This tool lets you check and correct GOP time code errors, audio PTS
(presentation time stamp) errors, and DVD GOP size violations, which
may be present in an MPEG data file.
GOP Time Code Error
In MPEG compression, a video sequence is divided into groups of image frames.
A typical example is coded pictures of "I B B P B B P B B P B B P B B".
This is called a GOP (group of pictures). A header is usually stored
before the I-picture, and is called a GOP header, and it’s usually an
8-byte data segment. One of the important information stored in this
header is a 25-bit integer, called a SMPTE time code (SMPTE: Society
of Motion Picture and Television Engineers). This time code refers to
the first picture of the GOP in display order.
This GOP header and its time code provide very convenient points for managing
a coded MPEG movie. It allows our MPEG editor to randomly access a
lengthy MPEG movie very efficiently. However, this also makes the
editor highly dependent on the accuracy of time codes in the GOP
header, especially dependent on the continuity of the time codes
in a complete MPEG movie. Unfortunately, an MPEG movie may have errors
in the time code, or the time code may be disrupted, or the time code
may not correctly reflect the coded image sequence. When any of these
happens to an MPEG file, we consider it a GOP time code error.
The GOP fixer tool will correct the GOP time code errors based on its
calculation from the complete video sequence. It will only modify those
errorous 25-bit integers.
Audio PTS Error
Audio Presentation Time Stamps are 33-bit integers inserted in an MPEG
stream to allow an MPEG decoder to synchronize the decoded audio sound
with the decoded video image display. Those PTS values should match
the playback time length of the decoded audio sound. When they are
in error, the MPEG movie cannot be played correctly, and the audio and
the video will be out of sync.
The GOP fixer tool will correct the audio PTS errors based on its
calculation from the complete audio data. It will only modify those
errorous 33-bit integers.
DVD GOP Size Violation
The DVD standard has special restrictions on the size of a GOP for
a movie to be compressed into MPEG. Specifically, it requires that no
GOP should have more than 18 frames for NTSC TV system and 15 frames
for PAL TV system, even though the MPEG standards have no
To make an MPEG file compliant with the DVD restriction on GOP
size, the GOP fixer tool will find the violator and break it into
multiple GOP’s of small sizes. For other GOP’s, it will simply copy over.