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 About Digital Video |
Once the video frame is digitized, it can be accessed by software and saved to disk (a process called image capture or frame grabbing. The saving of the digitized video image to disk does take system resources, primarily in the formating of the raw video image into a standard format like BMP, TIFF, GIF or JPEG. The larger the video image, the longer it takes to convert the image to the standard format.
You can also create a digital movies with all Hauppauge digital video boards (except for the WinTV-Prism). A digital movie is simply a sequence of video frames which are stored to disk, rather than just one frame performed with frame grabbing. Video is transmitted at either 30 frames per second in North America or 25 frames per second in Europe and other parts of the world.
The WinTV-Celebrity, HighQ and CinemaPro use a method called analog chroma keying to mix the digitized video image with the VGA sceen image. They have a frame buffer to hold the digitized video image while it is being mixed with the VGA screen image. They also have hardware to resize the video image (called image scaling) and to put the video image in a window on your VGA screen. Since the hardware to do this is on the WinTV board, no system resources are used and your computer continues to run at full speed.
The WinCast and WinTV-pci boards move the digitized video image directly into the VGA memory so that the live vide can be seen on the VGA monitor. The resizing of the video image (called image scaling) is done on the WinCast/WinTVpci board when running in Primary Surface mode, or done by the VGA card when running in Overlay mode (see the WinCast Specification page for a description of Primary Surface and Overlay modes). No CPU resources are required during the movement of video from the WinCast/WinTVpci boards into the VGA memory, since these boards are PCI bus masters.
The VideoMagic and WinMotion60 move the video image into main system memory, where it is copied by software onto the VGA screen. This is called Preview mode, and results in a video picture which updates only 2-3 times per second. CPU resources are required to move the video during the Preview mode.
The WinTV Capture program creates digital video movies on your hard disk, combining a stream of video frames and audio together in one file. The video frames can have different sizes and frame rates, specified in the number of horizontal and vertical pixels saved and the number of frames per second saved. For example, a 320x240 video clip will have 320 horizontal pixels and 240 lines in each video frame. A video clip captured at 30 frames per second will have 30 digitazed video frames saved each second.
WinTV-Capture moves digitized video frames along with the captured audio (audio capture requires the use of a sound card) to disk in a special format created by Microsoft for digital movies, called the AVI format (audio video interleaved). This format specifies how the individual digital video frames are stored along with audio on disk. Click here to see how WinTV Capture program looks.
Your PC imposes limitations on the maximum image size and frame rate captured by WinTV-Capture. To get an idea on how much data must be saved (and then played back) by your PC, you can multiply the image size by the frame rate, and then divide by the compression ratio. For example, with the WinTV-CinemaPro:
320x240 x 2bytes per frame x 24 frames per second / 3 compression ratio = 1.2MBytes per second to hard disk
The WinCast/WinTVpci boards move raw YUV video (uncompressed) into the AVI file directly over the PCI bus. This gives excellent image quality and excellent performance, but consumes alot of hard disk space (4.5Mbytes per second when capturing 320x240 at 30 frames per second!). The performance is not limited by the speed of your PC's processor since the WinCast/WinTVpci are PCI bus masters, but the high data rates might exceed your hard disk's tranfer rate capacity.
With the WinTV-Celebrity, CinemaPro or HighQ, the video is compressed by the WinTV hardware in a 3:1 compression ratio and then transfered by your PC's processor into the AVI file. This gives very good image quality but requires data to be assembled by your PC's processor into the AVI format. Therefore if your PC's processor is slow, with the Celebrity, CinemaPro and HighQ you will not be able to process one digital image before the next one is digitized so you cannot capture 30 video frames per seconds. In general, a 90MHz Pentium can save a digital video movie at 15 frames per second if the image size is below a size of 320x240.
The VideoMagic and WinMotion60 can compress videos frames more than the Celebrity, CinemaPro and HighQ. With WinTV-Capture and the VideoMagic and WinMotion60, compression ratios can range up to 50:1. This lowers the data transfer requirements of your PC's processor, and therefore you can create larger video frames at higher frame rates.
If you want to capture full frame video movies to disk, you can use Hauppauge's Win/Motion60. This board has hardware motion JPEG compression to allow continuous video to be captured to disk in an AVI file format. The Win/Motion60 has compression ratios of between 6:1 to about 50:1. With compression ratios this high, the processor in your PC has less work to do and therefore can process full frame video (up to 640x480) continuously to disk. Note though, with higher compression ratios there is a loss of image qualtiy so it is best to still optimize your PC for video capture. There are some notes on optimizing your PC for video clip capture in the Tips section. Click here to see it now.
A popular question is "Why does my saved image have lines in it?" This normally happens when a fast moving image is frozen on top of a stationary background. This is due to the way video is broadcast. Video from most sources such as video camera, television and VCR is sent "interlaced", where a complete video image has a set of odd lines (called a "field") and a set of even lines. In the U.S. and other areas that use a 60 cycle line power, it takes 1/60 of a second to get the odd lines, then another 1/60 of a second to get the even lines. This means it takes 2/60 of a second (or 1/30 second) to get a complete video image. If the object that the video camera was pointing to moved during the 1/30 of a second it takes to send a complete video image, then on a frozen image you will see the position difference between the object in the odd field and even field. Click here to see an example of an object (the baseball player) that has moved between the odd field and the even field.
The reason you do not see this on home TV set is that a TV set does not freeze the TV picture. Also, TV sets are made with what is called a "slow phosphor", which smooths the image out, frame to frame.
Some WinTV boards have a control which can eliminate this effect, which is called "motion artifacts". The "Vertical Interpolation" mode, available on the high end WinTV-HighQ and the WinTV-Celebrity, can be enabled (in hardware so it is very fast) to throw away the even field and only use the odd field. The WinTV then recontructs the even field by averaging pixels above and below the line being reconstructed, hence the name "Vertical Interpolation". By displaying only the odd field, the "motion artifact" is eliminated!
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