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Is my TV broken? - The limitations of technology

What really constitutes a fault?

by Steve Withers Dec 21, 2015


  • There's no such thing as a perfect mass-produced product and despite the TV manufacturers' best efforts there will always be faulty units.
    However there is a fundamental difference between a faulty TV and the inherent limitations of the technology it uses. Whilst an actual fault will prevent the TV performing in the manner that it was intended and therefore should be replaced, there are certain aspects of a TV's performance that won't be covered by any warranty. These aspects are essentially limitations of the TV's technology that result in issues that are inherently part of its performance and thus don't constitute an actual fault. These sorts of issues are usually identified in any review conducted by AVForums and are almost never considered by the manufacturers as reasons to replace the TV.

    This often puts retailers in a difficult position, as they try to sell TVs that the manufacturers feel are as good as they can make them to consumers who expect perfection despite tumbling prices in stores. The end result is that in an attempt to keep customers satisfied, stores often replace TVs for reasons that the manufacturers simply don't consider a fault and thus won't replace themselves. That cuts into retailers profits in a highly competitive industry that is already struggling with wafer thin margins. The situation isn't helped by customers who use distance selling regulations as a way of continually replacing a TV until they get what they want or those that abuse them in order to upgrade their TV when something better is released.

    In this article we'll identify the three key TV technologies and their inherent limitations, so that anyone buying a new TV understands exactly what kind of performance they can expect from their latest purchase. For the sake of completeness we have included plasma TVs in this article, even though they are no longer produced. This is partly because there are still plenty of plasma TVs in circulation and also because the technology is often held up to be the pinnacle of picture quality. However as you'll see, even plasma had plenty of limitations that could result in visible issues, none of which could be considered actual faults. We'll begin by looking at something that could affect any modern TV regardless of whether it is plasma, LCD or OLED.

    Dead Pixels - Any modern TV, regardless of the technology it uses, creates its images by combining millions of individual pixels. In fact if your TV is Full HD it has a resolution of 1920 by 1080 which equates to a total of 2,073,600 pixels and if it's Ultra HD 4K then the resolution is 3840 x 2160 which uses 8,294,400 pixels. That means that even if the TV manufacturers used a 99.9999% level of accuracy on an Ultra HD TV, there would still be 8 dead pixels. In fact the number of actual dead pixels is normally even less than that but you still might find the odd dead pixel and whether a TV warrants replacement depends on the resolution and where any dead pixel is located.

    On an Ultra HD 4K TV you might not even notice a dead pixel because they are so small but with a Full HD TV it would probably be more noticeable. However, even if you can see a dead pixel, whether a manufacturer considers that grounds to replace the TV will depend on where the dead pixel is located. In general TV manufacturers would consider a single dead pixel to be within tolerance levels but if it's in the middle of the screen they are more likely to replace the TV than if the dead pixel was at the edge of the panel. So ultimately dead pixels fall into that grey area where they could be considered a genuine fault or an acceptable failure level on a mass produced product.

    Plasma


    Sadly plasma TVs are no longer produced, so their strengths and limitations are largely academic these days but since they were, and to a certain extent still are, held up as the pinnacle of image quality it's worth addressing the technology. Plasma is a self-emitting technology, which means that when energy is passed through the cells that make up each pixel they glow and produce an image. Since they are self-emitting, plasmas are capable of extremely good blacks with excellent shadow detail and gradations in darker parts of the image. In fact in terms of those gradations just above black, as well as motion handling, a plasma TV is still superior to even OLED, showing just how good the technology was in its prime. The motion handling on a plasma TV is also excellent and is still superior to both LCD and OLED. In addition the colour accuracy can also be impressive and a plasma TV is able to deliver an image that has a more film-like quality, especially when compared to an LCD TV which often has a slightly digital appearance.

