Copyright Michael Karbo, Denmark, Europe.


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    Chapter 19. Video on a computer

    Video­ recordings have to be able to be picked up by the computer. We have a video player (it can also be a video camera), which can pick up either analog or digital signals. These signals are picked up through one or several plugs, and there has, of course, to be corresponding plugs on the computer.

    Some video cameras can be directly connected to the computer with a single cable. Such as the small webcams, which often can’t work without being connected to a computer. In other models the connection can be more complicated.

    When the computer is connected to the video player, it is necessary to have software, which can get hold of the video signal. This process is called capturing. When the data stream has been captured by the program, you can process ”the film” if you want to. Afterwards you can store it in a suitable format.

     

    Video source

    Connection

    Webcam

    USB. The digital video signal is directly lead in the camera’s software.

    Digital camera

    USB or another interface. The films are processed by the camera’s software, and transferred as AVI files.

    Analog
    (e.g. VHS)

    Captured with a so-called grabber card, which converts analog signals to digital data.

    Digital
    (e.g. DV)

    Fetched directly in the computer via a FireWire cable

    Figure 120. The images have to be captured before the computer’s software can process the video recordings.

    So the first step is to capture the video film into the computer. Figure 120 shows the most usual ways of doing this. In the following sections, we are going to look closer at these forms of video and the software you can use.

    Image size and amount of data

    I have earlier mentioned that sound data takes a lot of room when the quality is good. And the problem doesn’t get any better at all, with video. Both sound and images have to be recorded here – and as most of us know, images need a lot of room! This is why it is necessary to think about the relation between quality, file size and compression.

    Video is often first recorded as ”raw data”, which is to say, there isn’t any compression, either of the sound or the image. The sound is digitised in a PCM format (see page 3). This means that the higher the sampling frequency we use, the better the sound quality will be. But the amount of data will be larger.

    The images are recorded with a determined frequency of, for example, 15 or 25 frames per second. Every image consists of a bit map, with a certain resolution. To get a good video quality, the image has to have a sizeable resolution and 25 images recorded per second – this gives a terrible lot of data.

    Compression is required for handling all this data, and a lot of compression if the video files are to be at all manageable. A practical example of this an be seen in the next section.

     

    Data type

    Parameters for file sizes

    Audio

    Sampling quaility (e.g. 8, 22 or 48 kHz).
    Compression.

    Image

    Image size (e.g. 352 x 288 or 480 x 576).
    Number of images (frames) per second.
    Compression

    Figure 121.The better the required quality, the larger the amount of data, and the more effective the compression, the smaller the file size.

    Low resolutions

    Luckily, relatively low resolutions are used for video images, because a television screen’s resolution is much worse than a modern computer’s.

    With so-called VHS quality we can make digital videos, which have an image quality corresponding to a television program, which has been recorded with a normal VHS tape recorder. The resolution is only 352 x 288 pixels. This is a very small image compared with what we are used to with digital cameras.

    The quality is much better with the digital video formats DV and Digital8, because the image size is strikingly higher (720 x 576 pixels).


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