Overcoming video design constraints in tablets and notebooks
Article By : Akeem Chen
Innovations in camera module design allow notebook and tablet designers to offer best combination of video performance and device size.
The pandemic has forever changed the way we work, attend school, and even seek medical care. Now that millions of people have experienced the advantages and conveniences of video conferencing, they will continue to use this technology even after they return to offices and classrooms. The number of computers per household has increased, and people are now accustomed to the idea that every family member needs his or her own dedicated device.
Demand for portable clamshell devices such as notebooks, 2-in-1s, and tablets has spiked in the past year and a half. This trend is especially notable for tablets, a market that had been flat and is now in high demand. According to an April 29, 2021 report by Strategy Analytics, the tablet market posted a 44% year-over-year increase in Q1 2021―the highest in eight years―as more people seek the flexibility and portability that tablets offer.
But not all devices perform equally. Users—from preteens to retirees and everywhere in between—have high expectations for crisp video quality, long battery life, and ease of use. Engineers can design compact notebooks and tablets that fulfill these requirements if they choose the right architecture and components, beginning with the image sensor.
Three design challenges
Compact clamshell devices pose a particular design challenge because of the limited space for the front-facing camera module. Edge-to-edge displays with bezels that are as narrow as possible constrain the design. There is typically a trade-off between size of the camera module and video performance. As a result, many devices on the market suffer from poor video quality.
Second, working virtually does not mean being tied to a desk. Especially for tablets, consumers expect their devices to run for hours without being plugged in. An ideal solution will offer high-resolution, responsive video at either 30 or 60 frames per second (fps) while not sacrificing battery life.
Third, portable devices also need to be protected from overheating without resorting to fans or large heatsinks, as there is no available space for those parts. Power consumption of every component is, therefore, an important consideration for any design.
Several innovations in camera module design and features give designers the best combination of performance and size. The trick is to leverage technology from the mobile phone and security product sectors to provide the industry’s smallest 1080-pixel high-definition image sensor.
- Shrinking and stacking
The number and size of each pixel determines, in large part, the X and Y dimensions of an image sensor. Increasing the resolution while maintaining sensor size requires shrinking the pixels. Doing so reduces the number of photons hitting each pixel, which decreases full-well capacity and can induce blooming issues. The signal-to-noise ratio (SNR) drops, resulting in poor video quality.
The pixel technology borrowed from the security camera sector solves this dilemma. Full-color security cameras must operate reliably in a wide range of lighting conditions from bright sunlight to full cloud cover or shade. Excellent low-light sensitivity and high dynamic range are crucial parameters. Users require crisp, clear color images both indoors and outdoors.
Accurate security footage does not tolerate blooming, even in bright light. Blooming is becoming more of a concern in the notebook and tablet market as people venture out of their homes and bring their devices with them. Many people still feel safer outdoors than inside a building with strangers, and they want to be able to access work or entertainment on the go.
OmniVision has developed the PureCel Plus-S stacked die pixel technology to meet the requirements of security and mobile phone applications (Figure 1). Stacking enables maximum functionality in the smallest possible die size. Resolution doubles from 1 to 2 megapixels without increasing the X or Y dimensions.
Figure 1 PureCel Plus-S stacked die pixel technology improves sensitivity, angular response, and full-well capacity. Source: OmniVision
This next-generation pixel technology provides higher color fidelity and robust low-light sensitivity, along with a high SNR of 37.5 dB for crisper images. Additionally, PureCel Plus-S provides higher full-well capacity, zero blooming and lower power consumption.
It makes sense to incorporate this stacked die technology into clamshell devices. It can accommodate the Y size restriction of 3 mm for ultra-thin-bezel devices. A 1/7-inch optical format allows for maximum field of view while keeping with the required footprint. The Z height remains within expected thickness restrictions for portable devices.
In addition to a more compact pixel configuration, the pad geometry also maximizes the available space. All pads are located on the left and right sides of the die rather than around the entire periphery (Figure 2).
Figure 2 A compact pixel configuration helps optimize design space. Source: OmniVision
- Power consumption
Any feature that reduces overall power consumption extends battery life and reduces the risk of devices overheating. One such innovation is the “always on” setting for image sensors. The camera remains in a low-power streaming mode when not in active use. This allows it to respond quickly when activated while saving energy the rest of the time.
The “always on” feature provides the additional benefit of enhanced security (Figure 3). The camera can, for example, detect movement and blur the image of someone walking by in the background. This protects privacy when devices are used in public settings and also minimizes background distractions during video calls.
Figure 3 The “always on” feature offers a major venue for power savings. Source: OmniVision
- Signal integrity
Clamshell devices incorporate embedded MIPI cables to transmit data from the camera to the microprocessor. For optimum video quality, the signal should be as noise-free as possible.
However, electromagnetic interference (EMI) from antennas and other components can compromise signal integrity. One way to limit EMI is to bring the camera closer to the microprocessor to reduce cable length, but this doesn’t provide a positive user experience.
Since the microprocessor is located under the keyboard, placing the camera at the bottom of the screen allows for shorter cables. However, it’s a poor choice for videoconference applications, as the camera points up the participant’s nose and there is no way to achieve a good viewing angle. Anyone who has used a laptop for video calls probably agrees that the best position for the camera is at the top center of the screen. The designer, therefore, needs to work within this constraint.
One way to reduce noise is with multi-frame processing. For most notebook and tablet applications, a frame rate of 30 fps is sufficient. Designers can select a 60-fps image sensor—an option primarily aimed at applications that require complex image processing—but use multi-frame processing to screen out noise and output high-quality 30 fps video to the user.
A second solution involves spread spectrum clocking (SSC), a common way to reduce EMI. The SSC technology is usually incorporated at the system level to modulate data transmission through long cables. The problem is that clocking chips take up valuable space in an already constrained geometry and add cost.
By choosing image sensors with embedded SSC feature, designers can make MIPI cables as long as needed to position the camera and other components while keeping EMI low and avoiding the need to add more discrete components.
Future trends in portable video
The demand for high-quality, portable video is not going away. Here, image sensors are a necessary piece of the design puzzle as camera space for thin-bezel tablets keeps shrinking and performance expectations grow.
We expect to see continued increases in resolution to 4K and the challenge will be to shrink pixels further to keep the size small. This can expand camera capabilities in tablets, for example, enabling new features like center stage, which lets the speaker move around a room without walking out of the frame during video calls. Additionally, more advanced “always on” features will further extend battery life and may even enable monitoring of user performance and screen time.
Demand for portable clamshell devices will continue to grow. Fortunately, designers have tools at their disposal to meet the performance expectations of their users, whether they be students, medical professionals, or business leaders.
This article was originally published on EDN.
Akeem Chen is product marketing manager at OmniVision Technologies Inc.
