How Does a Video Capture Card Work?

Video capture cards come in two main types: internal circuit boards you install inside your computer and external boxes that connect via USB or another interface. Internal cards plug into standard PCI and PCI Express slots on a desktop computer's motherboard. Both types of video capture devices have one or more input jacks that accept analog or digital video. Computer chips on the card process the video input and send a stream of data through the PCI slot or USB connector.

The video capture card's input jacks take in video from a variety of sources. Depending on the card, it may be digital-only or it may have inputs for older analog sources such as DVD players. For example, RCA and S-video jacks support analog video; an HDMI connector accepts high-definition video from digital sources. Some cards have tuner circuits that select channels from an antenna or broadband cable TV signal. Using a video capture card with a tuner, you can watch TV shows in one window on your computer screen while checking your email in another window.

A specialized high-speed chip on the capture card analyzes the incoming video signal, turning it into a stream of digital images. The source largely determines the quality of the image and frame rate; generally, digital sources produce higher-resolution images at faster rates. Because analog signals have lower quality to begin with, a capture card typically processes these at a VGA-standard 640-by-480 resolution at 30 frames per second. HD signals have more picture information; the 1080p standard, for example, has a resolution of 1,920 by 1,080 pixels at up to 60 frames per second.

Once the video is converted into data, the processor chip on the capture card stores the video images into a memory area called a buffer. The buffer acts as a reservoir, keeping a certain amount of video data ready to send to the PC. The capture card may produce more data than the computer can handle at a given moment, so the buffer helps the PC stay in sync with incoming video images.

The video capture card manufacturer provides software called drivers along with the card itself. When you install the capture card, Windows automatically loads the drivers, which become part of the computer's operating system. The drivers enable Windows to recognize the card and handle the data coming from it. To use the card, you connect it to a video source and start a video editing program. The program displays the video on your computer screen and helps you add, remove or rearrange scenes and then save the results to a file on the hard drive.