in nature and are needed to maintain the integrity of the sub-pixel gas cells, so a certain portion of the screen area will always be unable to generate an image, resulting in the well-known screen door effect that requires viewers to stay beyond a certain viewing distance to avoid detecting the inter-pixel grid. These barrier ribs prevent light leakage from one pixel to the next—hence PDP's point-to-point contrast excellence—but they also consume surface area, reducing the maximum amount of light the panel can display. Hence, PDPs have the lowest full-screen brightness of the competing technologies. And, because the percentage of panel area consumed by the barrier ribs goes up with resolution, panel brightness goes down with resolution.
This, folks, is why many Plasma connoisseurs say that ED is better than HD—at least with PDPs: with less pixels in the same surface area, each pixel is bigger and hence brighter (more emissive area), so pixel-to-pixel contrast is higher and full-panel brightness is higher. Plus, the panels are easier to manufacture and are likely to be cheaper.
Technology-wise, PDPs are an oddity in that their core technology doesn't come from the semiconductor industries like DLP and the various LCD variants, but rather from the analog TV industry. And where the others are transmissive or reflective technologies that rely on separate light sources, PDPs are emissive and produce their own light. Most of the science behind these displays has its roots in the earliest B&W CRT displays. As a result, in the PDP marketing world the term HD doesn't mean the same thing it does elsewhere, particularly when it comes to resolution. In the analog TV era and especially the B&W days, horizontal resolution was not a critical design element; you didn’t design for a given resolution—you took whatever you could extract from the received broadcast signal and what you got is what you got. Even after the advent of color and stripe-based displays, analog TV designers never bothered much with the horizontal resolution of a display - that was strictly a concern of the computer monitor manufacturers, not the designers. As a result, most consumer-grade CRT TVs are rated by their lines of resolution (usually 600 for the low and mid-range, 800 for the better displays) and any attempt to divine the dot-pitch of a TV tube (and hence its horizontal resolution) usually gets blanks stares or is just shrugged off.
PDP manufacturers seem to share this lack of concern over horizontal resolution and their customers don’t seem to mind at all. As recently as mid-2005 over half the PDPs sold were ED, not HD TVs, and of the ones billed as HD, an unspecified but significant (over 50%?) fraction are actually 1024x768 stretched XGA, rather than native HD displays. This does not hamper sales. Indeed, it is not uncommon to hear from PDP proponents that ED image quality is better than HD. (They have a point, too, but not the one they think).
PDP buyers just take what they can get and are happy to do so. Indeed, if you look carefully at recent sales data (2005), it becomes apparent that 90% of all PDP sales are for just two sizes, 42" and 50". If you're looking for HD-native resolutions, you're most likely looking at 50"+, as most of the 42" models are stretched-XGA, not 720p-native.
Which is to say, size-wise, PDPs don't offer consumers much choice.
What they do offer is the most CRT-like image quality among HD displays, and some tend to think of PDPs as "flat CRTs", which isn't technically accurate. PDPs create their images by indirectly stimulating a glowing phosphor layer instead of direct stimulation, as CRTs do—but it does reflect the "flavor" of the displays pretty accurately. For a real “flat CRT” display, we’ll all have to wait for the mythical SED flat panels, if they ever do get to market this decade. Their expected ‘06 arrival has recently been pushed back at least a year.
The CRT-like nature of PDPs is both the source of their strengths and weaknesses. Consider that PDPs generate their images by electrically exciting a low pressure gas into a high-energy state of matter, called a Plasma. This then causes it to emit ultraviolet photons that in turn stimulate a layer of phosphors to emit visible-light photons—somewhat like fluorescent bulbs do—which then go through a colored filter layer to produce a visible, colored pixel. While PDPs are fixed-pixel digital displays at the panel level, the individual pixels are basically analog in nature—the intensity of the ultraviolet glow can be controlled with great precision so that each sub-pixel channel can have a different response curve. A very nice attribute. The price of this? It takes a lot of energy to excite a gas into a light-emitting Plasma; hence PDP's reputation as high power-consumption devices. Current PDPs aren’t as bad as early models, though, and they only average about 300-350 watts. A second side effect is that the phosphors being excited by the hot Plasma degrade with time and that degradation is a function of the power being applied. The higher the stimulation, the brighter the image, the faster the phosphor degrades. Current phosphor formulations have good half-lives (the time it takes for the panel to lose half its initial maximum brightness) of over 10 years of normal usage.
PDPs do offer the best pixel-to-pixel contrast and the best black levels of current (2005-06) HD displays. This is due to the presence of—indeed, need for—dark barrier ribs separating the individual sub-pixel gas cells, that are analogous to the shadow mask that defines the pixel borders in CRT displays. It is these necessary barrier ribs that constrain PDP panel sizes and limit manufacturers’ ability to increase resolution or reduce HD panel size below 42”. With other HD display technologies, there are no physical sub-pixel borders (microdisplays) or the inter-pixel area is very small compared to the lighted screen area (LCOS, direct-view LCD). Not so with PDPs, where the barrier ribs are structural in nature and are needed to maintain the integrity of the sub-pixel gas cells, so a certain portion of the screen area will always be unable to generate an image, resulting in the well-known screen door effect that requires viewers to stay beyond a certain viewing distance to avoid detecting the inter-pixel grid. These barrier ribs prevent light leakage from one pixel to the next—hence PDP's point-to-point contrast excellence—but they also consume surface area, reducing the maximum amount of light the panel can display. Hence, PDPs have the lowest full-screen brightness of the competing technologies. And, because the percentage of panel area consumed by the barrier ribs goes up with resolution, panel brightness goes down with resolution.
This, folks, is why many Plasma connoisseurs say that ED is better than HD—at least with PDPs: with less pixels in the same surface area, each pixel is bigger and hence brighter (more emissive area), so pixel-to-pixel contrast is higher and full-panel brightness is higher. Plus, the panels are easier to manufacture and are likely to be cheaper.
