Technical How to comply with the Energy Star 6.0 standard for LED TVs: a demonstration of reduced power consumption with improved picture quality Markus Luidolt and David Gamperl
How to comply with the Energy Star 6.0 standard for LED TVs: a demonstration of reduced power consumption with improved picture quality Markus Luidolt and David Gamperl A constant tightening of the regulations governing the power consumption of TVs has had its intended effect: modern LED TVs today consume around half as much energy as LCD TVs with a standard CCFL (cold cathode fluorescent lamp) backlight consumed even as recently as two years ago. The squeeze on power usage continues, however, with the next revision of the US Energy Star regulation, bringing into play the technique of adjustable display brightness optimised for varying ambient light conditions. In a room with subdued lighting, the TV viewer will be more comfortable with a relatively dim display output. In a brightly sunlit room, the display screen must be backlit at high power, in order for the viewer to be able to see the picture in clear detail. But the ambient light sensors (ALS) generally used in today s TVs are not able to provide a finely graduated backlight control, and so fail to take full advantage of the opportunity to reduce power inputs to LED backlights. In fact, intelligent backlight dimming local backlight dimming synchronised with picture content, as well as finely graduated dimming in response to ambient light conditions looks to be a promising method for markedly reducing the power usage of LED TVs. This article describes a design project by ams to implement both local dimming and ambient light display control in a mainstream LED TV on the market today. How large an effect on average power consumption is advanced dimming able to make? Changes to energy standards The most important energy standards for TVs are the EU Energy Label, the Chinese energy consumption standard for TVs, and the best known standard, the US s Energy Star. The current Energy Star standard for TVs, version 5.3, already sets tight power limits, but version 6.0 planned for 2013 will further cut the maximum allowed power consumption (see Figure 1). Under v6.0, the maximum allowed power usage for a TV of 46 falls by 20.5% from 93,9W to 74,6W. Page 2 / 9
Fig. 1: Energy Star on-power limits for various TV screen sizes It is already hard enough to reach this standard. But the regulators have also made the standard tougher in another way. Until now, the TV s power consumption would be measured under two conditions: in the dark (0 lux) and in a bright room (300 lux). With version 6.0, the Automatic Brightness Control (ABC) function must be validated as follows, to define the maximum allowed on power: Power measurements must be taken at three different luminance levels: 10, 50, 100 lux If the average power measured at 10lux is 5% lower than at 50lux, and the average power at 50lux is 5% lower than at 100lux,a 10% On mode power allowance is granted (See green curve in Figure 1). The ON mode power is measured with ABC disabled, and with default home settings. Without having an ABC, all TVs will not work above the red energy star mode (Figure 1) Many TVs on the market today fail to comply with this new test (see Figure 2). Page 3 / 9
Fig. 2: Energy Star on-power maximum allowed power vs reported power This is largely because they were designed to comply with the 5.3 standard, using cheap photodiodes to implement a crude ALS that reduces TV backlight power at 0 lux. Many manufacturers have used this ABC to reduce backlight brightness to an unrealistic low level, producing poor picture quality (in order to reduce reported Energy Star power consumption), in the quite justified belief that viewers rarely watch TV in a completely dark room (0 lux). Page 4 / 9
Ideal TV Green: LED,LCD Blue: CCFL, LCD Fig. 3: power consumption of TVs at measured ambient light conditions. As the TVs are similar but not identical in size, the diagram shows power consumption normalised by measuring Watts per square inch. As Figure 2 shows, such crude implementations of backlight power control in response to ambient light will not be sufficient to support compliance with version 6.0. So how much difference is made when more intelligent, fine-grained dimming is implemented? Figure 3, a study of TVs on the market today, shows that many TVs fail to take advantage of an opportunity to save power at low room light conditions of between 10 and 100 lux. This is important not only for compliance, but also for customers. Three studies carried out independently have come to the same conclusion: viewers spend more than 80% of their viewing time in ambient light of less than 85 lux. Figure 3 also shows that intelligent local edge dimming offers dramatic power savings: of the models studied, only the LG 47W5600 divides the LED edge light into 12 segments, and automatically dims each of the 12 segments in tune with the picture content. Local dimming means that where the video content is dark, the backlight output is reduced. This results not only in lower power consump- Page 5 / 9
tion but also a better contrast ratio and hence better picture quality. (LG s precise implementation of local dimming is achieved using an advanced LED driver from ams, the AS3695C.) In the same way, only the Vizio TV implements intelligent ABC (using an ams ambient light sensor). Fine-grained response to ambient light conditions again offers both power savings and an improved viewing experience the display brightness is automatically reduced to comfortable levels when viewing in a relatively dark room. And while a black picture can often appear grey in today s LCD TVs, a dimmed backlight permits the production of a deep black and true colour reproduction. So can the ideal TV shown in Figure 3 combining both forms of intelligent backlight dimming provide a means to comply with Energy Star v6.0? Combining smart local dimming and ambient light sensing As a demonstration, ams modified a mainstream 42 LG TV (model number LW5500), implementing intelligent ambient light sensing alongside the TV s local edgelight dimming (see Figure 4). The modified TV was then submitted to an independent Energy Star-certified laboratory for Energy Star tests, using the standard 10-minute video clip in the three room light conditions laid down by the v6.0 standard. Fig. 4: power consumption of the LG LW5500 modified by ams Page 6 / 9
