Since digital video is both a sequence of images over time and (sometimes)
an audio track, it is not surprising that file sizes can grow very rapidly.
Compression is a must. Most video capture and editing software allows
the user to choose to compress more than once: while digitizing, perhaps
also while editing, and again when saving the final result. Since each
compression potentially could bring a loss of quality, it is best to
save compression for the last step. If possible, digitize and edit your
video with little or no compression.
Digital video compression uses two techniques: spatial and temporal
compression. Spatial compression was introduced in the section on digital
images. It involves removing or reordering information about a field
of colored pixels to conserve file space. Temporal compression, as the
name might suggest, operates across time. It compares one still frame
with an adjoining frame and, instead of saving all the information about
each frame into the digital video file, only saves information about
the differences between frames (frame differencing). This type of compression
relies on periodic keyframes.
At each keyframe, the entire still image is saved, and these complete
pictures are used as the comparison frames for frame differencing.
Another key concept introduced when compressing digital video (but
also used with other time-based media, such as audio) is data rate.
Data rate refers to the speed at which information must be read from
the delivery source (a file off of a hard drive, a file from a CD-ROM,
a file over the network, etc). When compressing video, most applications
allow the user to specify a data
rate, so that playback will be smooth for users on, for example,
a 56 Kb/second modem.
Some commonly used digital video codecs are:
MPEG: both a file format and a compression algorithm. Generally
requires higher-end networks and clients. MPEG-1 designed for data rates
of up to 1.5Mb/s and can handle 30fps video at 352x288 window size.
Sorenson: built in to QuickTime 3.0. Best for video under 100kB/s
data rate and 320x240 window. Asynchronous
compression (slow to compress, decompresses for playback quickly). There
is a regular and a Professional version of the Sorenson codec. Not supported
in older versions of QuickTime.
Cinepak: designed for multimedia CD-ROM use, Cinepak was also
the most commonly used WWW-video codec before Sorenson was released
in 1997. OK for intermediate data rates (30kB/s - 100kB/s), not good
for higher or lower. Image quality inferior to Sorenson's. Asynchronous
compression. Best choice for backwards compatibility (older versions
Indeo4 : very nice image quality, but not supported on the MacOS.
(older versions of Indeo are, but are no longer supported in QuickTime3
for the PC). Available in QuickTime for Windows, AVI (Video for Windows).