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


"dhs" <dhs@xxxxxxxxxxxxx> wrote in message
news:e2tmet$feq$1@xxxxxxxxxxxxxxxxxxxx
In article <jtc45258drbt2r6oa39tegn3kc3he5oi0d@xxxxxxx>,

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 ..)?

Most often video cameras are interlaced, so each field is taken in
in a separate exposure. If I understand correctly, the array on a CCD
is exposed all at the same time, then the image is passed to a CCD
buffer that sits right behind the one that sees the light (on the same
chip).
It is then clocked out into an analog to digital converter and then to a DSP
that processes the raw image into the appropriate format (NTSC or PAL)
and then passed on to the recording section of the camera. In the mean
time the front CCD is flushed and takes the next exposure. How long the
exposure is is dependant on the time between the CCDs being flushed
and the time the image gets passed to the buffer.

This may not be totally accurate,
but I think it speaks to the spirit of the process.


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?

Yes tubes scanned a "target" inside the tube in real time.
That is to say that if you were watching a live show at home,
your TV would be scanning out the top line of video at almost
the same time as the camera in the studio is scanning the
top line in the camera. It don't get more LIVE than that :-)

In contrast, CCDs grab the entire image at the same moment,
but then it gets scanned out of a buffer on it's way to be processed.
In the early days CCDs matched the format of the media to be recorded.
Now the size of the CCD array are pretty arbitrary. In most cases
the array is somewhat larger than the output signal calls for. At least for
standard definition that is true. I don't think all of the relatively
inexpensive
High Def cameras have arrays that are oversized yet. I think they are
counting on the DSP to make stuff up :-)

David

Thanks,
Doug


.

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