Re: CCD scanning/exposure (was Re: Canopus ADVC300 ... )

On 4/28/2006, dhs posted this:
In article <jtc45258drbt2r6oa39tegn3kc3he5oi0d@xxxxxxx>,
Martin Heffels <mot@xxxxxxxxx> wrote:
On Wed, 26 Apr 2006 15:37:36 GMT, "David McCall" <david.mccall@xxxxxxxxxxx>
wrote:

Well sort-of. If you are talking about those new fangled video cameras
with their CCDs, then yes.
Not sort of... With PAL interlaced video, the CCD/CMOS is refreshed every
1/50th of a second. Film when shooting for cinema, is only exposed for
1/48th of a second when you use the standard 180 degree shutter. So the
difference in exposure-time between these two is negligible, and makes no
difference in motion-blur. NTSC will have a little less blur in this
instance, but to compensate the actors should move faster, and the pans
need to be done faster ;-)
When shooting television commercials or series on film, there will be no
difference anymore in motion-blur, as these are shot at 25/30 fps.

This may be a stupid/frequent question, but I'm going to ask
anyway. For digital cameras, how are the CCDs exposed/scanned?
Film, of course, is exposed (and "scanned") nominally a frame at
a time, modulo the speed at which the shutter moves. But what about video cameras? Are the pixels in a given frame
all exposed at about the same time as with film and then scanned
over 1/30s (or 1/60 or 1/50 or ..)?

The original video cameras were raster-scanned devices in which
exposure = scanning. Are the CCDs in digital cameras more like
this or are they more like film?

Thanks,
Doug

(A non-expert view)

The sequence goes like this:
1. Light gathering is turned on in the sensors (pixels).
2. Electrons accumulate in each pixel in proportion to the amount of light falling on it.
3. Light gathering is turned off in the sensors.
4. For each pixel, a signal proportional to its number of electrons is transferred to wherever it goes (the A to D onverter, for a digital camera). This probably takes only a millisecond or two for the whole CCD or CMOS.
5. The process idles until it's time to start over at #1.

The time from #1 to the next #1 is the appropriate field or frame time for the video method used.

The time from #1 to #3 is the shutter speed.

I would expect that if the shutter speed grow beyond a field time or frame time, the rep rate must slow down (because I can't figure out how to do otherwise!).

Those who actually know, please correct the above.

Gino

--
Gene E. Bloch (Gino)
letters617blochg3251
(replace the numbers by "at" and "dotcom")


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