The master clock used for item timebases.
- iOS 6.0+
- macOS 10.8+
- tvOS 9.0+
The default value of this property is
NULL, which means that the master clock is automatically chosen. When non-
NULL, this property overrides the automatic choice of master clock for item timebases. This is most useful when you are synchronizing video-only movies with audio played by another source.