[opendtv] Re: CMOS sensors and rolling shutters

  • From: Tom Barry <trbarry@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 01 Feb 2008 22:15:02 -0500

Meanwhile, if you are compressing the picture in progressive MPEG-whatever then you are making an assumption of a series of stills. Likewise doing a frame rate up conversion or displaying each image multiple times, say 120p.

So I hope compatibility issues with legacy slow scanning cameras don't get overly built into our new digital capture culture. It could end up as bad as interlace.

Imagine SMPTE standards stating that until forever the lower half of an image must always represent a time 1/24 second later than the top. Yuk!

Of course faster rates would still help.

- Tom

Jeroen Stessen wrote:


Tom Barry:
 > So it would seem that any frame-at-a-time capture designed for a fixed
 > pixel display would not look right on a scanning CRT, and vice versa.
 > Why isn't this more of a problem?

Because, and this is important, also most matrix displays are
scanned from top to bottom. It takes almost a frame period to build
up a picture on an LCD. The only exception are displays that use
pulse width modulation by means of weighted sub-fields to create a
grey scale. The two types that I know of are plasma and DLP.
They build up the sub-fields at a much higher frame rate, so that
for the eye it seems as if it is generated instantaneously.
But they come with their own class of motion artefacts, probably
bigger than the one we are discussing here...

So, IF the camera and the display are scanned synchronously, then
there should not be any motion artefacts. But the artefacts will
appear if you display the pictures in an unintended way, like
freezing an individual frame. Then it appears to the eye as not
being scanned at all, and you can see (only) the camera artefacts.
Also zooming a picture (showing it in a small window on your
monitor) disrupts the synchronisation between source and display,
not to mention the fact that the refresh rate of the PC monitor
is usually not synchronized to anything external.

This is btw also how interlace gets a bad name, because people
are capturing stills from a moving program, instead of showing
the fields at their intended refresh rate. You don't see quite as
many artefacts if you just view 1080i on a 1080i CRT (or ALIS).

BTW^2, there are 3 scanning speeds to consider: the camera, the
display, and the eye tracking. Complicated stuff....

-- Jeroen

| From:    Jeroen H. Stessen                |
| Deptmt.: Philips Consumer Lifestyle       |
|          Innovation Lab Eindhoven         |
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Tom Barry                  trbarry@xxxxxxxxxxx  

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