How does a Global Shutter Work?

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?”

global shutter VS rolling shutter

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 SensorCMOS 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 SensorCCD 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.

CCD vs CMOS image pathAPS / 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.

Kodak Truesense FamilyMost 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!

CCD-VS-CMOS layoutThis 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

Sensor TrendsBecause 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 vs CMOS demandCCD 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.

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joerubinstein

About joerubinstein

Joe Rubinstein is one of the founders and CEO of Digital Bolex. At Polite in Public, a photo marketing company he also co-founded, Joe was the Chief Technology Officer who worked with electronics developers and software developers to create the Polite in Public Photobooth which helped define modern photo marketing services.

31 thoughts on “How does a Global Shutter Work?

  1. Pingback: Rolling Shutter vs Global Shutter: What's the difference?

  2. Pingback: The Sony F55 & My First Anamorphic Experience | Behind the Scenes Pictures

  3. Good article, I was looking for a simple description of CMOS vs CCD technology and the way that global shutter is achieved for CMOS. I’d suggest you are wrong to say that APS sensors in CMOS don’t store charge however- TV cameras would be massively insensitive if they didn’t (Bairds first “cameras” couldn’t integrate light as charge). APS sensors do store charge – the rolling shutter effect is caused because the period that they integrate (store charge) for- the “shutter speed” of the camera is not the same instance in time for all points in the picture

  4. Hi
    I looking for a good CCd camera and it is excellent article for me, but I am not clear in some specifications like Signal to Noise Ratio Maximum(SNR) and Absolute Sensitivity Threshold. Is there anybody who knows about thoese?
    for example I found one with 38.08 dB in SNR,Is it good?
    Thanks

  5. I have an interesting take on CCDs….Light has different frequencies/wavelengths in the colour spectrum. What happens in a CMOS sensor is the clock which is designed to pick up the frequency, picks up only a singular frequency at a given point of time. But in reality, Red has a longer wavelength and every exposure of Red frequency will be an equivalent of several blue frequencies. CMOS is not designed to capture all the blue frequencies at the same time because it’s clocked, and it can only pick up one signal at any given point. CCD on the other hand retains the charge for all the frequencies; giving an exact value of colours. That’s why Blue looks like Cyan in a CMOS and the cannot handle variations in Colour Temperature/Green-Magenta Shift; and why CCDs look so better. CCD is closer to what the film does. It’s be interesting too see if CCDs can somehow handle the overloaded charge into exact values (rather than dumping them). Please correct me if I’m wrong.

  6. Hi…to all, I liked the conversation and I thank joe for giving valuable inputs. ..i hope CCD will capture the market again with global shutter in future cameras(fingers crossed)

  7. Joe,
    I just shot some footage with the Weisscam (single Cmos but global shutter) at 1000FPS of fruit falling. We played with several shutter angles to help blurring but still felt it was too much. Perhaps this was more an issue of the LCD we were watching it on than the capture device.

  8. Hi Joe,
    I have shot 16mm film for years and am really drawn to the look of the D16 over the Blackmagic though I have never seen either in person. I did see the 4k Blackmagic price point for the camera was not so bad, BUT the cost of getting lenses for it to replicate what I already own in c-mount lenses about gave me stroke. I know it is early, but is there any talk (or room in the D16 camera body) to “up gun” from 2k to a D16 4k model? Or do you think it would even be needed for independent film shooting using vintage film lenses? PS I assume you have shot some test footage with a vintage Kern/ Switar lens or two. How did it look?

    Lance

    • Just adding more why D16 is of great interest to me, when I shoot a living history, airshow or other live event I often use my vintage c-mount 12-240mm Angenieux zoom to cover whatever I need ( no time for changing primes), so I am jazzed it will fit a D16. To replicate this zoom range ( well not quite as great a the range) on a Blackmagic I saw lens priced in $20,000 to $40,000! Ugh! So in this type of shooting the Blackmagic camera could be free but the lenses ( I like to own my equipment) put it out of site pricewise from what I have seen so far.

      Lance

    • Hi Lance, I am right now preparing a blog post about 2K vs 4K for independent cinema, so look for it soon, but I do not think 4K is necessary for most independent films. We are going in early July to test the lenses we designed on the camera, and some vintage lenses too, including an Angenieux zoom, and of course some Switars 🙂

  9. Vertical smear should only be an occasional problem with extreme overexposure. The smear suppression spec is 300x full white clip point, or the equivalent of 6,000,000 electron volts over the 20,000 ev clip point.

    • Exactly true! And modern CCD have “channel wells” intended to reduce vertical streaking even at that point, but people will point the camera at the sun and still get it sometimes.

  10. Hi there, hope all is well.

    When are you going to start releasing more footage from the camera? Its coming out this year yet theres nearly no footage available. I’ve looked at what is available on your site and Vimeo.

    Thanks

    • Hi Jimmy,

      Very soon! If you take a look at our last post we are now getting clean images off the sensor, but there is one or two more small steps before we can show footage, but it will be soon!

  11. I’m waiting for the rental houses to carry the D16 so personal testing can be done. This camera is being built by filmmakers and the design shows it! However, the Blackmagic Production Camera 4K also has a global shutter. I’m waiting for the rental houses to carry this camera so personal testing can be done. I evaluated the original BMCC last January and while the weather did not cooperate, the camera’s firmware sadly under performed, almost being useless.

    • Hi Frank, it may be a little while before the D16 is readily available at rental houses. The D16 and the 4K Blackmagic will have very different aesthetics and I believe both will be good tools, but maybe for different projects 🙂

  12. Excellent post. I agree that the vertical smear is preferable to the unbearable rolling shutter artifacts of at least some cameras. To me it almost has some cinematic quality to it. Even though this doesn’t make any sense. Maybe because the smearing resembles a little of the anamorphic lens flaring, but tuned 90 degrees? ;-]

  13. I’m all for CCD sensors except for the vertical smearing when bright lights are in the frame. Is that still a problem with CCDs and present in the D16? And if so, how evident is it?

    • Yeah I’m sure there will still be vertical smearing with the sensor chosen for the D16 as there is with almost all CCDs, but because of excess charge dispersion channels it is significantly reduced compared to older sensors. And to me if the choice is between vertical smearing and rolling shutter I’ll take the vertical smear every time 🙂

      • In he ’90s, I owned and operated several Ikegami broadcast cameras. They were equipped with three FIT CCDs (Field interline Transfer) and these were totally immune to vertical smear. Our competition in the broadcast TV commercial market used Sony cameras and many clients preferred the ‘look’ of our Ikegamis.

  14. I love that you’ve posted about why CCDs are better than CMOS sensors. I’ve always loved CCD cameras, and it’s no wonder that my old CCD-equipped Panasonic HPX2000 ENG cam is still my go-to body for high-end jobs. The look it provides is far more organic and “film-like” than any of my other digital cinema cameras.

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