Why video can make engineers' eyes water
01 October 2007
Everyone wants mobile video, delivered to PDAs, phones and media players, but it can make a grown design engineer cry, as it throws-up power management, display driver, form factor and audio integration horrors, writes CAROLINE HAYES
Sound is no longer enough, consumers want video as well as quality audio on handsets. The trend is a strong one, with In-Stat predicting that sales of 300million video-enabled devices will increase to one billion by 2011. Service providers are keen to find new revenue streams, as are conventional broadcasters, TV and film makers. All are promoting video and broadcast services to handheld devices.
Today there are over one million subscribers worldwide and in the next three years, this is expected to rocket to around 250 million mobile video subscribers in 2010. This brings design challenges, from power consumption which has to be minimised; EMI levels due to the design complexity of display, audio, digital processing, media storage and RF uplink integration; to the need for high- resolution displays, usually a minimum resolution of 240 x 320 is specified, to enjoy the results. While Hollywood is keen to promote its content for mobile video, it is Japan that has embraced these feature phones. Last year, 98 new 3G models were introduced in Japan alone, and sales reached 43.5million.
First, decode the video The first step in mobile video is to decode and process the video data for it to be displayed. This is one of the main bugbears in the power budget, as the application processor that decodes and processes the data is power-heavy and can be a major factor to limiting battery life on mobile terminals.
National Semiconductor’s PowerWise technology can intelligently manage the processor, using adaptive voltage scaling (AVS). The PowerWise IP integrated on the processor with compatible power management ICs, for example the company’s LP5552, can reduce energy consumption by up to 70 per cent, extending the battery life of the portable device. The IC supports AVS Consumers want high quality video with audio on handsets can adjust its voltage to reduce energy. It includes two 800mA buck regulators and five LDO regulators in a 36bump micro SMD package.
Sending video
wirelessly is another drain on battery power that can reduce power supplies optimised for 3G RF power amplifiers. Power supplies, like the new LM3207 DC/DC power supply, also from National Semiconductor, reduce energy consumption by regulating the RF power amplifier supply voltage to the lowest possible level while maintaining linearity in the power amplifier. (www.national.com/pf/LM/ LM3207.html) Serialising video data National Semiconductor’s Mobile Pixel Link (MPL) serial interface family streams data, in the form of images, video and graphics, to the display while conserving battery life. MPL is an open industry standard which defines the interface between displays and baseband or application processors in mobile devices. It consists of a physical layer, clocking and bit ordering for transmission. The physical layer is made up of just three wires, lowering the IC pin count, size of packages and width of flex cables in handheld devices. A narrow flex cable, for example, can reduce the size of a clamshell phone as the video source can be connected underneath the keypad and linked to the display in the top half of the open ‘shell’. The operating voltages can be different, as MPL is based on current mode signalling, preventing level shifting, which can occur when using low voltage baseband processors and higher voltages on displays and camera peripherals in handheld devices.
This handling of the voltage level translation between the host and display eliminates the need for an external level shifter. The video data is serialised to reduce the wire count, which in turn decreases the connector size and flexible PCB. The interface reduces the number of signals and pin count from 28 to three, depending on which interface in a mobile handheld device is replaced. Power consumption is also claimed to be reduced and is comparable to traditional parallel LVCMOS solutions. The low power signalling method can significantly reduce EMI and, according to National, reduces power consumption to half that of competing interfaces. The latest addition to the company’s MPL offering is the LM2512, a serial host device that supports a video mode interface bridge between the processor and the display. It has a look up table that allows for colour correction so the designer can optimise displays from different manufacturers. A dithering function allows 24bit video to be displayed on 18bit displays, lowering the power consumption to levels akin to those of 6bit RGB data converters. It is packaged in a 49-bump UFBGA. (www.national.com/appinfo/mpl/)
The increasing demand for memory capacity and storage in mobile video devices requires logic level shifting power, EMI filtering and ESD protection. Secure digital (miniSD/SD) card interface IC, the LP3929 is, claims National Semiconductor, the industry’s first to provide an interface by combining level-shifter, power supply, EMI filtering and 15kV of ESD protection in a single device. It is packaged in a 2mm x 2mm 24bump micro SMD package. (www.national.com/pf/LM/ LP3929.html)
Power regulation
Mobile video brings LDO (low drop out) regulators to the fore together with LVDS (low voltage differential signalling) for display quality. Data is processed from non-volatile storage devices, such as a secure digital card, a flash or micro hard drive. Each source poses energy consumption challenges. National’s new LM3668 buck-boost regulator provides a 3.3V output voltage to power the driver motor. It is claimed to offer more than 90 per cent efficiency in buck and boost modes, essential for driving high power applications. The regulator creates a seamless transition between the two operation modes to provide a stable output for error-free operation.
The stable output and high efficiency levels, packaged in a 12pin LPP mean that the regulator can be used as a pre-regulator for high-voltage, handheld applications. Driving displays The company’s FPD95120 is a low temperature polycrystalline silicon display driver that is claimed to be the narrowest glass-mounted device available for half-VGA format at 0.9mm wide. The slim device can be used to maximize the active area of the display glass so that more video can be displayed effectively on the size of screen demanded by the mobile device’s form factor.
The driver integrates an MPL deserialiser, an inductive DC/DC switcher and RAM for low power and standby operation. An EEPROM is also integrated in the device, which stores module calibration data to match flicker and colour parameters across a host of manufacturers’ displays. It can also programme product identification for security. (www.national.com/pf/FPD/ FP9120.html)
Backlighting is an effective way to make the most of a display, but this brings its own challenges in terms of the power consumption this can require. National has also released four RGB LED drivers to lower power consumption compared to white LEDs and provide clear, bright, defined colours on screen. One, the LP5520 does not need an optical feedback to produce a true white light and is claimed to improve the colour gamut from 70 to 100 per cent of the NTSC standard. The driver’s circuitry automatically adjusts when it is in adaptive mode, to deliver the output voltage for the lowest possible power consumption. It is packaged in a 25 micro-bump SMD package.
Sounding off
Video glitch noise can be eliminated with video filters, which also extend battery life in handheld devices. Intersil has recently introduced the ISL59118, 9MHz, dual channel, low pass filter that uses nearly 50 per cent less bias current than previous generations, claims the company. www.intersil.com/fn/fn6317.pdf)
It can be used where there are two composite video signal inputs or outputs and as an input anti-aliasing filter before an ADC to remove high-frequency, non imaging forming noise from the signal before it is digitised. If positioned after a DAC, it can be used as an output construction filter to smooth out the digital output waveform and to remove any glitches and noise. The filter operates from just 4.5mA bias current, and uses 10 times less current than competitive devices, according to the company. In power- down mode it consumes just 0.5mA. It operates from a 2.5V to 3.6V supply. It is housed in a 10lead ìTQFN package.
The other part of the mobile broadcast and video experience is audio. Audio subsystems such as National’s LM49100 allows routing of mono voice or stereo music signals to a mono speaker driver or stereo ground-referenced headphone amplifiers, or both. These can be selected via pre-set options. A headphone ground-sensing function and improved noise rejection are included to reduce form factors in mobile devices. The subsystem consumes less than 5mA, with all channels active, claimed to be the industry’s lowest quiescent current. (www.national.com/pf/LM/ LM49100.html)
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