TimeScale property

Karl Petersen karlpet at mac.com
Fri Jun 28 12:25:01 CDT 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:
http://developer.apple.com/quicktime/

One of the intro pages has an overview of time concepts -- including 
the time scale.
http://developer.apple.com/techpubs/quicktime/qtdevdocs/RM/frameset.htm

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."


Karl



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