    However plasma has its limitations and for a start the technology isn't that bright, especially when compared to an LCD TV. A plasma TV is certainly bright enough to produce a good image for viewing in the evening but calibrators often struggle to get a bright enough image for an effective daytime setting in well lit surroundings. Although plasmas aren't inherently that bright, manufacturers often deliberately limited the brightness in order to reduce the energy consumption, which is also very high compared to an LCD TV using LED backlighting. In fact a plasma's energy usage was a contributory factor in the technology's demise. It was also prohibitively expensive to produce an Ultra HD 4K plasma, so the technology is effectively limited to Full HD resolution. In addition, although plasma TVs can produce very accurate colours, they can struggle to produce a fully saturated red when compared to the Rec.709 colour gamut. As technology progressed so the venerable plasma was left behind by a world of higher resolutions, brighter images with high dynamic range and quantum dot with wider colour gamuts.
    Although plasmas are no longer produced, there are still plenty around. So it's worth reminding ourselves of their strengths and weaknesses.
    Even in its prime, plasma had plenty of other limitations that often caused owners to wonder if their new TV was broken. Probably the most famous was the problem of image retention and screen burn. There's no doubt that this was a genuine issue in the early days of plasma and unfortunately it often put people off buying into the technology. However over time the manufacturers found ways to mitigate the problem and certainly towards the end of its life cycle genuine screen burn was very rare and any image retention would usually go after a short while. If you wanted to prevent image retention, you just needed to avoid leaving a static image up for too long. Of course with today's station logos in the corner of the screen that's not always possible and the same goes for most video games, although the latter might be worth it when you consider the low input lag on a plasma TV.

    There were other issues as well such as PWM (pulse width modulation) noise in the image, which is a side-effect of how the technology works and some people prefer the cleaner look of an LCD for this reason. The power supply on a plasma TV can also produce a buzzing sound that can be annoying; although how noticeable it is will often be a consequence of the individual unit, the room's acoustics and the acuity of a person's hearing. Plasmas can also get quite hot and the larger screen sizes often need to be cooled with fans, which again people can hear. There are other issues such as line bleed, where horizontal lines can appear, usually in connection with scrolling credits; banding where you could see vertical banding in the image on camera pans across a uniform image like a football pitch; and dirty screen effect where the image can lack uniformity when its a large block of a single colour, often making a white screen appear 'dirty'.

    There was also the problem of rising blacks, where the black levels would gradually rise over time or fluctuate within a scene, although this was fixed in later generations. Finally there is a problem that only effects later generation Panasonics called dynamic false contouring (DFC). This will manifest as coloured fringing around moving objects and was often most obvious in close-ups of faces as they moved across the frame. DFC was a side effect of how Panasonic were driving their panels and over time they managed to mitigate the issue, although even on the reference status ZT65 it could still be seen if you knew where to look. So even plasma TVs have their issues but most of them aren't actual faults with the TV, rather limitations in the technology. How annoying these issues can be alternates from TV to TV and consumer to consumer but we always tried to identify them in our reviews..

    LCD


    The humble LCD panel has become the dominant television technology over the last decade, especially now that plasmas are no longer produced, and although there's OLED as well these days, the majority of TVs sold use an LCD panel. Why is this so? Well primarily because LCD panels are cheap to produce when compared to plasma and especially OLED. A bargain will always catch a shopper's attention and in stores the inherently bright and over-saturated images produced by LCD TVs will also catch their eye. Many consumers also like the clean and detailed images produced by an LCD panel, no PWM noise, and there's no screen burn or image retention to worry about. The other big reason that LCD panels are so popular is that the manufacturers have been very clever in taking the inherent limitations of the technology and turning them in to marketing features.

    That's not to say that LCD TVs don't have strengths because they do and many of them have proved useful recently as the standards for TV have been updated to reflect advances in technology. First of all producing LCD TVs that use 4K panels has proved relatively easy, driving sales of LCD Ultra HD TVs over the last few years. As we mentioned LCD TVs can also be very bright, this not only catches consumers' eyes in stores but also means they lend themselves to the way that many people watch TV these days with plenty of ambient light in the room. In addition the move towards high dynamic range (HDR) means that TVs have to deliver far more peak brightness than they ever have before. We have also seen a move to wider colour gamuts and again the development of quantum dot means that LCD TVs are perfectly positioned to embrace these changing standards.

    The biggest weakness of any LCD panel is that it uses a backlight for illumination. This means it can be bright but since the backlight shines through the panel it struggles to deliver really deep blacks because even when the pixels are closed, some light still leaks through. In the past an LCD TV used a CCFL backlight, which was essentially a series of fluorescent tubes behind the panel. This approach tended to deliver an even backlight but resulted in a fairly deep chassis. So more recently the TV manufacturers have been using LEDs as backlights, which results in a number of advantages. The TVs can go brighter, their colours are purer and the energy consumption is improved. However perhaps the biggest advantage was that the TV manufacturers could produce much thinner TVs.

    These ultra-slim TVs proved hugely popular with consumers and the popularity of LED LCD TVs was helped by some clever marketing that referred to them as 'LED' TVs, implying that this was a new technology. It wasn't of course but the name stuck and the use of LED backlights helped drive the popularity of LCD TVs. However, as always there's a trade-off and in order to make the TVs so thin the manufacturers put the LEDs at the sides or top and bottom of the screen. The light is then effectively bounced off mirrored surfaces behind the panel in order to illuminate it. This approach works and allows for some very attractive and very thin LCD panels but often results in an uneven or cloudy backlight with bright corners or edges where the LEDs are actually positioned.

