An external parallel interface
An external USB/IEEE 1394 interfaceThe distinction between internal and external video capture devices may be blurred
by the fact that many capture cards use abreakout box, a separate piece of hardware that
attaches to the rear of the card and contains all of the connectors for interfacing with the
input device (video camera, VCR). Some video cards also double as video capture de-
vices, with varying capabilities.
Digital vs. Analog
Some video capture devices only accept an analog signal like that supplied by a legacy
camcorder or VCR through a Composite or S-Video input port. Digital video capture de-
vices use high-speed IEEE 1394 interfaces and accept data directly from digital video cam-
eras.Therearevideocapturecardsthatincludeacombinationofdigitalandanaloginputs.
CODEC
As with digital audio, the file sizes associated with digital video are huge. One second of
uncompressed, full-motion video and audio captured at 24-bit, 640 × 480 resolution will
take up approximately 30MB of disk space. Because of this, all video capture devices use
at least one compression method to reduce the amount of storage space required. The
compression method used has a direct bearing upon the applications for which the cap-
tured video can be used, so it should be considered carefully.
These are the most common compression schemes used by video capture devices:
MJPEG A motion video compression method based on the JPEG (Joint
Photographic Experts Group) still image compression method. MJPEG
(moving JPEG) is optimized for transfer to and from videotape but is used
less for multimedia and Internet applications because it requires specialized
hardware for playback. Image quality is high, but like most lossy schemes, the
quality varies with the amount of compression used.
MPEG-1 One of two common video compression schemes developed by
the Moving Pictures Experts Group. MPEG-1 is popular for multimedia and
Internet video because playback is software-based and file sizes can be reduced
while maintaining a good image quality.
DV Digital video (DV) is the compression method used by digital video cameras,
which perform their own compression during recording. DV capture cards connect
to digital cameras over an IEEE 1394 interface, which is able to transfer the digital
video at very high speeds with no signal loss.
MPEG-2 The newest compression scheme that supports image resolutions
up to four times higher than MPEG-1. MPEG-2 compression is scalable, so it
can be used for multimedia or Web-based applications with broadcast quality
video. Of course, higher data rates translate to larger file sizes.Chapter 21: Audio/Visual Devices^565