TiLT Media Production
  temporal and spatial compression

Frames per second,
sound sample

Video file formats

Temporal and spatial
compression

Streaming video

Video capture boards,
FireWire, etc.

Premiere and other
software

Hands-on lesson

 

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 of QuickTime).

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).