APPENDIX B
350
Compressing Video and Audio
Lossless and lossy compression
Some codecs use
lossless
compression, which ensures that all of the information in the original
clip is preserved after compression. This maintains the full quality of the original, which makes
lossless compression useful for final-cut editing or moving clips between systems. However,
preserving the original level of quality limits the degree to which you can lower the data rate
and file size, and the resulting data rate may be too high for smooth playback on many systems.
Other compression methods discard some of the original data during compression. This is
called
lossy
compression. For example, if the pixels making up a sky actually contain 78 shades
of blue, a lossy codec set for less-than-best quality may record 60 shades of blue. Lossy codecs
usually let you specify how much picture quality you want to trade to lower the data rate and
file size so that you can tailor playback for your audience. Lossy compression allows much
lower data rates and file sizes than lossless compression, so lossy codecs are commonly used for
final production of video delivered using CD-ROM or the Internet. Some codecs are always
lossy, such as JPEG, or always lossless, such as Planar RGB. Other codecs may or may not be
lossy, usually depending on the settings you specify for the Quality and Data Rate options—
lowering the value for these options saves more space by discarding more data.
Asymmetrical and symmetrical compression
The codec you choose affects your production workflow, not just in file size or playback speed,
but in the time required for a codec to compress a given number of frames. Fast compression
helps video production, and fast decompression makes viewing easier, but many codecs take
far more time to compress frames than to decompress them during playback. This is why a 30-
second program may take a few minutes to process before playback. Compressing video is like
packing a suitcase—you can pack as fast as you unpack by simply throwing clothes into the
suitcase, but if you spend more time to fold and organize the clothes in the suitcase, you can fit
more clothes in the same space.
Similarly, different codecs require various amounts of time to compress or decompress video.
A codec is considered
symmetrical
when it requires the same amount of time to compress as to
decompress a clip. A codec is
asymmetrical
when the times required to compress and decom-
press a clip are significantly different. For example, the Cinepak asymmetrical codec decom-
c00.book for PS Page 350 Tuesday, March 31, 1998 1:28 PM
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