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Building a Digital Video Capture System - Part I

Let's Face It, Video Is Expensive

So even though prices have come way down over the years a reasonably good consumer-level video capture board will cost you between $250 and $900. And unfortunately, that's just the beginning.

As I pointed out earlier, video requires a lot of bandwidth and storage space . While it is possible to do video work with an EIDE drive you will be pushing the limits of the bus and the hard drive. Just about every video professional or serious hobbyist I know uses an Ultra SCSI 2 controller and drives...big, big drives.

I personally like the Adaptec AHA 2940UW, (even though it's a bit long in the tooth) and 5,000 to 7,500rpm IBM or Seagate Cheetah drives . There are two sidenotes here. First, not all SCSI interface controllers are created equal. Some of the less expensive controllers don't play nice with the system bus. They basically take control of the entire bus and can choke off the rest of your system. While video capture is 99% a matter of capture card, bus, controller, and hard drive talking to each other there are still a few functions that require at least some CPU and memory system bus access. Your SCSI controller and drives may be able to handle the video stream but could still end up waiting for a bus-starved CPU to tell if you clicked the mouse somewhere. I have also seen motherboards with lousy system busses that prevent the SCSI controller from achieving its full potential.

Second, hard drives (and file systems) are designed for lots of little reads and writes with time to rest in-between. They were not designed to handle very, very large files that absolutely must have continuous, uninterrupted data transfers both to and from the drive. Because of this many hard disk drives use a caching system to boost speed. Unfortunately, those caches are too small to do any good when you're trying to save or read a 10 GB video file. If you find that you are dropping frames while capturing (the most common problem) then you might have to disable the hard drive's buffering.

Also, some hard drives perform a thermal recalibration every few seconds. It only takes an instant but it can be enough to cause dropped frames. Newer drives and so called "AV drives" don't have this problem but you should be aware that your old drives might not be up to the strain of video capture.

Lastly, depending on the version of your file system you may hit a 2 GB limit on file sizes. The FAT-16 (file allocation table) uses a linked list of 16 bit values to keep track of clusters, the minimum amounts of disk space the operating system will allocate. The maximum number of such entries (65,536) multiplied by the maximum size of a cluster (32,768 Bytes) is 2,147,483,648 Bytes, or 2 GBLike so many things in computer design, programmers of the original FAT file system structures never dreamed that anyone would ever want to (or be able to) store files over 2 GB. This problem occures in PC FAT 16 (or lower), Linux NFS, and Apple Macintosh (prior to OS X) but not in NTFS or FAT 32 (although problems can still occur). Most capture systems and accompanying software will offer a workaround to this problem but not all. If you find that you can't capture sequences longer than a few minutes (or a few seconds depending on your compression settings) this limit might be your problem and you might consider moving up to FAT-32 or NTFS.