    The alternative to edge-lit LED LCD TVs is to position the LEDs directly behind the panel in an array, which will result in a more even backlight. The chassis is slightly deeper but recently TV manufacturers have used the direct LED backlight approach as a way of delivering a more even and brighter backlight, especially on their higher-end models. However even direct LED backlights aren't perfect and often the LEDs themselves can be visible as banding that usually manifests as the camera pans across a uniform image like a football pitch. As with edge-lighting, most of the problems experienced with direct LED backlights are just limitations in the technology itself, rather than genuine faults. The same goes for other commonly reported issues like dirty screen effect (DSE) and, especially with edge-lit LED LCD TVs, the uniformity of the backlight can be something of a lottery.
    LCDs have become the dominant panel technology by often making marketing features out of a perceived weaknesses.
    In an attempt to address the poor blacks associated with LCD TVs the manufacturers have developed various technologies that go hand-in-hand with the adoption of LED backlighting. Initially there was what is often referred to as global dimming, when if the image was a black screen the LEDs would turn off creating a completely black image. However this didn't improve the actual blacks in an image and merely drew attention to itself when the screen suddenly became much blacker, so local dimming was developed. This approach breaks the screen down into a number of zones, the more zones the more effective it is, and each zone can effectively be turned off. This allows for deeper blacks within the actual image itself and has proved quite effective but there are issues associated with the technology.

    The processing is reducing the light in dark parts of the image and this can result in a loss of detail in shadows and dark areas just above black. This is an area where LCD panels are inherently weak but the local dimming just amplifies the problem. Conversely there can also be a loss of detail in the bright parts of the image where the amount of light is boosted in order to maximise the dynamic range (the difference between black and white) of the image. The other common problem with local dimming is haloing, where bright images appear to glow against a dark background. The number of zones and the processing used can help mitigate this issue but the only way to completely eliminate it would be to have local dimming zones for every single pixel which, in a sense, is exactly how both plasma and OLED work because of their self-emitting technologies.

    One of the limiting factors of any local dimming system is the native black levels of the panel itself, the darker the blacks without local dimming, the better the effect when it's applied. There are two basic types of LCD panel and they have been developed to deliver distinctly different advantages. The VA (vertical alignment) panel allows for better native blacks but has a limited optimal viewing angle, so anyone watching off centre will experience reduced contrast performance and washed-out colours. The alternative is an IPS (In-Plane Switching) panel where the optimal viewing angles are much better but the native blacks are weaker. Which is best for you will depend on your viewing position and how many people are watching TV but if you're using a TV with an IPS panel then local dimming is essential to get a decent black performance, especially when watching at night.

    The other big limitation of LCD TVs is their motion handling because they use a form of 'sample and hold', where one frame is held on screen whilst the next frame is sampled, which results in reduced resolution with motion. The manufacturers have developed a number of technologies to address this limitation, all based around the idea of frame interpolation. This approach essentially analyses the first frame and the frame after it and then adds additional frames in between that have been interpolated (an educated guess essentially) in order to improve the motion resolution. As the processing has become more powerful and faster, so the number of additional frames has increased and in some respects the technology is very successful. However it can introduce artefacts as a result of the processing and film content will appear overly smooth and more like video. We generally find that frame interpolation or motion smoothing can be quite effective with fast-paced sports action but we always leave it off with film-based content.

    This frame interpolation technology proved very popular with consumers and was even included on plasma TVs, which frankly didn't need it since they already had excellent motion handling. It's certainly the best example of TV manufacturers taking a limitation of a technology and turning it into a marketing feature. In fact LCD is proof of this because despite a number of inherent limitations, all of which we identify in our reviews, LCD has become the most popular TV technology inspite of, or perhaps because of, these limitations.

    OLED


    OLED is the new boy on the block and whilst this TV technology is still in its infancy, we feel that it will ultimately be the future of television. Despite the similarities in their names, OLED and LED TVs are completely different. Whilst an LED TV is just an LCD TV with an LED backlight, an OLED TV is a completely new technology that uses an organic compound that glows when electricity is passed through it. In that sense OLED has more in common with plasma because both are self-emitting and thus have many of the same strengths and weaknesses. Until recently it has been incredibly hard to make large screen OLED panels and only LG has really developed the technology to date but their investment is beginning to pay off. The manufacturer has managed to produce large screen Full HD models in numbers and it can now make Ultra HD 4K OLED TVs in sufficient quantities for prices to start falling. At the moment OLED TVs are still expensive but they will approach a more mass market price point soon.

