Video/Imaging DesignLine | Serial video screen link shrinks wire count and mobile power consumption

June 29, 2007

Mobile video serial screen link: Part of a healthy power consumption diet

Raw power consumption for a competent mobile video display must be reduced by at least 40 percent in order to maintain the same battery life that current cell phone users have become accustomed to.

By Todd D. Whitaker
National Semiconductor

A generation ago we realized the vision of bringing electronic entertainment into our homes. Today, the vision is about bringing television, the Internet, and new types of video content to the cell phones that we carry with us wherever we go. This new vision is shared by both optimists and pragmatists.

The optimist sees an accelerating progression of trends making the transition to mobile video imminent.

The insatiable appetite of consumers for video entertainment
The growth and acceptance of digital video recorders freeing us from the time constraints imposed by television programmers
Breakthrough business models enabling access to both old and new types of content over the internet
Increasing flash storage capacity in ever-smaller form factors
Dynamic changes in the handheld market exhibited by features that cross over traditional product categories (GPS in cell phones)
The entry of exciting new industry players to shake up the market

Where the optimist sees change already in process, the pragmatist sees significant hurdles to a rapid transition.

Carrier readiness (and willingness) to provide the necessary bandwidth and infrastructure
Creation of compelling content for the presumed mobile usage model
New business models to capture the value and fund development for this new medium
The lack of a successful leadership product in this space
Hardware changes to allow satisfactory viewing of video content without compromising battery life and "cool" handheld device aesthetics.

The hardware challenges of mobile video
User experiences watching large screen TVs have set lofty expectations. LCD TVs have been pushing the envelope on very large screen sizes (larger than 50"), higher resolutions (high definition), increased brightness, contrast, color depth (and breadth), and enhanced motion compensation.

Standard cell phone displays have to undergo a number of changes to view video content in a satisfactory way.

Screen size
Traditional cell phone, MP3, and PDA screens are too small to enjoy most video content created for the large screen. Users expect to be able to see things such as the score in a basketball game. With the expectation of mobile video, display sizes are now increasing with the minimum size likely to be 3" or larger.

Format
Cell phone handsets have traditionally been implemented in a portrait orientation with room for a keypad below the display. Video content has a landscape orientation and typically a wide screen aspect ratio (16:9). Handheld displays must either change orientations or be capable of switching to landscape on demand. Touch panel technologies can replace the keypad in order make more of the primary face of the device available for the display.

Next: Color depth, managing power in mobile video displays

Resolution
Current screen resolutions (QVGA or less) will not suffice to show details that users expect to see in video content. The half VGA format is here now and is the minimum acceptable format for video content. Long term, the industry is likely to concentrate around Wide-VGA or WVGA format as the standard for mobile video.

Brightness
Higher resolutions limit the amount of light escaping the display module so overall brightness is decreased as resolution is increased. This is the opposite of what needs to happen to actually view large screen video content. Television content needs be shown at minimum of 400 nits or roughly twice the brightness currently available in high resolution mobile screens. By increasing the backlighting, some cell phones have achieved the required brightness, but they do so only at very low resolution.

Color depth
Cell phones use 6-bit color depth. That's all that has been necessary. Notebook computers use 8-bit color depth and LCD TVs are currently transitioning from 8-bit to 10-bit color depth. Users can tell the difference in color depth and the difference between 6-bit and 8-bit is dramatic. The handset transition from 6-bit color depth to 8-bit color depth was already under way and will be accelerated by mobile video requirements.

Individually these changes can all be accomplished. Taken altogether, these requirements have very serious implications for the handset power budget. A 3"- 4", landscape-oriented display with WVGA resolution, 8-bit color depth, twice the current brightness, and perhaps a touch panel interface presents a very serious challenge to battery life. Add in the aesthetic requirements driving smaller, lighter, thinner devices (including smaller batteries) and the design challenge to match display performance with battery life becomes quite daunting.

