[opendtv] Re: CMOS sensors and rolling shutters

  • From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Thu, 31 Jan 2008 09:18:57 -0500

Wobble was actually caused by a disconnect (for the first time) between the portion of the frame being exposed in a tube camera and the portion being presented on a picture tube. The moving electron beam in a camera tube discharged the target at the point of beam contact. Exposure then began at that point until it was discharged at the next sweep. There was no line-by-line exposure; it was instantaneous along the scanning line as well. But, regardless of whether the video was transmitted live or recorded, the light appeared on a picture tube in the same beam-sweep order. If a camera panned, the same thing that happened at the target of the camera tube happened at the faceplate of the picture tube, with the same positional timing.


When CCDs captured images, they did so all at once (and the first CCD cameras had mechanical shutters very similar to motion-picture-camera shutters). But picture tubes continued to start at the upper left and draw scanning lines. That's what led to the wobble effect, the discontinuity between the timing at the image sensor and the image display.

CCDs transfer charge in different ways. Frame-transfer chips transfer through the photosensitive area, requiring a mechanical shutter to prevent exposure during transfer. Interline-transfer chips have theoretically non-photosensitive areas between scanning lines, eliminating the need for a mechanical shutter. But excessive exposure can stress them, which is where the vertical smear described comes from (though later designs have essentially eliminated it).

TTFN,
Mark


John Shutt wrote:
According to the article that Dave posted, CMOS devices don't have a physical shutter, either. The way the image is clocked out of the device line by line while letting subsequent lines continue to gather light information is referred to as a "rolling shutter" and is the cause of wobble.

A quote:

"A ROLLING SHUTTER is very different. The rolling shutter actually exposes different portions of the frame at different points in time, "rolling" through the frame. Again, it's not an actual physical moving shutter that's doing this (as opposed to a movie camera, which actually has a moving physical shutter). Instead, the sensor is telling different portions to become light-sensitive at different moments in time, and as this process proceeds down the course of the full frame, until the entire frame is exposed."

This to me sounds exactly the same as how a tube camera's electron beam scans the target and produces a signal. Hence Dave's question.

John

----- Original Message ----- From: "Mark Schubin" <tvmark@xxxxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Wednesday, January 30, 2008 6:03 PM
Subject: [opendtv] Re: CMOS sensors and rolling shutters


Tube cameras did not use shutters.

Some CCD cameras did (and do). And skew and wobble may be seen on CCD cameras, too.

TTFN,
Mark




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