Of the two Macintosh AV systems (the Centris/Quadra 660AV and the Quadra
840AV), the Quadra 840AV has better capture characteristics largely due to
the option to have an additional 1MB of VRAM installed. Given the fact, here
are the answers to your questions:
1) The black band is most likely the blanking part of the video frame.
The AV digitizer expects a certain number of lines and we've have seen
this occur on other video digitizers. The signal contains the blanking
information and is normally outside the display area of the typical
television monitors.
Due to the vast amount of history with the NTSC standard, there is some
"freedom" about exactly how blanking works. If the number of image
lines the AV digitizer is expecting is more than what the source
supplies, there will be visible blanking info at the bottom of the image
(or at times, on one of the edges).
When capturing still frames it is relatively easy to fix these captured
images using an application like Photoshop or Painter. Simply use their
tools to Crop out the unwanted area. While not an ideal solution, it
is only one available to eliminate this part of the display. Some video
capture applications provide a crop rectangle so that cropping can be
specified prior to the capture.
2) Based on what the AV expects in a video signal, it is possible to
over saturate the bright areas of an image. Over saturatation is easier
to introduce with S-video because you have a stronger signal available.
Choosing the VCR option (instead of the TV/video disc option) in the
video input control panel provides the best setting in such situations.
The best way to control the brightness is during the shooting of the
video. Keeping the contrast ratio as low as possible will provide the
best results.
3) There are two issues to consider in the area of video resolution
versus computer pixels. First, the output of the analog video device
does not always map directly to the pixel count of the digital device.
Second, concerns the number of pixels that are captured by the AV
system. Based on VRAM of 1MB, a true 640x480 image can not be captured.
A 640x480 image can be created, but it will use line-doubling from
single field to produce the full screen image. To get a full 640x480
image using both fields 2MB of VRAM is required. The 1MB limits the
dual field/undoubled line image to 496x372 pixels.
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Some additional thoughts to consider on single field versus single
frame, which uses both fields: If the capture image is 240 or less
lines vertical 1 field is dropped to scale the image. Two examples:
* First case, 1MB VRAM - if the capture image is greater than 240 lines
vertical or is equal to/less than 372 lines vertical, then both fields
are used. A scalar chip is used to scale the information from the two
fields.
* Second case, 2MB VRAM (840AV only) - if the capture image is greater
than 240 lines vertical, up to 480 lines vertical, both fields are
used. Again a scalar chip is used to scale both fields to the selected
image size.