Incremental changes in display power consumption are simply not enough. Raw power consumption for a display implementing all of the necessary requirements for mobile video must be reduced by at least 40 percent in order to maintain the same battery life that current cell phone users have become accustomed to. While small improvements in power efficiency can add up, real innovative breakthroughs are required to solve this hardware challenge.

Managing power in mobile video displays
Some techniques for conserving and efficiently managing power can be adapted from notebook computers or large panel LCD TV innovations. Others have been newly developed specifically to address the battery life challenges created by the mobile video transition.

Most mobile video displays will utilize a technology called Low Temperature Poli-Silicon or LTPS. LTPS display module makers can integrate some circuits directly into the glass to reduce the number of semiconductor parts requires to support the module. This reduces power as well as the overall size of the module. The remaining semiconductors can be integrated into a single monolithic display driver which includes the column drivers, timing controller, module power supply (DC/DC conversion), the EEPROM used to store the module settings, and the DRAM used to present a partial display mode image while the host processor is sleeping.

Charge sharing
Charge sharing is a technology initially developed for the large-panel column drivers used in notebook computers, LCD monitors and TVs. Charge sharing can be described as a form of electrical ju-jitsu in which much of the energy used to charge and discharge column address lines is conserved through charge conservation. This feature can be implemented in either the driver or in the display glass itself. National has made this technology available to the industry through a variety of cross-license agreements.

Demand for thinner, more stylish phones has led the industry to replace the traditional RGB parallel interface between a host processor and the LCD module with a serial interface. Serial interfaces reduce the number of wires and extend the distance of the link allowing for more flexibility in the design of a handset.

Next: Mobile Pixel Link

Mobile Pixel Link
National has developed a serial interface technology specifically for handheld devices called Mobile Pixel Link (MPL). MPL is an open specification allowing for host processor companies to integrate the transmitter function and driver companies to integrate the receiver function thus removing the need for additional bridge and hub devices.

MPL reduces the wire count from as many as 28 wires to only 3 wires. MPL is based on a current mode signaling method so operating voltages on each end of the link can be different This "built-in" level shifting capability solves a problem between low voltage baseband processors and higher voltage display or camera peripherals.

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Figure 2: To minimize space consumption MPL is built along edge of display panel.

The primary objective in developing MPL was to achieve a serial link with the lowest power in the industry. Currently when active, the MPL physical layer link consumes 15 milliwatts. The nearest competing serial interface uses twice that amount of power.

Integrated LTPS display drivers with the MPL interface (FPD95120) as well as MPL serial bridges (LM2512) are currently available from National Semiconductor in volume production.

Next: MIPI Alliance, Dynamic Backlight Control

Going forward, the industry is developing a standard specification for mobile serial interfaces under the auspices of Mobile Industry Processor Interface (MIPI) Alliance. MIPI has released a 0.65 version of the MIPI DSI standard specification, and many companies have begun early development based on this preliminary standard. National Semiconductor is a key contributor to this effort and is developing a low-power version of these devices.

Another method to radically reduce the power consumption of a mobile video display is to manage the power used by the backlighting system based upon the content that is being displayed. Simply put, some content, such as black on white text can be viewed at much lower brightness levels without impacting the user experience.

Dynamic Backlight Control uses algorithms to determine the type of content being presented and to smoothly increase or decrease the backlight power. It is estimated that in normal use, this technology can reduce power consumption by as much as 25 percent.

Much like the transition from film to television, or more recently, the migration from CDs to MP3 players, the migration of video content to mobile platforms presents both significant challenges and wholly new experiences. There is risk as well as reward for those who can envision and lead the development of technologies that will make these new experiences possible.

About the author
Todd D. Whitaker manages National Semiconductor's Displays Division. Prior to National, he worked for Blaze Network Products, Next Level Communications (now Motorola) and Intel Corporation. He has a bachelor's degree from UC Berkeley and an MBA from UC Davis. He can be reached at todd.d.whitaker@whitaker.com.