[Product Innovation]
Digital-Camera Chip Set Delivers High-Quality Images At 50 Mpixels/s
A front-end/post-processing duo enables low-cost, low-power still or video CCD and CMOS cameras without compromising performance.
Customers are demanding film-quality digital cameras and high-definition digital camcorders at progressively lower prices. They also want to see cameras with longer battery operating life. In fact, consumers would like to carry just one camera with the ability to capture and then supply both high-quality still pictures and high-quality video. To meet these requirements, NuCore has introduced a two-chip set, comprising the NDX-1250 front-end device and the SiP-1250 smart image processor (SiP).
These two chips allow the design of low-cost and low-power dissipating digital cameras that use CCD imagers, without sacrificing image-processing speeds. In addition, the two-chip set is being aimed at newer cameras that use CMOS image sensors.
Applications for the NuCore chip set include digital video camcorders, digital still cameras, PC video teleconferencing cameras, PDA cameras, machine-vision front ends, medical imagers, and security cameras. The chip set is compatible with progressive as well as interlaced RGB complementary-color CCD and CMOS imagers.
In professional digital camcorders, where three CCDs are typically used for image sensing (one for each primary color), the chip set allows a single CCD imager to be used instead. No compromise is made in the image quality or in the image-processing speed. With most video cameras, excellent still-frame performance is seldom possible because of their low resolution. But, if video images are recorded at megapixel resolution, then it becomes much easier to derive excellent still images from existing footage.
This chip set enables designers to build a single camera that can acquire, enhance, compress, display, and store 4-Mpixel, silver-halide-quality digital still pictures continuously, at a rate of 12/s ("paparazzi-like"). The chip set also is able to readily capture 1 million-pixel video at 30 frames/s with 12-bit accuracy extended throughout the entire chip set. Therefore, this chip set is well suited for high-end consumer and professional applications.
Just One CCD Sensor NuCore's solution requires just a single CCD imager. In the past, the only way to obtain performance approaching that of the NuCore chip set required three separate signal-processing chains—one each for the red, green, and blue colors (RGB). In this design, R, G, and B are amplified individually despite the fact that the traditional, three distinct signal-processing chains have been merged into one.
Handling this extraordinarily high pixel rate required a specially designed, pipelined embedded processor, developed solely to perform image processing. The NDX-1250/SiP-1250 chip pair is said to be the only chip set that combines a mixed-signal, front-end chip with a back-end device, as a system. Virtually all of the necessary functions and algorithms are implemented in hardware on every pixel, one by one, at 20-ns intervals (Fig. 1).
Competing designs tend to combine one company's analog front end with another company's digital processing unit, which uses off-the-shelf digital signal processors or embedded cores. Such arrangements, however, don't come anywhere near the 50-Mpixel/s rate realized by NuCore's chip set. One reason is that the speed of these other solutions is hindered by the need to implement most of the image-processing algorithms in software, rather than hardware.
One major contribution to NuCore's high-performance solution is the way that the company successfully overcame the inherent limitations of the CCD. First of all, the dynamic range of a CCD is less than that of the human eye. A human observer can see both shadowed and unshadowed features in a scene that's illuminated by sunlight. But, that's not the case in single-CCD designs to date. Human observers have been unable to simultaneously capture details—in both the brightest and darkest parts of a scene. Without a high dynamic range, much of the detail is usually hidden by shadows or extreme brightness. In other words, a higher dynamic range yields more detail.
In most single-CCD cameras, the dynamic range problem is at its worst after the green adjustment, which also covers the color yellow. Once adjusted for yellow, when the CCD then tries to adjust for blue, there can be a tenfold difference. In such situations, the blue comes out almost transparent. So, this dynamic-range problem must be compensated for in software, after the analog-to-digital conversion, in the digital back-end chip.
NuCore's approach is different. It normalizes and changes the gain, pixel by pixel, at a very high speed (which at 50 Mpixels/s is every 20 ns). This, in effect, is an on-chip, dynamic-range expansion technique, to 32 dB, that enhances image quality under varying ambient conditions.
For portable, consumer battery-powered camera applications, low power is a critical requirement. Energy consumption per image for this chip set is very low. NuCore believes that its chip set extends battery life by a factor of three to five, compared to its closest competitor. Plus, sleep and decimation modes can be programmed to lower power consumption even more.
Figure 2 shows a block diagram of a typical digital camera using the two chips. Images sensed by the CCD (or the CMOS sensor) are first fed to the NDX-1250 analog front-end chip. This device performs various corrections before converting the analog information to a digital data signal. The data representing the image is then transmitted to the image processor, which performs additional signal conditioning and compression. From there, the data can be sent to a number of outputs stored in memory (hard-disk or flash), displayed on a TV or LCD, or fed to a PC over a USB interface. These interfaces are all built into the SiP-1250.
As shown, the timing signal supplied via an external clock is fed to the CCD, the NDX-1250 analog front end, and the SiP-1250 smart image processor. So, all three stay synchronized.
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