Preview Time is the point in a movie at which its preview begins. A movie preview is a very short version of the movie, typically less than 5 seconds long.
The file defines this preview with 2 values, a start and a length. Preview Time is the start, and Preview Duration is the length. A flag on each track says whether the preview uses that track.
Which files carry it
Preview Time sits in the QuickTime movie header, which describes the whole movie. The header also holds the movie's time scale and its Duration.
QuickTime calls this part of the file an atom with the type mvhd. The MP4 format has a header of the same type and calls it a box.
The metadata reader ExifTool reads this header in QuickTime files and in the formats it calls QuickTime-based. These files, for example, can show Preview Time:
- MOV and QT, the Apple QuickTime movie format
- MP4, the MPEG-4 file format
- M4A, M4B, M4P, and M4V audio and video files
- 3GP and 3GPP audio and video files
- F4A, F4B, F4P, and F4V files for Adobe Flash Player
- AAX, the Audible Enhanced Audiobook format
So an AAX audiobook can show Preview Time even though it is not a video. AVI files are different. ExifTool describes AVI as RIFF-based, and its lists of AVI values do not include Preview Time.
What the value looks like
Inside the file, Preview Time is a 4-byte number. The number counts units of the movie's time scale, not seconds.
The time scale is the number of units per second. With a time scale of 60, 60 units make 1 second.
ExifTool divides the stored number by the time scale. It then shows the result as a time, in one of 3 forms:
0 swhen the stored number is 0- seconds with 2 decimal places and the letter s, for a time under 30 seconds
- hours, minutes, and seconds separated by colons, for a time of 30 seconds or more
Where the value comes from
The software that writes the movie file sets Preview Time. Apple's QuickTime documentation describes a function that lets an application define the preview's start and length.
MP4 files are based on the standard ISO/IEC 14496-12. This standard names the same place in the header pre_defined and sets it to 0. A metadata reader can still show these bytes as Preview Time, so an MP4 file usually shows 0 s.
Some software writes 0 here. FFmpeg writes 0 into this place and labels it as reserved. The MP4 writer in the Chromium browser project also writes 0.
FFmpeg also skips the value when it reads a file. A report that depends only on FFmpeg therefore has no Preview Time row.
Why it matters for your file
Read Preview Time together with Preview Duration. A value of 0 s alone does not tell you whether the movie has a preview.
In Apple's QuickTime documentation, a function returns 0 for both values when a movie has no preview. Apple's sample code also tests for a duration above 0 to decide whether a movie has a preview. So if Preview Duration is also 0 s, the movie has no preview.
How it differs from similar values
Other values sit next to Preview Time or have a similar name. Each one answers a different question:
- Preview Duration is the length of the preview, in the same time scale units.
- Poster Time is the time of the movie poster, a still frame that represents the movie.
- Preview Image Size is the width and height of a preview picture inside a file, not a time.