TITLE
    Centris, Quadra 660AV: Video Capture Issues (12/93)
Article ID:
Created:
Modified: 		14300
12/14/93
12/14/93
TOPIC


    I'm attempting to capture quality screen snapshots using the Video Monitor
    application on my Centris 660AV. I'm experiencing the following strange
    effects:

    1) A black band line (about 3 pixels wide) appears at the bottom of the
    picture when taking screen snapshots with Video Monitor. This occurs
    when using both Composite In and S-Video In as the video source. Is
    there a work around?

    2) When capturing S-Video, I get an extremely bright picture when using the
    lowest video setting in S-Video. When using Composite Video In, it
    works fine. Is there any way to adjust the brightness beyond the Video
    Settings?

    3) I'm concerned that the Centris 660AV is not utilizing all of the
    resolution lines that my still camera outputs. The camera is capable
    of putting out 450 Resolution lines. Does the Centris 660AV utilize
    all of these lines. Customer is concerned that the Centris 660AV is
    digitizing with field resolution as opposed to frame resolution. How
    many of the resolution lines does the Centris 660AV utilize in the
    Video Monitor application?

DISCUSSION


    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.
    ╩
    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.


Document Information
Product Area: Computers
Category: Macintosh Quadra & Centris
Sub Category: Quadra/Centris 660AV

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