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CMOS chips away CCD costs
by David Fox
When cameras switched from tubes to CCDs, the business case was overwhelming. CCDs were smaller, cheaper, and so much more reliable. Even if they didn't initially offer better quality, they rapidly became ubiquitous. Soon, CCDs could go the same way as tube technology, thanks to the arrival of CMOS (complementary metal-oxide semiconductor) image sensors, which could dramatically cut the cost of camera heads, and offer higher resolutions.
The future for CMOS
CMOS has disadvantages, but they are being overcome. Indeed, Arri Digital Systems, which is developing a high-end digital camera using CMOS, believes the new chip will enable it to offer a viable alternative to film, for HD production and beyond, for the first time.
Other manufacturers also see an HD future for CMOS. JVC unveiled an HD CMOS camera at IBC 2003, while Ikegami had a prototype model on its stand.
Thomson also recognises the potential of the technology and is working on putting CMOS sensors in its camera line, although Jan Eveleens, head of the camera business unit at Thomson Broadcast and Media Solutions, has reservations. "It is too early to predict whether CMOS will completely overhaul CCD technology, which is also still making substantial progress. CMOS sensors that are available today still have some serious performance issues which effectively blocks implementation in high quality cameras where picture performance should be without compromises," he states.
However, for the smaller companies that don't have their own CCD production lines, the benefits of CMOS are notable - particularly that it can be produced economically in small quantities. CMOS is also more flexible than CCDs, as every pixel can be read individually, at any time.
Once it saw what the chip could do, Arri, which envisaged CMOS purely for use in a camera, used it to create its first film scanner.
Arri's experimental D20 camera is designed to feel like a film camera, with optical viewfinder, a chip the size of 35mm film, variable speed recording, and PL-mount lenses. Although the CMOS imaging chip could capture up to 6k images, in the camera its effective resolution is 3k, using Bayer mask oversampling, with high dynamic range and good colour fidelity. It was shown at IBC to get feedback from the market, with a product expected in 2005.
"It is an alternative concept for digital acquisition. It is also a test bed for a cine-style digital camera for TV-oriented applications," says Marc Shipman-Müller, camera technical marketing, Arri. He is not convinced that even with CMOS, digital can completely replace film, but for television, where cost and turnaround times are a consideration, he feels it is a viable alternative.
He doesn't believe that current HD cameras are adequate for film-style production. "You can't just take a news camera, give it a bigger chip and call it a movie camera."
The D20, for which development funding has come from the EU MetaVision project, has a modular design. It has a future-proof internal 10Gbps data bus and currently has three different backs: outputting real-time HD ("Which still looks better than current HD"); SD output; and a raw 24 frames per second data output, requiring post processing but giving even better images. There is also a special 72fps mode, using a Snell and Wilcox encoder, where the motion data from the additional frames help create higher quality slow motion and speed ramps.
CMOS can give higher image quality, because it can be read at any point. So, the camera can read one exposure after 10% of the light has fallen on the chip and another when all the light has, to give a higher contrast ratio. The D20 already offers a couple of stops more contrast than an HD camera (almost matching a film camera), but Arri is still working on this, so the final product should be even better.
The chip is bigger than the film frame, as this extra area is required by the Arriscan scanner to see the perforations for optical registration. With a CCD, the camera would have to read out all the chip, but with CMOS, "you have essentially random access to every pixel," says Shipman-Müller.
The Arriscan is not using the Bayer mask. "We need all the resolution that's in there," explains Arriscan product manager, Elfie Bernt.
The Arriscan 16mm and 35mm film scanner uses the CMOS chip to its full 3648x2162-pixel resolution. As the chip can see the perforations, it is ideal for archive film that could be damaged by pin registration. The scanner uses LED illumination, which can be flashed briefly at 10% to capture highlights - adding more detail to the full image capture. Each additional flash (up to four exposures - useful for high density print film) adds more information, going from 14-bit to 16-bit - Bernt says this would not be possible without CMOS, which is a non-destructive device and can capture the full dynamic range. It captures one frame per second at 2k or 3k, or 0.25fps at 4k or 6k.
