Re: Canopus ADVC300 Issue/Question (too long answer :-)


"Nate" <nnord@xxxxxxxxxx> wrote in message
news:1146054830.889565.284750@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
When converting VHS tapes to AVI with my ADVC300, I notice horizontal
lines (kind of breaks up a little) whenever the camera pans or there's
rapid movement in the shot. For example, in one scene, my son is
feeding seagulls that are hovering above him... the gulls wings have
the lines and appear to be breaking up. It's not terrible but I
definitely notice it. Is this normal? Any idea what this may be?

Thanks in advance.

If you are talking about interlace artifacts, then yes it's normal.

Standard video is interlaced. In the US, and other places that
use NTSC video, there are 60 images displayed per second.
We call them fields. These images occupy every other line.
It takes 2 of these images to make a frame. In Europe and
many other parts of the world they use 50 fields per second.

Most computer software can only display frames (2 fields each)
rather than displaying the fields sequentially the way television
is intended to be viewed.

The images are each taken at separate times. If nothing is moving,
then a video frame looks fine displayed on a computer. Unfortunately,
things that are moving will look broken up because you see 2 fields
at a time on the computer. the 2 fields do not match up because
they are recorded at slightly different times.

Why would they do this? There are several good reasons. If you display
images at much less than 50 per second, the viewer will perceive a
flickering effect. Another issue is perceived motion. You have to change
the images at more than 20 times a second for the motion to appear smooth.

In the past televisions scanned an electron beam across a plane of
phosphors that glow when the beam hits them, but then they fade away.
If television had been created as 30 full images per second, then the
top of the image would be completely faded out by the time the scan
got toward the bottom of the frame and you would definitely see a
flickering picture.It would have been difficult to display 40 or more
images of decent resolution each second with the available technology,
Plus, TVs used to be synchronized to the powerline frequency (60cps
in the US, 50 in Europe).What they could do 60 of per second was too
low of a resolution to be acceptable.

The compromise was to do the lower res images at 60 per second,
but to offset them slightly so that they would be interlaced. Odd lines
are one field, and even lines were the other field. When things didn't
move you got what appeared to the eye as twice the resolution of the
actual fields. It was a brilliant solution. In fact it was much better than
24 frame per second film in terms of being able to reproduce
smooth motion.

So, what about film? It only displays a pathetic 24 images per second,
yet most people can't detect any flickering, and the motion appears
OK for the most part. The flicker fusion is handled by splitting the shutter
of the projector so that it displays 2 or more flashes per frame thus
upping the flicker rate. The low frame rate is compensated for by the
natural motion blur. Anything that moves in film gets blurry. This helps
smooth out the motion a little. Beyond that cinematographers know
how to minimize the motion by the way they shoot it. Most often they
will adjust the shot so that the motion is moving away from or toward
the camera, or they will try to keep the moving object relatively stable
in the frame and let the background go blurry. If you go to a camera
store that sells video cameras, try setting the camera to 24 fps
(progressive mode), and then pan the camera fairly fast. You will
see a jittery picture. Now switch to a normal video mode and make
the same move. The motion will be substantially smother.

Back to the computer. Because you see both fields at the same
time you will see that combing effect. If you dump it to DVD and
watch it on a television, that effect will go away. If you intend to
mostly watch it on a computer, then you may want to remove the
interlacing. Unfortunately that means throwing away every other
field and thus chucking half of your image resolution, as well as half
of your temporal resolution.This effects the smoothness of the motion.
There are a few programs that are smart enough to keep all of the
detail on things that don't move, and only deinterlace the parts of the
picture where motion is occurring. There are even programs
(proprietary as far as I know) that will interpret the interlace information
to something like motion blur. This technique is sometimes done
when converting video to film.

David


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