在移动电话上实现真正 24 位色的枢纽(英)

Video on the move has arrived. Terrestrial digital TV broadcasting to mobile phones is happening now in Korea and Japan and is undergoing trials in Europe and the United States. Consumers are downloading popular network TV shows to multimedia players and playing movies on portable gaming consoles. However, the end-user still has to contend with a disappointing viewing experience, where on some of the larger 2.2-inch portable screen sizes available today, pictures are coarse and video motion is rough. Improvements in the capabilities of application processors, liquid crystal display (LCD) modules and the interface that connects them will make video smoother and pictures more realistic.

To date, the largest resolution found in mobile phones has been QVGA, but higher resolution HVGA and VGA LCD modules, including new driver ICs, are now becoming available. Today抯 advertising lauds 64k colors and 256k colors. True 24-bit color enables more than 16 million colors, resulting in much more realistic pictures. Whereas the refresh rate has been 30 frames per second (fps), new technology will support 60 fps. All these improvements will bring about dramatically improved viewing quality to end consumers.

Figure 1. A superimposed image of three EMI measurments with the device operating at f(PCLK)=5 MHz, f(PCLK)=22 MHz and f(PCLK)= 65 MHz. This excellent performance meets the system requirements of dense mobile designs with a noise floor of ~2 dBuV (-105 dBm).

Most applications or graphics processors in phones have used the conventional processor-to-memory-type interface methods when sending data to LCD modules, which include some memory that acts as a frame buffer. This is efficient for mostly static displays and has been sufficient for slowly changing video. It offers little benefit when the display is used for full-motion video. Additionally, as processing power improves and allows the main applications processor to output red/green/blue (RGB) video directly, the need for additional silicon real estate in the form of specialized graphics processors and frame buffering memory is diminished. Video data can be transmitted directly from the processor to the LCD driver IC. This should help reduce manufacturing costs and simplify designs.

Figure 2. Wide parallel data is serialized into a much-reduced number of serial differential lines.  (click image to enlarge)

These improvements do bring with them some technical challenges, however. Remember when laptop computers had monochrome screens and processor speeds were measured in the tens of MHz? As laptop computer screens improved, it became untenable to transfer data using the usual parallel single-ended bus schemes. Higher resolution, more color depth, faster screen refresh rates and shorter LCD response times meant a very high data throughput requirement, and in turn a thick bundle of wires going through the hinge from the electronics under the keyboard to the screen. The same problem exists today for clamshell phones.

To meet this need, serializer/deserializer (SerDes) solutions for laptop LCD screens were introduced in the late 1990s . FlatLink?G from Texas Instruments is an example of a more modern SerDes interface for today抯 3G phones. It avails of advances in the semiconductor industry to provide a scalable solution with selectable throughput for resolution sizes from QVGA through XGA, sub-low voltage differential signaling (subLVDS) for the physical layer interface, full 24-bit RGB color depth and a 1.8V power supply. The new serial interface is featured in the TI OMAP application processor platform and as a discrete bridging chip, the SN65LVDS301 serializer-transmitter. Several major manufacturers of LCD IC driver and LCD modules are making FlatLink3G-compatible components.

Interfaces have a long history of becoming standardized over time. RS-232, TIA/EIA-644A (LVDS), IEEE1394 are some well-known examples. Standards allow system developers many benefits, such as eliminating the need for them to develop their own proprietary interfaces. This accelerates time to market and allows engineers to focus on more value-added core competency areas. Buyers like the increased availability of components and modules with common interfaces as this reduces costs of moving from one supplier to another and will tend to reduce actual component costs as a point of product differentiation is removed. The Mobile Industry Processor Interface MIPI Alliance, founded by ARM, Nokia, STMicroelectronics and Texas Instruments, is an open membership organization that includes over 90 companies in the mobile phone industry. It is expected that following on from the Camera Serial Interface 2 (CSI-2), the Display Serial Interface standard will become widely used once it becomes finalized.

Serial interfaces bring solid benefits to phone manufacturers. Flexible printed circuits (FPC) are used to carry data across the hinge in clamshell or flip phones. Today phone makers use FPCs that are many layers thick and carry upwards of 30 signals with many ground traces and vias. The size of this cable can become cause for concern in handheld hinged equipment where successive closing and opening, or twisting in the case of swivel-hinged phones, can create mechanical stress in the FPC and sometimes cause wire fractures and short circuits leading to display failures. This is actually a major contributor of phone returns by end users and is very costly to manufacturers. By serializing this wide parallel data into a much-reduced number of high speed differential signals, the FPC width can become thinner in terms of number of signal wires and layers needed. This can allow for more slack in the confined space of the plastic enclosures that comprise the hinge. The lesser number of wires can lead to reduced costs of both the FPC itself and smaller connectors at both ends.

Another major benefit is lower radiated electromagnetic emissions. The low voltage differential signaling schemes used in most SerDes solutions allows for very low electromagnetic interference (EMI) in two major ways. First, by being differential in nature, the electrical and magnetic fields associated with both parts of the signal will in large part cancel each other out. Second, whereas single-ended signals see voltage changes in multiples of 1V, differential signals typically swing in fractions of a volt. For example, subLVDS has a 150-mV swing. For mobile phones, the consideration of EMI is obviously critical, given the potential to interfere with radio reception and transmission. Standardized test methodologies have been developed such as the SAE J1234/5, which uses a GTEM cell. Such tests are very helpful in allowing engineers to make unbiased comparisons between different technology offerings.

Processors, LCD modules and the supporting bridging serial interfaces are now available to support true 24-bit color in screen resolutions of VGA and higher in mobile handheld equipment. Video now really does mean moving pictures.