It will also deliver workflow benefits by allowing colour grading to be done without tieing up the scanner. Available next Spring, prices will start at euro 330,000.
Pattern masking and signal to noise ratios
Ikegami is looking to CMOS for its next generation HD cameras. "One problem was that we couldn't manufacture multi-format cameras as we wanted to do. Sony, Panasonic and Thomson made their own CCDs, so they could do it. We do not," explained Michael Lätzsch, chief engineer for broadcast, Ikegami Electronics (Europe). This meant it was limited to single format HD cameras.
Although he says that CMOS has a signal-to-noise ratio too high for Pal or NTSC, HDTV has a poorer s/n ratio (about 54dB) so CMOS can be used.
Theoretically, it can handle any format, as it can even read out a single pixel, and can pan and zoom within the signal. It does have problems, such as fixed pattern noise. Because it is "fixed", it might seem simple to it remove in the digital signal processor, but it does alter gradually over time as the chip's temperature changes (which CMOS is prone to do), which makes it more difficult to deal with.
Arri uses special algorithms to get rid of the fixed pattern masking. It captures an image of any noise during the dark phase of the shutter and uses that to remove the noise from the subsequent image. It requires a lot of digital signal processing in the head. Although the current version of the chip is not temperature regulated, Arri's next version will be.
But, the key consideration is that CMOS costs only about 5% of the price of a CCD sensor, says Lätzsch. Given that a CCD head is about half the cost of a standard definition camera, and probably even more of an HD camera, he believes that the potential for cost reduction is significant.
"Our goal is not to have parallel structures for HD and SD, instead we'll just have multiformat cameras offering everything," he says. The small camera shown at IBC was a prototype, but he expects it to become a product during 2004.
The Ikegami chip should also find its way into the new hybrid film/HDTV camera created by Joe Dunton Cameras, which can use both the Mitchell Digital Magazine and high-speed 16mm film in the same camera - an Arriflex 16SR3. The magazine is being developed with Ikegami.
The camera would benefit considerably from moving to CMOS as it would reduce the size of the magazine and enable progressive shooting.
JVC already has a CMOS 1080i HD camera, the KH-F870, launched at IBC 2003, using three 2/3-inch 2.1 megapixel ProCam HD image sensors developed by Rockwell Scientific. Yasukazu Tanabe, JVC's general manager for planning, maintains that CMOS sensors are improving rapidly in terms of signal-to-noise ratio and sensitivity, and already outperform CCDs in some areas, particularly power consumption. The KH-F870 requires just 200mW, about 20% that of a comparable CCD.
"Additionally, SOC (silicon on chip) enables cameras to be more compact and smaller. As a result this facilitates us to design a wider variety of camera products. This is the reason why JVC is considering to use the CMOS sensor - particularly for HDTV cameras," states Tanabe.
The compact KH-F870 is an HDTV system camera that can be used for rostrum, remote and studio use, and outputs 1080i60/50 and 720p60/50. It has on chip 12-bit analogue-to-digital conversion and a high data rate (75MHz at full HD resolution). The sensor is claimed to have a high dynamic range (more than 60dB) and sensitivity, negligible blooming, no visible smear and low fixed pattern noise. "We are planning to use CMOS sensors in HDTV systems first while still using CCD for other camera products such as DV camcorders," he adds.
He believes that CMOS will affect the future of broadcasting, but it will take some time. "At the moment the sensitivity of the HDTV CMOS sensor requires some technical development to make it compatible with the sensitivity of CCDs. However, as can be seen in the photo industry, CMOS, which has higher resolution, as much as six million or more [pixels], is becoming increasingly popular. With the cost advantage that CMOS has over CCD, as CMOS sensor's general performance improves so will their popularity and wide-scale appeal. In time, the effect of these advantages will be felt across all industries."