Less expensive video capture boards use a two-step process, because they don't have the processing power to compress the video as it is captured. In the first step, these boards capture and store the raw digitized video information on a hard disk. In the second step, a software routine compresses this file off-line. This two-step process requires disk space not only to hold the uncompressed video file, but also the compressed file (for a total of 45 to 60 MB for one minute of video in a 160 x 120 window). In addition, the off-line software compression step takes a lot of extra time--as much as 15 to 30 minutes for every minute of video. During that time, the PC is completely dedicated to compression, and cannot be used for other work.
Newer compression technologies use a combination of software and hardware to compress video much more quickly in a single-step approach. This single-step process offers important advantages. It minimizes the cost of PC video because it uses far less hard disk space (9 MB versus 45 to 60 MB for one minute of video). This often eliminates the need to add large and fast hard disk capacity to your PC. It also offers convenience and time savings because there is no lengthy compression process involved.
Single-step capture and compression has an important advantage in that video quality is not sensitive to the hardware configuration as it is in two-step capture and compression. With two-step capture and compression, all uncompressed video data must be transferred directly to the hard disk. In many configurations, the speed of the microprocessor and hard disk cannot keep pace with the large volume of incoming data. The system becomes overloaded and begins to "drop" frames. So, you may not be able to achieve the full frame rate (quality) that is specified for the video capture board. For example, a two-step video capture board rated at 30 frames per second, may in reality capture far fewer frames per second on less powerful hardware configurations.
One-step capture and compression, on the other hand, compresses video data before it is transferred to the hard disk. As a result, the amount of data that the hard disk must handle is dramatically reduced--by a factor of up to 6 to 1. Thus, few frames, if any, are "dropped" during capture. A single-step video capture board rated at 30 frames per second will deliver 30 frames per second even on less powerful system configurations.