Once again, this depends on the codecs involved. The QT Player uses the AVFoundation structure for playback. If there is a playback problem (i.e., the codecs, file container, or combination of the two is not supported), then the data is analyzed by the Modernizer structure to determine if it can be converted to compression formats natively supported by QT X. (I.e., the Modernizer checks to see if the your system is indeed configured with the required codecs for QT 7 use). If it can be converted (the required codecs are available), then the Converter structure is called to perform the actual conversion. Unfortunately, this doesn't always work correctly. For instance, under Mavericks, QT X with not attempt to convert a file containing DTS audio if Perian is not installed but will attempt the conversion if it is installed. Unhappily, following the conversion, the DTS sourced audio is still not playable. (Have yet to see if this has been corrected under El Capitan.) In most cases, the conversion is successful. However, in some cases the resulting quality may be much lower than expected due to the absence of user control and the quality of the source file.