CMOS: New Imager on the Block
In the same way that only the old-timers talk today about shooting with tube-type video cameras, in a decade it may be only the old-timers who remember CCDs. At least that's the scenario developers of CMOS technology are pursuing.
If the name CMOS (pronounced SEE-moss) rings a bell, it's because this Complementary Metal Oxide Semiconductor is the technology popularly used to make computer processors such as the Pentium.
As a substitute for CCD chips, CMOS imagers allow a camera with lower power consumption (and less resultant heat to dissipate), lower signal-to-noise ratio and smaller overall design. CMOS sensors are also more resistant to cosmic radiation, and allow for creation of multiformat cameras.
And because CMOS manufacturing is far more widespread than that for CCDs, and because the processes necessary to make CMOS imagers are not much different from microprocessors used in computers, they can be expected to soon cost less than CCDs.
"We have to purchase from a very limited number of CCD vendors because there just aren't that many companies out there that make the CCDs," said Dave Walton, JVC Professional Products national marketing communications manager.
Walton sees CMOS imagers as a viable alternative to CCDs, and said he feels their capabilities may push CCD manufacturers harder. "Competition never hurt anybody, and we feel as a manufacturer we should be willing to take a look at everything out there, and CMOS definitely has some advantages."
To that end, both JVC and Ikegami debuted three-chip, high definition CMOS imager cameras at NAB2003. Both were box-style cameras, taking advantage of the space-saving character of the CMOS imagers.
IMAGERS FROM SPACE
Rockwell Scientific Co., which has previously made CMOS imagers for specialized applications such as astronomy (including the Hubble Telescope) and other governmental and military tasks, supplied its ProCamHD sensors to JVC and Ikegami for their cameras.
Rockwell's manager of CMOS sensor design, Markus Loose, described the difference between CMOS and CCD sensors. "On a CMOS chip you do much more than on a CCD chip," he said. "You have analog-to-digital converters on it. You have the control logic on it. It is, in fact, a single-chip camera, so to speak."
Douglas Howe, Rockwell director of business development added that CCDs need two or three additional external chips for all processing. With CMOS sensors, a lot of that processing happens directly on that die. "So that saves a lot of room, [has] lower power consumption, and it makes a smaller footprint."
Another difference between CMOS imagers and their CCD predecessors is the number of amplifiers used. A CCD (Charge Coupled Device) passes the charge received from sensing light from pixel to pixel and finally through an amplifier. A CMOS sensor has a separate amplifier for each pixel. An amplifier for each pixel allows the CMOS sensor to avoid a problem CCDs experience when the number of pixels, and thus resolution, is increased. "If you have to feed all the pixels of a CCD through one amplifier, the amplifier has to run at the whole pixel rate, which makes it a very noisy amplifier," said Loose.
"In CMOS, every pixel has its own amplifier, so all the amplifiers are working in parallel and they can run much, much slower," he said. "So their noise will be much less and that becomes very significant when you go to high frequencies."
With all these advantages, it's fair to ask where CMOS image sensor technology has been hiding all these years. Loose points out that people have been thinking about CMOS sensors for many years, but had to wait for the manufacturing process to become capable of making more microscopic components on the silicon chip.
"Five years ago, people were [manufacturing at] 0.8 microns and 0.2 microns, and maybe smaller ones. These processes didn't allow building of very small pixels and overall small designs," he said. "If a lot of the transistors are big and bulky, you can't build small pixels.
"With the shrinking of the processes to quarter micron, 0.18 micron and so on, suddenly it became feasible to build small pixels and integrate all the functionality that you need in order to achieve good performance that's comparable to CCDs."
Without initial demand for CMOS sensors from the broadcast, professional, industrial or consumer marketplace, developing the devices also took a well-heeled customer with a special need.
"When NASA would bring back a video camera from the Space Shuttle using CCDs, they would have to replace the CCDs before they sent it up again because the CCDs would be destroyed by the radiation," said Howe.
The villain is cosmic radiation. "If that hits a single CCD pixel, that pixel becomes inoperable, and that means you have to ship all the charge from the other pixels through that one particular pixel," said Loose. CMOS imagers are characteristically less susceptible to radiation damage and can be further hardened against it.
"Rockwell Scientific has a little over a decade's experience in CMOS design and implementation in very large, high-performance arrays for scientific applications," said Howe. "Very low volume, very expensive sensors."
The company has been able to take the CMOS technology it developed for these specialized applications and pair it with advances in CMOS manufacturing to enter the HD video market.
"Our strategy is to enter at the high end, and studio broadcasting, that's about the highest-quality requirements today in the video marketplace," said Howe, "and then move toward the middle-market applications, which are a little bit higher-volume and can help us to reduce costs even further."
To get its CMOS imager product ready for NAB, Ikegami replaced high-definition CCDs in an existing product, the HDL-40. Small as the HDL-40 is at approximately 3.5x4x7 inches and 4 pounds, company VP of Engineering Alan Keil says it doesn't fully take advantage of CMOS's ability to reduce size.
"We fitted [the CMOS sensors] into the HDL-40 because it was convenient and available for NAB, but the final product will be somewhat further reduced in size, also distinctly lower in power consumption," he said.
At least one other camera maker is working with CMOS imagers. "We have working CMOS camera prototypes in our labs," said Jan Eveleens, Acquisition and Production business unit manager for Thomson Broadcast and Media Solutions.
"There are still lot of issues with the current state-of-the-art sensors and technology, which prevents application in high-end, high-quality broadcast cameras," he said. "We believe these issues will be addressed in the coming years and thus it is likely that CMOS sensors will finally also be used in broadcast cameras."
Manufacturers say the it's too soon to write the epitaph for CCDs. But with clear advantages of lower power consumption, possible lower manufacturing costs and high performance, CMOS is definitely about to make its mark on the video camera industry.
"Based on some of the response we got at the [NAB] show, we're looking at some other capabilities of including the CMOS in some other types of cameras," said Walton.
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