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Capture basics

Even though the RealCap software module is simple and easy to use many thing can go wrong in the capture process. Sometimes it is not on the user side and sometimes it is. To give you a better understanding of capturing video this small guide will explanation about the various aspects of the capturing process. Maybe this could solve the problem you might experience.

One of the most important issues in real-time video capturing is that the PC system is capable of storing the amount of data onto the harddrive. In full size and full speed (NTSC mode) the amount of data that has to be streamed onto the harddrive is approximately 27Mb (24bit color * 640 * 480 * 30 frames/sec). This amount of data can only be streamed onto a harddrive using very expensive equipment. Not only has the harddrive to be very fast but also the data bus on the mainboard. I know that today's PCI bus and harddrive with Ultra DMA is capable of up to 33Mb/sec of data through put. But it must be remembered that is this in extreme cases where the system does not interfere on the bus, which is hardly the case for any Windows operating system. The real through put on these system (in situation where the system has been optimized) is about 8-9Mb/sec which is still pretty much. In order to stream the video data onto the harddrive the amount of data much be reduced. In our system the reduction is done using Motion JPEG format. There is also the MPEG standard but unfortunately the equipment for full speed and size video capturing is still to expensive.
The Motion JPEG format is similar to the all known JPEG format which
is extensively used on the WEB for single images. The amount of compression can be varied depending on the capturing hardware. The Iomega Jaz Buz is capable of compressing from the normal size 900Kb/frame to as much as 4kb/frame i.e. 120K/sec or as less as 200kb/frame i.e. 6Mb/sec. Off course the image quality depends on the amount of compression. A higher compression means that you have to throw away a lot information in order to use less space for each image.
Choosing small compression the system will have to stream an amount of data to the harddrive which is close to the actual through put of the harddrive system. In cases where the harddrive has not been defracmentated (cleaned up) for some time the head of the drive will have to jump around on the drive to find space for the video data. The RealCap module uses a preallocated data which has been created contiguously. Meaning that the head of the harddrive does not have to move to much jumping in order to store the video data on the drive.

When displaying the video data on the monitor there is two options that can do the trick. First option is called preview which means that the computer forcibly extracts a video image from the video stream each .5 sec and renders the image on the screen/monitor. This is off course time consuming because the processor is part of the whole process. Also when searching on the video it is not always easy to find the location from where you want to capture using only 2 Images/sec. The second option is called overlay mode and is a special no processor depended operation where the capture card is capable of streaming the video data directly to the display card memory. This is a very powerful option and leaves a lot of idle processor time for other programs and the current running application - The RealCap.
In order to have this overlay mode it is important that the graphics adapter in the computer has enough video memory and is capable of running a refresh rate that is high enough. Also the graphics card must support the DirectX technology from Microsoft. Using the Iomega Jaz Buz there has been reported some difficulties using various graphical adapters. Please check the following link to ensure that your hardware is compatible with the Iomega Jaz Buz:

wpe2.jpg (3403 bytes) Iomega Hardware compatibility!

After the video has been captured it is unlikely that the start position of the captured video will mach any other captured video sequence from another angle/view. In 3D biomechanical analysis it is required to use more than 2 view is order for transform image coordinates to spatial coordinates. Even if taking advantage of VCR synchronization the VCR mechanism will not always respond with the same time delay. This means that the start position will have to be modified/trimmed in order for them to correspond. After the video sequence has been captured the RealCap proceeds directly to a trimming process which enables you to modify the video sequence, selecting a synchronization image and the amount of pre and post images.


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This page was last modified on 12/07/2008 at 22:47 PST. Copyright � 1994 - 2002, all rights reserved, Ariel Dynamics Inc. Please send your comments or feedback to or proceed to our feedback form. This page has been accessed many times since Dec 12, 2002. Our privacy policy is here.