karlpet at mac.com
Fri Jun 28 12:25:01 EDT 2002
At 4:24 PM -0400 6/27/02, Bob Arnold wrote:
>This may be a Quicktime issue, but can someone explain the Timescale
>property to me?
Apple has tons of useful QuickTime documentation for developers at:
One of the intro pages has an overview of time concepts -- including
the time scale.
In part, the page says this:
"Time management is an important and sometimes complex part of the
implementation of QuickTime movies. For many movies, the correct play
rate is the rate at which human actions appear natural and objects
fall with normal acceleration. But what is the play rate of a movie
that shows spreadsheet data charted over time or a map of the earth
that recapitulates continental drift? This problem is deepened by the
differing clock speeds of various platforms and the need to
decompress data in real time, all of which affect time scales.
To manage the time dimension of movies, QuickTime defines time
coordinate systems, which anchor movies and their media data
structures to a common temporal reality, the second. A time
coordinate system contains a time scale that provides the translation
between real time and the apparent time in a movie. Time scales are
marked in time units. The number of units that pass per second
quantifies the scale--that is, a time scale of 26 means that 26 units
pass per second and each time unit is 1/26 of a second. A time
coordinate system also contains a duration, which is the length of a
movie or a media structure in terms of the number of time units it
contains. A particular point in a movie can then be identified by the
number of time units elapsed to that point. Each track in a movie
contains a time offset and a duration, which determine when the track
begins playing and for how long.
Each media structure has its own time scale, which determines its
number of samples per second. The Movie Toolbox maps each type of
media data from the movie's time coordinate system to the media
structure's time coordinate system."
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