Sources for video capture normally include VCRs and camcorders. Some capture cards include an RF demodulator and a TV tuner so that video can be captured from a television broadcast signal or a cable TV input.

The output from these video-producing devices tends to be composite TV or analog S-video signals. A video decoder circuit is used to convert the analog signal into a stream of digital signals. However, these are not the RGB digital signals useful to the VGA card. The characteristics of the decoded TV signal are defined in television industry terms as YUV. The Y portion of the term refers to the luminance of the signal color, and the UV portion describes the color component of the signal.

The digitized output from the A-to-D converter is applied to a video compression Application Specific Integrated Circuit (ASIC). The compression chip reduces the size of the file by removing redundant information from consecutive frames. This reduction is necessary due to the extreme size of typical digitized video files. Video-compression schemes can reduce the size of a video file by a ratio of up to 200:1. As the sections of video are compressed, the reduced files may be applied to the system RAM memory or may be routed directly to the hard-disk drive. The audio signal is not compressed, but it is synchronized to the video signal so that it will play in the right places when the video is run.

When the digitized video is recalled for output purposes, the file is reapplied to the compression chip, which restores the redundant information to the frames. The output from the compression chip is applied to the digital-to-analog portion of the video-processing circuitry. The analog signals are converted back into the proper VGA format and are applied to the video-out connector at the back plate of the card.