Quality Analysis of Video Output Signals of Handheld Devices

Humans are visually guided, so it's no surprise that there's a market for high-resolution plasma TVs. Even though these 40-inch flat-panel behemoths cost 10 times as much as cathode ray tube (CRT) displays, consumers are willing to pay for the increased resolution and contrast that the new technology brings.

Likewise, consumer expectations for video and displays in handheld devices are rising. Cell phones, PDAs, and even MP3 players like the iPod can provide images as sharp as those on larger display devices just a few years ago. Today's portable devices are equipped with brighter, more colorful, and more user-friendly displays, but they are susceptible to noise that degrades the quality of the video signal.

While the overall quality of video output signals has steadily improved, display errors caused by noise and electrical interference have become more noticeable. At the same time, as cell phones, microwave ovens, and wireless networks have become more widespread, potential sources of interference have become more powerful and more common. The same conveniences that have made cell phones so popular also apply to other wireless devices.

These trends present great challenges to designers who wish to integrate high-quality video into portable devices. High consumer expectations, compatibility with multiple formats, limited battery life, user misuse and a variety of external signal interference all mean that today's video drivers must have multiple features and the ability to resist multiple interference sources.

The trend toward single-chip integration has made handheld devices small, reliable, and versatile. Today's video drivers are smaller than 1mm3, but still provide high-quality video output signals without significant interference. In addition to advances in semiconductor manufacturing technology, the popularity of small-sized chips and surface-mount chips also means that more high-tech components can be made into the smallest volume. Surface-mount chips have more advantages than through-hole models, such as simple automatic assembly with pick-and-place machines, and more flexibility in space-saving double-sided circuit board design.

Using fewer components is another trend to save space and energy, which can make portable devices smaller while extending battery life. Fully integrated systems on a chip mean that external resistors, capacitors, and inductors are no longer helpful for system operation. For example, integrated filters can save a dozen external components, and a third-order filter can save 14 components. In addition, compatibility with multiple video signals means that multiple filters must coexist and they must maintain their own operating modes. In addition, the tolerance of integrated filters is half that of discrete devices, making operation more reliable and stable.

These filters need to be switched in a way that the switching process appears transparent or invisible to the user. Switching from an 8MHz standard resolution output to a 15MHz progressive scan or 32MHz high resolution output means that the video signal is suddenly placed on a completely different bandwidth interface, so the filters must compensate accordingly. A 3 channel signal like RGB requires different filters than a 2 channel signal like Y/C. With so many different signal formats on the market, the question is whether the chip can support all of the formats that customers need and whether there will be problems with the switching process.

AC coupled video signal output

DC coupled video signal output

With so many options to support, some of the less frequently used features in a product also consume some energy, whether it is a filter or an external coax driver. Coupled with the fact that product functions, product features and consumer demands have far outpaced the development of battery technology, efficiency is a key factor for any handheld device. Today's lithium-ion battery technology is a considerable improvement over the nickel-metal hydride and nickel-cadmium battery technology of the past, but who spends a lot of time discussing how much power is left in a mobile phone? Users are certainly not satisfied with having to recharge because the power does not continue to be supplied within the specified interval.

For a system designer, he may not be able to do anything about the battery capacity, but he can choose more efficient components. Obviously, when the chip is in working state, it is necessary to pay attention to its power consumption, but the power consumption of these devices in standby state is equally important. Many devices have standby power consumption. Even if the standby power consumption of a single device is small, the standby power consumption of the entire system will greatly reduce the battery life. Fortunately, the standby power consumption of video drivers on the market is very low, and the current is only 250μA when not working.

Compared with other portable devices, mobile phones present additional design challenges due to their physical structure. Although more and more new features make mobile phones more practical and more expensive, people will not hesitate to use mobile phones in harsh environments where they would not think they could use MP3 players and digital cameras. This puts higher demands on robustness than before.

The robustness of electronic products is indeed more important. Compared with the past, today's portable products have higher chip density, lower power consumption, and smaller size, but they are also more vulnerable to electric shock damage. A person can generate several kilowatts of charge just by walking on a carpet, which can easily be transferred to the mobile device he is holding. Although the spark current that may be generated is very low, its extremely high voltage is extremely dangerous for small devices. Therefore, for components such as video driver chips that are connected to the outside world, it is very necessary to have integrated protection functions or external protection circuits. Devices with high-quality protection functions must be able to withstand high voltages of at least 12kV. [page]

Despite such high demands on video drivers, all efforts will be in vain if the final video signal does not meet consumer expectations. In order to maintain the highest possible video output quality, the video driver needs to perform the function of a reconfigurable filter. Noise can only be unnoticeable if it is reduced to below -20dB (visible level), which can be achieved through 3rd to 6th order filters.

Types of noise include artificial high-frequency external electromagnetic interference and sideband swings caused by low-frequency oscillations. External EMI may come from any nearby wireless device, even a microwave oven. The space around us is full of radio signals, so the system must be designed to adapt to the harshest environment. Signal crosstalk within the device is also a problem, especially for devices such as mobile phones and GPS receivers that also transmit and receive signals as part of their own operation, which need to provide high-quality video display. Because the human eye is very sensitive to these noise effects, the filter requirements in all video applications are very demanding.

End users may never understand how difficult it is to meet their demands for more features, longer battery life, better compatibility, and clearer audio and video. Fortunately, device manufacturers and chip designers continue to work hard to provide system designers with more cost-effective components.