Recently I had lunch with a friend who’s new to cinematography. The D16 inevitably came up, and while we were discussing some of the aspects of the camera, he asked me, “how does a global shutter work?”
I thought this might be an interesting topic to talk about here. During this project, many people have asked us what the difference is between CCD and CMOS sensors, and why a CCD sensor usually has a global shutter and a CMOS sensor usually has a rolling shutter.
CMOS stands for Complementary Metal Oxide Semiconductor, which actually refers to the process for making CMOS sensors. The term for what we call CMOS sensors in the tech world is actually APS, or Active Pixel Sensor. This is a little confusing because APS-C (Advanced Photo System type-C) is a common sensor size in DSLRs so it’s easy to mix up the two terms.
Active Pixel Sensors read all of their pixels linearly from top left to bottom right while the shutter is open. The pixels don’t store any charge, they simply read how much light is hitting that pixel at the exact moment and convert that into an electrical signal. A rolling shutter (as opposed to a global shutter) is always active and “rolling” through pixels from top to bottom. This can result in the now-familiar motion artifacts often referred to as “jello.”
CCD stands for Charge Coupled Device, and unlike the APS sensors, the pixels in a CCD store their charge until it has been depleted. A camera that has a CCD sensor almost always also has a shutter system, which can be electronic or mechanical. While this shutter system is “closed” (or off) the pixels can still be read because they store their charges. While the shutter is open, the sensor is collecting light, and after the shutter closes, the AFE (Analog Front End) reads the pixel charges one by one, dumps any excess charge, and gets the pixels ready for the next frame. In another words, the CCD captures the entire image at the same time and then reads the information after the capture is completed, rather than reading top to bottom during the exposure. Because it captures everything at once, the shutter is considered “global”. The result is an image with no motion artifacts.
CCD sensors are double layered and transfer their charges from one layer to the next before the read out is conducted. It’s almost like two sensors sandwiched together. The manufacturing process for creating this kind of sensor is very complicated and often results in a significant portion of manufactured sensors having errors, so they must be checked very carefully at the factory level.
APS / CMOS sensors require less specialized manufacturing and because of this often cost a fraction of the price of CCD sensors of the same size. A CMOS sensor of a similar size to the CCD inside the D16 costs 4 times less than our sensor.
Most CMOS sensors have image processing functions within the sensor circuitry. Many CMOS sensors have built in functionally like signal amplification, noise reduction, and some even do the analog to digital conversion right there on the sensor. Programming the firmware for a CMOS sensor is much easier than programming for a CCD. With a CCD you must not only drive the sensor, but also setup a separate read time, shutter times, and an excess charge dump time. It has many more clocks running and they all must be synchronized, and not just once, but for every frame rate and every shutter speed / angle!
This also explains why CMOS sensors can have higher frame rates too, as a CMOS sensor can just keep reading images in an ever faster loop, where as CCD sensors must stop to read the pixels and dump the excess charges.
So you may ask yourself: if CCD sensors are four times more expensive, are significantly more complicated to work with, and have lower frame rates, why would any camera maker decide to work with them? Because of the look.
The pixels that are next to each other on CCD sensors effect each other. When one pixel overflows with energy it, effects the pixels around it. The pixels work together as a unit, much the way chemicals on a film plane do. We believe this gives the sensor a more organic look.
Although CCDs are not the only technology to allow for light detection, CCD image sensors are widely used in professional, medical, and scientific applications where high-quality image data is required. In applications where a somewhat lower quality can be tolerated, such as webcams, cheaper active pixel sensors (CMOS) are generally used. -Wikipedia.org
Because of the lower cost and ease of use of CMOS sensors, they have taken over the consumer imaging market, even if the images these sensors create can be wobbly and unclear. Since 2009, the market share for CCDs has been declining steadily as CMOS sensors have popped up in almost all of our personal devices, from cell phones and computers to back up cameras in your car.
CCD technology is reliable, and produces images that are precise and steady. And while CCD sensors might not be necessary for all applications, for cameras like ours that needs stable motion and the utmost clarity, there is no substitute.