There is only one file type that uses the .TTXT extension:
MPEG-4 Timed Text Subtitles File
Movie subtitles file created in the MPEG-4 Timed Text (TTXT) format, which is used for MPEG-4 and 3GP videos and mobile phone movies; uses an XML format and supports textual positioning and styles as well as timing precisions of 1 millisecond.
Timed Text subtitles are often mixed within the video file instead of being delivered as a separate ".ttxt" file.
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Updated: November 16, 2012