ELPH 115 IS uses a CCD?

I've been using lots of different PowerShots with CHDK for many years, and have had students do many projects programming them.  Last year, I got a fleet of A4000 IS for student use in my "Cameras as Computer Systems" course... but with more students this semester, I added a fleet of the apparently very similar ELPH 115 IS.  However, in preparing background materials for my students, I noticed that Canon's WWW site claims the ELPH 115 IS uses a CCD!

Now I'd have bet it uses a CMOS sensor, and that's also what I assumed was in the A4000 IS (which Canon does not say uses a CCD).  However, I'd really like to know.  Is the ELPH 115 IS sensor a CCD? Are other PowerShots, including the A4000 IS, CMOS sensors?

I asked Canon tech support and they said:

It is true that both the PowerShot ELPH 115 IS and the A4000 IS have the same sensor:

16.0 Megapixel 1/2.3-inch CCD

Ok. I'm still a little surprised if it's really a CCD....

Gonini wrote:

Whats wrong if it uses a CCD?.... Quite a few P/S still use CCD.

It's not so much that it's wrong to use a CCD; they might be an older technology, but still can make excellent sensors with high QE (quantum efficiency) and no rolling shutter effects. However, CCDs tend to be more expensive to fab and it is harder to do things like live view and video.

A CCD is an array of analog shift registers in which you generally have to read every cell to read any cell in a line. In contrast, CMOS sensors easily could have individual pixels (sensels) directly addressable. The live view on the camera is nowhere near the (16MP) sensor resolution, so the refresh rate can be increased dramatically by only sampling a tiny fraction of the sensor pixels (sensels) for it. The same is typically true of video -- you don't need 16MP for 720 HD video, and very few cameras shooting video actually look at all the pixels and scale the image down.  Judging by the performance of the raw sensor system as exposed using CHDK, the A4000 IS and ELPH 115 IS apparently are not capable of filling the sensor's raw buffer at the live view or video frame rates. Further, video and live view image capture go through very different code paths from still capture in the camera's firmware and live view doesn't overwrite the sensor's raw frame buffer (and there isn't space for a second full-resolution raw frame buffer in the camera's memory)... which strongly suggests that only a tiny fraction of the sensels on the  sensor are being sampled for live view -- again, something CCDs normally can't do.

In sum, if it really is a CCD, there must be some interesting readout logic somewhere. Perhaps the readout is fast enough that just skipping lines works, but that would imply still reading an entire line when you only needed a tiny fraction of its values. I believe that's usually been the trick, but that's also why a lot of early cameras had terrible live view refresh rates. Maybe line readout is now fast enough? I don't see image quality signs of live view and video being computationally scaled down -- scaling would improve S/N ratio, but noise levels appear no better than if only a sparse sampling of sensels was performed, so perhaps digital logic is only saving the values for the desired pixels?

I'm using these cameras in a computer engineering course I'm teaching (largely using CHDK) about the computing aspects of cameras, so I'd really like to know what is going on at this low level. Obviously, from an ordinary user perspective, you shouldn't much care what sensor technology is used as long as the camera does its job well enough....  Perhaps I should have posted this in the tech forum or on the CHDK wiki?

The PowerShot cameras with the "HS System" have the CMOS sensors, and generally have the "HS" as part of their name; e.g., "ELPH 330 HS IS".

However to add to the confusion Canon does not include the "HS" in the camera's name for all PowerShot cameras with the HS System, i.e., Current D-Series and N-Series.

In just looking at the Current PowerShot Cameras none of the current ELPH 1xx cameras have the HS System/ CMOS sensor. I still have my ELPH 100 HS.

BTW per the PowerShot A4000 IS Specs it has a CCD Sensor.

Other item I noticed is the CCD sensor is only 720p HD video, whereas the HS/ CMOS sensor is full 1080p HD video.  See the camera's specs for info on video capabilities.