Saturday, March 15, 2008

“Why CMOS over CCD?” I ask my toaster for answers.

Rolling shutters and noise: why do we tolerate CMOS sensors?

The other day I found Harry, the Civil Toaster ("civil" not as in kind, but because he was in the civil service) in the driveway tossing corn chips.

“Harry… dare I ask what you’re doing?”

“It’s very simple, white bread,” he said. (Are toasters naturally rude, or is it something they develop over time?) “I read that CMOS chips have a rolling shudder, so I’m trying to find one by rolling chips and seeing if I can induce a seizure. If a chip starts shuddering, I’m halfway toward making my own camera!”

“Harry, you’re smart for a toaster. Which means if you heat up when you’re plugged in you’re already at the median for toaster intelligence, so I’m not saying much.

“What I think you’re talking about is that CMOS chips have a rolling shutter. CCD chips generally grab data in one fell swoop, after which the data is read out of the chip, the chip is blanked, and it’s then ready for another exposure.

“CMOS chips expose one horizontal line at a time, from the top of the frame to the bottom, and while they do it very quickly it’s not unusual to see artifacts from this type of scanning. Generally this shows up as slanted verticals when a camera is panned quickly. That’s because a vertical line will appear to be in one spot when the scan starts at the top of the frame, but will appear in different spots as the camera continues to pan and the shutter moves down and exposes successive lines.

“They can also be a bit noisy. CMOS sensors tend to have a lot of blue noise in the shadows.”

“Huh,” said Harry, taking his bag of chips into the shadow of the garage and looking for any that exhibit blue speckles. “So why use CMOS chips, then?”

“Well, they tend to require less power than CCD chips do. The Sony EX1, for example, would run extremely hot if it used CCD chips instead of CMOS. It deals with noise by putting the analog-to-digital (A/D) converter, which converts the chip’s analog signals to digital, right on the chip. Traditionally the A/D converter is separate from the chip and connected by wires, which can act like antennas and pick up transient signals from inside the camera and make the noise problem worse. By putting the A/D converter right on the chip the noise floor is reduced dramatically. There are solutions developing for these issues, and I’m sure we’ll see huge advancements in the near future.”

“I’m going to beat them all,” said Harry. “All I need to do is find a corn chip with low noise on the floor and I’ll be way ahead of the game.” And so he set out to find a quiet chip, snapping them one at a time while laying in the shade of the garage door.

“By the way,” he said, “from now on I’m changing my name. I’m going to be Harry the Video Toaster. Once I build my camera I’ll be famous, and everyone will shiver and quake at my name! I’m even considering joining the ‘League of Unlikely Media Superheroes.’***”

“Harry,” I said, shaking my head, “you’re twenty years too late. The original Video Toaster has long gone”

Harry looked me straight in the eye: “The Toaster will rise again!” I couldn’t tell if he was serious or if I was the victim of what he calls his “rye humor.”

***Foreshadowing alert. The reader is informed that this will pay off in time.

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