## QMedia v1.5 | ## Introducing QuickTime

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## Time Management

Underlying all aspects of QuickTime operation is the notion of time.

Standard QuickTime movies have their own time coordinate system that is defined by the movie's time scale and duration. A time coordinate system provides the context for evaluating the passage of time in the movie. If you think of the time coordinate system as defining an axis for measuring time, it is only natural that this axis would be marked with a scale that defines a basic unit of measurement. That measurement system is the time scale.

A QuickTime time scale defines the number of time units that pass each second in a given time coordinate system. A time coordinate system that has a time scale of 1, measures time in seconds. Similarly, a time coordinate system that has a time scale of 60, measures sixtieths of a second. In general, each time unit in a time coordinate system is equal to (1/time scale) seconds. Some common time scales are:

Time scale | Absolute time measured
1 | Seconds
60 | Sixtieths of a second (4D ticks)
1000 | Milliseconds

A particular point in time in a time coordinate system is represented using a time value. A time value is expressed in terms of the time scale of its time coordinate system. Without an appropriate time scale, a time value is meaningless. For example, in a time coordinate system with a time scale of 300, a time value of 1200 translates to 4 seconds. All time coordinate systems tie back to absolute time (that is, time as we measure it in seconds).

Time coordinate systems have a finite maximum duration that defines the maximum time value for a time coordinate system (the minimum time value is always 0). Note that as a QuickTime movie is edited, the duration changes.

- Movies always start at time 0.
- The time scale specifies the number of time units in a second.
- The duration specifies the length of the movie, expressed in the movie's time units.