    In many respects an OLED TV combines many of the advantages of plasma and LCD into a single technology. The self-emitting nature of OLED means that the panels can be incredibly thin (less than 5mm) but without all the backlight issues associated with ultra-slim LCD TVs. The black levels on an OLED TV are also exceptional because, like a plasma, when the pixel stops glowing it goes completely black. In fact OLED TVs are capable of even better blacks than a plasma because, unlike a plasma, the cells that make up each pixel don't have a slight after glow. However an OLED TV can struggle with fine gradations of detail just above black, so that's one area where plasma still has the edge. However an OLED TV is capable of an incredible dynamic range because not only can it deliver absolute blacks but it's also very bright. Not as bright as the best LCD TVs but certainly far brighter than a plasma, which means that OLED can support High Dynamic Range (HDR).

    OLED TVs can also deliver a much wider colour gamut, again not quite as wide as the best LCD TVs but certainly much wider than a plasma TV and the size of the colour gamut is increasing with each generation of OLED. If there is one area where plasma is still superior to OLED is in terms of its motion handling because despite OLED's incredibly fast response times, the current TVs still use a variation on the 'sample and hold' approach used by LCD TVs. As a result the motion resolution is better than LCD but not as good as plasma, which means that for fast-paced sports action you might want to consider using the frame interpolation feature. However when watching film-based content we would, as always, recommend turning any frame interpolation feature off.
    OLED is the TV panel of the future but is it perfect and what are the limitations of this exciting new technology?
    Since OLED is self-emitting, it could potentially suffer from a similar issue as plasma when it comes to image retention and screen burn. However, although we have seen signs of screen burn on pre-production samples at shows and have heard reports of image retention, we should stress that neither issue was present on any of the OLED TVs that we have tested to date. The manufacturers have included a number of features to mitigate these issues and it would appear that they have been successful but we would still recommend caution when it comes to leaving static images for long periods of time on an OLED TV.

    So far you might be thinking that OLED sounds perfect but, as we said at the start of this article, no technology is perfect. OLED is still in its infancy and has had a number of issues that whilst annoying don't necessarily detract from the impressive overall picture quality. It's likely that these issues will gradually be addressed in later generations and to some extent are just the price you pay for being an early adopter.

    The biggest issue to date has been the dark edges seen on many OLED TVs, which is sometimes referred to as 'vignetting'. It isn't always visible, varies from panel to panel and some people might not even notice it but it usually manifests in a very dark scene or as the screen fades to black. These dark edges just above black are probably a result of how the Ultra HD OLED panels are driven and whilst it has been reported on some of the LG OLED TVs, Panasonic went to great lengths to eliminate it on their CZ952 OLED TV (which actually uses an LG panel). The issue is less apparent on the most recent generation of LG OLED TVs and we would expect the company to eventually eliminate it.

    Another widely reported issue with early generations of OLED TVs is banding visible in dark images just above black. This has been apparent on all of the LG and Panasonic Ultra HD OLED TVs but, like the vignetting, it is only apparent with very specific material and most people might not even notice it. LG are aware of the issue but at present it appears to be a limitation of the OLED technology. As mentioned in the section on plasma TVs, OLEDs can struggle with gradations in areas just above black, resulting in the crushing of some shadow detail. It's possible that this is another limitation of the technology but careful setup can help in this area.

    There have been reports of discolouration or a tint to some OLED screens, which might well be down to variances in production lines and are probably the result of the difficulty in making OLED panels. There have been reports of minor faults in some OLED units sold, which again probably just reflects the inherent difficulties in their production, and as the technology matures these will undoubtedly become less common. That's why we feel that OLED is the future of television because most of the issues will eventually be eliminated and even now, an OLED TV is capable of a picture that is generally superior to both LCD and plasma.

    So there you have it, three completely different TV technologies, all of which have their limitations and most of which can't be considered genuine faults. So what can you do to avoid disappointment or frustration when buying a new TV? Well forewarned is forearmed, so we would recommend researching your choices thoroughly. Thanks to the internet this couldn't be easier and AVForums is here to help with both reviews and buyers guides. Once you have decided on a short list of TVs that meet your needs at your budget, we strongly recommended demoing them before making decision on which one to buy. This is especially important if you're spending a lot of money so, if you think a certain issue might annoy you, go and look at the TV yourself to make sure. Finally once you have made your decision and bought your TV, go to our PicturePerfect page to find out how to get the best from your new purchase.

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