Local dimming is not implemented in most TVs on the market today. When turned off in the LW5500, the TV consumes an average of 93W in the standard TV HOME setting of the Energy Star 6.0 test. By enabling 12-segment local dimming, the TV benefits from a power reduction of 16%. Picture quality and contrast are also enhanced. For result 3 in Figure 4, crude ambient light sensing was also turned on, however due to bad implementation, no ALS allowance is granted according to the v6.0 tests. Result 4 shows the improvement made when precise ambient light sensing is implemented, graduating the backlight output in response to changes in room lighting. The TV s least aggressive internal power-saving mode, which slightly reduces the backlight output in certain video picture conditions with a negligible effect on picture quality, was enabled during this measurement. Through implementing ALS in accordance with the energy star specification, a 10% maximum on power allowance is granted, which is mandatory to achieve energy star 6.0. The tests showed that, with only a combination of local edge light dimming, precise ambient light sensing and a mild power-saving mode, an existing mainstream TV can be modified to comply with the stringent requirements of the Energy Star standard v6.0, while improving the viewing experience. What is more, this has been achieved without any modification to the TV s architecture or power circuit. Design engineering considerations This dramatic improvement in power consumption makes particularly demanding calls on the performance of the ALS. The dimming of the display panel s brightness must match very closely the response of the human eye to changes in ambient light levels. The ALS used in the modified LW5500 demonstration is superior in two main respects to the crude ALS devices typically found in TVs today: first, it maintains high accuracy and precision across its measurement range, even at low light levels. Second, it features filters which reject the IR portion of the light spectrum incident on the sensor. IR is invisible to the human eye, but distorts the measurement results of ambient light sensors unless filtered out. The LW5500 demonstration design uses a CT820 ALS from ams, which also offers high sensitivity. This means it can operate from a concealed location behind the TV s bezel, which makes drills through the TV bezels obsolete. Page 7 / 9
The choice of ALS device, then, is extremely important in the implementation of this energy-saving design. Another key element of the design is local dimming. This requires the LED driver to be synchronised with the TV picture, using VSYNC and HSYNC input signals. In order to make the LED light output match precisely the video signals, the demonstration design uses an ams LED driver, the [AS3695C / AS3820] with completely independent PWM generators for each LED channel; the PWM generators are synchronised with VSYNC and HSYNC signals. Each PWM generator can be programmed independently in respect of High time (that is, the duty cycle) and delay. This gives a great deal of flexibility to the system designer, and supports all LED backlight architectures (3D, edgelight, direct backlighting). With this scheme, interaction between the main video processor (TCON) and the LED driver chipset can be minimised, allowing the use of a relatively slow SPI interface operating at well below 1MHz. TCON local dimming information SPI LED Backlight TCON local dimming information SPI LED Backlight Driver ambient light information analog dimming ALS 12C MCU Algorithm I2C ALS Fig. 5: ambient light control bypassing the video processor Fig. 6: ambient light control via the video processor In the LW5500 demonstration design, the ALS is connected to a separate MCU, which reads out the ambient light information, processes this information via a proprietary algorithm and converts it this into an analog signal (see Figure 5). This analog signal then directly dims the LED backlight. The advantage of this topology is that dimming control can be implemented completely independently of the main video processor s software, and the ABC function is autonomous of the local dimming protocol. Figure 6 shows a possible different topology, in which the ambient light signal is routed via the main video processor. Page 8 / 9
Conclusion The LW5500 demonstration design from ams shows that relatively simple methods of intelligent backlight dimming can produce power savings of ~30% in an LED TV, which already featured an advanced power system design. Reducing a TVs energy consumption by 30% would eliminate around 8 megatons of CO 2 emissions per year worldwide. What is more, this energy saving can be combined with better picture quality and an improved viewing experience. ams is the leading provider of driver and lighting sensor solutions for LED TVs and has a complete portfolio of LED driver and lighting sensor products supporting edge lighting, direct lighting, ambilight and 3D TVs. The aim of ams is to provide its customers with outstanding performance in power-efficient systems with outstanding LED TV picture quality. For more information on power saving in LED TVs, including a video demonstrating the effect of the design described in this article, http://www.ams.com/eng/lighting/60watttv For further information ams AG Tel: +43 (0) 3136 500 info@ams.com www.ams.com Page 9 / 9