National Semiconductor
Santa Clara, CA
http://www.national.com
With the demand for ever higher quality full motion video on handsets and portable devices, engineers are challenged with providing the end users with viable solutions. This includes high quality real-time video playback, a compact package, long battery life, and more. Using analog techniques for moving video, driving the LCD glass, and managing power can greatly enhance the quality of the end users experience. In this blog installment, we will look at several technologies for enabling small, high performance displays, moving data around, and managing the ever dwindling power resources found in today’s modern handsets and personal media players.
As with any technology, consumers drive the next generation. After all, technology is defined by how people use it. In the case of video, we have gone from the early days of extremely heavy and expensive black and white, tube based “television” devices circa 1950s to today’s modern flat panel LCD and Plasma Television / Monitors. With the increasing number of PDAs, Personal Media Players (PMP) and cellular phones that incorporate full motion cameras, it was inevitable that users would desire higher quality, full motion video on small, hand-held devices. This comes with a large amount of complexity not found in larger, land-powered devices. Power consumption, which directly relates to play time, is a major limiting factor. In a world of technology where thinner is better, batteries must remain small. Video quality as well as audio quality also drive consumers to make brand / model decisions. In this BLOG entry, I will explore these problems and some possible solutions to improve the end users video experience.
POWER, POWER, POWER
There are three things to remember when it comes to portable, battery powered devices. They are, in no particular order, Run Time, Run Time, and Run Time. If a portable device lasts one hour on batteries, it is essentially useless. Even a flash light (which is a portable, battery powered device) is required to run for many hours without a recharge or battery replacement. As full speed (24+ frames per second) video is added to a portable device, there are new functions that require additional power over static displays.
An example of increased power requirements of video is the stream processing such as H.264 decoding. This requires more cycles from either a dedicated processor or a DSP than decompressing a static image. Additionally, there is the problem with the decompressed video stream itself – it must be displayed. Driving the LCD glass in a portable device along with the color management required for good quality video requires an increase in power. Add the problems with managing the back light, and it becomes imperative that the designer be conscious of the power budget.
There are several solutions available that make managing power in these video enabled mobile devices much easier. The LM3919 from National Semiconductor is a complete portable power management unit (PMU) with support for many of the subsystems found in portable devices. Adaptive Voltage Scaling is another technique that can greatly reduce power consumption. The idea is to minimize the supply voltage of the processor core in a closed loop manner as to minimize the energy requirements in a real time fashion. This technique is available on processors that incorporate the PowerWise™ open standard. This technology can be found in many ARM based processors. Special Energy Management Units (EMU) such as the LP5552 communicate with the processor to dynamically scale the supply voltage as required greatly reducing the power consumed.
The back light is another critical area for power management, and even more so when trying to provide high quality video. The image quality is critical and many consumers are becoming ever savvier when comparing similar devices. This is driving designers to move away from the traditional white LED type backlights due to poor color control and process variances. Back lights that now have red, green, and blue LEDs are preferred for better color saturation (over 100% NTSC) and allow better control of the overall video image. Devices such as the LP5520 are used to drive the RGB LEDs. When used in adaptive mode, this device even drops the drive current back to the minimum level required for the best quality image at the lowest power level.
VIDEO STREAMING AND DISPLAY QUALITY
Now that we’ve discussed some of the issues with minimizing the power requirements for full motion video, let’s discuss what it takes to move that uncompressed video to a display. There are several issues that plague video streams which include Electro Magnetic Interference (EMI), routing, and driving LCD glass. Let’s take a look at the routing problem first.
Many of today’s personal media players have VGA resolution screens even though they are only 2.5 inches diagonal. These displays provide the sharpest possible picture in a compact size; however, it also means a large number of rows and columns to drive in a very small space. Display drivers, such as the FPD95120, are typically mounted directly on the LCD glass to minimize connections and reduce EMI. It also greatly improves reliability.
Since the drivers are on the glass, the most logical next step is to serialize the data driving the display reducing the number of off-glass interconnects. This has been done for some time for large LCD displays using Low Voltage Differential Signaling (LVDS). LVDS uses current switching instead of voltage switching which allows much higher speeds and lower EMI. However, in a portable device, power is a major concern and LVDS, RSDS and other interface technologies were simply too high a power. An open standard called Mobil Pixel Link or MPL was developed to directly address this issue in portable devices with LCD displays. It reduces the current level to one tenth that of LVDS (300 uA for MPL versus 3.5mA for LVDS) and also reduces the voltage swings to greatly reduce power and EMI. MPL also reduces the wires greatly – there are only 2 active lines which make routing through a hinge or around a second display much easier and more reliable.
Since MPL uses current levels as the signaling method, the voltage levels at each end can actually be different and still work fine. This provides a method for level shifting between low voltage processors and higher voltages used on the displays. Today, MPL devices such as the LM2512 carry even more functionality such as a dithering function which converts 24 bit RGB data to 18 bits and still provides an excellent video experience without the higher power consumption of the full 24 bit stream.
OTHER THINGS TO CONSIDER
As strange as it sounds (no pun intended here), if the audio of a high performance video device is poor, people will tend to complain about the display quality, unaware the problem is in the audio. Humans are very sensitive to sound quality as well as synchronization with the video. A mere 20 to 40 mS delay between the audio and video will cause a viewer to notice and be distracted. Distortion or poor audio quality will distract from the video quality and must be avoided.
Where there is video and audio, there is some kind of storage media. Some types of media require special level translation and static protection – especially if the media is removable. Good examples of this are Secure Digital (SD) cards. Secure Digital or SD cards can range in capacity from as small as 8 megabytes to 2 gigabytes (cards range from 4 to 32 gigabytes in the newer SD 2.0 or SDHC standard). Since the connector is exposed to the outside world, and users will come in contact with it, the sensitive internal electronics need to be protected from electrostatic discharge (ESD). Additionally, SD cards operate at 2.85V where many portable CPU’s I/O bus works at 1.8V. There needs to be both level translation and ESD protection as well as methods to limit EMI from the connector. Components like the LM3929 do all of this in a single, tiny device and will withstand +/- 8KV direct contact discharges (per IEC61000-4-2 level 4 ESD).
As you can see, integrating full motion video has challenges such as the new processing requirements of video streams, image and display quality, power management and run time, as well as audio quality and large removable media. With careful engineering practice, high quality full motion video can be integrated into small, handheld devices. The technology is available and many manufacturers are already producing incredible devices with world class performance. ■
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