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Optoelectronics/Displays??

Less pixels mean better Webcam performance

Posted: 16 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:Webcams? Shone Tran?

With the proliferation of integrated notebook Webcams, designers face the challenge of balancing the actual consumer need vs. marketing seesaw. Webcams risk heading down a path similar to digicams: the drive for more pixels (higher resolution) when there really isn't a compelling need for one. In reality, going down this route can severely degrade the end-user experience because packing more pixels into smaller sensors translates into smaller pixels, which decreases low-light performance.

This article discusses key issues related to integrating webcams into consumer notebooks, including how to maximize the performance attributes affecting image quality: size, low-light performance, resolution and frame rate, while keeping in mind the limited broadband infrastructure and the typical applications/environment in which consumers operate their Webcams.

Designers must consider the two typical characteristics about the environment in which consumers use integrated notebook Webcams: over the Internet on instant messaging (IM) and in unfavorable lighting conditions like the home or office.

Notebook PC vendors are rushing to integrate Webcams as online IM achieves critical mass. While the Internet is powerful enough to connect users, it presents one major restriction to the performance of a webcam: limited bandwidth. A corollary to limited bandwidth is the fact that different users have varying connection speeds. IM clients take this into account by compressing already low-resolution video down to manageable sizes to transport through the Internet for the average connection speed.

Now, let's evaluate the user's typical environment!the home or office. The typical camera flash emits 2,000W of light to properly expose a picture. A typical home or office environment uses lighting from 100-150W light bulbs, perhaps with multiple bulbs. This order of magnitude difference illustrates that users are operating webcams in unfavorable lighting conditions.

Make or break
The designer should probably start from the mechanical constraints and choose a Webcam architecture that will meet the size constraints put on by the laptop bezel, while satisfying performance requirements. Choosing the proper architecture can make or break a Webcam design, as each has its pros and cons. Consider the following architectures:

SoC + USB!This is a two-chip solution that uses an image sensor integrated with image processing (SoC) and a high-speed USB peripheral controller.

Image sensor + back-end chip (ISP + JPEG + USB)!Also a two-chip solution, its difference is having the image sensor standalone while the ISP portion is integrated (sometimes with JPEG compression) with the USB controller.

Image sensor + ISP + USB!This is a one-chip integrated solution that has currently gained little traction because it is limited to full-speed USB, ultimately limiting the resolution. These one-chip solutions are potentially limited to Full-Speed USB, because Hi-Speed USB uses extremely fast signaling, which generates significant heat.

The following are key considerations in Webcam design.

Resolution!A major consideration is whether there is a need for videoconferencing in high resolution. Higher resolution enables blowing up an image for editing or printing, which are moot points when the webcam is used for video capture. Consider the average laptop or LCD, which run on 1024-by-768. Such a display would not be able to show an entire 1.3Mpixel picture. Also consider the usage model of Webconferencing, where users are usually multitasking. Users wouldn't want a full-screen video, because they need to access e-mail or view a document that they're discussing.

Optical format!This attribute simply specifies the diagonal length of an image sensor. It is an important consideration, as notebook PC vendors are trying to fit Webcams into the small bezel of a notebook LCD. Ideally, a designer will choose the largest optical format that will fit within physical limits, while using the largest pixel size at a reasonable resolution.

Frame rate!Typical VGA CMOS sensors found in Webcams can capture video at 30fps in full light. But increasing integration time can easily reduce the frame rate below 20fps, where the human eye, which sees fluid motion at 24fps, will perceive choppiness. Minimizing the need for integration time to improve low light will keep the camera from having to run lower frame rates to compensate for poor low-light performance.

Low-light performance!This is a characteristic of Webcams that is difficult to measure. Designing for maximum low-light performance requires pinpointing the factors that most directly affect it. Pixel size is directly related to low-light performance because the bigger the pixel, the more light it can collect, thus improving exposure. Given a constant optical format, typical pixel sizes for VGA sensors are in the 5.6?m range, while 1.3Mpixel sensors are in the 2.8?m range.

USB throughput!Webcams are ubiquitously connected to PCs via USB connections and is the method of choice even in integrated notebook Webcams. To ensure smooth video, the USB 2.0 specification allows for dedicated bandwidth via isochronous transfers. The maximum theoretical throughput using isochronous transfers is 24MBps. While Hi-Speed USB 2.0 is a standard, not all USB controllers are created equal, and not all of them will produce robust, uninterrupted 24MBps isochronous transfers. Choosing a programmable and reliable, high-speed USB peripheral controller ensures maximum frame rate and easy upgrading to accommodate the ever-improving offering of image sensors. Most importantly, USB becomes a bottleneck above VGA resolution.

Heat!An often subtle and overlooked consideration is the problem of heat and its effect on image quality in a Webcam. Heat often causes unwanted noise, showing up as graininess and blurry edges on images. Chip selection will be an important consideration to battle the effects of heat on image quality. The USB controller generates tremendous amounts of heat due to high-speed signaling. Given the confined space, designers need to design the board to separate the USB controller as far as possible from the image sensor, and choose a USB controller with the lowest current consumption.

Dynamic range!This refers to how well the image sensor can handle extremely low and bright light. When evaluating image sensors, this is an important characteristic designers should benchmark because it is very likely that a lamp in the background can ruin the video capture. An image sensor with poor dynamic range can wash out on brighter parts of the picture while an image sensor with good dynamic range maintains composure.

- Shone Tran
Product Manager, Consumer and Computation Division
Cypress Semiconductor Corp.




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