This page contains a Flash digital edition of a book.
52 TVBEurope The Workflow April 2012

What’s Grass Valley showcasing at NAB? See page 63>> and at

Time to move to CMOS imaging?

CMOS is ready as a technology to challenge CCD in broadcast cameras, says Klaus Weber, director of Product Marketing, Cameras, for Grass Valley

SOME MAY find the question posed in the title of this article strange. To be sure, CMOS imagers are widely used in cameras today. It is certainly true that nearly all still cameras and camera phones use CMOS sensors. So too do the latest breed of 35mm equivalent digital cinematography cameras. The thing they all have in common is that they are single chip designs with colour separation on the chip, usually by means of the Beyer pattern. What I propose for your consideration, however, is the high-performance broadcast camera — the system camera we rely on for crystal clear pictures of The X Factor or rugby internationals or music festivals. These have always relied on three discrete imaging channels — red, green, and blue — and we believe they will for some time to come. But today, the latest generation

of CCDs found in system cameras is the last generation of CCDs. Since its invention in 1969, and its use in our broadcast

cameras since 1987, CCD technology has gone through much development, but it has now reached its practical limit. There are a number of

reasons for this, but just one for our discussion: the design of a CCD means that all of its information — the number of electrons in each photosite — is read from a single point. For a

at 330MHz, and that is impractical. We have reached the end of the line with this aspect of the CCD architecture. The alternative is the CMOS

imager. Its construction has an amplifier and output for each photosite, so the bandwidth problems disappear. All ultra motion cameras used in broadcast today have CMOS

While the CCD has been taken about as far as it can go, the CMOS imager is still blazing a trail and will get better in the very near future

full HD picture, that is around 2.2 million values. Multiply that by 50, for 50fps, and you need a bandwidth at the output node of 110MHz.

Super slo-mo cameras are

now the mainstay of sport and other television. To serve 1080p with triple speed cameras means sampling each imager

imagers for this very reason: there is no practical limitation on imager speed. The history of the CMOS

device actually goes back as far as the CCD, but its development as an imaging device is very much more recent, which means we are at a different point in its evolutionary curve. While the

CMOS with full exposure

CCD has been taken about as far as it can go, the CMOS imager is still blazing a trail and will get better and better in the very near future. In 2007 we developed our

own CMOS imager, the Xensium. It contains 2.4 million photosites, for a full 1920x1080 pixel raster. Physically, the target

CMOS with short exposure

is the same size as the CCD, so fits into the same optical path with no impact on lenses. It is used in our three chip multi- format LDK 3000 HD camera. The way that a CMOS device

is fabricated is very different from a CCD. CMOS devices are made very much as other silicon chips are made, which means that other components can be built onto the same substrate. As already noted, there is an amplifier for each photosite. We also incorporate the

analogue-to-digital conversion on the same chip. To reduce the clock rate two 14-bit A/D- converters are used — one for the odd rows and the other for even rows. For an improved FPN (fixed-pattern-noise) behaviour we use a newly- developed technique called double digital sampling (DDS). This quantises the charge on the photosite at the beginning and end of each frame and subtracts the first from the second. This eliminates any bias in the pixel due to residual or leaky charge, and ensures a much more accurate image.

Powered up This integrated design means that overall the chip uses less power than the CCD with its high-speed switching. In a chip, power means heat and our CMOS imager runs several degrees cooler than our CCD imager. In turn, reduced heat

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84