[Leapfrog: First Look]
Camera Sensors Capture The Cell-Phone Landscape
Equipped with higher-resolution image sensors, camera-based cell phones deliver higher-quality images and work at lower light levels.
Go ahead. Try and find a new cell phone without a built-in camera. It's not going to be easy. In fact, with these phones offering resolutions of 1 to as many as 5 Mpixels, they're replacing digital cameras in many applications.
All of this has been made possible through novel schemes in imaging chips that improve image capture in low-light conditions. They also provide enhanced image processing, which is critical due to the typical cell phone's limited compute power. The key element behind these schemes is a new crop of CMOS multi-megapixel image sensors.
For instance, the 3.1-Mpixel CYIWOSC3000A offers a 1/3-in. optical format (2048 horizontal by 1536 vertical elements) using 2.54 µm2 pixels and a HiSENS sensitivity mode for lowlight conditions (Fig. 1). Developed by Cypress Semiconductor, it's based on a three-transistor-per-pixel architecture that provides low-light sensitivity without increasing the pixel transistor count or sacrificing image quality.
The CYIWOSC3000A incorporates the company's Autobrite technology, which delivers clear bright and dark areas in the same image thanks to a 72-dB dynamic range. It cranks out from seven to 30 frames/s at full QXGA resolution-and up to 83 frames/s at VGA resolution. Also, its analog-to-digital converter provides a 12-bit/pixel output that's directed to a parallel data port. The port can be set to deliver 8-, 10-, or 12-bit data.
The core of the sensor is the activepixel array. The on-chip timing and control circuitry sequences through the rows of the array, resetting and then reading each row. In the time interval between resetting and reading a row, the pixels in that row integrate the incident light.
Varying the time interval between reset and readout controls the exposure. After a row is read, the data from the columns is sequenced through an analog signal chain (providing offset correction and gain) and then through the analog-to-digital converter. The pixel array contains optically active and light-shielded "black" pixels. These black pixels provide reference points for on-chip offset corrention algorithms (black-level control).
Programmable controls include frame size, frame rate, gain exposure, blanking, flip and mirroring, windowing, auto black-level offset correction, and panning. Pixels can be horizontally and vertically binned to improve sensitivity. A demo kit runs about $1000. In volume, the CYIWOSC3000A costs less than $10 each.
Not to be outdone, Eastman Kodak offers two imagers. The KAC-3100 is a 1/2.7-in. format CMOS sensor with 3.1-Mpixel resolution designed for cell phones (Fig. 2). The KAC-5000 offers a 1/1.8-in. optical format and 5 Mpixels for digital still cameras and digital video. Both employ 2.7- µm2 pixels and Kodak's Pixelux technology, which combines the use of pinned photodiodes, four-transistor pixels, and a shared-pixel architecture for high sensitivity under low-light conditions.
The pinned photodiodes are referenced back to ground, giving the pixels a reduced dark current. The sensors' true correlated double sampling scheme removes thermally generated noise, which improves the spectral response. With the shared-pixel architecture, four adjacent pixels can be grouped to create a larger pixel to better capture images in low-light situations. However, this scheme reduces the image resolution.
A thin copper-metal manufacturing process improves the optical angle response and achieves a high quantum efficiency in the sensor elements. Additional features in the chip include progressive programmable scan, electronic rolling shutter, and a digital video output with flip and mirror output options. In volume, the 3.1-Mpixel sensors sell for less than $10.
Another development comes from Pixelplus. Its 2-Mpixel PO1200N delivers 8-bit YCbCr, RGB565, 10-bit RGB Bayer, or 12-bit RGB888 data. This 1/2.7-in. format chip employs 3.3 µm2 pixels and has a 1600-by-1200 active pixel array with color filters and micro lenses. It can deliver up to 12 frames/s at full UXGA resolution and up to 30 frames/s at reduced SVGA resolution. One unusual feature integrated on the chip is its four-channel motor driver, which controls mechanical shutter, iris, and autofocus functions.
Proprietary algorithms cancel fixedpattern noise, eliminate smearing, and significantly reduce blooming. On-chip signal processing includes gamma correction, defect correction, a low-pass filter, color interpolation and correction, white balance, exposure control, and backlight compensation. Contact the company for volume pricing.
These developments follow the 2-Mpixel MT9D111 from Micron Technology (see "Low-Power Image Sensor Also Processes Pix," www.elecdesign. com, ED Online 10188). This sensor captures 15 frames/s at full 1600- by 1200-pixel resolution and 30 frames/s at 800 by 600 pixels. An on-chip microcontroller improves its ability to adjust color and perform other image-processing functions. Integrated autofocus and JPEG compression save system design cost and board space. It costs around $9 in lots of 10,000.
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